transition metal catalyzed reactions of carbohydrates: a .../67531/metadc...transition metal...
Post on 09-Jun-2020
11 Views
Preview:
TRANSCRIPT
+
Preliminary Draft of Chapter for “Green Chemistry” Book – 12/16/96h BNL-63763
Transition Metal Catalyzed Reactions of Carbohydrates:
A Nonoxidative Approach to Oxygenated Organics
Mark Andrews, Chemistry Department
Brookhaven National Laboratory, Upton, NY 11973-5000
TABLE OF CONTENTS
I - Background and Introduction ................................ ........................................ 2
Oxygenated Organics: Problems with Hydrocarbon Oxidation Approaches .....2
Oxygenated Organics: Advantages of Non-Oxidative Biomass Approaches .....3
Oxygenated Organics from Biomass: Methods and Research Opportunities ....4
Carbohydrate Catalysis Research Needs ................................ .............................5
II - Progress to Date . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . **o* . . . . ..*.. *o**oo.. *.. o* . . . . . . . . . . . . ..o . . . ..o* . . ..*oo . . ..o... o ‘7
Prior Literature ................................ ................................................................ ......7
Aldose Decarbonylation ................................ ......................................................... 8
Catalytic Hydrocracking of Sugars ................................ ....................................... 9
Pt and Pd Diolate and Alditolate Complexes ................................ ....................... 10
Catalytic Diol Deoxydehydration ................................ .......................................... 14
III - Opportunities for the Future ................................ ....................................... 15
Complexation of Cyclic and Disaccharide Sugars/ Other Metals ....................... 15
Development of Catalytic Polyol Disproportionation ................................ ........... 18
Development of Catalytic Carbohydrate Ionic Hydrogenolysis ........................... 19
Development of Aqueous Organometallic Carbohydrate Chemistry ..................22
N- Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
N- Bibliography *.. *... *.. o.*.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ●..0. 24
1
I - BACKGROUND AND INTRODUCTION
Oxygenated Organics: Problems with Hydrocarbon Oxidation
A prime target for new, environmentally benign,
Approaches
organic synthesis
technologies is the production ofoxygenated organics, key compounds in the fuel,
commodity, andspecialty chemical markets.1 Most of these products are currently
derived from the partial air oxidation of petroleum or natural gas fossil feedstocks.
While hydrocarbons and oxygen are presently cheap raw materials, their utilization
poses long-standing, recalcitrant problems, many with unfavorable environmental
repercussions. The following commercial example is illustrative. Adipic acid, a two
billion lb/yr nylon precursor, is currently produced2’3 by a multi-step process which
begins with the hydrogenation of carcinogenic benzene to cyclohexane, followed by
catalytic air-oxidation to give a cyclohexanone / cyclohexanol mixture in about 75%
yield at 5% conversion. After recycle of unreacted starting material by distillation,
this ketone-alcohol mixture must be further oxidized with corrosive nitric acid to
produce not only the desired adipic acid, but nitrogen oxides, which are only
partially recyclable. Thus, the overall process involves a reduction, two oxidations,
and a distillation, and generates several environmentally hazardous waste streams.
Furthermore, the air oxidation of organic materials almost always involves
potentially explosive conditions, which require expensive, specially designed reactor
systems to account for this latent safety/environmental problem.
The fundamental problematic issues, which adversely affect not only process
economics but the environment and public perception, are summarized in
Highlight 1. While solutions to some of these problems may come from current
research efforts, other problems are inherent in the approach itself. For this reason,
we believe that alternative “green chemistry” strategies merit serious exploration.
DISCLAIMER
This report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products. Images are
produced from the best available original
document.
Highlight 1. Fundamental Issues in Hydrocarbon Oxidation
Technological Issues
● The chemical inertness of hydrocarbons leads to high reaction activation
energies, often resulting in poor selectivities due to greaterreactivity of primary oxidation products.
● Poor air-oxidation selectivity wastes valuable feedstock.● Other oxidants, while frequently more selective, are much more
expensive or, if produced from oxygen (e.g., R02H), generatestoichiometric co-products when used.
● Oxidations with air or peroxides pose explosion hazards.
Soc ietal Issues
● Hydrocarbons are a non-renewable, fossil resource.
● Crude oil production is fraught with political and environmental concerns.● Transportation of natural gas and petroleum products poses safety and
environmental hazards.● Poor air-oxidation selectivity generates significant toxic waste streams.
Oxygenated Organics: Advantages of Non-Oxidative Biomass Approaches
One such alternative approach is the production of oxygenated organics from
biomass. Biomass carbohydrates are not only our most abundant organic feedstock,
but they are renewable and “pre-oxygenated”. This leads to a complete shift in
strategies for producing oxygenated organics compared to hydrocarbon oxidation.
The objective now generally becomes the partial removal or redistribution of the
oxygen already present in these feedstocks, rather than its problematic addition.
Advantages of this non-oxidative approach are summarized in Highlight 2. The
societal advantages of a biomass approach are particularly significant, as these
issues are ever-growing contributors to governmental regulations and industrial
decision-making considerations. In particular, environmental costs can radically
change the overall economics of a process, such that feedstock prices no longer
dominate the equation. While there are potential
associated with biomass production, such as monocrop
environmental concerns
plantings and the use of
3
.
