Push-pull alkenes

17 Pages·2006·0.27 MB·English

by  Milan Bogdanovic

J.Serb.Chem.Soc.71(1)1–17(2006) UDC547.313+54–724:543.422.25
JSCS–3394 Reviewpaper
REVIEW
Push-pull alkenes: structure and (cid:1)-electron distribution *
ERICHKLEINPETER
UniversitätPotsdam,ChemischesInstitut,Postfach601553,D-14415Potsdam,Germany
(e-mail:[email protected])
(Received12July2005)
Abstract:Push-pullalkenesaresubstitutedalkeneswithoneortwoelectron-donat-
ingsubstituentsononeendofC=Cdoublebondandwithoneortwoelectron-ac-
ceptingsubstituentsattheotherend.Allowancefor(cid:1) -electrondelocalizationleads
to the central C=C double bond becoming ever more polarized and with rising
push-pullcharacter,the(cid:1) -bondorderofthisdoublebondisreducedand,conversely,
thecorresponding(cid:1) -bondordersoftheC–DonandC–Accbondsareaccordinglyin-
creased.Thispush-pulleffectisofdecisiveinfluenceonboththedynamicbehavior
andthechemicalreactivityofthisclassofcompoundsandthusitisofconsiderable
interesttobothdetermineandtoquantifytheinherentpush-pulleffect.Previously,
thebarrierstorotationabouttheC=C,C–Donand/orC–Accpartialdoublebonds
((cid:2) G(cid:3) ,asdeterminedbydynamicNMRspectroscopy)orthe13Cchemicalshiftdif-
ferenceofthepolarizedC=Cpartialdoublebond((cid:2) (cid:1) )wereemployedforthis
C=C
purpose.However,theseparameterscanhaveseriouslimitations,viz.thebarriers
canbeimmeasurableontheNMRtimescale(eitherbybeingtoohighortoolow;
heavily-biased conformers are present, etc.) or (cid:2) (cid:1) behaves in a non-additive
C=C
mannerwithrespecttothecombinationofthefoursubstituents.Hence,ageneralpa-
rametertoquantifythepush-pulleffectisnotyetavailable.AbinitioMOcalcula-
tionsonacollectionofcompounds,togetherwithNBOanalysis,providedvaluable
informationonthestructure,bondenergies,electronoccupanciesandbonding/anti-
bondinginteractions.Inadditionto(cid:2) G(cid:3) (eitherexperimentallydeterminedor
C=C
theoreticallycalculated)and(cid:2) (cid:1) ,thebondlengthoftheC=Cpartialdoublebond
C=C
wasalsoexaminedanditprovedtobeareliableparametertoquantifythepush-pull
effect.Equallyso,thequotientoftheoccupationnumbersoftheantibondingand
bonding(cid:1) orbitalsofthecentralC=Cpartialdoublebond((cid:1) * /(cid:1) )couldalso
C=C C=C
beemployedforthispurpose.
Keywords:push-pullalkenes,barrierstorotation,13Cchemicalshiftdifferences,ab
initioMOcalculations,NBOanalysis,quantificationofthepush-pulleffect.
* PlenarylecturepresentedattheXLIIIthAnnualMeetingoftheSerbianChemicalSociety,Jan-
uary24th–25th,2005,Belgrade,SerbiaandMontenegro.
doi:10.2298/JSC0601001K
1
2
KLEINPETER
CONTENTS
1.Introduction
2.Quantificationofthepush-pulleffect
2.1.BarriertorotationaboutthepartialC=Cdoublebond
2.2.13Cchemicalshiftdifferencesoftheolefiniccarbonatoms
2.3.AbinitioMOcalculationstoquantitythepush-pulleffect
3.Thepush-pulleffect
4.Peculiaritiesofpush-pullalkenes
4.1.Thepseudochalcogenoanalogyprinciple
4.2.Thioamide/vinylogousthioamide"resonance"
1.INTRODUCTION
The(cid:1) -bondorderofethyleneisca.1,however,substituents,dueeithertotheirin-
ductiveormesomericeffects,canincreaseordecrease,respectively,thedoublebond
character.Inthisrespect,push-pullalkenesareofspecialinterestastheyhave1or2
electron donating substituents on the one carbon atom together with 1 or 2 electron
withdrawingsubstituentsonthesecondcarbonatom.Duetothecontradictorycharac-
terofthesubstituents,theolefinicC=Cdoublebondorderisreducedonthebehalfof
Scheme1.
