Preparation of (E)‐1‐Alkenylboronic Acid Pinacol Esters via Transfer of Alkenyl Group from Boron to Boron.

Abstract

For Abstract see ChemInform Abstract in Full Text.

A Comparison of Two Zinc Hydride Catalysts for Terminal Alkyne C–H Borylation/Hydroboration and the Formation of 1,1,1-Triborylalkanes by Tandem Catalysis Using Zn–H and B–H Compounds

“…BEt 3 was initially explored as it reacts in aromatic solvents with HBPin to form H y BEt x ( x + y = 3) species, and we surmised that low steric bulk, base-free R 2 BH/RBH 2 species may be required for catalyzing hydroboration of bulky 3a via a mechanism related to that recently identified . Substoichiometric amounts of BEt 3 were added to the standard reaction mixture (which still contains excess HBPin) after the formation of diborylated alkene 3a using I (or II ).…”

“…It was hypothesized that the failure of BH 3 -THF to effect catalytic hydroboration of 6a with HBPin is due to the reaction of BH 3 -THF with multiple equivalents of 6a to form sterically hindered compounds such as A / B (and isomers thereof, Scheme ). Hindered R 3 B compounds have been demonstrated to not undergo metathesis with HBPin; thus, it is feasible that species like A or B would not react (or only react very slowly) with HBPin even on heating. The reaction of BH 3 -THF with 2 equiv of 6a in the absence of HBPin at ambient temperature led to a complex mixture, but key observations include the following: the complete consumption of 6a ; the formation of alkylBpin species (indicated by δ 11B = 34.1 ppm); and the persistence of some BH 3 -THF.…”

Kinetics and Mechanism of the Arase-Hoshi R2BH-Catalyzed Alkyne Hydroboration: Alkenylboronate Generation via B–H/C–B Metathesis

“…The latter is a known catalyst for alkyne hydroboration, , and thus impure or aged solutions, of 1 pin , or those prepared in situ from pinacol and H 3 B · SMe 2 , can contain sufficient BH 3 to undergo efficient but indirect (i.e., catalyzed) reaction with alkynes. We began by confirming that, in the absence of a deliberately introduced catalyst, , 1 pin does not react with alkyne 2 in solution at 23 °C, or undergo disproportionation to generate B 2 pin 3 + BH 3 , and hydroboration products thereof. Of a range of solvents explored, a mixture of dioxane and CHCl 3 (1%) emerged as highly effective at stabilizing 1 pin , and there was no detectable reaction of 1 pin with 2 (600 mM) over a period of 24 h at ambient temperature (99.9% 2 , 19 F NMR spectroscopy; > 99% 1 pin , < 0.2% B 2 pin 3 , 11 B NMR spectroscopy).…”

“…Although many B–H species hydroborate alkynes at ambient temperature, 1,3,2-dioxaborinanes, and 1,3,2-dioxaborolanes, such as catecholborane ( 1 cat ) and pinacolborane ( 1 pin ), are inert . This has prompted the development of numerous catalysts , and initiators, for the process, including Ca, Ti, Zr, Fe, Ru, Ni, Co, Rh, Ir, Cu, Au, and Ag complexes/salts, − carboxylic acids, amides, NaOH, Et 3 P, Al hydrides and alkyls, , and a wide range of boron species. − …”

Hidden Boron Catalysis: Nucleophile-Promoted Decomposition of HBpin

“…The sum of [ n Bu 3 B] obs and [BH 3 ] obs equaled [BH 3 ] calc . Although trialkylboranes have not been reported to catalyze hydroboration reactions, they are known to redistribute to catalytically active dialkyl- and monoalkylboranes, , e.g., n Bu 2 BH and n BuBH 2 , in the presence of BH 3 . Hydroboration using Et 3 B (10 mol %) was successful (entry 7), presumably through redistribution with HBpin to form R 3– n BH n species, albeit at a much lower rate than that using BH 3 .…”