+
fertilizers, herbicides, and pesticides, these issues should be more tractable than
those associated with fossil feedstocks.
Highlight 2. Advantages of Biomass Route to Oxygenated Organics
Technological Advantaae~
● Rich functionality in biomass fosters low reaction activation energies,
permitting mild reaction conditions and hence lower capital andoperating costs.
● Hydrogen-bonding characteristics of carbohydrates should be veryhelpful in promoting highly selective reactions.
● Conversion processes will primarily involve reductive reactions, which
are typically more selective than are oxidative reactions.
I Adv~
● Biomass is an abundant, widespread, sustainable resource.● Production and transportation of biomass are relatively safe and
environmentally friendly.● Biomass process solvents and waste streams are likely to be relatively
biocompatible.
Oxygenated Organics from Biomass: Methods and Research Opportunities
There are two major traditional approaches to biomass conversion
(Highlight 3), fermentation and high-temperature pyrolysis.4 “We believe that both
of these will be significant contributors to future oxygenated organics technologies.
While both approaches can effectively utilize raw, multi-component biomass, they
sufTer from limitations on the range of products that can be obtained and require
substantial energy input. Recently, we have proposed a new strategy based on
homogeneous transition-metal catalysis,5-7 a technology that has become
increasingly significant in the latter part of this century. 2
1----—Highlight 3. Biomass Conversion Strategies
● Fermentation● High-temperature pyrolysis* Homogeneous transition-metal catalysis
4
.
.
Our approach recognizes that the high selectivity and reactivity of homogeneous
transition-metal catalysts are an excellent match to the rich and fragile
functionalities (polyhydroxyl, carbonyl, and acetal) present in biomass
carbohydrates. As such, homogeneous catalysis should be an effective means of
converting specific carbohydrates, obtainable from sugar-rich crops and biomass
fermentation, into valuable oxygenated organics. The potential advantages of a
homogeneous catalysis approach are its greater product versatility and lower energy
requirements compared to pyrolysis and straight fermentation. At present,
however, the design of appropriate catalytic conversion systems is strongly inhibited
by a lack of information on the interactions of native carbohydrates with
organometallic homogeneous catalysts. We believe that this situation presents a
unique basic research opportunity. Highlight 4 gives illustrate ve examples of the
types of carbohydrate conversions that we have begun to explore that might be
feasible using transition-metal catalysis:
Highlight 4. Potential Metal-Catalyzed Carbohydrate Conversions
● Hydrocracking to glycerol, ethylene glycol, and methanol.
● Deoxydehydration to allylic alcohols, e.g., xylitol to pentadienol.● Disproportionation to unsaturated aldehydes or acids, e.g., glycerol to
acrolei n.● Dehydroxylation to cx,~-diols and derived compounds, e.g. glucose to
adipic acid, erythritol to tetrahydrofuran.
Carbohydrate Catalysis Research Needs
Highlight 5 gives the fundamental objectives identified as being essential to
effective attainment of transition-metal catalyzed transformations of carbohydrates.
While our primary focus has been on exploring and understanding the interactions
of organotransition metal complexes with carbohydrates, we have also found it
advantageous to model these interactions by studying the simpler case of metal–diol
5
Highlight 5. Fundamental Metal–Carbohydrate Research Objectives
● Synthesis and characterization of organometallic carbohydrate anddiolate complexes as models for catalytic intermediates.
. Understanding factors affecting complexation stabilities and selectivities.● Exploring reactivities of carbohydrate/diolate complexes.● Determining solvent effects on complexation and reaction chemistries.● Discovering catalytic cycles to effect carbohydrate/diol redox chemistry.
chemistry.s’g
concerns here
Solvent issues are also a significant secondary focal point.
include simple volubility problems, interference with catalysis
The
from
solvent coordination via donor atoms or hydroxyl groups, effects of solvent
hydrogen-bonding interactions, and product isolation problems. Some of the
carbohydrate redox transformations and catalytic intermediates that we envision
and will describe later include the following:
/
LmM’H +Hz
+H2 / –H20
r
RH2C-CH2R
~1HO R
I
LmM’H +H2
HO R2 RCH20H
.
.
For sugars, R = CH20H, etc.
6
.
II - PROGRESS TO DATE
Prior Literature
One of the first reported homogeneously catalyzed reactions of carbohydrates
was by Kruse of ICI in 1976, the RuHCl(PPhs )3 mediated hydrogenation of aldose
and ketose sugars to the corresponding alditols in amide solvents: I“’ll
0“H o
HOH
OHH—HO
H
H, ~0c
H
*
OH
HO H
H OH
A :,~’owfip:t:Kruse(ICI) GLUCOSE
H2*
RUHCL3
CHZOH
H
$
OH
HO H
H OH
H OH
CHZOH
GLUCITOL
Note that, despite the small proportion of open-chain aldehydo-glucose present in
solution, 12 the metal catalyst is capable of hydrogenating the “latent” carbonyl
group. Prior to our studies, there were also several related reports of catalytic
transfer hydrogenation and disproportionation reactions of sugars, 13-1* and one
example of an isolated organometallic carbohydrate complex, (1,2-
CGHA(P*P~e)2)Pt(CsHsOs),19 derived from the simplest possible carbohydrate,
glycerol. While diastereoselectivity is observed in the formation of this complex,
complexation of glycerol poses minimal regioselectivity considerations (only 1,2- vs
1,3-) compared to up to 15 possibilities in an asymmetric C fj alditol. In the reported
study, only 1,2-glycerol complexation was observed:
.
