reaction mechanisms - harned research group · pdf fileof the more importatn reaction...
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Reaction MechanismsBefore we get into the synthetic chemistry it is a good idea to first become familiar with someof the more importatn reaction mechanisms available to transition metals. We will see these
again and again as we continue in the course.
I. Ligand Substitution
II. Oxidative Addition/Reductive Elimination
M L1 L2+ M L2 L1+
M(n) + A Boxidative addition
reductive eliminationM(n+2)A
B
usually low-valent (n = 0,1),"nucleophilic" metal
coordinatively unsaturated
often polarized,"electrophilic"
M–A and M–B bonds areusually strong, complexcoordinatively saturated
metal has beenformally oxidized
Both associative (SN2-like) and dissociative (SN1-like) mechanisms are possible
M
Reaction Mechanisms
III. Migratory Insertion & Elimination
IV. Nucleophilic Attack on Ligands Coordinated to Metal
M Y M YX
X M Y XL L
note cisrelationship
note emptycoordination site
M X Y+ X Y Nuc–
M X Y Nuc
unreactive tonucleophiles
(electron-rich)
reactive tonucleophiles
(electron-deficient)
reactivity increasedif electron-deficeint
very reactive to other electrophiles,
often this process results in "reductive elimination" of the metal
Reaction Mechanisms
V. Transmetallation
M1 R M2 X+ M2 R M1 X+
M1 = Mg, Zn, Zr, B, Hg, Si, Sn, GeM2 = transition metal
almost always the rate-limiting step,usually the culpret when catalyticprocesses fail
VI. Electrophilic Attack on Metal Coordinated Ligands
Several different reaction modes are known, will explore further later
M R E+ M R R NucENuc–
inverstion at Rreductiveelimination
E R
retention at R
attack can directly cleave M–R bond orcan happen α, β, or γ to the metal
Ligand SubstitutionThough we will be concerning ourselves more with the reactivity and synthetic utility of organimetallic
complexes, understanding the mechanisms available for ligand substitution is critical to understanding how the complexes react.
Associative Mechansim (SN2-like) – typically occurs with coordinatively unsaturated complexes; exemplified by 16-electron, square planar, d8 metals (Ni(II), Pd(II), Pt(II), Rh (I), Ir (I))
M L1 L2+ M L2 L1+
MLT Lc
Lc X + Y
apicalattack
MLT Lc
Lc XY
MYX
Lc
Lc
LT MLT Lc
Lc Y
XM
LT Lc
Lc Y
X
– X
apicalexit
Factors that influence the rate:
– identity of the metal– identy of incoming and outgoing ligands– identy of the trans ligand ("trans effect")
squareplanar(16 e–)
squarepyramidal
(18 e–)
trigonalbipyramidal
(18 e–)
Ligand SubstitutionThough we will be concerning ourselves more with the reactivity and synthetic utility of organimetallic
complexes, understanding the mechanisms available for ligand substitution is critical to understanding how the complexes react.
Dissociative Mechansim (SN1-like) – typically occurs with 18 electron coordinatively saturated complexes; often slower that associative substitution; exemplified by M(0) metal carbonyl complexes
M L1 L2+ M L2 L1+
Ni(CO)4
(d10, 18 e–)
– CONi(CO)3
(d10, 16 e–)
+ LLNi(CO)3
(d10, 18 e–)
The rate can be accelerated by bulky ligands (loss of labile ligand relieves steric strain). This is particularly noticeable with phosphines and can be measured by the "cone angle". The
electronics of the phosphine can be changed (idenpendently from sterics) by substitution.
M
PR R
R
cone angle (Θ)
R θ
OMe 107OPh 128Ph 145
o-tolyl 194Cy 170
t-Bu 182
νco (cm-1)207920852069
–20562056
νco (cm-1) is determined with Ni(CO)3L and is a measurement of the amount of backbonding. More donating L, more backbonding and νco decreases.
