chiral induction in photochemical reactions. 14

0957416691 s3.w.00 pergmm Pm plc

Linear Free Ettergy Relationships as an Xnstrume~t for

the Pr~dict~~~ of &her de values or C5~furrnat~~~

Energies of the auxiliary in the ~i~ter~~lectiv~

Patern~-cocci R~ction*

HEIST BUSC~~ANN, NORBERT HOF~A~ AND FANS-~IRT~R SCRAP*

Institi ftir Drg~ Ckeznie der RWTK Aaches

~~.-~~et-~r. 1, D-5100 Aachen F B G , . f .

Pi = a * Qi

1429

1430 H. BUSCHM~ et al.

Pi representsthepropertyofasystemunderthe~fluenceofafactori, whereasQiisacharacteristicfigureof its respond on i. p1 is a specific pmpo~o~ity factor. CMer examples which obey the Relation (If are for instance the ~r~~ted~ OT the ~arnm~tt relation.6

For the diastereoselective P~m~-R~chire~tiony according to(2)8*9*1 1 aH~e~-~~ogous relation was ob served, when phenyl substituted-phenylglyoxyhttes of type 1 wereconverted photochemically with furan 3 as the ole- fiic partner. The logarithmic parameters of selectivity (k/k’)(kHKH) for this reaction correlate with the a-Hammett constants of me pura substituents R” of phenyl gro~p.‘~

However a corresponding relation for the auxiliary in (2) is not observed, ifparu substituted trans-‘I-phe- nylcyclohexanols of type 2 are used as chial

C R* = chiral Auxilq

1 2

The compatison of the auxiliary structure of the cyc~ohex~ol derivatives shows no cormlation of the selectivities with the stereic demand of the shield~g su~i~n~. %tt Acing to common conceptions a steric ~l~ge~ut of the auxiriary should favour for high diasteremeric excesses in a~eement with the “conc~~~o~v~-~~~e~ of Helm&en and Schmierer9,t2, but however very often the contrary happens.

This is demonstrated by the dependence of the diastereoselectivities from the auxiliary structure and the olefmic partner in the investigated reaction (2) (Table 1).

Conformation Energies of Substituted Cyclohexanols To fiid a Linear Free Energy Relationship between structuml parameters ofthe cyclohexanol auxiliaries and the

~lectivitiesof~ere~t~onwe triedtocorrefatethefreeenthalpyofactivationAG#ofasystemtothefreeenthalpyAG” of the ground state conf~e~ ~co~~g to Equation (3)P

6 (AC*) = p * li(AG")

The operator 6 describes the influence of the substituent variation in Equation (3). The differences of free en- thalpies of activation for the Reaction (2) on the basis of the product ratio of the di~te~ome~c oxetanes for a given tem~ratu~ are available by AAG” v~ues.*19~11

Chiral induction in photochemical reactions-XIV 1431

Table 1: ~i~e~o~l~v~ty in terms of $b de values[al for ~phot~h~~~ly oxetane axon (2}, if chiral phenyl~yoxyl~~ are added to olefmes of type 8 (T = 14’T),

tran&substituted cyclohexanols[a]

de value (96)

5 6 CH3 7 3

1 49,s 55,4 52,4 67,O 65,4

2 40,7 39,8 35,6 40,4 50%

3 -XI- / \ CH3 454 46,O 42,6 52,4 59,4 -

55,O 43,l 71,0 63,6

w 47,4 61,O 72,6

42,0 37,2 49,2 59,s

no reaction

1432 H. BUSCHMANN et al.

ii-

11

12

13

14

15

16

17

-CH,

-CH2CH2

p3

-CH&i

‘CH,

p3

-CH

‘CH,

CH3

I -C-CH3

I CH3

CH:, 38,3 39,8 252 51,2 61,8

-Cl

-Br

35,6 41,8 253 41p 44,2

45,7 4877 22,0 w 52,l

39,2 39,l 31,4 48,6 548

35,6 38,6 19,0 39,2 47,6

36,8 43,2 2394 43,6 46,4

4874 59,l 27,8 641 59,o

91,0 92,0 946 95,4 95,6

45,8 49,3 rq2 51,8 51,4

33,8 40,4 33,4 53,0 41,0

32,4 23,8 29,s 36,0 42,8

29,4 4w 43,0 48,8

Choral induction in ~hot~he~~ reactions-XIV

[aI The ratio ofd~~~rne~ is derived from the%%MR spectra of the axetane mixture; tbl far the photo- chemi~~y ad~tion to olefmes 47 benzene as solvent WB used; fc] in the case of ~3 the olefine itself WaS used as solvent; [d] axon time 6 hours; [e] axon time 15 hours (mercury high pressure lamp: Philips HPK 1251.

