Cyclic boranes, four-membered bonding

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

Addition of borane to the alkene tt bond occurs in a single step through a cyclic four-membered-ring transition state. The dotted lines indicate partial bonds that are breaking or forming. 

A prediction of four-membered cyclic boranes is presented. Structure and bonding of planar and puckered rings optimized at MBPT(2)/PVDZ level are discussed. Singlet-triplet gaps and thermodynamic stabilities of the rings based on Coupled Cluster calculations are given. 

Hydroboration occurs by a concerted process and takes place through a four-membered cyclic transition state, formed by addition of a polarized B—H bond (boron is the more positive) to the alkene double bond (5.2). This is supported by the fact that the reaction is stereospecific, with syn addition of the boron and hydrogen atoms. The reaction can also be stereoselective, with hydroboration taking place preferentially on the less hindered side of the double bond. Stereospecific addition of borane to a 1-alkylcycloalkene such as 1-methylcyclohexene, gives, after oxidation of the organoborane product (see Scheme 5.21), almost exclusively the trans alcohol product (5.3). 

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