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Carbenoid Alkene Insertion Reactions of Oxiranyllithiums
journal contribution
posted on 2012-10-05, 00:00 authored by Lawrence
M. Pratt, Binh Khanh Mai, B. Ramu RamachandranThe first computational investigations of the carbenoid
reactions of α-lithiated dimethyl ether (methoxymethyllithium)
and the intramolecular and intermolecular reactions of lithiated epoxides
with the alkene double bond to yield cyclopropane rings are presented.
These reactions represent the full spectrum of known carbenoid pathways
to cyclopropanation. The reaction of Li–CH2–O–CH3 with ethylene proceeds exclusively through a two-step carbolithiation
pathway, the intramolecular reaction of 1,2-epoxy-5-hexene follows
either the carbometalation or a concerted methylene transfer pathway
(the former is energetically more favorable), and the reaction of
lithiated ethylene oxide (oxiranyllithium) with ethylene, the main
focus of this paper, appears to proceed exclusively by the methylene
transfer mechanism. In the case of these latter reactions, the free
energy of activation for cyclopropanation tends to decrease with the
higher aggregation states. Formation of tetramers or higher aggregates
is favorable in nonpolar solvents, but in strongly coordinating solvents
such as tetrahydrofuran (THF), steric factors appear to limit aggregate
sizes to the dimer. In the case of 1,2-epoxy-5-hexene, consideration
of competing reaction pathways provide an explanation for the observed
product distribution.
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Keywords
carbenoid pathwayscarbolithiation pathwaylatter reactionsmethylene transfer mechanismCarbenoid Alkene Insertion Reactionsmethylene transfer pathwaylithiated epoxidesnonpolar solventsintramolecular reactionethylene proceedscarbenoid reactionsproduct distributionreaction pathwaysTHFlithiated ethylene oxidecyclopropane ringsaggregation states