Hartwig, Organotransition Metal Chemistry, 2010, pp 37–38.
M
Ligand SubstitutionA "full dissociation" is not always necessary to open coordination site on an 18-electron complex.
Sometimes a polydendate ligand can "slip" and free up a coordination site.
This can explain some observations seen with ligands such as η3-allyl, η5-cyclopentadienyl, and η6-arene complexes. By slipping to a lower hapticity, a coordination site (or two) is opened.
M M
η3-allyl(2 sites)
η1-allyl(2 sites) η6-arene
(3 sites)
Mη4-arene(2 sites)
Mη2-arene(1 site)
Mn(CO)3Mn(CO)3
Mn(I), d6
18 e–Mn(I), d6
16 e–
+ L
Mn(CO)3L
– CO
Mn(CO)2L
Oxidative Addition/Reductive EliminationReactions of this type are central to the synthetic utility of transition metals complexes and relies
on the ability of metals to easily and reversably change oxidation states (compare to what is takes to change oxidation state of C).
M(n) + A Boxidative addition
reductive eliminationM(n+2)A
B
The terms "oxidative addition" and "reductive elimination" are generic and refer only to the process of changing the oxidation state of the metal. The exact mechanism by which this occurs can vary.
Oxidative Addition (OA)
Metal must be coordinatively unsaturated and relatively electron rich (nucleophilic) and usually in low oxidation state (0, +1). σ-Donor ligands (PR3, R–, and H–) facilitate OA. π-Acceptor ligands
(CO, CN–, alkenes) suppress OA.
By the formalism used to assign oxidation state, the metal has lost two electrons during the above process (the metal has been oxidized)
Metals that most commonly undergo OA reactions (other are certainly known):
d10: Ni(0), Pd(0) → d8: Ni(II), Pd(II)d8: Rh(I), Ir(I) → d6: Rh(III), Ir(III)
Exact mechanism by which the OA occurs depends on the nature of the substrate.
Oxidative Addition/Reductive Elimination
Nonpolar Electrophiles
Common examples: H2, R–H, Ar–H, R3Si–H, R3Sn–H, R2B–H, R3Sn–SnR3, R2B–BR2,
Generally undergo OA by concerted, one-step "insertion" mechanism. The configuration of any stereocenters would be expected to be retained. May require dissociation of a ligand from the
initial complex.
LnMA–B
LnMB
A
"agostic" interaction(2 e–, 3 center bond)
cis stereochemistry(kinetic)
Examples:
LnMA
BLnM
A
B
RCO
H
Ph3PIr
ClOC PPh3 H2
Ph3PIr
Cl
H PPh3H
COPh3P
RhCl
Ph3P PPh3
Ph3PRh
BR2
Ph3P PPh3H
Cl
R2BH
Ph3PRh
ClPh3P PPh3
Ph3PRh
Ph3P PPh3H
Cl
RCHO
O
R
Oxidative Addition/Reductive EliminationPolar Electrophiles
Common examples: HX, X2, R–X, R(O)X, Ar–X,
Two mechanisms are possible. One is analagous to reactions with nonpolar electrophiles (direct insertion). The other is an ionic, two-step SN2 mechanism, where the metal functions as a
nucleophile and donates two electrons in the process. The configuration of any stereocenters would be expected to be inverted in this case. The structure of the electrophile determines which is active.
Mn C X C XM CM(n+2) CMX X
relative rates:
Me > primary > secondary >> tertiary
I > Br ~ OTs > Cl >> F
phosphines promote with greater basicity giving faster rates
Oxidative Addition/Reductive EliminationPolar Electrophiles, cont'd
Examples:
OCIr
LL Cl
OCIr
L
L ClCH3
I
CH3I
trans(kinetic)
TsOt-Bu
D H
H DL2Pd
t-BuH D
H DTsO
Pd(0)Pt-Bu2Me
L2Pd PhBr+
inversion
L2PdBr
PhHtrans
PhL2Pd
Br
trans(retention)
Fe(CO)5
d8, 18 e–
PhPhONa
Na2[Fe(CO)4]2–
Collman's reagent"supernucleophile"
R XNa[RFe(CO)4]–
Further reactions possible
Oxidative Addition/Reductive EliminationPolar Electrophiles, cont'd
There are also examples of reactions that cannot be explained by either of these mechanisms (concerted or SN2). These have been rationalized by a radical-chain mechanism.
