Acyclic organoboranes

The oxidation by amine oxides provides a basis for selection among non-equivalent groups on boron. In acyclic organoboranes, the order of reaction is tertiary > secondary > primary. In cyclic boranes, stereoelectronic factors dominate. With 9-BBN derivatives, for example, preferential migration of a C—B bond which is part of the bicylic ring structure occurs. 

Organoboranes derived from internal acyclic olefins by hydroboration with BH3 undergo thermal isomerization at elevated temperature (100-160 °C). The Boron atom readily moves down the chain, past a single branch, but not past a quaternary carbon atom [1]. 

This rearrangement can be also performed with acyclic tetrasubstituted olefins, enabling control of two adjacent carbon centers. Thus, hydroboration of Z- and E-2,3-dimethylstilbene (43) with BH3.THF and subsequent heating at 70 °C for 12 h furnishes stereo selectively the syn and anti organoboranes 44, which after oxidative workup provide the corresponding alcohols syn-45 and anti-45 in 90 % yield and dr > 99.5 % (Scheme 7) [7-9]. 

BBN-H tolerates many functional groups, and this, coupled with its high regioselectivity, allows the clean synthesis of a number of functionalized organoboranes e.g. equation 26), including many derived from unsaturated heterocyclic compounds. It also shows impressive stereofacial selectivity in the hydroboration of cyclic alkenes (e.g. equations 27-29), - and sometimes in the cases of acyclic alkenes. ... 

Two equivalents 1-alkyne with thexylborane produces the thexyldialkenylboranes . Thexylalkylalkenylboranes can be obtained either by sequential hydroboration of an alkene-alkyne pair with thexylborane [Eq. (ct)] or via hydroboration of an alkene with thexylchloroborane, followed by reduction and hydroboration of an alkyne [Eq. (cw)]. Cyclic trialkylboranes are synthesized via hydroboration. Thus, HjB-THF reacts 2 3 with simple acyclic dienes to give predominantly the dumbbell-shaped organoboranes. When heated, they undergo isomerization to the more thermodynamically stable isomers. These empirical rules predict their thermal behavior (a) the thermal... 

Hydroboration of acyclic trienes with 1 1 H3B THF at 0°C leads to insoluble products. Monomeric bicyclic boranes are produced in low yields. These organoboranes are obtained, however, either by thermal treatment of the insoluble products or directly by hydroboration of trienes with borane-triethylamine complex . ... 

Alkane synthesis via coupling of organoboranes was discovered very early during the development of organoborane chemistry. The procedure is simple and involves the hydroboration of an alkene followed by treatment with basic silver nitrate (equations 108-110). Both acyclic and cyclic alkanes can be prepared through this procedure. 

Michael-type additions to alkenylmetallics have serious defects because of the inapplicability due to readily-polymerizability of acyclic a,p-unsaturated ketones [17], Organoboranes, on the other hand, give good results, even for acyclic enones [18], but there is no direct and general method for the synthesis of p,p-disubstituted alkenylboranes. However, cross-coupling reaction [1-4] between 6-halo-y,6-unsaturated ketones and organometallics do afford the 6,6-disubstituted-Y,6-unsaturated ketones. 

Since the 1990s, the olefin metathesis reaction has become a major synthetic tool in organic chemistry. Organoboranes were first employed in the construction of carbo-cyclic and heterocyclic alkenylboronates by ring closure of the corresponding acyclic precursors [104], Ruthenium-catalyzed enyne metathesis of acetylenic boronates was later demonstrated as a concise route for the construction of cyclic 1,3-dienylboronic esters, which can be further engaged in [4-i-2] cycloadditions (Scheme 9.51) [86]. 

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