Approximationat
thecoalescence (cid:2) G(cid:3) (cid:2) H(cid:3) (cid:2) (cid:3) S
temperatureTc k (cid:4) (cid:3)(cid:6)(cid:8) kBT(cid:9)(cid:11) e RT (cid:4) (cid:3)(cid:6)(cid:8) kBT(cid:9)(cid:11) e RT e R
(cid:7) h (cid:10) (cid:7) h (cid:10)
(cid:1) *(cid:1)(cid:2)
k (cid:4) c
c
2
(cid:2) G(cid:3) =19,14Tc(cid:6)(cid:8)(cid:8) 10,32(cid:5) logTc(cid:9)(cid:11)(cid:11)
(cid:7) k (cid:10)
c
Restrictedrotationabout
theN-Phenylbond
(cid:2) G(cid:3) =60.4kJ/mol
(cid:2) H(cid:3) =38.1(cid:12) 1.1kJ/mol
(cid:2) S(cid:3) =55.5(cid:12) 4.27JmolK-1
Fig.1.Estimationofrotationalbarriersby(i)approximationatthecoalescencetemperatureT /K
c
and(ii)bycompletelineshapeanalysis.
3
PUSH-PULLALKENES
theincreasedbondordersofthebondsbetweenthetwoolefiniccarbonatomsandtheir
respectiveelectrondonorandacceptorsubstituents.Thiseffectisreadilyapparentby
therestrictedrotationsabouttheC–DonandC–Accpartialdoublebonds(depictionsB
andCinScheme1)whichcanbestudiedquantitativelybydynamicNMRspectros-
copy.Forafeasibleexperimentalexaminationofthedynamicbehaviorofthecentral
olefinicC=Cdoublebond(depictionA),betterrepresentedbyapartialdoublebond
(depictionD),thepresenceoftwoelectronacceptorsubstituentsandatleastonedonor
substituent is necessary.1 The barrier to rotation ((cid:2) G(cid:3) ) about this C=C double bond
canbereadilyemployedasameasureofthepush-pullcharacterofthealkene.1,2Rota-
tionalbarrierscanbemeasuredatornearthecoalescencetemperatureofthedynamic
rotational process, or further afield by complete line shape analysis (Fig. 1).1–4 2D
EXSYNMRexperimentshavealsobeenemployedforquantifyingthisprocess.5
However, the dynamic NMR window is rather small (ca. 30–100 kJ/mol,
dependingontheavailableNMRequipment)6andoften,evenforcasesexpected
withinthisrange,therotationalbarriercannotbedeterminedduetoheavily-biased
conformerseitherduetoelectronicorstericeffects,orstructuralpeculiaritiessuch
asring-closedsystemsorsymmetryrelationships.Asaresulttherehavebeensev-
eral proposals to replace the barrier to rotation as a quantitative measure of the
push-pulleffect.Inthiscontext,theclassificationofpush-pullalkenesaccordingto
thenumber and character of substituentspresent and somerepresentativeand in-
terestingconclusionswillbereportedinthismicroreview.
2.QUANTIFICATIONOFTHEPUSH-PULLEFFECT
2.1.BarriertorotationaboutthepartialC=Cdoublebond
Thestaticanddynamicstereochemistryofpush-pullalkenespublishedupto
1983 has been reviewed by Sandström;1 subsequent literature on this topic has
Scheme2.