1434

Generally molecules exist in different conformers .Thus the conformation energy13 seems to us to be a suitable figure for the representation of the ground state enthalpy AC” which would characterize the influence of the auxiliary in acertaiu sence. Aconformation of amolecule designates the alteration of itsstructure by simply rotation aroundsin- gle bonds.‘3a

Monosubstituted cyclohexanols exist as two stable conformers which are distinguished by either axial orequa- torialpositions of a substituent. They ~terch~gee~ily by rotation of single bonds, weak defo~~ion of bond angles and by su~u~g torsional stress of hydrogen atoms. The two chair confo~ations 9 and 9’ are disc~in~ed by a con- siderable amount of energy ~~nd~g whether the substi~ent X is in an axial or in an ~uato~~ position, however. ~~~e~~3ade~es in general the confo~ation energy of a cyclohex~ol derivative as the free energy AG” of aconform- er relative to the item of lowest energy.

Forexamplethe negative value of the free energy AG” for the equilibrium Q/Q’ is the conformation energy of the axial conformer 9’. These energy values are available in literature at least for monosubstituted cyclohexanol derivatives (Table 2).14

Table~Confo~ationener~es[aj(ch~g~of~eeeo~~pie~ the~i~equ~o~~~uilib~umofmonosubsti- tuted cyciohexane derivatives 919’ in aprotic s&ems).

X -AC”

kJmol-l ’

-AG”

kJmol_’

F 0.53 Ph 125.40

Cl 1.80 cycle-C,H, 34 1 9.20

Br 1.57 OH 2.17

I 1.80 0 AC 2.51

CN 0.71 OCHJ 2.51

CH3 7.11 OCH,CH, 3.72

CHzCH3 7.35 OTs 2.09

~H~CH~CH3~ 8.79 SH 3.75

CH,CH,CH,CH,34 8.79 SCH, 2.93

CfWH3)z 8.99 SPh 3.34

WH,), 50.20

[al Literature data (“the best values” determined by different methods) taken from Ref. 14 if not otherwise noted.

ItisobviousfromTable2~~confo~~onenergies~nlyde~ndon~eatomofasubstiuentwhichis~tly hnhed to the cyclohex~ol ring. t3aCon~quendycyc~ohex~ol derivatives ha~gsu~tituents like: OH, OAc, OCH,, 0CH,CH30r 0% resamble in their conformation energy. The same is true for derivatives having groups lie: SH, SCH3, SPh, respectively CH,, CH&H3, CH(CH&

Chiiai induction in phol~~rnic~ reactions-XIV 1435

As long asthe direct linked atom carries at least one H-atom m~~vely an electron lone pair the syn-axialre- puision does not increase. If the rert butyl group is in au ~~~ition~ any case one ~~~1~~ of it isdirected to- wards the middle of the ring. Cons~uently its repulsion potential with syn-axialH-atoms increaseconsiderably. This explains the strong aversion of the tert butyl group of acyclohexane ring to occupy an axial position (anchor function of a tert butyl group).15916 The collected values in Table Zrepresent the differences iu free enthalpy. There are only a few investigations known whiih give iuformation about the share ofthese values in terms of enthalpy and entropy. An entropy differences is certainly expected for unsymmetric substituents. For instance the differences in AGo values for methyl, ethyl and iso propyl cyclo~x~e is completeIy reduced to entropy diffemnc~.17

