Abstract
Regioselective hydroboration of vinylarenes catalyzed by a Rh(I)-DPPB
complex proceeded rapidly when DMAP was used as an additive. The
catalyst loading could be reduced to 0.4 mol% Rh(I) to
furnish the desired products in good to excellent yield with high
regioselectivity.
Key words
rhodium - hydroborations - alkenes - boron - regioselectivity
References and Notes
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4 The preliminary examination for hydroboration
typically proceeds regioselectively in the presence of DPPB as a ligand.
Other ligands often diminished the regioselectivity; see refs. 1c,
2, and 3e.
5 The reaction with catecholborane (HBcat)
for 10 min gave a regioisomeric mixture of 1-phenylethanol and 2-phenylethanol
in 83% yield with 77:23 ratio after the oxidation of the
crude boronate compounds.
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7 The reaction of 1,2-disubstituted olefin,
such as cis -stilbene, gave lower yield
along with the formation of reduction product. The reaction of 1-hexene
under the same reaction conditions for 10 min as entry 6 in Table
[¹
8 These reactions without DMAP additive
gave the corresponding products as a mixture of 2g and 3g in 29% yield with 21:79 ratio
and 2h and 3h in
8% yield with 13:87 ratio.
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10 The ESI-MS analyses of the mixture
of [RhCl(cod)]2 , DPPB (2 equiv), and
DMAP (4 equiv) gave a single peak as a [Rh + DPPB + (DMAP)2 ]+ (m/z calcd: 773.2 (100%);
found: 773.3). Thus, the coordination of DMAP to Rh(I) center is probable
under the present catalysis. The ³¹ P
NMR analysis in CDCl3 (phosphoric acid as external reference)
of a mixture of [RhCl(cod)]2 , DPPB
(2 equiv), and DMAP (4 equiv) gave a single peak at δ = 18.64
(d, J
Rh-P = 99.2
Hz, 2 P). A mixture of [RhCl(cod)]2 ,
DPPB (2 equiv), and DMAP (2 equiv) gave four peaks at δ = 20.85
(d, J
Rh-P = 110.2
Hz), 20.69 (d, J
Rh-P = 110.2
Hz), 17.98 (d, J
Rh-P = 101 Hz),
17.82 (d, J
Rh-P = 101
Hz). In contrast, a mixture of [RhCl(cod)]2 and
DPPB (2 equiv) gave many peaks (not specified). These results offer
the formation of monomeric [Rh(dppb)(dmap)2 ]Cl
as an intermediate. We assume that the strong σ-donation
ability of DMAP is favorable, but the details should be examined
to determine the role of DMAP.
