Hydroboration Of Vinylarenes Research Articles

Facile synthesis of alkyl- and arylboronate esters enabled by a carbon nanotube supported copper catalyst

An efficient and reusable copper nanoparticle stabilised on carbon nanotubes system has been developed for the synthesis of alkyl and aryl boronate esters via C–X borylation of alkyl and aryl halides and hydroboration of vinylarenes with B2pin2.

Markovnikov‐Selective Hydroboration of Aryl Alkenes Enabled by a Simple Nickel Salt

Comprehensive SummaryThe sustainable development of synthetic reactions catalyzed by earth‐abundant metals is one of the principal goals in homogeneous catalysis. However, so far most of the protocols are still plagued by sophisticated ligands, hazardous activators, high catalyst loading (1—10 mol%) and/or multistep synthesis of the metal complexes in the process development. Consequently, the development of earth‐abundant metal catalysts with high activity from commercially available metal salts is highly desirable for practical utilization of base metal catalyzed synthetic methodology. Herein, we report the catalyst generated in situ from a mixture of catalytic amounts of Ni(acac)2 (as low as 0.005 mol%) and CsF, which is found highly active for Markovnikov‐selective hydroboration of vinylarenes, including 1,1‐disubstituted vinylarenes and internal olefins, affording a wide range of secondary and tertiary alkyl boronates in excellent yields. Mechanistic experiments indicate that the key to the success of this catalysis is the use of CsF, which in combination with pinacolborane acts as an effective activator for Ni(acac)2, probably generating metastable Ni nanoparticles in situ that demonstrate high activity in the catalytic hydroboration (TON up to 18800).

Synthesis and Post‐Resolution Modification of New Axially Chiral Ligands for Asymmetric Catalysis

AbstractThe synthesis of four new members of the Quinazolinap series of ligands is described. Three of these ligands were prepared by post‐resolution modification of the known ligand (R)‐7‐chloro‐2‐isopropyl‐Quinazolinap, a new approach which offers an expedient route to a range of enantiopure ligands as it precludes the need for resolution of each ligand prepared. The remaining ligand, 7‐chloro‐2‐methyl‐Quinazolinap, was prepared in a seven‐step synthetic sequence incorporating palladium‐ and nickel‐catalyzed transformations as the key steps. A diastereomerically pure palladacycle of this ligand was characterised by X‐ray crystallography. (R)‐7‐Chloro‐2‐isopropyl‐Quinazolinap was applied to the rhodium‐catalyzed hydroboration of vinylarenes with regioselectivities of up to > 99:1 and ee values of up to 68 %. Each of the Quinazolinap ligands prepared were applied to the palladium‐catalyzed allylic alkylation of 1,3‐diphenylprop‐2‐enyl acetate resulting in conversions of up to 100 % and ee values of up to 85 %. Solution‐phase NMR studies on a palladium complex of one of the ligands provided a rationale for the sense of asymmetric induction.

Cross Coupling Reactions of Chiral Secondary Organoboronic Esters With Retention of Configuration

We report the first example of a coupling reaction of chiral secondary boronic esters generated by the hydroboration of vinyl arenes. In order for the reaction to take place in high yields, the use of silver oxide as a base and the presence of at least 8 equiv of triphenyl phosphine per Pd are required. The reaction proceeds with >90% retention of configuration in all cases except one. Remarkably, the linear boronate ester does not react under these conditions.

Regioselectivity of the Rhodium‐Catalyzed Hydroboration of Vinyl Arenes: Electronic Twists and Mechanistic Shifts

Any substituent does it: The hydroboration of vinyl arenes with pinacol borane (HBPin) and cationic rhodium complexes selectively placed the boron proximal to the aryl rather than phenyl ring, regardless of whether this ring bears electron-donating or electron-withdrawing substituents. In competition experiments between styrene and various vinyl arenes, preferential hydroboration also occured at the substituted arene (see scheme). Hammett plots indicate a break in the mechanism.

Regioselectivity of the Rhodium‐Catalyzed Hydroboration of Vinyl Arenes: Electronic Twists and Mechanistic Shifts

Jeder Substituent tut's: Bei der Hydroborierung von Vinylarenen mit Pinacolboran (HBPin) in Gegenwart kationischer Rhodiumkomplexe bindet das Boratom nicht auf der Seite des Phenylrests, sondern selektiv auf der Seite des Arylrests – gleich ob dieser einen elektronenschiebenden oder -ziehenden Substituenten trägt. Bei Konkurrenzexperimenten mit Styrol und Vinylarenen reagiert ebenfalls das substituierte Aren (siehe Schema). Hammett-Auftragungen sprechen für einen Wechsel des Mechanismus.

The Development of Bidentate P,N Ligands for Asymmetric Catalysis

AbstractIn this review, an attempt is made to systemise the rôle which bidentate phosphinamine (P,N) ligands play in asymmetric catalysis. The ligands will be classified, not by the reaction to which their metal complexes have been applied, but by the nature of their donor atoms. In this manner the development of ligand architectural design can be more easily monitored. The asymmetric transformations to which metal complexes of these ligands have been applied include among others, palladium‐catalysed allylic substitutions, copper‐catalysed 1,4‐additions to enones and rhodium‐catalysed hydroboration of vinylarenes. Excellent enantioselectivities, regioselectivities and reactivities have been achieved in each of these processes.

New planar chiral P,N-ligands containing tricarbonyl(arene)chromium for enantioselective asymmetric hydroboration of styrenes

Optically active (1,2-disubstituted arene) chromium tricarbonyl complexes 2A– 2D with a pyridyl and a phosphorus group in the two ortho benzylic positions have been stereoselectively synthesized from a commercially available (+)-(4,6- O-benzylidene)methyl-α- d-glucopyranoside. These chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts for the hydroboration of styrene derivatives. High enantioselectivities were observed in the hydroboration of vinylarenes.

Synthesis of Planar Chiral (1,2-Disubstituted arene)chromium Tricarbonyl Compounds and Their Application in Asymmetric Hydroboration

Optically active (1,2-disubstituted arene)chromium tricarbonyl complexes with a diamine and a phosphorus group in the two ortho benzylic positions were stereoselectively synthesized from a commercially available (+)-(4,6-O-benzylidene)methyl-α-D-glucopyranoside. These chromium complexes have been used as chiral ligands in the preparation of rhodium catalysts for the hydroboration of styrene derivatives. Moderate enantioselectivities were observed in the hydroboration of vinylarenes.