Aldose Decarbonylation
Our first work in metal-mediated carbohydrate chemistry involved a novel
approach to carbohydrate “descent of series” chemistry, 20 the decarbonylation of
aldose sugars (e.g., glucose, R = H), by Wilkinson’s catalyst, Rh(PPhJsCl: 21’22
7H20H H. Oc+ $H20H
H+
OH CH20H
tH20H +CH20H
Q
oRhCl(CO)L2
HO ~H3
~= OH NOSolvent = u (NMP)
OHAndrews,Klaeren&Goul(
While the reaction is not catalytic nor particularly suited to the production of
oxygenated organics, the study confirmed Kruse’s early observation 1 that amide
solvents, such as lV-methyl-2-pyrrolidinone (NMP), are an effective medium for
conducting organometallic transformations of native carbohydrates. Again, the
metal reagent has the notable ability to selectively react with the tiny fraction
(< 0.01%) of open-chain, aldehyde form of the sugar present without resort to
temporary protecting groups. 23 The reactions are clean and predictable, offering a
convenient and economical synthesis of authentic reference samples of certain types,
of alditols (sugar alcohols) for comparison with natural produ~ts. For example,
glycosylpentitols can be prepared in a simple single step from readily available
disaccharides (as illustrated above for lactose), a process that would otherwise
constitute a multi-step synthetic challenge with generation of concomitant waste
8
.
*
streams. With ketose sugars, we observed decarbonylation and dehydration
products, including a variety of furans. 24 We have also observed formation of
furandimethanol (FDM) when the Kruse hydrogenation system is applied to
fi-uctose.6 FDM is a difunctional aromatic monomer used to manufacture flame-
resistant polymers, and is also a precursor to tetrahydrofurandimethanol, an
increasingly important diol.25
Catalytic Hydrocracking of Sugars
Our second major study also involved chemistry dependent upon the latent
carbonyl group present in monosaccharide sugars. This work demonstrated that
aldose and ketose carbon–carbon single bonds could be catalytically hydrocracked
by H2Ru(PPhs)A via a combination of a retro-aldol reaction and carbonyl group
hydrogenation, resulting in the formation of lower polyols: 5J6
CH20H Hexitols(64%) CH20H
$0
t
\HOH
RUH2L4 (2%)CH20H
HO H
+2H OH +
HO H
H OH Hz (300 psi) H OH
100 “C, 24 hCH’20H
H OH H OH
CHZOHNMP Glycerol (15%)
CH’20H----- ----- ----- ----- ----- ------ ------ --
The hydrocracking of fmctose is very selective for glycerol; the only other significant
J- ORUH ~2
0
HO HO+
/ RUH2
species present are unreacted fructose and the two hexitols derived from simple
9
.
hydrogenation of the fructose carbonyl group. Addition of a basic co-catalyst (KOH)
accelerates the reaction and increases the selectivity for hydrocracking over simple
hydrogenation (3 l% yield ofglycerol). Unfortunately the mass balance and cracking
selectivities are then somewhat reduced, due to sugar degradation and enhanced
formation of Cz, Cd and C5 fragments. Key findings of these studies are that the
transformation occurs under much milder conditions (100 ‘C/ 300 psi Hz) than with
previously patented heterogeneous catalysts (2OO ‘C / 2000 psi Hz )26-34 and with
greater selectivity (no formation of partially deoxygenated products, e.g., 1,.2-
propanediol). An improved version of this chemistry would be desirable as glycerol
is potentially a good precursor to a variety of oxygenated organics.
Pt and Pd Diolate and Alditolate Complexes
Though it is clearly possible to do catalytic chemistry involving carbohydrate
carbonyl groups, the most characteristic functionality present in these
polyhydroxylic compounds is the diol unit. This grouping is found even in
polysaccharides such as starch and cellulose where the carbonyl group is fully
masked as an acetal, rather than as the equilibrating hemiacetal found in
monosaccharides. Building on the brief literature reports of (Lz)Pt(diolate)
complexes, 19’35we have now made an extensive study of bis(phosphine) platinum(H)
diolate and sugar alcohol complexes. 8’9 The key to this entire study was provided by
the development of a new method for synthesizing diolate complexes, reaction of a
bis(phosphine) platinum carbonate, L2Pt(COs),36 with the diol or alciitol:
10
This reaction is nearly thermoneutral, hence various exchange reactions allow
determinationof relative complexation constants as afunction ofthesubstituents
on the diol and ancillary phosphine ligand. For different 1,2-diols with Lz =
l,3-bis(diphenylphosphino)propane (dppp), these cover arangeof almost 100. For
different phosphines with ethane-l,2-diol ,therelative complexation constants cover
a similar range. The total range, from (1,2 -bis(dicyclophexylphosphino)ethane)-
Pt(pinacolate) at the low end to (cis-1,2-bis(diphenylphosphino)ethene)-
Pt(phenylethane-l,2-diolate)at the high end, is estimated tobe over 10s. In all
cases, electron-donating groups on both the diol and phosphine lead to lower diol
binding constants. Complexationof l,3-diols is about afactorof100–1000 lower
than that of 1,2-diols.
