Boranes fragmentation reactions

A process that involves an unprecedented B—B bond cleavage in B2cat2 promoted by two Ni(I) centers in the dimer [Ni(p -PNP)]2 has been described by Meyer and Mindiola to proceed plausibly via a binuclear oxidative addition reaction (Scheme 20A). The B NMR spectrum of [Ni(Bcat)(PNP)] clearly reveals the formation of a rare example of a nickel-boryl (47 ppm). Alternatively, the [Ni(Bcat)(PNP)] complex can be prepared by o-bond metathesis from [Ni(0 Bu)(PNP)] and B2cat2 (Scheme 20B) A proposed intermediate could be detected by NMR spectroscopy and confirmed by DFT calculations as the isomer that conducts the CT-bond metathesis. The weak interaction between the nickel center and the tethered borane fragment in B2cat2 is rationaHzed as a consequence of the high energy required to access the empty dj(2 y2-orbital in a stericaUy encumbered square planar environment. 

Similar fragmentations to produce S-cyclodecen-l-ones and 1,6-cyclodecadienes have employed l-tosyloxy-4a-decalols and 5-mesyloxy-l-decalyl boranes as educts. The ringfusing carbon-carbon bond was smoothly cleaved and new n-bonds were thereby formed in the macrocycle (P.S. Wharton, 1965 J.A. Marshall, 1966). The mechanism of these reactions is probably E2, and the positions of the leaving groups determine the stereochemistry of the olefinic product. 

Compounds with P—C—N—B fragments Compounds containing the P—C—N—B fragment can be obtained in the borylation reactions of functionally substituted phosphines with boranes (88IZV2190 89IZV1375). As a rule, the first stage is characterized by the formation of complexes in all cases. 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

In the above preparations of BioHm, B5H11, and Bi+Hio, abstraction of H from a borane anion gives an unstable neutral fragment which is believed to obtain a BH3 unit from another like fragment to form the desired borane product. One hundred per cent conversion of the starting material to the desired boron hydride cannot be achieved. On the other hand if BH3 could be "funneled" into the reaction mixture to react with the unstable intermediate, then 100% conversion could be achieved in principle. 

Boranes undergo a variety of reactions, such as proton abstraction, electrophilic substitution, fragmentation and adduct formation. Some of these reactions are highlighted below with selective examples. 

Only monomeric boranes can undergo a cis-addition to alkenes. In principle, every B—H bond of a monomeric borane can be involved in this kind of reaction. It effects a one-step addition of formerly joined fragments BR2 and H to the C=C double bond. This addition is therefore called the hydroboration of a C=C double bond. 

Another significant aspect of the approach of Parry and Edwards was based on the types of reactions most easily participated in by the various classes of boranes. Species which contain a BH2 unit (which included B2H6 and the series BnHn+6) all reacted with nucleophiles to generate either molecular or ionic fragments. These two reaction modes were described as symmetrical and non-symmetrical cleavage respectively. The two modes of cleavage are illustrated in Figure 2 for... 

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