R Xhν or
O2RR
R + LnMn R M(n+1)Ln
R M(n+1)Ln RX+ R M(n+2)Ln
XR+
sequential 1e– oxidations,net 2e– oxidation of metal
Oxidative Addition/Reductive Elimination
M(n) + A Boxidative addition
reductive eliminationM(n+2)A
BReductive Elimination (RE)
The reverse of oxidative addition. Concerted mechanism proceeds with retention of any stereochemical information. Nucleophilic attack on the ligand would invert the configuration.
Factors that influence:– First row metals faster than second row, faster than third row– Electron-poor complexes react faster than electron-rich– Sterically hindered complexes reacter faster– H reacts faster than R– complexes with 1 or 3 L-type ligands faster than 2 or 4
Geometry of the complex is also quite important
PPd
P Me
Me
Ph Ph
Ph Ph
fastMe Me PPh2Ph2P Pd
Me
MeΔ
no reaction
Migratory Insertion & Eliminations
Migratory Insertion
M Y M YX
X M Y XL L
In this process an unsaturated ligand (CO, RNC, alkene, alkyne) inserts into an existing M-ligand bond. The two ligands involved must be cis to one another. These are usually reversible processes. At the end
of the reaction the metal is left with an empty coordination site.
General examples:
LnMR
CO
LnML
C
R = aryl, alkyl, H
+ L
OR
LnMR LnM
L+ L
A B A H B
RH
trans trans
LnMR
LnML+ L
B
A
R
BA
cis
Migratory Insertion & Eliminations
β-Hydride Elimination (BHE)
If an alkyl metal complex has hydrogens b to the metal, then this type of elimination is likely to occur. However, the β-hydrogens usually must be syn coplanar to the metal. Also the metal
usually must have an open coordination site.
Eliminations are the reverse reaction of migratory insertion and can occur one after the other. The group being eliminated does not have to be the one that participated in the insertion.
There are several types of eliminations.
HLnM
syn coplanar
LnM HLnM H
BHE from transition metal-alkoxides and -amines are also important
OHLnM
MeMe
LnM H
OMeMe LnM H
L+ L
Me
O
Me+
M–H without using H2
β-Eliminations of alkoxides and halides are known.
Migratory Insertion & Eliminations
α-Hydride Elimination (AHE)Elimination of an α-hydrogen from metal alkyl complexes. This forms a carbene. Much slower than β-elimination processes and usually only occur when BHE is not possible. More common
with early transition metals (d0, group 4 and 6), but can happen with later metals.
Eliminations are the reverse reaction of migratory insertion and can occur one after the other. The group being eliminated does not have to be the one that participated in the insertion.
There are several types of eliminations.
LnMH
HLnM
HH
Often induced by ligand exchange processes.
V
Cp
Me3P
t-Bu
t-Bu
PMe2Me2P+ V
Cp
PP
MeMe
MeMe
t-Bu + tBuCH3 + PMe3
MM
Nucleophilic Attack on Coordinated Ligands
Attack on Metal-Bound Carbonyl – The nucleophile is typically strong nucleophiles, like RLi
Many different kinds of examples of this. From our prespective the more important ones involve attack on M–CO complexes and M–alkene/alkyne complexes.
LnM C O
Ln is good π-acceptor(another CO)
RLi
LnM R
O
acyl "ate" complex
usually quite stable and can be further manipulated
Attack on M–C σ-Bonds – Such bonds are often intermediates in catalytic reactions. The carbon can be sp2 or sp3 hybridized. Nucleophilc reactions with η3-allyl complexes fall in this category. Can also be considered as a "reductive elimination" process.