With alditols, isomeric complexes are possible, even after requiring
complexation to occur via an a, ~-diol linkage. In practice, significant
regioselectivities are observed, which vary strongly with alditol stereochemistry,
favoring coordination to internal threo diol units (e.g., 2,3-galactitol, 3,4-mannitol)
over erythro and terminal diol units (3,4-galactitol, 2,3- and 1,2-mannitol):
11
.
—
P
CH20H HoH2y~ 83% 1,2-Isomer 14%~ ~ 1670
:: H+C)H~17% 2.3-Isomer 86-
H+OH . . .
H+
OH
@El cH@H
[
Ph Ph\/
~p ~ RI\ /0/p(\
o R2/p\
Ph ‘Ph
] “+OHHOH2C
EZl
= 1 J @zmlcH@H~11%
HOH2C7% ~
+
1,2-Isomer 1%~
$
~ 3%H OH HO H
89V0~ ~ 71~0 2,3-Isomer 17%~ ~ 5%HO H
470 ~ ~ 18% 3,4-Isomer 82%~
t
HO H
H ‘H t_cD,a2_J ‘: ~ “r
yridine CH20H HOH2CAndrews& VOSS Pyridine
These ratios are thermodynamic, not kinetic, as isomerization is fast on the
laboratory time scale but slow on the NMR time scale. Hydrogen-bonding
interactions are important contributors to these regioselectivities,as illustrated by
the x-ray diffraction determined structure of (dppp)Pt(3,4-mannitolate ):9
~h ~h Hs. ?
: , .O.. OH
< >J
‘\?’ ),,,,\\i
OH/ptJ \
/! ] .0‘h Ph Ho”
Detailed studies show that the hydrogen-bond acceptor (HBA) strength of the
platinum diolate oxygens is comparable to that of the strongest neutral 13BA’s.~7
The intramolecular hydrogen-bonding interactions in the Pt alditolate complexes
are thus sufficiently strong that they are retained to a large extent even in neat
12
HBA solvents such as pyridine, leading to only minor changes in complexation
isomer ratios as a fi.mction of solvent (cf. comparisons in above Figure).
While the (L2)Pt diolate and alditolate complexes are remarkably stable
thermally, they undergo facile photochemical oxidative-cleavage of the diol C–C!
bond to give two carbonyl fragments and a reactive (L2 )PtO species.g When the
photolyses are run in the presence of hydrogen and catalytic HzRu(PPhs)A, the
ketones oraldehydes can bereduced tothe corresponding alcohol. Synthesis of the
starting diolate ii-omthe platinum carbonate canalso be integrated into the overall
reaction. The net transformation then corresponds to hydrocracking of the starting
diol carbon–carbon single bond. Thus it is possible to selectively convert mannitol
into glycerol: 6
$OH
HOit I
OH
HO
‘“OHHO
Hz,hv~2
1.0 (dppe)Pt(COs)
0.1 HzRu(PPhs)4
NMP
EOH
OH
OH
While this reaction is clearly not of practical utility, the results demonstrate that
carbohydrate complexation selectivity, facilitated by the rich functionality of
carbohydrate substrates, can be converted into reaction selectivity.
Since Pd(II) is a much more catalytically active metal than Pt(II), we have
investigated palladium carbohydrate chemistry. Initial results (S. K. Mandal and
K. S. Koenig) were disappointing, however, as competition studies show that diols
and alditols bind about a factor of 100 times poorer to (L2 )Pd(II) than to the
corresponding (L2 )Pt(II) center. Furthermore, while (dppp)Pd(3,4-mannitolate) has
been isolated, it is very sensitive to decomposition by water and shows no sign of
thermal oxidative-cleavage of the central C–C bond, possibly due to Woodward -
Hoffiann symmetry constraints. 8
13
Catalytic Diol Deoxydehydration
Since Pt(II) and Pd(II) diolate complexes exhibited no signs of thermal redox
activity, we looked elsewhere for possible systems to effect catalytic carbohydrate
transformations. Based on several stoichiometric reactions reported by Herrmann38
and Gable39 in conjunction with their studies of alkene oxidation, we have now
designed and implemented the catalytic cycle shown to the left below:7
R,mdrews & Cook
HO OH
)--’
-2~0 Cp*Re03* ~hF
Ph + 90“c +PPh~ O=PPh3
50, t“ 1
II ttt4.2 “~.O.I h ,“ A
.,. r i- 7 ‘/—\
6 4 8 12 16
Time (h)
For the simple diol phenyl- l,2-ethanediol, the reaction to give styrene proceeds very
well in solvents such as benzene or chlorobenzene using triphenylphosphine as a
sacrificial reductant (above right). The initial rate of the reaction is essentially
equal to that observed by Gable for alkene extrusion from pure
Cp*ReO(phenylethanediolate),40 suggesting that extrusion is the rate-limiting step
in the catalytic cycle. In donor solvents, such as tetrahydrofuran (T13F) and NMP,
the catalyst dies after a few turnovers. A primary cause of this has been identified
14
(over-reduction of the catalyst to a Re(III) species) and ways to prevent this from
occurring have been discovered (use of an acid co-catalyst or a weaker sacrificial
reducing agent). The reaction is stereospecific as shown by the conversion of the
protected sugar l,2:5,6-diisopropylidenemannitol to the corresponding trans alkene.
