The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins

Alkoxide-catalyzed reduction of ketones with pinacolborane

The reduction of ketones with pinacolborane (4,4,5,5-tetramethyl-1,3,2-dioxaborolane) is catalyzed by 5 mol % NaOt-Bu at ambient temperature. The reaction is high yielding and general, providing complete conversion of aryl and dialkyl ketones. Although spectroscopic studies of the active hydride source in benzene-d(6) were complicated due to poor solubility, the data are consistent with the active hydride source being the trialkoxyborohydride, which is believed to be present in low concentration under the reaction conditions. Performing analogous studies in tetrahydrofuran resulted in a complex equilibrium between several different boron-containing species in which the trialkoxyborohydride compound was the major species.

Remarkable levels of enantioswitching in catalytic asymmetric hydroboration

TADDOL-derived phosphites and phosphoramidites are effective ligands for rhodium-catalyzed asymmetric hydroborations of β,γ-unsaturated amides, achieving up to 99% ee. However, the sense of stereoinduction, R or S, is surprisingly dependent on rather subtle features of the ligand. For example, catalysts employing a TADDOL phenylphosphite and those using the closely related N-methylaniline-derived phosphoramidite of the same configuration give opposite enantiomers of the product. Those derived from optical antipodes give the same product with virtually the same enantioselectivity as illustrated above. The different stereochemical outcomes may reflect fundamental differences in catalyst structure, reactivity, or reaction mechanism.

Efficient boron-copper additions to aryl-substituted alkenes promoted by NHC-based catalysts. enantioselective Cu-catalyzed hydroboration reactions

A Cu-catalyzed method for efficient boron-copper addition processes involving acyclic and cyclic disubstituted aryl olefins are reported. Reactions are promoted with 0.5-5 mol % of a readily available N-heterocyclic carbene (NHC) complex; the presence of MeOH promotes in situ protonation of the C-Cu bond and leads to efficient catalyst turnover, constituting a net Cu-catalyzed hydroboration process. Reactions proceed in >98:<2 site selectivity and furnish secondary organoborane isomers that complement those obtained through reactions of boron-hydride reagents or by Rh- or Ir-catalyzed hydroborations (benzylic secondary C-B bonds). Initial observations regarding processes catalyzed by chiral NHC complexes, delivering products in up to 99:1 enantiomeric ratio, are disclosed.

Phosphoramidites: privileged ligands in asymmetric catalysis

Asymmetric catalysis with transition-metal complexes is the basis for a vast array of stereoselective transformations and has changed the face of modern synthetic chemistry. Key to this success has been the design of chiral ligands to control the regio-, diastereo-, and enantioselectivity. Phosphoramidites have emerged as a highly versatile and readily accessible class of chiral ligands. Their modular structure enables the formation of ligand libraries and easy fine-tuning for a specific catalytic reaction. Phosphoramidites frequently show exceptional levels of stereocontrol, and their monodentate nature is essential in combinatorial catalysis, where a ligand-mixture approach is used. In this Review, recent developments in asymmetric catalysis with phosphoramidites used as ligands are discussed, with a focus on the formation of carbon-carbon and carbon-heteroatom bonds.

Amide-directed catalytic asymmetric hydroboration of trisubstituted alkenes

Rhodium-catalyzed hydroborations of trisubstituted alkenes are generally slow and often suffer from competing alkene isomerization. In contrast, the trisubstituted alkene moieties contained within the framework of a beta,gamma-unsaturated amide undergo facile reaction, perhaps facilitated by carbonyl directing effects and two-point binding of the substrate to the rhodium catalyst. Stereoisomeric substrates, for example, (E)- and (Z)-3, cleanly give rise to diastereomeric products, and thus the rhodium-catalyzed reaction is stereospecific. In addition, simple TADDOL-derived phenyl monophosphite ligands in combination with Rh(nbd)(2)BF(4) afford highly enantioselective catalysts (seven examples, 91-98% ee). These catalysts provide an alternative methodology to prepare Felkin or anti-Felkin acetate-aldol products and related derivatives that are obtainable from the intermediate chiral organoboranes.

Vicinal diboronates in high enantiomeric purity through tandem site-selective NHC-Cu-catalyzed boron-copper additions to terminal alkynes

A Cu-catalyzed protocol for conversion of terminal alkynes to enantiomerically enriched diboronates is reported. In a single vessel, a site-selective hydroboration of an alkyne leads to the corresponding terminal vinylboronate, which undergoes a second site-selective and enantioselective hydroboration. Reactions proceed in the presence of 2 equiv of commercially available bis(pinacolato)diboron [B(2)(pin)(2)] and 5-7.5 mol % loading of a chiral bidentate imidazolinium salt, affording diboronates in 60-93% yield and up to 97.5:2.5 enantiomeric ratio (er). The enantiomerically enriched products can be functionalized to afford an assortment of versatile organic molecules. Enynes are converted to unsaturated diboronates with high chemo- (>98% reaction of alkyne; <2% at alkene) and enantioselectivity (e.g., 94.5:5.5 er).

Iridium-catalyzed enantioselective hydrogenation of vinyl boronates

The first Ir-catalyzed asymmetric hydrogenations of vinyl boronates have been performed using low catalyst loadings (0.5 mol%) and pressure (as low as 1 bar). Good selectivities (76-98% ee) were obtained for a range of substrates.

Exploring the utility of a new chiral phosphoramidite P,N-ligand derived from (R)-BINOL and 7-azaindole in asymmetric catalysis

Lithiation of 7-azaindole, followed by quenching with [(R)-(1,1′-binaphthalene-2,2′-diyl)]chlorophosphite afforded the new chiral phosphoramidite 1 in 92% isolated yield. Treatment of 0.5 equiv. of [(COD)MCl]2 (M = Rh, Ir; COD = η4-1,5-cyclooctadiene) with 2 equiv. of 1 afforded the corresponding [(κ1-P,N-1)(κ2-P,N-1)MCl] complexes in 68% (2a, Rh) and 72% (2b, Ir) yield, while treatment of (PPh3)3RhCl with 1 equiv. of 1 afforded [(PPh3)(κ2-P,N-1)RhCl] (3) as an analytically pure solid in 74% isolated yield. In examining the reaction of 1 with a mixture of 0.5 equiv. of [(COD)RhCl]2 and 1 equiv. of AgBF4, an inseparable mixture of [(κ2-P,N-1)2Rh]+BF4– (4a) and [(COD)(κ2-P,N-1)Rh]+BF4– (5a) was generated. Under analogous conditions employing [(COD)IrCl]2, the complex [(COD)(κ2-P,N-1)Ir]+BF4– (5b) was obtained in 86% isolated yield. Treatment of 0.5 equiv. of [(η3-allyl)PdCl]2 with 1 afforded [(η3-allyl)(κ2-P,N-1)Pd]+Cl– (6) in 86% isolated yield. The application of 1 in platinum group metal-mediated asymmetric chemical transformations, including alkene hydrogenation and hydroboration, ketone hydrosilylation, and allylic alkylation, was examined.Key words: chiral, ligand, phosphoramidite, asymmetric catalysis.