ArPd
O
L
LX ROH
PdLn ArCO2R+ + HX
Nuc–Nuc
Nucleophilic Attack on Coordinated Ligands
Many different kinds of examples of this. From our prespective the more important ones involve attack on M–CO complexes and M–alkene/alkyne complexes.
Attack on M–C π-Bonds – By ligating the metal, alkenes and alkynes usually become electrophilic. This makes then susceptible to nucelophilic attack. Depending on how the nucleophile reacts, the addition can be syn or anti.
M
Nuc–
M
Nuc"external" addition of nucleophileproduct of anti addition(most common pathway)
M Nuc
insertion
M
"internal" addition of nucleophileproduct of syn addition
Nuc
Other nucleophilic reactions will be covered as needed
Nuc–
Transmetallation
M1 R M2 X+ M2 R M1 X+
M1 = Mg, Zn, Zr, B, Hg, Si, Sn, GeM2 = transition metal
Importance is growing as this is a key step in useful methods for constructing C–C bonds, particularly such bonds that are difficult to forge by other means. However, the exact mechanism by which
transmetallation occurs is not well understood and seems to be quite dependent on the metal species.
Generally speaking, transmetallation involves replacing the halide or pseudohalide in a transition metal (M2) complex with the organic group of a "main group" organometallic (M1) reagent. This step is almost always the rate-limiting step and is usually the culpret when cross-coupling reactions fail.
This is an equilibrium, so to ensure success both partners must gain some thermodynamic benefit. Often this can be enhanced by appropriate "activation" of the main group element.
Isomeric integrity (cis, trans) is usually maintained when R is an olefin. With alkyl metals the situation is more complicated. With polar solvents, alkylstannanes can transmetallate with inversion of
configuration (open transition state?), but in less polar solvents retention is seen (closed transition state?). However, aliphatic organoboron reagents tend to proeed with retention.
Pd
R
XL
LC SnBu3
proposed open t.s.leading to inversion
Pd
R
XL
LC
SnBu3
proposed open t.s.leading to retention
similar mechanisms could be drawn with other metals under apprpriate
activation
Fe
Electrophilic Attack on Coordinated LigandsSeveral different reactivity modes depending on the metal, ligand, and electrophile involved. More
specific examples will be discussed as needed.
Electrophilic cleavage of σ-alkyl metal bonds – Note metal is removed.
R M + E+ R E + M+ retention at R
Me Fe(CO)2Cp Me DDCl CpFe(CO)2Cl+
Attack at α-position – Forms carbenes
M CHPhH
+ M CH
PhM C R + M C
H
Ph
H+Ph3C+
OCOC CH2OH
Fe
OCOC CH2
TMSOTf
CH2Cl2–90 ºC
TfO–
+ Me3SiOH
Electrophilic Attack on Coordinated LigandsSeveral different reactivity modes depending on the metal, ligand, and electrophile involved. More
specific examples will be discussed as needed.
Attack at β-position
+ + E+Ph3C+H
M M M ME
M R + E+ M CE
R
vinylidene
Mn
OCOC
Mn
OCOC C
MeOTf
CO2Me
Me
CO2Me
(OC)5W Ph
O
(OC)5W Ph
O Me3OBF4(OC)5W
Ph
OMe
Electrophilic Attack on Coordinated LigandsSeveral different reactivity modes depending on the metal, ligand, and electrophile involved. More
specific examples will be discussed as needed.
Attack at γ-position
+ E+M ME
+M MA
A
BB
MA
B
SnBu3
R2
R1 + R3CHO PdCl2(PPh3)2
R3
OH
R1
R2
likely involves formation of η1-allyl intermediate
Cp(CO)3Mo
Me
ArSO2NCO+N
Me
Cp(CO)3Mo
O
SO2Ar