Alditols are also deoxydehydrated, glycerol yielding allyl alcohol and the C4 alditol
erythritol giving not only 3-buten- 1,2-diol and cis -2-buten-l ,4-diol, but the fully
deoxygenated product butadiene as well. This technique is at least as good as most
current methods for converting diols to alkenes, the typical approach being reaction
with thiophosgene to give a thiocarbonate, which is then heated with a phosphine to
give the alkene, phosphine sulfide, and carbon dioxide.41 For carbohydrate
substrates, the reaction shows promise of selectively converting specific hydroxyl
groups into another reactive but readily differentiated functional group, an alkene,
or more specifically, an allyl alcohol. This methodology could significantly reduce
the need for inefficient, waste generating temporary protecting groups in
carbohydrate syntheses. Future implementations that would be even more
environmentally benign could potentially be developed based on carbon monoxide as
the sacrificial reductant. Improvements of this sort might then lead to viable routes
to biomass-derived commodity oxygenated organics as well fine chemicals.
Ill – OPPORTUNITIES FOR THE FUTURE
Complexation of Cyclic and Disaccharide Sugars/ Other Metals
A key extension of our previous (L2)PtII alditol complexation studies is the
preparation of compounds derived from cyclic and disaccharide sugars, which are
more typical of primary biomass sugars such as glucose, fructose, and sucrose, as
well as being better models for the biomass polymers starch and cellulose. They are
also more complex and will provide a demanding test of complexation stereo- and
15
regio-selectivity since the a- and ~ -anomers of both pyranose and furanose ring
forms are in equilibrium via the open-chain form of the sugar:
yH20H 7H20H
~-Pyranose
$
HO H a-Pyranose
H OH
HO 1 /
H+OH ~
CH20H HOT
HO
e
0>0OH aldehydo- HO
Glucose
b
0>0
OH
fl-Furanose ‘HOH
a-Furanose
Our prior model studies with cis - and trans-cycloalkanediols show that
complexation of the cis isomer is favored over the trans isomer by a factor of seven
for cyclohexane- 1,2-diol.9 A somewhat smaller ratio (ea. 2:1) is observed for the one
sugar prototype we have examined (Z. H. Shriver), internal cis/trans competition in
methyl- cx-mamopyranoside. For cyclopentane- 1,2-diol, complexation of the cis
isomer is four times stronger than for cis -cyclohexane - 1,2-diol, while the trans
isomer is essentially unreactive, suggesting that very high selectivities should be
achievable for cis vs trans coordination in furanose sugars. For underivatized C G
sugars such as glucose, there are theoretically twelve possible isomeric a, ~-diolate
complexes that can be formed, not counting the four from the open-chain form.
Based on our previously observed trends, the higher acidity of the anomeric
hemiacetal hydroxyl protons should favor complexation involving this site. It is also
probable that complexation to the metal will significantly alter the equilibrium
16
.
ratios of the sugar isomers present, for example, increasing the furanose form over
the pyranose form by preferential binding to a cis-furanose diol unit or to the exo -
cyclic dihydroxyethyl side-chain in a C (j furanose sugar.
The common disaccharide sucrose will actually be a much simpler case since
its carbonyl group is masked as a full acetal and only the two trans- (x,&diol units in
the glucopyranose fragment would be expected to coordinate, the trans-diol unit in
the fructofuranose ring presumably being unreactive. Two products are
experimentally observed in a 3:1 ratio (M. A. Andrews). The common
polysaccharides cellulose and starch have only one unique U,&diol unit to complex,
a 2,3-trans-diol glucopyranose unit.
HOH2C
~
ct,~-Diol Units in Sucrose, Starch and Cellulose
O.O ~ ~
HO 2 trans-Pyranose1 m.wzs-Furanose I
OHo
HOH2C
w
O>OH ~os”)
nCH20H
OH 1 ?rans-Pyranose / Repeat Unit
Another essential future objective is determining the complexation and
reactivity characteristics of carbohydrates with other metals besides Ptll,
particularly those that are more redox active and those from other parts of the
periodic table. In particular, the carbohydrate coordination chemistry of early,
oxophilic transition metals should be quite different from that of late metals with
have studied to date. Lone pairs on the coordinated diolate oxygens would be
expected to participate in n-backbonding to the electron-deficient metal center,
greatly reducing their hydrogen-bond acceptor ability. This, and the decreased
polarization of the metal–oxygen bond, should significantly alter the coordination
17
selectivities. Results from both studies with more biomass relevant carbohydrates
and with other metals will be helpful in predicting and understanding the kinds of
catalytic reaction selectivities that may be achievable with native sugar substrates.
