Wd. Wulff et Ts. Powers, STEREOCHEMICAL CONTROL IN INTRAMOLECULAR DIELS-ALDER REACTIONS WITH CARBENE COMPLEXES AS ESTER SYNTHONS, Journal of organic chemistry, 58(9), 1993, pp. 2381-2393
An evaluation of the intramolecular Diels-Alder reactions of Fischer c
arbene complexes was made for complexes which have an all-carbon tethe
r between the diene and dienophile. Specifically, both the cis- and tr
ans-isomers of the deca-2,7,9-trienyl carbene complexes (26-cis, 24-tr
ans) and the undeca-2,8,10-trienyl carbene complexes (27-cis, 25-trans
) were prepared and the rates and stereoselectivities of their intramo
lecular Diels-Alder reactions were in each case compared with those of
the known reactions of their corresponding methyl esters. For the dec
a-2,7,9-trienyl complexes 24 and 26, the stereoselectivities are compa
rable to those observed for the Lewis-acid-catalyzed reactions of thei
r corresponding methyl esters and much higher than the thermal reactio
ns of the methyl esters which are completely unselective. The undeca-2
,8,10-trienyl complex 25 undergoes intramolecular Diels-Alder reaction
with a 93:7 endo/exo selectivity whereas the corresponding methyl est
er 7 is known to give a 51:49 selectivity under thermal conditions and
to fail with attempts at Lewis acid catalysis. The cis-substituted co
mplex 27 also undergoes a selective reaction where the corresponding r
eaction of the ester fails. In addition no trace of isomerization of t
he cis-complex 27 could be observed during its cycloaddition. The trip
henylphosphine and isopropoxy complexes 28-30 were prepared, and the s
tereoselectivity of their intramolecular Diels-Alder reactions is cons
istent with an s-trans conformation of the vinyl carbene funtionality
in the transition state. In all cases the carbene complex cycloadducts
can be oxidatively cleaved in high yield to their corresponding ester
s. The results show that the value of these complexes as synthons for
esters is a result of tolerance of the carbene complex functional grou
p to sensitive diene units which do not tolerate traditional Lewis aci
ds utilized in accelerating intramolecular Diels-Alder reactions.