DMAP-Accelerated Rhodium(I) Chloride Catalyzed Hydroboration of Vinylarenes

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.

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] gave the linear product in 65% yield (unoptimized).

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.

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 ^³¹P 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.