Samarium(III)-catalyzed hydroboration of olefins with catecholborane. A general approach to the synthesis of boronate esters

Abstract

A broad selection of olefins undergo hydroborationwith catecholborane in a reaction that is catalyzed by a range of samarium(II1) as well as other lanthanide complexes.The utility of the hydroboration reaction has been significantly extended by the development of transition metal-catalyzed As a consequence of the mechanistic differences between the catalyzed and uncatalyzed processes,3 complementary regio-and diastereoselectivities have been observed. Marks has recently reported a catalyzed hydroboration where the proposed catalytic species is bis(pentamethylcyclopentadieny1)-samarium hydride? The purpose of this paper is to disclose that lanthanide hydroboration catalysis with catecholborane (CB) can be extended beyond lanthanide cyclopentadienyl complexes, to demonstrate that few requirements need be imposed on the catalyst ligand architecture, and to report some of the important mechanistic details of the Sm(II1)-catalyzed process.In the initial set of experiments, various trivalent samarium species were screened as catalysts in the hydroboration of 1-decene with CB to determine whether there are specific ligand requirements for catalyst activity (Table I). Samarium triiodide? (t-BuO)SmI2,6 and (iPr0)3Sm7 were all found to be competent catalysts (entries 1-3) which afforded 1-demo1 in good yield after an 18-h reaction period (25 "C, THF) and subsequent peroxide oxidation. On the other hand, SmBr3, SmCls, and SmF3, as well as Sm(OTf)3,8 failed to catalyze the reaction. In order to evaluate the possibility that samarium is uniquely required for catalysis, other trivalent lanthanides were screened and found to be efficient catalysts (entries 5-8). Regioselectivity for the hydroboration of 1-decene was found to be dependent on both ligand and metal, with ratios of primaryhecondary alcohols ranging from 11-501. Although the results in Table I document a range of catalyst options, SmI3 was selected as the focus of further study.The regioselectivity of the SmI3-catalyzed reaction was discovered to be time-dependent (eq 1). This modulation in regioselectivity during the course of the reaction implies either that formation of the alkylboronate is reversible (vide infra) or that the catalyst is being chemically + Postdodoral Fellow of the Studienstiftung des Deutachen Volkes * Abstract published in Aduance ACS Abstracts, September 1,1993. (1) Burgess, K.; O b e y e r , M. J. Chem. Rev. 1991,91,1179-1191 and (2) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem. SOC. 1992, (3) Evans, D. A.; Fu, G. C.; Anderson, B. A. J. Am. Chem. SOC. 1992, (4) Harrison, K. N.; Marks, T. J. J. Am. Chem. SOC. 1992,114,9220-(5) Imamoto, T.; Ono, M. Chem. Lett. 1987,501-602. (6) Namy, J. L.; Souppe, J.; Collin, J.;Kagan, H. B. J. Org. Chem. 1984, and the BASF AG. references cited therein. 114,6671-6679. 114,6679-6685. 9221. 49,2045-2049. Table I. Hydroboration of 1-Decene with Trivalent Lanthanides. ~ ratio: entry catalyst conv,b % primarrsecondtuy 1 SmIs 2 (t-BuO)SmIz 3 Sm(Oi-Pr)s 98 501 94 46A 98 421 78 261 69 439 92 21:l 84 11:l 92 221...

Iridium-catalyzed hydroboration of alkenes with pinacolborane

“…9,14b Indeed, most neutral and cationic iridium-phosphine complexes mainly afforded a terminal boron product (2) also for pinacolborane (entries 12-24). Among them, a combination of [IrCl(cod)] 2 and dppe, dppp or dppb was recognized to be the best catalyst for achieving high yields and high selectivities (entries [13][14][15]. Analogous catalysts prepared from a cationic iridium(I) precursor also predominated the formation of 2, but they were less selective than were neutral complexes (entries [17][18][19][20][21][22][23][24].…”

Some Synthetic Uses of Double‐Bonded Functional Groups

“…Hydroboration of a wide range of alkenes using catecholborane (and similar boranes), catalyzed by samarium(III) 136,137 , palladium 138,139 , iridium 140,141 and other transition metal and lanthanide complexes 142 145 , gives good to excellent yields of alcohols (equation 26). The process is regioselective and is dependent upon the catalyst used 146 .…”

Hydroboration, Diboration, Silylboration, and Stannylboration

“…The cyclopentadienyl complexes such as Cp 2 TiMe 2 [30] and Cp* 2 Sm(THF) [31] are excellent catalysts for the addition of boron to the terminal carbon (entries 15 and 16). Such high terminal selectivity can be accounted for by steric hindrance of the cyclopentadienyl ligand, since the Cp* complex exhibits higher terminal selectivity than that of the Cp ligand and SmI 3 [32] results in a mixture of both isomers. The steric effect of borane reagents also plays an important role in selectivity.…”

“…SmI 3 exhibits a unique regioselectivity depending on the reaction time (entries 5 and 6) [32]. The reaction initially yields a mixture of internal and terminal product, but the catalyst slowly isomerizes secondary alkylboronate to the primary one on prolongation of reaction time, thus suggesting reversible formation of the C-B bond.…”