What about the ~onfo~~on energy of mult~b~~~d cyclohexanes? For the fust approach Eliat assumes, that the conformation energies behaves additively. t3a The confotmative behaviour (4) of tranf-1,2-disubstimted cyclohexanes 11 demonstrates the preferential occupation of the bisequatorial position. Thii can be additionally sta- biked or destabilii by gauche-interactions of X and Y considerabIy.18*19

According to Zefirov I9 the total conformation energy AGO of the trans-1,2disubstituted cyclohexanes constit- ~ntsiscompos~ ~ditiveiyof~e~~en~~~y of~~onf~~on~u~~~aof~~~po~mgmon~u~i~~ cy~lohex~es AC*, and AGor hording to Equation (5) conside~g also the g~c~-~ter~~ AGox~p of the substituents in the bisequatorial position 1k20

AGO = AG; t A";+ AG& (3

Inthecaseof~pu~i~tlleenergyofguuche-inreractionbecomes positiv. Itisneg~v,~~~tion~~~nthe comments occur (e.g. in the case of strongly electronegative sub~imen~). The dete~~on of guuc~~-~te~ti~s is howeververy diffkult to achieve. Therefore it has been executed for a few examples only. Since the conformation energy of the applied auxiliaries (Table 1) were unknown, we used the additivity principle for their calculation as the first approach.

~epbeny~lyoxy~e~sidueof~ea~~~~~Table 1 istfreu~h~gedsuu~featoreoftbis~mpoimds, soits gu~~-~~~tions with ~eadjacent substiments(~~or~u~o~~) miserly ~~a~orn~~~e orderof magnitude. Thiiailowsus to compare the conformation energies with each other. t3a In Table 3 we present these energies which have been calculated by the additive procedure. Instead of the unknown conformation energy of the cyclo- hexylphenylglyoxylaweusedtheAGo-vfueoftheacetateresidue14becauseofthes~eprimarilytothering~ atom. Therefore the conformation energy difference of the cyclohexylesters should be low, *ja The influence of tem- pera&ire and solvents on the AG” term has heeu neglected.

Con~quendy the values in Table 3 have the character of st~i~~l~ve figures with rhe ~v~~~ to be comparable.

The (MG?AG’+Relation for the Diastereoselevtive Paternb-&hi Reaction The AC’-values of the auxiliaries, determinatedaccording to this procedure and summerizedin Table 3 yield for

a given olefiu 4-7 at constant temperature in the -AAG%AC”-diagram a linear plot which is demonstrated for 2,2~~~yl-l~~oxol5 as a typical olefinic partner for T = 14OC iu Schetue 1.

Fromthisdiagramw~chreffectsdte~~nc~oftheolefinic~ersaswell~;ihereactionconditionsthein- ~r~ept~ve~~eMG#~v~ue andthep constant isev~u~~~~eslo~. Forthe olefines3-7 the values havebeen smmnerized in Table 4.

Theinducedabsolut configurationofthemajoroxetanedepends~theconfigurationofthec-atomofthecyclo- hex~e~gc~~g~e OH-group. 1 A21 fluency a factor “a” is defined tl for (1R) configurated ~x~i~es~d -1 for ~yclo~x~e derivatives with (l~)~nfi~on.

1436 H. BUSCH&INN er al.

TableA Conforrnatione~r~esAG’~a]fortheauxitiaTieswithcyclohexanestructureapptiedinthephot~em- *“&a “~~L”‘.~ IYII......“.. ,-,.

C R*-OX -AC* C R*-OX -AGO

;;mcH3 19.20 3

__. _ . . . . . . [a] The AGo values are composed of literature data shown in Table 2 accordmg to the addrtrvrty-princupte (AG” = &AGoi,at=+l forequatorialsubstituemsXandat=-1 foraxialgroupsXofthemoststablechaircon- formation); gauche interactions described by Zej?rov*9 are neglected. Instead of the AGot-value for the phenyl- glyoxylate substituent the conformation energy of the acetate is used. t4The temperature and solventdependen-

ties of the AGO-term are neglected too.

