Scope and Mechanistic Analysis of the Enantioselective Synthesis of Allenes by Rhodium-Catalyzed Tandem Ylide Formation/[2,3]-Sigmatropic Rearrangement between Donor/Acceptor Carbenoids and Propargylic Alcohols

Rhodium-catalyzed reactions of tertiary propargylic alcohols
with
methyl aryl- and styryldiazoacetates result in tandem reactions, consisting
of oxonium ylide formation followed by [2,3]-sigmatropic rearrangement.
This process competes favorably with the standard O–H insertion
reaction of carbenoids. The resulting allenes are produced with high
enantioselectivity (88–98% ee) when the reaction is catalyzed
by the dirhodium tetraprolinate complex, Rh2(S-DOSP)4. Kinetic resolution is possible when racemic tertiary
propargylic alcohols are used as substrates. Under the kinetic resolution
conditions, the allenes are formed with good diastereoselectivity
and enantioselectivity (up to 6.1:1 dr, 88–93% ee), and the
unreacted alcohols are enantioenriched to 65–95% ee. Computational
studies reveal that the high asymmetric induction is obtained via
an organized transition state involving a two-point attachment: ylide
formation between the alcohol oxygen and the carbenoid and hydrogen
bonding of the alcohol to a carboxylate ligand. The 2,3-sigmatropic
rearrangement proceeds through initial cleavage of the O–H
bond to generate an intermediate with close-lying open-shell singlet,
triplet, and closed-shell singlet electronic states. This intermediate
would have significant diradical character, which is consistent with
the observation that the 2,3-sigmatropic rearrangement is favored
with donor/acceptor carbenoids and more highly functionalized propargylic
alcohols.