Development of Catalytic Polyol Disproportionation
We have intriguing preliminary results (G. K. Cook) which suggest that it
may be possible to effect catalytic diol deoxydehydration without addition of an
external reducing agent via a disproportionation reaction. Thus 1,2-
phenylethanediol is converted to styrene in about 60% yield in the presence of only
catalytic Cp*Re03 and a suitable “initiator”, also present in catalytic amounts. One
of the most effective initiators is the product styrene itself, hence the reaction
exhibits auto catalytic behavior. Cross-over experiments using ct-methylstyrene as
the initiator and 1,2-phenylethanediol as substrate show that the a-methylstyrene
is not consumed.
presence of styrene
these experiments,
Similarly, conversion of 1,2-hexanediol to l-hexene in the
as an initiator proceeds without oxidation of the styrene. From
we conclude that the formation of alkene from the diol must
proceed via diol disproportionation, though we have not yet identified the oxidized
diol product(s). While there is obviously still much that we don’t understand about
these reactions, development of the following type of catalytic polyol
disproportionation reaction seems potentially attainable and is certainly
stoichimetrically achievable with a suitable catalyst:
‘07-0” - -“ +2“20OH L02=+%x”
Proposed
18
This chemistry could be used to produce valuable unsaturated aldehydes and
carboxylic acids from biomass feedstocks, the glycerol coming from either fatty acid
esters or from the hydrocracking of fructose or other carbohydrates.
Development of Catalytic Carbohydrate Ionic Hydrogenolysis
Our colleagues at Brookhaven National Laboratory have developed an ionic
hydrogenation chemistry that accomplishes the stoichiometric hydrogenation of a
number of organic substrates, ranging from alkenes42 and alkynes43 to ketones, 44
acetals,45 and alcohols,4G utilizing a metal hydride and a strong acid, e.g.
(C&0JW(CO)3H + triflic acid (HOTf). For alcohols, this reaction accomplishes the
net removal of oxygen, presumably by a mechanism involving protonation of the
alcohol, loss of water, and hydride transfer to the resulting carbenium ion to
generate the hydrocarbon product:
OH1.2 CF3S03H
+
22 ‘C, 5 rninuws+ Cp(CO)3WH ~
[i,,] -*P,::)3W,0TDBullock & .%ng
The qualitative reaction rates reflect the stabilities of the intermediate carbenium
ions. Thus, deoxygenation of tertiary alcohols (above) occurs within minutes at
room temperature. Deoxygenation of secondary alcohols requires hours at room
temperature, while most primary alcohols are inert under these conditions.
Based on these results and other organometallic literature, catalytic versions
of these reactions are very plausible, as illustrated for a diol:
19
OH
\R
H2
YOxidativeAddition
LnM@
4
@LnM
,H
‘H
OHR
<
Y
OH
ProtonTransfer H20
\
Proposed R
There are a large number of cationic dihydrides / molecular dihydrogen complexes
now known,47-49 many of which have sufficient acidity to protonate substrates such
as diols. The resulting intermediate carbenium ion in this case would be stabilized
by lone-pair donation from the adjacent hydroxyl group. Subsequent hydride attack
should occur at the more positively charged internal carbon, leading to a primary
alcohol that should be stable towards further reduction. With polyols as substrates,
this reactivity would lead to cx,co-diols, which are valuable compounds in their own
right or they may be converted to other products such as tetrahydrofuran (from the
C4 alditol erythritol) or adipic acid (from CG alditols, e.g., sorbitol and mannitol,
readily available from the hydrogenation of glucose and/or fructose). Such a
synthesis of adipic acid would clearly be a much more environmentally benign route
to this product than the current route described in the introduction.
A number of problems will need to be addressed by experimentation to
develop this chemistry. The first is to show that diols and polyols will undergo this
20
type of ionic hydrogenation., Preliminary experiments (N. M. Brunkan) show that
phenylethane- 1,2-diol is reduced by (C5H5)W(CO)3H + HOTf in dichloromethane
solution at room temperature to give the expected 2-phenylethanol as the primary
product (via the intermediate complex [Cp(CO)s(HOCH2CH 2Ph)]+[OTfJ-). Styrene
epoxide, a more direct route to the proposed pronated epoxide intermediate, is
reduced to 2-phenylethanol more quickly under the same conditions. Simple diols,
such as 1,2-hexanediol, were not reduced, however, NMR spectra suggesting that
the reaction stopped at the protonated diol stage. Propylene oxide was reduced to
n-propanol, further indicating that dissociation of water from the protonated diol is
the rate inhibiting step. The second problem will be to conduct the ionic
hydrogenation in solvents that will dissolve sugars. The problem here is to avoid
excessive leveling of the acidity of the acid by protonation of the solvent. A good
choice might be acetic acid, whose conjugate acid [CH3C(OH)2]+ has a pKa = –6, ca.
four orders of magnitude more acidic than protonated alcohols. Here test
experiments indicate that the problem will be esterification of the diol or polyol.