4
KLEINPETER
sincebeencoveredinourlastrelevantpaper.7Asalreadymentioned,twoacceptor
andatleastonedonorsubstituentarerequiredtoattainabarriertorotationmeasur-
ableontheNMRtimescaleandinthisrespect,NR aloneasthedonorprovedmost
2
effective,1 OR as a single donor was determined at the limit8 [e.g.,
MeCOO(MeOOC)C=C(Me)OMe,(cid:2) G(cid:3) =107.6kJ/mol],butSRwasatleast7.5kJ/mol
higher than this and the restricted C=C rotation could not be adequately measured9
[MeCOO(MeOOC)C=C(Me)SMe,(cid:1)G(cid:3) >115.2kJ/mol].Theintroductionofasecond
donorreducesthebarriertoC=Crotationsufficientlyandabroadstructuralcollection
hasbeenexamined.1,7Thesmallestbarriers,ca.40kJ/mol,werefoundwhentwoamino
substituentswerepresent,e.g.,PhC(O,S)(CN)C=C(NR ) .1,2,7
2 2
AnothercaseforlowbarrierstoC=Crotation–butstillmeasurableontheNMR
timescale–iswhenbothgroundstates(GS)andtransitionstates(TS)arestabilizedby
pseudoaromaticstructures,similartotheeffectsobservedinvariousfulvenes.1Inthis
regard, westudied anumber of formylmethylenethiopyranes 1 and thiazinederiva-
tives of Meldrum’s acid 3 (Scheme 2).10,11 In each case, besides (cid:1)G(cid:4) , another
C=C
barriertorotationwasalsoobtained[(cid:1)G(cid:4)(C –C )in1and(cid:1)G(cid:4)(C –N)in3]andin
6 7 2
generaltheC=Cbarriertorotationwasthelargerofthetwo.10,11Thedifferentiation
betweenthetwobarrierswasstraightforwardin3asdifferentsitesallowedforfollow-
ingthelineshape,butthedistinctionwasdifficultinthecaseof1whereonlytheline
shapevariationofthealdehydeprotoncouldbefollowed.Finally,wesucceededsyn-
theticallyandstudiedthering-closedderivative2and,byapplicationofLISreagents,
assignedtheisomers/conformersthatwerepresent.10
2.2.13CChemicalshiftdifferencesoftheolefiniccarbonatoms
The practical application of the barrier to C=C rotation as a measure of the
push-pulleffectislimitedtothetypesofcompoundsmentionedabove.Thezwit-
Fig.2.Linearcorrelationofthebar-
rier to restricted rotation about the
centralpush-pullpartialC=Cdouble
bond(cid:1)G(cid:3) withthe13Cchemical
C=C
shift difference (cid:1)(cid:1) of the two
C=C
carbonatoms.
5
PUSH-PULLALKENES
terionic structure (depiction D in Scheme 1) of these compounds was confirmed
experimentally quiterecently by very low-temperatureX-ray diffraction analysis
ofthepush-pullalkene3-(1,3-diisopropyl-2-imidazolidinylidene)-2,4-pentanedi-
one.12 However, the push-pull effect is already present if only one donor sub-
stituentandoneacceptorsubstituentareattachedtothetwoendsofethylene;anin-
dicationofthiscomesfromtherestrictedrotationsofthedonorandacceptorsub-
stituents(depictionsBandCinScheme1).13Thisstrongpolarizationofthedou-
blebondisalsoreadilydiscernibleby13C-NMRduetotheextremelow-fieldposi-
tionoftheolefiniccarbononthedonorsideandthecontrastinglyhigh-fieldposi-
tion of the carbon atomon the acceptor side of the push-pull alkene (D).2 There-
fore,inadditiontothebarriertorotation((cid:2) G(cid:1))aboutthepartialC=Cdoublebond,
thechemicalshiftdifference((cid:1)(cid:5) )ofthetwosp2-hybridizedcarbonsconstitut-
C=C
ingthedoublebondhavealsobeenemployedtoquantifythepush-pulleffect.Ifthe
compounds compared are rather similar, e.g., aroylcyanoketene-S,S-acetals with
varyingsubstitutiononthephenylring,then(cid:2) G(cid:3) and(cid:2) (cid:1) canbelinearlycorre-
C=C
lated2 (Fig. 2) and (cid:2) (cid:1) can even be satisfactorily employed as a substitute for
C=C
(cid:2) G(cid:3) incaseswhere(cid:2) G(cid:3) cannotbedetermined,e.g.,eitherbecauseitliesoutside
the bounds set by the NMR timescale2,14 or because if forms part of a ring.15,16
The push-pull effect has been satisfactorily assessed by both these parameters in
variousclassesofcompoundsincludingpush-pullalkenes,17–22push-pullbutadie-
nes,23 nitroenamines,24 1-methyl-4-(2’-methylthiovinyl)pyridinium iodide25 and
push-pullenynes.26
Other parameters that have been used to describe the push-pull effect include
boththebondlengthandthe(cid:1)-bondorderofthecentralC=Cpartialdoublebond.
However,onlypoorcorrelationsfortheseparameterstothecorrespondingbarrierto
rotationhaveresulted(e.g.,qualitativedependenceof(cid:2) G(cid:3) vs.p hasbeen
(C=C) (C=C)
reported27).Thisisduetosterichindranceeffects,hydrogenbondingandtheabili-
tiesofthedonorand/oracceptorsubstituentstostabilize/destabilizeeithertheGSor
theTSoftherestrictedrotation.1Thesefacts,togetherwiththegenerallyobserved
non-additivityofthesubstituentinfluencesonthepush-pulleffect,14wasthereason
thatMOcalculationsofpush-pullalkenesatvariouslevelsoftheorywereperformed
inordertoquantifythepush-pulleffectfromatheoreticalpointofview.
