Published on Web 03/28/2007
BH3-Catalyzed Oligomerization of Ethyl Diazoacetate: The
Role of C-Boron Enolates
Jie Bai, Lonnie D. Burke, and Kenneth J. Shea*
Contribution from the Department of Chemistry, UniVersity of California, IrVine,
IrVine, California 92697
Received October 10, 2006; E-mail: kjshea@uci.edu
Abstract: In contrast to trialkyl boranes, the reaction of borane (BH3) and ethyl diazoacetate (EDA) generates
dimer, trimer, and oligomers of EDA. The products arise from double, triple, and multiple insertions of
CHCO2Et groups in B-C bonds. On the basis of NMR spectroscopic data, trapping experiments, and
computational studies, a novel C-boron enolate has been identified as a key intermediate in this reaction.
This C-boron enolate species is calculated to be 7.1 kcal/mol (gas phase) more stable than its isomeric
O-boron enolate form. Both spectroscopic data and trapping results also reveal the formation of a doubly
borylated enolate generated as a side product by a proton transfer between the C- and O-boron
monoenolates.
Introduction
reaction of EDA 1 with borane (BH3, 2). In contrast to
trialkylboranes, we observed that the reaction gives a mixture
The reaction of organoboranes with diazo compounds is a
valuable sp3-sp3 carbon-carbon bond forming method.1 Ex-
amples include the homologation reaction of trialkylboranes with
ethyl diazoacetate (EDA, 1).2 Scheme 1 illustrates the reaction
of EDA 1 with tri-n-propylborane. Ethyl pentanoate is formed
by a single insertion of the ethoxycarbonylmethylidene group
(CHCO2Et) with elimination of N2.2 This reaction stops after a
single insertion; products from multiple carbon-carbon bond
insertions were not observed.
of products, including oligomers derived from multiple ethoxy-
carbonylmethylidene group (CHCO2Et) insertions. A mecha-
nistic investigation was undertaken to elucidate the origin of
oligomer formation. This study has led to the discovery of a
novel C-boron enolate, which serves as the resting state for the
reaction and provides insight to the repetitive sp3-sp3 carbon-
carbon bond formation in the polymerization reaction of
diazocarbonyl compounds.
The oligomerization of alkyl diazoacetates has been achieved
by transition metal catalysts including Cu powder3 and Pd
complexes4 resulting in the synthesis of low molecular weight
poly(alkyl 2-ylidene-acetate) (viscous oil with degree of po-
lymerization up to 100), while high molecular weight polymers
with a number average molecular weight (Mn) up to 57 kDa
were obtained when rhodium catalysts were employed.5 Related
polymer structures have been prepared by free radical polym-
erization of dialkyl fumarates, a rare example of polymerization
of 1,2-disubstituted olefins.6
Results
Reaction of Borane 2 and EDA 1. Based on earlier reports
that trialkylboranes react with EDA 1 to give monohomologated
products2 and that borane 2 reacts with an excess of trimeth-
ylsilyl diazomethane to generate tris(trimethylsilylmethyl)borane
(82% yield),8 the reaction of BH3 2 and an excess of EDA 1
was anticipated to produce either ethyl acetate 3 or bis- or tris-
(ethoxycarbonylmethyl)borane. However, upon reaction, a
number of products were isolated that included dimer, trimer,
and oligomers arising from multiple insertions of CHCO2Et
groups. The reaction is shown in Scheme 2. EDA 1 (3 equiv)
As part of our study of repetitive sp3-sp3 carbon-carbon
bond forming reactions,7 we had occasion to examine the
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