Thus a Linear Free Energy Relation (Equation (6))cnn he formulated forthe stereoelectronic influences of substituents on the cyclohexane auxiliaries. This is in accordance with the Ug~~~c~ mode1.35

f - 1, for (1R) configurated cyclohexanols

a= + 1, for (1s) configurated cyclohexauols

ChiraI induction in ph~~he~c~ reactions-XIV

Sdremel:(-MG’/-AGo)-~i~forthereacti~~ofphenylglyoxyl~sofsubstitutedcyclohe~o~with~- dime~yl-1~3dioxol$ in benzene at T = 14*C1’

10 20 30 40 50 60

Table 4: The AAG**- and pvalues for a given olefine ~~v~~orn dse co~espon~mg MG#lAG*~~.

According of this relation MG* is a function of Mf, and AGo (7)!6

Scheme2&~snatesthederivationoftherelevantp~te~of~uatian(6)forthePatemb-Biichireaetion, Principally this procedure should be extended also to other reactions as far as diasteteoselectivity in a stoicho-

1438 II. BUS~HM~ et al.

metric asymmetric synthesis is induced by a chiral auxiliary of the cyclohexane typ . The atmlactinat formation (8)‘“~2~z3 is a farther example in this line,37

With the reaction specific parameters p and MG$ and sub~uent application of Equation (6) the extent of dia- s~re~lectivit y and the diction of chii ~duction should be c~culabl~ as far as the confo~~on energies, tbeole- fiie and the reaction conditions are known. On the other hand ~uation (6) and Reaction ~2)~~ be appIi~ for the cai- c&ion of an uaknown confo~~ion energy AC? of a mono emitted cyclohexane auxiliary.

Beside numerous spectroscopic and kinetic methods for the determination of conformation energies it is now possible to use additionally a selectivity parameter of a stereoselective reaction.

In Table 5 we have collected the hithero mown ACP-values for monosubsti~t~ cyclohex~e derivatives which we have evaluated with Equ~ion (6). The items in Table 5 are averages rest&g from ~de~ndent determinations for the olefines 3-7. As publish~ data from Table 2 were used 5~s basis for calculated values, both data sets are comparable.

Additionally an electronic influence on the conformation energies is observed, since steric interactions remain more or less unchanged. However, the steric effect seems to be large enough that a Hammett relationship is not ob- tained,

Furthermore the increase of the stericinteraction for the mesityl- and naphthyl substiituents compared to the phenyl residue is evident. If one compares the similar conformation energies of the benzyl, 2-phenylethyl and iso butyl group with each other, again the dominance of the atom primarily iinked to the ~yclohex~e ring becomes obvious.

In line with the MG#/A~o#~el~ion one can conclude that the selectivity of the Reaction (2) exhibits not a simple ~nction~ity of the sterical proprties of the shielding group but is essentially determined by inte~ol~l~ stereoelectronic effects in the auxiliary - as it definitely should be expected.

Origins and Reasons fur Selectivity Control Caused by Conformational Influence of the Auxiliary

The product ratio is either influenced by the populati~ of conformers in the educts or by those in the reactive in- foliates wirhii a cock rea~tion.~~~,26~ 27 For the fo~r~ere an: only a few examples known, however. As an example Still and Julynkzr 28described the educt confo~a~ion controled selectivity in the ~etic~ly controled hydrogenation of macrocyclic exo-me~y~neketones. The basis of ~e~~urnentsis a quantum mechanical study of equilibria of educt conformations. In the vast number of kinetically controled stereoselective reactions in the ground state the Curtin-Hammett principle 24, 26is valid. This principle affirms that the major product is formed via the transi- tion state of the lowest energy. The principle explains, why the prefered confonnarion of an educt is not necessarily its most reactive one. If the free enthalpy of activation is much higher than the energy of conformation interchange for the educt, the prediction ofthe product ratio on the basis of the prefered educt conformation isnomore possible. This is expected for most reactions,

~ot~hernic~ and photophy~~~ processes should be typical examples for app~c~ion of the C~~~~n-~a~e~r purple, because in this cases the excitation energies are much larger than the energy of ~tivation for confo~~ion interchange. Nevertheless, examples are known, where different photoreac~ons result from different educt conformations,29* 3o

A provedexample hasbeeninvestigatedbyNavinga.31 It is thephotocyclzation of 1,35hexatrienederivatives. Further arguments have been collected by Lewis and Johnson (“Can Molecular Co&wmation Controf Pkotochemi- cal Behavior ?“).32 If two conformers A and B according Equation (9) would lead to two diffe=nt photopr~u~s X resp. Y one can discuss two ~~erline cases.