2.3.AbinitioMOcalculationstoquantifythepush-pulleffect
The study of the electronic structure of push-pull diazenes28 and push-pull
dyes containing malononitrile dimer as the electron-acceptor29 have previously
beenperformedusingabinitioHFcalculationsanda6–31G*basisset.FortheTSs
ofthepush-pullalkenesstudiedhere(Fig.3),sincetheyhavetwodonorandtwo
acceptorsubstituentsoneithersideofthecentralC=Cdoublebond,theyaresuffi-
cientlypolarizedtostillbetackledbysingledeterminantHFwavefunctionsasthe
diradicalcontributiontotheelectronicstructureoftheTSisnegligibleforstrongly
6
KLEINPETER
Fig.3.DepictionoftheGSsandTSsfortherestrictedrotationaboutthecentralC=Cpartialdou-
blebondincompounds4aand4b.
polarizedpush-pullcompounds.18,19,30,31However,ifthebarriertorotationistoo
hightobestudiedbyDNMR,thentherotationalbarriercannot,infact,becalcu-
lated at the HF level of theory as the system then needs to take into account the
diradicalnatureoftherotationalTSandthecalculationsmustbeperformedusing
multi-configurationalmethods.31aAlternatively,onecanemployanextrapolation
procedurewhichestimatestheTSenergy((cid:2)=90º)fromtheHFenergiescalculated
forconformationswhicharetwistedtosomedegree((cid:2)<60º).32
Inthismanner,thebarriertoC=Crotationinavarietyofpush-pullalkenes4,
aswellasmodelpush-pullalkenes5(Scheme3),couldbedescribedwelltheoreti-
cally7andsuccessfulcorrelationbetweentheoreticallycalculatedandexperimen-
talbarrierstorotationwereobtainedinbothcases.Inthisvein,thecorrelationof
thepresentbarrierstoC=Crotationwiththebondlengthofthecorrespondingpar-
tialC=Cdoublebond(whichwasonlyfoundtobepoorpreviously1)wasagainex-
amined.Indeed,itwasfoundthatthecorrelationremainedpoor;however,ifcom-
Scheme3.
7
PUSH-PULLALKENES
Scheme4.
parable compounds only were correlated then fine linear dependences resulted.
DuetodifferentlevelsofsterichindranceandtheadditionalpossibilityofGSsta-
bilizationbyhydrogenbonding,differentascentsforthelineardependencieswere
obtainedforthevarioussimilargroupings.7Withthisencouragingresultinhand,
Kleinpeteretal.7studiedthemodelcompounds5furtherbyNBOanalysis.33By
NBOanalysis,theoccupationnumbersofthevariousmolecularorbitalsforboth
bondsandlonepairsinthecompoundswereassessedandthedifferentdonor/ac-
ceptorcombinationscomparedaccordingly.Theconclusiondrawnfromthesethe-
oreticalcalculationsisthattheacceptoractivityinpush-pullalkenescanbecharac-
terizedbestbytheoccupancyofthe(cid:2)orbitaloftheC=Cpartialdoublebondand
thecorrespondingdonoractivitybytheoccupancyofthe(cid:2)*orbitalsofthissame
bond. Both parameters are of decisive influence on the corresponding C=C bond
lengthandforthisreasonthequotientofthetwooccupanciescouldbecorrelated
successfullywiththeC=Cbondlength(Scheme4).Indeed,thecorrelationisvery
clear: strong donors increase (cid:2)* orbital occupation, thereby increasing the bond
length,andstrongacceptorsreduce(cid:2)orbitaloccupationtherebyalsolengthening
the C=C partial double bond. It is clearly evident therefore that the length of the
centralC=Cpartialdoublebondofpush-pullalkenesisasimplecharacterizingpa-
rameterforthepush-pulleffectandthequotientoftheoccupancynumbersofthe(cid:2)
and(cid:2)*orbitalsofthisbondisanother.Actually,itisamuchbetterone,moresensi-
