I was not surprised that you have found 'additional stabilisation (of the
organolithium) by the double bond'. This tallies with our results on
anionic cyclizations (JACS, 1996, 5322) and those of Bailey (JACS, 1991,
5720) and recently Hoffmann (JCSPT2, 1995, 1953) who has NMR evidence of
Li-pi coordination. What is surprising is the extent of retention with
benzylbromide. If the N-allyl-2-lithiopyrrolidine is indeed more
kinetically stable than the N-methyl analogue then it would be less prone
to undergo single electron transfer with benzylbromide and may explain why
racemic products no longer result. It is more difficult to explain the
difference between benzylbromide and alkylhalides.
Do you think that the initial interaction of all electrophiles (including
alkylhalides) is via coordination to the lithium? If so, then the more
reactive benzylbromide may react rapidly with retention, whereas the less
reactive alkylhalides transfer to the opposide face (maybe for steric
reasons due to the possible dimeric organolithium?).
It would be interesting to discover the result with allylbromide or
electrophiles of reactivity intermediate between benzylbromide and
alkylhalides.
Iain Coldham
______________________________________________________
Dr Iain Coldham
Department of Chemistry, University of Exeter,
Stocker Road, Exeter EX4 4QD
Phone 01392 263465 (44 1392 263465); Fax 01392 263434
E.mail I.Coldham@exeter.ac.uk
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