(9)

k AP

Due totheFranck-Condonprinciplethepopulationof A* andB* is first determined bythe populationofthecon- formers A and B in the ground state and by the extinction coefficients of each one.

Scheme2 Schematicpresentationofthederivationofa,AG’,MG#~dPinEquation(6).AC’andaarespecif- ic constants of the auxiliary. AGo is compares of literature data14 according to the additivity-principle (AGO = pi AGoi). a may be derived empirically from comparative considerations of the structure of the auxiliary and the absolute configuration of the major diastereomer: a = +I if the auxiliary is (l!+conQurated (oxetaue 10 is the major diastereomer), a = -1 if the auxiliary is (lS)-configurated (oxetane 10’ is the major diastereomer). MG* is deduced from the product ratio (In (k/k’) =-RT AA@, k/k’ = [lO]/[lO’] fork&‘). The oletinic partner, the temperature and further reaction conditions keep constant. From the MG*/AG”-plot the reaction specific parameters MG$ and p are derived. They depend on the reaction conditions and the olefmic partner.The product ratio can be predicted, if the AC” is known. On the otherhandthe de valuescan be used todetenninthe conformation energy AG” of mono substituted cyclohexane derivatives with substituents X.

- AGO -)


Table 5: The &*-values [a] for monosu~im~ cycohexane derivatives with a su~~ent X derived from Equation (6) because of the determined de values in the phot~he~~ oxetane fo~ation (2).

[a] The given AG%alues are averages, which are confmed by independent determinations for the oiefines3.7

In the fit case the conformation barrier (AC OAB)* in the excited state is lower than the energy of activation for the formation of the products X and Y (k *AB >> k,, ke). The Cur#in-Hamme~~~~ciple is valid since the product dis- ~bution is detents by the difference of the energies of activ~on for the products X and Y.

In the second case the energy of ~tivation of the equilib~~ of the conformers At/B* i&&her as the energy of activation forte fo~~on oftbe pr~ucts X and Y (k *+ k,, kB). In thiscase the product dis~~don is deceive by the relative population of the ~onfa~m A* and B*. The lifetime of the latter caa be napery different.

The ~G~/AGO~o~el~on doesn’t imply, that the di~temo~l~tivi~ is directly contra&i by refound state confo~ation equ~b~um, because this ~fo~~ian ratio doesn’t determine the de value in the oxetane mixture. The

observed AAG*/AG’- co~l~on means at least, that a certain of n%.tlon between the confound energies of the auxiliary and the observed ratio ofdiastereomers in the product exista’lbe confo~tions can influence the product ratio on different levels of diastereoselection.

Scheme 3

l.hv 2.1x!

i

In lie with the classifrcatlon of Lewis and Johnson %e photooxetane formation is thws assigned to the first caseifonlytheoveralprocessisconsidered-thlsm~sthatafastequilibr~ionofconfo~~andthede~ationof the product ratio by the diffetences of ~tiv~ion energies of the subs~u~t processes exists.

If one discusses the selection ~h~~rn in terms of the i~*~~y~rsi~n Pr~c~ie8~9 the Muence of ~ea~i~~ confo~ations can take place either on tbe first level (formation of the 1,4-b~~c~ ~~e~~s) or on the second level (~~l~v~e of the bodice steppes). Since the ~pul~~on of~xi~~ confo~o~ is not d~dy re- ~ctedintheratiooftheoxetanediastereomers,tbein8uenceontheIevelofselectionaccordingtoScheme3isof minor dance. Thii is a~pple~nt~ hint for the ~~rnpti~ that the C~~-~~~ei~ p~~ipl~ is not violated+

A control of the cycfisatio~e~~leavagerrttio on the level of the triplet biradicals by ~e~onf~ion energies of the auxiliaries is much more likely (Scheme 4)‘ Thii may happen through the control of biical confo~tions by the auxiliary which means that conformers which are energetically unfavorable for ring closure to the relevant oxetane better split into the educts whereas the favourable conformations of the biradicals easier undergo ring closure, Altema- tively the influence of auxiliary ~nf~ons at the lavel of the 1prbiradical.s can also be discussed in that way that ~~~i~es with bi~uato~ c~fo~e~ p~ferenti~ly split into the educts (s~ere~lec~o~c gcu&e e&c& which also would explain that selectivity is produced.

