Transfer reactions involving boron. XX. Disproportionation reactions of alkyl-, alkoxy-, and haloboranes

Abstract

The solution redistribution equilibria of borane with alkylboranes, alkoxyboranes, haloboranes, and arylmercaptoboranes have been studied. The results obtained with the various systems are discussed individually and compared with each other and, when available, with gas-phase and other data appearing in the literature. The redistribution of trialkylboranes with borane in tetrahydrofuran produces a mixture of mono-, di-, and triatkylboranes and borane, the first two being the predominant products. Complicating the systems are the presence of five hydrogen-bridged monomer-dimer equilibria. The equilibrium constants for these equilibria have been determined for the n-propyl-and isopropylborane systems. The reaction of 1 or 2 mol of alcohols with 1 mol of borane produces dialkoxyboranes in kinetically controlled reactions. The dialkoxyboranes undergo slow redistribution reactions to give mixtures of borane and di-and trialkoxyboranes. Similarly, trialkyl borates and borane undergo slow disproportionation reactions. The redistribution equilibrium constants are identical, within experimental error, for primary and secondary alkoxyborane systems; however, the t-butoxyborane equilibrium constant is significantly different. The rate of attainment of equilibrium is markedly dependent on the structure of the alkoxy group. Boron trichloride reacts with borane to give either mono-or dichloroborane depending on the stoichiometry of the starting reagents, whereas boron trifluoride does not react with borane.

Stereoselective Syntheses of Aza, Amino and Imino Sugar Derivatives by Hydroboration of 3,6‐Dihydro‐2H‐1,2‐oxazines as Key Reaction

“…[18] At this point two possible subsequent pathways of the monoalkoxyborane include: (a) reduction of two solvent molecules with formation of a trialkoxyborane via a dialkoxyborane, [18] (b) reaction with a second monoalkoxyborane to regenerate borane itself and to form a dialkoxyborane and subsequently a trialkoxy species (Scheme 4). [19] Monoalkoxyboranes are unstable compounds and readily undergo further transformations; therefore, the newly formed species should mainly be dialkoxy-or trialkoxyboranes. To test our hypothesis that alkoxyboranes are responsible for the unexpected stereochemical outcome during the hydroboration of 3,6-dihydro-2H-1,2-oxazine syn-1a a series of control experiments was performed.…”

“…Although we never obtained a 1:1 ratio as observed in the initial experiment with "degraded" borane-THF complex, our experiments clearly demonstrate that formation of alkoxyboranes is very likely the reason for the reduced stereoselectivity. Wojtkowski [19] had shown that 1:1 mixtures of alcohols and borane led to equilibria of dialkoxy-and trialkoxyborane species with values for the equilibrium constant K c = 2.9-7 in favour of the trialkoxy species for sterically less demanding alcohols and K c = 0.13-0.17 in favour of the dialkoxyborane species for tert-butyl alcohol. Given the facts, that monoalkoxyboranes are fairly unstable, that alkoxyboranes are significantly weaker reducing agents than borane itself, and that our hydroboration attempts with primary or secondary boranes such as thexylborane, disiamylborane, 9-BBN and catecholborane [14g] completely failed, we assume that in situ formed dialkoxyboranes are the key for shifting the ratio of diastereomers, but are not the reducing agents.…”

Effect of the Secondary Reduction on the Enantioselectivity and Function of Additives in the Chiral Oxazaborolidine‐Catalyzed Asymmetric Borane Reduction of Ketones

“…1), and dialkoxyborane 5 could react with monoalkoxyborane 4 to yield trialkyl borate · THF complex 6 and borane · THF complex (Eqn. 2) [17]. Because no monoalkoxyborane 4 was detected, they only determined the equilibrium constant of Eqn.…”

Kinetics of the reduction of pinacolone by borane–dimethyl sulfide and catecholborane in THF

“…7 Trialkoxyborane and borane are produced in a much slower redistribution reaction of dialkoxyboranes. 17, 19 With increasing steric demand this reaction becomes much slower. If the alkoxy group is tert-butoxy, the equilibrium is reached in THF only after more than one month at 25 ЊC.…”

“…If the alkoxy group is tert-butoxy, the equilibrium is reached in THF only after more than one month at 25 ЊC. 19 Therefore, the redistribution of DB = bis(3,3dimethylbutan-2-oxy)borane can be neglected on the short timescale of the reaction of P. Semi-empirical calculations suggest that MB and DB, like borane, coordinate with a solvent molecule to form the complexes MB-T and DB-T in THF. The oxygen atom of the ketone couples to the boron atom in the reaction with the borane reagents.…”