Catalytic Asymmetric Hydroboration Research Articles

Catalytic Asymmetric Hydroboration of α-Substituted Styrenyl Derivatives

1,1-Disubstituted olefins represent a group of challenging substrates for asymmetric transformations, generally providing lower stereoselectivities than other alkenes. Using these starting materials, the rhodium-mediated β-hydroboration was found to be suppressed by the α-pathway. In this paper, iridium catalysis is used to accomplish the enantioselective β-hydroboration of terminal alkenes with high catalytic activity, perfect regioselectivity, and ee values up to 92%. The catalytic asymmetric hydroboration remains one of the long-standing problems, which can hopefully be solved via iridium catalysis.

ChemInform Abstract: Highly Regio‐ and Enantioselective Catalytic Asymmetric Hydroboration of α‐Substituted Styrenyl Derivatives.

The catalytic asymmetric hydroboration of a variety of 1,1-disubstituted olefins has been achieved with excellent yields, perfect regioselectivity and in some cases, high levels of enantioselectivity using readily accessible iridium catalyst.

Highly regio- and enantioselective catalytic asymmetric hydroboration of α-substituted styrenyl derivatives

The catalytic asymmetric hydroboration of a variety of 1,1-disubstituted olefins has been achieved with excellent yields, perfect regioselectivity and in some cases, high levels of enantioselectivity using readily accessible iridium catalyst.

Catalytic asymmetric hydroboration of β,γ-unsaturated Weinreb amides: striking influence of the borane

Subtle differences in the structure of the borane strongly influence the catalytic efficiency and level of enantioselectivity in the catalytic asymmetric hydroboration of β,γ-unsaturated Weinreb amides.

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.

ChemInform Abstract: Catalytic Asymmetric Hydroboration with Oxazaborolidines.

Abstract The cyclic secondary boranes derived from ephedrine and pseudoephedrine undergo rhodium complex-catalysed hydroboration of styrenes; both the regiochemistry and derived enantiomeric excess depend strongly on the structure of the catalyst with the ferrocenebiphosphine complex (12d) the most promising to date.

ChemInform Abstract: Amide‐Directed Catalytic Asymmetric Hydroboration of Trisubstituted Alkenes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

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.

ChemInform Abstract: Efficient Amide‐Directed Catalytic Asymmetric Hydroboration.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Efficient Amide-Directed Catalytic Asymmetric Hydroboration

A series of acyclic beta,gamma-unsaturated amides are shown to undergo highly regio- (>95%) and enantioselective (93-99% ee) rhodium-catalyzed hydroboration with pinacolborane (PinBH) using simple chiral monophosphite or phosphoramidite ligands in combination with Rh(nbd)2BF4. The most effective ligands identified are phosphoramidite 4, derived from BINOL and N-methylaniline, and phosphite 5c, prepared from the (4'-tert-butyl)phenyl TADDOL analogue and phenol. For example, (E)-3-hexenoic acid phenylamide ((E)-1) undergoes rhodium-catalyzed hydroboration with PinBH (0.5 mol % Rh(nbd)2BF4, 1.1 mol % BINOL-derived phosphoramidite 4, THF, 40 degrees C, 2 h) affording an intermediate boronate ester which after oxidation with basic hydrogen peroxide gives the beta-hydroxy amide, (S)-3-hydroxyhexanoic acid phenylamide ((S)-3), in good yield (80%) and high enantiomeric purity (99% ee). Isomeric disubstituted (E)- and (Z)-alkenes give nearly identical results, and a trisubstituted alkene substrate is also shown to undergo efficient hydroboration (97% ee). Moderate catalyst loading (0.5 mol %) and reaction temperatures in 25-40 degrees C range are generally effective. N-Phenyl amides are generally more efficient than the corresponding N-benzyl or N,N-dibenzyl analogues. Pinacolborane is found to be more efficient than catecholborane.

Exploiting Self‐Assembly for Ligand‐Scaffold Optimization: Substrate‐Tailored Ligands for Efficient Catalytic Asymmetric Hydroboration

Eine Bibliothek aus selbstorganisierten Liganden (SAL XY) ergibt in der rhodiumkatalysierten asymmetrischen Hydroborierung breit gefächerte R/S-Verhältnisse (siehe Schema; nbd = 2,5-Norbornadien, R* = chiraler Substituent). Durch Optimierung des Ligandengerüsts gelangt man zu maßgeschneiderten Liganden für die hoch regio- und enantioselektive Umwandlung zahlreicher ortho-substituierter Styrolderivate.