bleandmuchbroaderinitsdescription,evenattheHF/6–31G*leveloftheory,of
thepush-pulleffectpresentinthepush-pullalkenesstudied.34
TABLEI.Occupationnumbersofthebondingandantibonding(cid:2)orbitalsofthepush-pulldouble
bondatogetherwiththebondlengthofthecorrespondingbondandthe13Cchemicalshiftdifference
oftheethylenecarbonatoms
Compounds EWG (cid:1)(cid:5) /ppm (cid:1) (cid:1) * (cid:1) */(cid:1) bondlengthp/Å
C=C C=C C=C
6a COPh 66.0 1.89135 0.2285 0.12081318 1.341
6b CO Et 69.1 1.90554 0.21892 0.11488607 1.337
2
6c CONH(CH )Ph 72.9 1.91414 0.21858 0.11419227 1.336
2
6d CN 93.3 1.92029 0.23301 0.12134105 1.337
7a COPh 66.7 1.88854 0.22905 0.12128417 1.34
7b CO Et 69.3 1.90487 0.21757 0.11421777 1.335
2
8
KLEINPETER
TABLEI.Continued
Compounds EWG (cid:1)(cid:5) /ppm (cid:1) (cid:1) * (cid:1) */(cid:1) bondlengthp/Å
C=C C=C C=C
7c CONH(CH )Ph 57.8 1.91140 0.21423 0.11208015 1.334
2
7d CN 93.6 1.92101 0.22703 0.11818262 1.334
7e CONHPh 60.5 1.90560 0.22173 0.11635705 1.337
8a COPh 78.0 1.87071 0.27132 0.14503584 1.352
8b CO Et 83.3 1.89226 0.25734 0.13599611 1.347
2
8c CONH(CH )Ph 83.8 1.90386 0.25097 0.13182167 1.345
2
8d CN 105.0 1.91628 0.25898 0.13514726 1.344
aQuantum chemical calculationswere performedonSGIOctane R12000andSGIOriginwork-
stationsusingtheGaussian98softwarepackage.47Themoleculeswereoptimizedatdifferentlev-
elsoftheoryusingthekeywordopt,optimizationofTSsoftherotationaboutcentraldoublebond
usingopt=tsandcalcfc.Chemicalshieldingswerecalculatedatdifferentlevelsoftheoryusingthe
GIAOmethodandreferencedtoTMSshieldingvalues(calculatedatthesameleveloftheory)toob-
tainchemicalshifts.TheNBO5.0populationanalysis48wasimplementedbylinkingtotheGaussi-
an98programpackage47withthekeywordsnlmoforNLMOanalysisandprintforgraphicalevalu-
ation.NRTanalysiswasperformedwithintheNBO5.0populationanalysiswithnrtandnrtthr=10.
ResultswereportrayedusingtheprogramSYBYL.49
As a further example, compounds 6–8 (Scheme 5) were calculated35 and in
TableIboththetheoreticallycalculatedC=Cbondlengths(p )andthe(cid:2)* /(cid:2)
C=C C=C C=C
Scheme5.
quotientsaregiventogetherwiththeexperimental13Cchemicalshiftdifferences
((cid:1)(cid:5) ). The barriers to rotation for 6–8 could not be experimentally measured
C=C
and,inanycase,atthisleveloftheoryarealsotooimprecise.7Theresultsremark-
ably corroborate both the (cid:2)* /(cid:2) quotient and the bond length to be very
C=C C=C
amenabletodescribingthepush-pulleffectpresentincompounds6–8andthelin-
earcorrelationobtainedispresentedinFig.4.The13Cchemicalshiftdifferences
((cid:1)(cid:1) ) correlated neither with the bond lengths nor the (cid:2)* /(cid:2) quotients
C=C C=C C=C
andthusthisparameterisusefulonlyinveryrarecircumstances.2Butnonetheless,
boththebondlengthandthe(cid:2)* /(cid:2) quotientaresensitive,particularlyuseful
C=C C=C
parameterstocharacterizethepush-pullcharacterofalkenes.
3.THEPUSH-PULLEFFECT
Theelectron density distribution (cid:3)(r) of apush-pull alkenewith two donor and
twoacceptorsubstituentswasrecentlydeterminedbylow-temperature(21K),high-re-
9
PUSH-PULLALKENES
Fig.4.Lineardependencyofthebondlengthofthepush-pullpartialdoublebondandthe
quotient(cid:2)* /(cid:2) incompounds6–8.