The dom~~ce of these both ~tem~ves may possibly be ~rn~~~ de~nden~


Scheme 4: Conformation control of the auxiliary in the triplett 1,4-biradical intermediates as a consequence of stereoelectronic regulation of the cycl.izatioo/retrocleavage ratio for the case of rrunr-2-substituted cyclohexa- no1 auxiliaries. The bisequatorial auxiliary conformation favors a cisoid 1,6biradicai conformation, which pre- dominantly gives oxetanes. In this case the bisaxial auxiliary conformation favors the traosoid geometry of the 1,4-biradical intermediate, which p~om~ently undergoes retn?cleavage. Alte~atively the ~~~q~ato~~ auxiliary co~o~~oo may also favor~e re~~leavage ofthe 1,4-biradicai. In this casethe selection isn’t by efficient retrocbavage.

ISC /J 1.

2.

,cO2R* 3

Ph

IZ 0

zi= 0

4 0 R J J ISC

-


Conclusion We could demonstrate that diitereomeric selectivities of the stoicbmetric asymmetric Patemb-Biichi reaction can be projected to structural items of an auxiliary of thecyclohexsnetypeviaaLinear Free Energy Relationship.Now, we are able to predict either de values or conformation energies of the auxiliary.

A more detailed ~vestig~onof the co~~n~g influence of entbalpy resp. entropy on the basis of t.beJ.%&- ve~s~n P r~~c~~e9 is possible and would give more detailed dicta the selection rn~~rn of this reaction. Fur- ther studies are on the way.

Acknowi~ge~~. - We thank the ~e~ts~he Fors~~~gsgerne~~~ (DFG) and the Fo& der chemisc~n In- dusrrie for the support of this work and we are very grateful to Prof. I. Ugi for valuable discussions.

References and Notes 1. Part 13: !&ar, C.; Scharf, H.-D., Ghan. Ber. 1991,967. 2. a) ~aprn~, N.B.; Shorter, J., C~~~~~o~ ~~~~s in C~~~-Re&e~ Adages; Plenum press: New York.

1978; b) Chapman, N-B.; Shorter, J., Advances in L&rr Free Energy Re~~~~s; Plenum Press: New York. 1972; c) Exner,O., Progr, P hys. Org. Chem. 1990,18,129; d) Exner, O., Collect. Czech. Chem. Comnuuz. 1974, 39,515; e) Johnson, CD.; Chem. Rev. 1975,75,755; f) Sj&trUm, M,; Wold, S.; Acta Chem. &and. 1981,835, 537.

3. Itisancommonruleinorg~icchemi~ttrarsimilarsystemsactinsimiIarways.Thirp~~plewasalreadycleat- Iy formulated in organic chemistry in the middle of the 19th cenhny. See, for instance: KekuI&, A., khrbuch der Organischen Chemie, Band 1, pp 124-132; Verlag van Ferdinand Enke: Erlangen 1861.

4. Klumpp, G.W.; Reakrivitiit in der Organischen Chemie, Vol. lt2, Thieme Verlag: Stuttgart 1977; English Edi- tion: Reactivity in Organic Chemistry, Wiley: New York. 1982.

5. a) B&n&, J.N.: Pedersen, K., 2. Phys. Chant. 1924,i08,185; b) Marcus, R.A.;J. Phys. C&m. 1968,72,891; C) Marcus, RA, J. Am. Chem. Sot, 1969,9f, 7224; d) Bell. R.P,, in Ref. 2a) pp. 55-84; ef Kresge, A.J., Chem. Sac, Rev. 1973,2,475.

