Mechanistic Studies of Vitamin B(12)-Catalyzed Dechlorination and Carbon-Carbon Bond-Forming Reactions
Shey, Justin
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https://hdl.handle.net/2142/84090
Description
Title
Mechanistic Studies of Vitamin B(12)-Catalyzed Dechlorination and Carbon-Carbon Bond-Forming Reactions
Author(s)
Shey, Justin
Issue Date
2002
Doctoral Committee Chair(s)
van der Donk, Wilfred A.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
In the presence of catalytic vitamin B12 and a reducing agent such as TI(III)citrate or Zn, arylalkenes can be dimerized with unusual regioselectivity forming a new carbon-carbon bond between the benzylic carbons of each coupling partner. Dimerization products were obtained in good to excellent yields for mono- and 1,1-disubstituted alkenes, and dienes containing one aryl alkene underwent intramolecular cyclization in good yields. 1,2-Disubstituted and trisubstituted alkenes were unreactive. Mechanistic investigations using radical traps suggest the involvement of benzylic radicals, and the lack of diastereoselectivity in the product distribution is consistent with dimerization of two such reactive intermediates. A strong reducing agent is required for the reaction and fulfills two roles. It returns the Co(II) form of the catalyst generated after the reaction to the active Co(I) state, and by removing Co(II) it also prevents the non-productive recombination of alkyl radicals with cob(II)alamin. The mechanism of the formation of benzylic radicals from arylalkenes and cob(I)alamin poses an interesting problem. The results with a one-electron transfer (et) probe indicate that the radical generation is not likely to involve an electron transfer. Several alternative mechanisms are discussed.
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