The final step of the catalytic cycle, reformation of the metal dihydride, could also
pose a problem. Thus, initial attempts (Song and Bullock) to react metal triflates
with Hz have not been very successful, apparently because the triflate counterion
binds too strongly to the vacant metal site required for effective oxidative-addition
of hydrogen. A potential solution to this problem is to use a much more weakly
coordinating counterion such as BAr’A– (Ar’ = 3,5-bis(trifluoromethyl)phenyl)).50 In
summary, while the preliminary results for ionic hydrogenation of diols discussed
here are somewhat disappointing, they represent very limited explorations of the
wide range of parameter space available, e.g., with respect to the metal and its
associated ligands which together are known to have a strong effect on the hydricity
of the metal hydride (a range of 10s in rates of reaction with trityl cation).5 I
21
Development of Aqueous Organometallic Carbohydrate Chemistry
Given the high water volubility of sugars and water’s biocompatibility and
low cost, the development of aqueous carbohydrate homogeneous catalysis systems
is a desirable goal for industrially benign organic synthesis applications. While
most organometallic chemistry has traditionally been done in organic solvents,
water-soluble complexes are now well-known 52-54and one aqueous process utilizing
them as catalysts, propylene hydroformylation, has been commercialized. 55 For
carbohydrate transformations that rely on the carbonyl functionality of sugars, such
as our fructose retro-aldol mediated hydrocracking, we see no inherent problems
associated with conversion to an aqueous medium. For catalytic cycles based on
coordination of a carbohydrate hydroxyl functionality, however, complexation
competition problems could arise, as water contains the same functionality. The
chelate effect operative with a carbohydrate diol functionality offers some entropic
advantage over water, but whether this is sufficient for the carbohydrate to
successfully compete with water for binding to the metal center remains to be
determined. For those systems for which water proves detrimental, there are still a
number of solvents, such as 2V-methyl-2-pyrrolidinone (NMP) and dimethylsulfoxide
(DMSO), which are well-suited to metal-carbohydrate chemistry and are considered
reasonably attractive under today’s health and environmental standards.
Some hydroxyl reactions, however, such as ionic alcohol hydrogenolysis, do
not depend on coordination. Recently an aqueous, homogeneously catalyzed alcohol
carbonylation system has been reported and applied to a carbohydrate-derived
substrate, hydroxymethylfurfural (HMF) to give the new compound 5 -formylfuan-
2-acetic acid. 56 Under different reaction conditions, the catalyst is also capable of
deoxygenating the alcohol group in HMF, giving 5-methylfurfural:
22
.
H+OH
LH20H
Fructose
Pdll J P(C~H4S03- )3+Co
H+/H20
A c02
‘“’CGCH”‘3CT5’CH;heldon
This latter carbonylative reduction chemistry may be an alternative to ionic
hydrogenolysis as amethod ofdehydroxylating carbohydrates.
IV - SUMMARY
There is a critical need for new environmentally friendly processes in the
United States chemical industry as legislative and economic pressures push the
industry to zero-waste and cradle-to-grave responsibility for the products they
produce. Carbohydrates represent a plentiful, renewable
processes might economically replace fossil feedstocks.
biomass to~uels, is still not generally economical,4 the
resource, which for some
While the conversion of
selective synthesis of a
commodity or fine chemical, however, could compete effectively if appropriate
catalytic conversion systems can be found. Oxygenated organics, found in avariety
ofproducts such as nylon and polyester, are particularly attractive targets.57 We
believe that with concerted research efforts, homogeneous transition metal
catalyzed reactions could play a significant role in bringing about this future green
chemistry technology.
23
.
,-
IV - BIBLIOGRAPHY
(1) KirsChner, E. M. Chem. Z3ng.News 1996, April 8, 16-20.
(2) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications and
Chemistry of Catalysis by Soluble Transition Metal Complexes; 2rd cd.;
Wiley: New York, 1992.
(3) Da~s, D. D.; Kemp, D. R. in Kirk-Othmer Encyclopedia of Chemical
Technology; 4th ed.;Wiley and Sons: New York, 1991; Vol. 1, pp 466-493.
(4) Bridgwater, A. V. Appl. CataL A-Gen 1994, 116,5-47.
(5) Andrews,M. A.; Klaeren, S. A. J. Am. Chem. Sot. 1989, 111,4131-4133.
(6) Andrew, M A.; Klaeren, S. A.; Gould, G. L. in Carbohydrates as Organic
Raw Materials II; Descotes, G., Ed.; VCH: New York, 1993; , pp 3-25.
(7) Cook, G. K.; Andrews, M. A. J. Am. Chem. Sot. 1996, 118, 9448-9449.
(8) Andrews, M. A; Gould, G. L. Organometallics 199310,387-389.
(9) Andrews,M A.; Voss, E. J.; Gould, G. L.; Klooster, W. T.; Koetzle, T. F. J.
Am. Chern. Sot. 1994, 116, 5730-5740.
(10) Kruse, W. M. U. S. Patent 3,935,284, Jan, 1976.
(11) Kruse, W. M.; Wright, L. W. Carbohydr. Res. 1978,64,293-296.
(12) Angyal,S. J. Adu. Carbohydr. Chem. Biochem. 1991,49,19-35.
(13) Ra.iwwal, S.; Vancheesan, S.; Rajaram, J.; Kuriacose, J. C. Indian J. Chem.
1979, 18B, 293.
(14) Pillai, S. M.; Vancheesan, S.; Rajaram, J.; Kuriacose, J. C. J. Mol. CataL
1982,16,349-358.
(15) Rajagopal,S.; Vancheesan,S.; Rajaram, J.; Kuriacose, J. C. J. Mol. CataL
1983,22, 137-144.
(16) RajagqA, S.; Vancheesan, S.; Rajaram, J.; Kuriacose, J. C. J. Mol. CataL
1983,22, 131-135.
24
(17) Wssoui, M.; Beaupere,. D.; Goethals, G.; Uzan, R. J. Mol. Catal. 1985,29,7-
12.
(18) Rajagopal, S.; Vancheesan, S.; Rajaram, J.; Kuriacose, J. C. J. Mol. CataL
1993,81,185-194.