6. a) Hammett, L.P.; PflugerH.L.,J.Am Chem. Sot. 1933,55,4079; b) Hatnmen, L.P., PhysicalOrganic Chemis- try, 1st. edn. McGraw-Hill: New York. 1940; 2nd. edn. McGraw-Hill: New York 1971; c) Exner, O., inltef. Zb, pp. 153,; d) Jaff& H.H. Chem.Rev. 1953,53, I9l;e)H~~h,~,; Leo, A,;Taft, R.W.,C~~.R~. 199191,165; f) SjCrstr6m, M.; WoId, S., Chemica Scripta 1976,9,200; g) Johnson, D.C., ~he~~e~t~~~~n~ Cambridge University Press: Cambridge. 1973; h) Leffler, J.E.; Grunwald, E., Rates and Equilibria ofOrganic Reactions; Wiley: New York. 1963.

7. a) Pate&, E.; Chief& G., Gm. Chim. Zful. 1909,39,341; b) B&hi, G.; Inman, G.G.; Lipinsky, E.S., J. Am. Chem. Sot. 1954,76,43X$ c) Arnold, D.R., Adv. Pholochem. 1968,301; d) Jones, G. II, Organic Phorochemis- try; Padwa, A. Ed.; Marcel Dekker: New York, 1981, Vol. 5, p. 1; e) Carless, H.A.J., Sorbic Org~c Photo- c~rnis~~; Horspool, W.M., Ed.; Plenum Press: New York. 1983, p. 425; Q Braun, U,Nachr. Chemm. Techn. l&b. 1985,33,213; g) Wilson,KM.,~rga~~~~~~ch~~js~~ P~w~A.,Ed.;M~lDe~er:New York 1985, Vol. 7, p. 339; b) Demuth, M.; M&bail, G., Synthesis 1989,145.

8. Bight, H; Scharf, H.-D.; Horn, N.; Plath, M-W.; Runsink, J.J. Am. Chem. Sot. 1989, Ifi, 5367. 9. Bu~hm~, H.; Scharf, H.-D.; Hog, N.; Esser, P., Angew. Chem, l991,103,480;Angew. Chem. ht.&+.

En& 1991,31,477. 10. Pelter, R.; Scharf, H.-D.; Buschmann, H.; Runsink, J., Chem. Ber. 1989,122,1187. 11. Buschmum, H., Dissertation, Tech&he Hochschule Aachen 1991. 12. a) Helmschen, G.; Schmierer, R., Angew. Chem. 1981,93,208; Angew. Chem. Inr. Ed. Engl. 1981,20,205; b)

Fi%ster, ft; V@tIe, F., Angew. &hem. 1977,89,443; A~gew. Chem. Int. Ed. Engf. 1977,16,449. 13. a) Etie1,E.L. A~gew.Che~. l96!!,77,184;Angew. C~rn.~~.~, Engl. 1965,4,761; b) Fe~p. H.;F~~,

NC., Angew. Chem. 19&T?, 798; Angew. Chem. fnt. Ed. Ertgi. 1965,4,774; c) Eliel, E.L., J. Chem. Edtic. 1975,52,162; d) Mann, G., 2. Chem. 1990,3D, I; e) EIiel, E.L.; Ailmger, N.L.; Angyal, S.Y.; Morrison, G.k, C~~o~~~ Abyss btescience Publishers, Wiley: New York. 1965,


14. Hirsch, J.A., Top. Stereo&em. 1968,1,199. 15. Allinger, N.L.; Freiberg, L.A.,J. Am. Chem. Sot. 1960,82,2393.

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Zefuov, N.S., Tetrahedron, 1977,33,3193. 20. a) Wertz, D.H.; Alliuger, N.L.. Tetrahedron 1974,30,1579; b) Osawa,E.;CoIlins, J.B.; Schleyer, P.v.R., Tetru-

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36. The dependence of the MG’ tenn on MG $ and AGO is comparable to empirically deduced Marcus equation (see Ref. 5b,c).

37. For R”’ = CH3 and T = 0°C : p = 0.08, -MGz = 0,25 kJmol_1 (further reaction conditions see Ret 23).

chiral induction in photochemical reactions. 14

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