Exploiting Self‐Assembly for Ligand‐Scaffold Optimization: Substrate‐Tailored Ligands for Efficient Catalytic Asymmetric Hydroboration

Mix and match: A self-assembled ligand library (SAL XY) affords a wide range of R/S ratios in Rh-catalyzed asymmetric hydroboration (see scheme; nbd=2,5-norbornadiene, R* is a chiral substituent). Ligand-scaffold optimization reveals “substrate-tailored” ligands that afford high regio- and enantioselectivity for a variety of ortho-substituted styrene derivatives.

Catalytic asymmetric hydroboration of heterofunctional allylic substrates: an efficient heterogenized version

The hydroboration of heterofunctional allylic systems with catecholborane (HBcat) using neutral and cationic rhodium complexes modified with P–P and P–N bidentate chiral ligands has been described in order to produce the secondary heteroorganoboronate ester as a major product with moderate enantioselectivity. The immobilization of cationic chiral rhodium complexes onto clays has beneficial effects on the recyclability and reuse of the catalytic system in particular for the hydroboration of allyl aryl sulfones.

Regiochemical Control of the Catalytic Asymmetric Hydroboration of 1,2‐Diarylalkenes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins

Rhodium-catalysed enantioselective hydroboration of olefins is a valuable synthetic transformation, typically employing a chiral catalyst and an achiral borane source. The pertinent chemo-, regio- and enantioselectivity issues of this reaction are discussed. However, the main emphasis of this review is on the evolution of catalytic asymmetric hydroboration. This has primarily relied upon the development and application of chiral bidentate P,P and P,N ligands which have exhibited varying degrees of success in this transformation.

Regiochemical control of the catalytic asymmetric hydroboration of 1,2-diarylalkenes

The hydroboration of stilbenes and related disubstituted alkenes catalysed by QUINAP complexes may proceed with high enantio- and regioselectivity; rhodium and iridium catalysts give the same product regioisomer but opposite enantiomers.

Catalytic Asymmetric Hydroboration: Recent Advances and Applications in Carbon—Carbon Bond‐Forming Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic Asymmetric Hydroboration: Recent Advances and Applications in Carbon−Carbon Bond‐Forming Reactions

AbstractThe metal‐catalyzed hydroboration reaction provides access to functionalized organoboron derivatives that cannot be easily prepared using traditional reagents. New developments in this reaction including neoteric reaction media and novel ligands are described in this review. A key aspect of the hydroboration reaction is the ability to further derivatize the product. Thus the conversion of boronate esters into amines, esters, alcohols, carboxylic acids and amino acids is described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

How to turn the catalytic asymmetric hydroboration reaction of vinylarenes into a recyclable process.

Optically pure rhodium(I) complexes [Rh(cod)(Lbond;L)]X (cod=cyclooctadiene; L-L= (R)-2,2'-bis(diphenylphosphino)1-1'-binaphthyl ((R)-BINAP), (S,S)-2,4-bis(diphenylphosphino)pentane ((S,S)-BDPP), 2-diphenylphosphino-1-(1'-isoquinolyl)naphthalene ((S)-QUINAP); X=BF(4), PF(6), SO(3)CF(3), BPh(4)) were immobilised onto smectite clays such as montmorillonite K-10 (MK-10) and bentonite (Na(+)-M). (19)F, (31)P and (11)B NMR experiments recorded in CDCl(3) during the impregnation process provided evidence that montmorillonite K-10 may immobilise ionic metal complexes throughout the cationic and anionic counterparts. However, when bentonite was used as the solid, only the cationic metal complex was immobilised through cationic exchange while the counteranion remained in solution. When we used these preformed catalytic systems in the hydroboration of prochiral vinylarenes, we obtained high activities and enantiomeric excess with (S)-1-(2-diphenylphosphino-1-naphthyl)isoquinoline-modified rhodium complexes. These activities and selectivities are competitive with the homogeneous counterparts. The significant features of this method are the simple separation and good retention of the active metal in the solid, which allows efficient recycling even on exposure to air.

Enantioselective Desymmetrization of Meso Bicyclic Hydrazines: A Novel Approach to the Asymmetric Synthesis of Polysubstituted Amino Cyclopentanic Cores.

AbstractFor Abstract see ChemInform Abstract in Full Text.