(19) Appelt, A.; Willis, A. C.; Wild, S. B. J. Chem. Sot., Chem. Commun. 1988,
938-940.
(20) 130ugh, L.; Robinson, A. C. in 27w Carbohydrates: Chemistry and
Biochemistry; Pigman, W. W. and Horton, D., Ed.; Academic Press: New
York, 1972; Vol. IA, pp 127-138.
(21) Andrews, M. A.; Klaeren, S. A. J. Chem. Sot., Chem. Commun. 1988, 1266-
1267.
(22) Andrews, M. A.; Gould, G. L.; Klaeren, S. A. J. Org. Chem. 1989,54,5257-
5264.
(23) Binkley, R. W. Modern Carbohydrate Chemistry; Marcel Dekker: New York,
1988, pp 113-166.
(24) Andrews, M. A. Organometallics 1989,8,2703-2708.
(25) McKillip, W. J.; Sherman, E. in Kirk -Othmer Encyclopedia of Chemical
Technology; 3rd ed.;Wiley: New York, 1980; Vol. 11, pp 499-527.
(26) van Ling, G.; Ruijterman, C.; Vlugter, J. C. Carbohydr. Res. 1967,4,380-386.
(27) Hanford, W. E. U. S. Patent 2,209,055, July, 1940.
(28) Conradin, F.; Bertossa, G.; Giesen, J. U. S. Patent 3,030,429, Apr, 1962.
(29) Kasehagen, L. U. S. Patent 3,369,199, Aug, 1968.
(30) Sirkar, A. K. U. S. Patent 4,380,678, Apr, 1983.
(31) Arena, B. J. U. S. Patent 4,401,823, Aug, 1983.
(32) Arena, B. J. U. S. Patent 4,496,780, Jan, 1985.
(33) Tanikella, M. S. S. R. U. S. Patent 4,404,411, Sep, 1983.
(34) Dubeck, M.; Knapp, G. G. U. S. Patent 4,476,331, Ott, 1984.
25
.
ii
(35) Bryndza, H. E.; Calabwse, J. C.; Marsi, M.; Roe, D. C.; Tam, W.; Bercaw, J.
E. J. Am. Chem. Sot. 1986,108,4805-4813.
(36) Andrew, M. A; Gould, G. L.; Klooster, W. T.; Koenig, K. S.; Voss, E. J. Inorg.
Chem. 1996,35,5478-5483.
(37) Andrews,M. A.; Cook, G. K.; Shriver, Z. H. J. Am. Chem. Sot. Submitted.
(38) Herrmann,W. A.; Marz, D.; Herdtweck, E.; Schafer, A.; Wagner, W.;
Kneuper, H.-J. Angew, Chem., Int. Ed. EngL 1987,26,462-464.
(39) Gable, K. P. Organometallics 1994,13,2486-2488.
(40) Gable, K. P.; Juliette, J. J. J. J. Am. C%em.Sot. 1995,117,955-962.
(41) Block, E. Org. Reactions 1984,30,457-566.
(42) Bullock, R. M.; Song, J.-S. J. Am. Chem. Sot. 1994,116,8602-8612.
(43) LUan, L.; Song, J.-S.; Bullock, R. M. J. Org. Chem. 1995,60,7170-7176.
(44) Song, J.-S.; Szalda, D. J.; Bullock, R. M.; Lawrie, C. J. C.; Rodkin, M. A.;
Norton, J. R. Angew. Chem., Int. Ed. EngL 1992,31, 1233-1235.
(45) Song, J.-S.; Szalda, D. J.; Bullock, R. M. J. Am. Chem. Sot. 1996,118, 11134-
11141.
(46) Song, J.-S.; Bullock, R. M. unpublished results.
(47) Heinekey, D. M.; Oldham, W. J., Jr. Chem. Rev. 1993,93,913-926.
(48) Jessop, P. G.; Morris, R. 1% Coord. Chem. Rev. 1992,121, 155-284.
(49) Kubas, G. J. Ace. Chem. Res. 1988,21, 120-128.
(50) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,11,3920-
3922.
(5U Cheng, T.-Y.; Bullock, R. M. Organometallics 1995,14,4031-4033.
(52) ROundhiLD. M. Adv. Organometal. Chem. 1995,38,155-188.
(53) Kalck, P.; Monteil, F. Adv. Organometal. Chem. 1992,34,219-284.
(54) Barton, M.; Atwood, J. D. J. Coord. Chem. 199L24,43-67.
(55) Haggin, J. Chem. Eng. News 1994, Oct. 10,28-36.
26
(56)
(57)
Papadogianakis, G.; Maat, L.; Sheldon, R. A. J. Chem. Sot., Chem. Commun.
1994,2659-2660.
Busche, R. M. Biotech. Prog. 1985,1, 165-179.
Acknowledgments. The author would like to thank his co-workers for their
major contributions, experimental and intellectual, to the studies discussed in this
Chapter: Nicole Brunkan, Gerald Cook, George Gould, Stephen Klaeren, Wim
Klooster, Kristina Koenig, Santosh Mandal, Zachary Shriver, and Eric Voss. This
research was carried out at Brookhaven National Laboratory under contract DE-
AC02-76CHOO016 with the U.S. Department of Energy and supported by its
Division of Chemical Sciences, OffIce of Basic Energy Research.
27
top related