C=C C=C
sulutionX-raydiffractionanalysis12and,asaresult,thetetrasubstitutedethylenecan
appropriatelybedescribedasazwitterion(depictionDinScheme1)withalargede-
greeofchargetransferfromthedonortotheacceptorsubstituents.Additionally,large
molecular dipole and quadrupole moments were also obtained.12 The topological
analysis of (cid:3)(r) revealed that the olefinic double bond is characterized by unusual
propertiesincomparisonwithstandardalkenes:thepartialdoublebondboreclosere-
semblance to a typical single bond interspacing two conjugated double bonds, al-
thoughthestructureimpliesalowdegreeofconjugationbetweenthedonorandaccep-
torsubstituents.12Thisremarkablepolarizationofpush-pullalkenesleadstospecial
reactivitywithnucleophilicorelectrophilicreagents,23be.g.,thesubstitutionofado-
norgroupbyanucleophileispossibleonlyifthelatterisabletobetterstabilizethepar-
tialpositivecharge,inducedbytheacceptorsubstituentspresent,thantheformerdo-
norsubstituent.Thisreactivitybehaviorandfurtherpeculiaritiesareverycharacteris-
ticforthepush-pulleffectpresentinthetetrasubstitutedethylenestudied.36
The push-pull effect is closely associated with the unusual properties of this
classofcompounds.Asidefromlargemoleculardipoles,extremelyhighhyperpo-
larizabilities30,37–40 and strong intramolecular charge-transfer absorption bands
have also been measured.41 On the whole, these properties render push-pull
alkenesto beof great promiseasmaterialsfor non-linear optical devices.37 Here
therequiredhighfirst-ordermolecularhyperpolarizabilities[(cid:4)(–2(cid:5),(cid:5),(cid:5))forsec-
ond-orderprocesses]provedtobedependentontheoptimaldonor/acceptorcom-
binationatthechromophore(inotherwords,onthepush-pulleffect).Atpresent,
thereisaninsufficientunderstandingofthecorrespondingeffectsofthesubstitu-
ents (number, positional isomerism, donor/acceptor strength and combination,
preferred conformations and, finally, solution)42 and more work is required be-
10
KLEINPETER
causeinthisrespect,thereisgenerallyonlyasingularcombinationofsubstituents
thatismosteffectiveandthereasonforthisisnotyetunderstood.
4.PECULIARITIESOFPUSH-PULLALKENES
4.1.Thepseudochalcogenoanalogyprinciple
In the dynamic NMR study of compounds 3 (see Scheme 2), in addition to
rotationabouttheC=Cdoublebond,thebarriertorotationabouttheexocyclicpar-
tialC–Ndoublebondwasalsostudied.Thevaluesof(cid:2) G(cid:3) obtainedforthislatter
rotation were compared with the corresponding barriers in their chalcogeno ana-
logs(Scheme6)andtheywerefoundtobeofapproximatelythesamesize.11This
observation suggests that there are very similar electronic effects for carbonyl,
thiocarbonylandthe=C(Acc) analogspresentincompounds3,thisisinlinewith
2
the pseudochalcogeno analogy principle.43 Consequently, the generality of the
pseudochalcogenoanalogyprinciple,whichhasbeenusedsuccessfullyforthees-
timationofthereactivityofrelevantcompoundsinpartlyanomalousreactions,43
isherebyremarkablycorroboratedfromanotherpointofview.
Scheme6.Pseudochalcogenoanalogyprinciple.
Thepush-pullcharactersofcompounds3havepreviouslybeenassessedbythe
C=Cbarrierstorotationandthedependenciesonthe=C(Acc) substituentswerewell
2
understood by way ofexcellentcorrelationsof(cid:1)G(cid:3) to the(cid:6) – constantsofthesub-
p
stituentsontheacceptormoieties.11However,theinfluenceofsterictwistand/orthe
positionofconformationalequilibria(E,E,E,ZandZ,Zconformers)remainedunclear.
Forthisreason,compounds3aand3b(Scheme7)wereabinitioMOcalculatedand
thevariousconformerscomparedenergeticallyandtheinfluenceoftheanisotropicef-
fectofthecarbonylgrouponthe1HchemicalshiftofH-5determinedquantitatively
byabinitiocalculation.44Thefollowingresultswereobtained:
(i)TheZ/Zconformerprovedtobethepreferredone(seeFig.5);theC=Osyn
totheringsulfurisin-plane(polarinteractions,seeabove)butthecarbonylgroup
syntoH-5,however(obviouslyduetosterichindrance),wasfoundtobestrongly
twistedawayfromtheplaneofresonance(dihedralangle(cid:2)=51º).
(ii)Energetically,thetwoE/Zconformerscomenextwithrespecttostability;
theconformerwithC=Osyntotheringsulfurwasslightlylessstable.