Monoborane

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

The experimental data of Bragg et ah88 on diborane decomposition, like those of previous investigators, have been interpreted in terms of a mechanism involving monoborane (BH3) as a reactive intermediate. Fehlner92 proposed that a different mechanism based on the initial dissociation reaction... 

This section reviews the developments in the chemistry of monoborane complexes of the transition metals especially borohydride and hydridoborate complexes. Although such complexes are not strictly metallaboranes in the sense that they are not cluster species, they are included here as they share many similarities with polyborane species of the transition metals such as three-center two-electron bonding. Additionally, as will be shown in Section 3.04.3.1 borohydride species can also be intermediates in the formation of larger M By clusters. In this chapter, three-coordinate monoborane species, which are best considered as cr-complexes between a transition metal and HBR2 or metal-boryl (M-B) species, are not considered. 

Abbreviations a, from monoboranes b, from diboranes(4) c, by borylene transfer d, from boryl complexes e, from borylene complexes x, no X-ray data available y, no "B-NMR data available. 

In monoborane (BH3), monoalkylboranes RBH2, or dialkylboranes R2BII there is only an electron sextet at the boron atom. In comparison to the more stable electron octet, the boron atom thus lacks two valence electrons. It obtains them by bonding with a suitable electron pair donor. When no better donor is available, the bonding electron pair of the B—H bond of a second borane molecule acts as the donor so that a two-electron, three-center bond is produced. Under these conditions, boranes are consequently present as dimers BH3, for example, as B2H6. Still, small fractions of the monomers appear as minor components in the dissociation equilibrium of the dimer B2H6, for example, thus contains some BH3. 

Only a few reports deal with the interaction of boron compounds with HFA. No addition products of HFA with monoborane have been detected in thermal 101) or in photochemical 264) reactions. However, co-photolysis of HFA with pentaborane(9) and 2,4-dicarbapentaborane(7) results in the insertion of HFA into B—H bonds 16a). Mechanisms are discussed in detail. 

Caution, Hydrazine-hishorane does not melt up to 100° and may explode violently if heated rapidly much beyond 100°. The hisborane may also be detonated by impact. Both the monoborane and the bisborane are extremely flammable but not pyrophoric. Shielding and remote-control manipulation are recommended. 

Goubeau has made a definitive study of the properties of hydrazine-monoborane. However, the cited melting point (61°) is difficult to reproduce, inasmuch as the degradation reaction can begin at temperatures as low as 50°. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

Piperazine- and triethylenediamine-monoborane (but not diborane) adducts of BH2F, BHF2, and BF3 are produced by displacement reactions between the diamine and the Me3N-fluoroboranes. H and B n.m.r., and i.r. data were given for these new species. ... 

Tetrafluorodiphosphine affords only a monoborane adduct when reacted with diborane at room temperature. The product which collects... 

The formation of only the monoborane adduct may result from partial double bond interaction (p d ) between the phosphorus atoms of P2F4 which enhances the basicity of one at the expense of the other, as was found for F2POPF2 (50). 

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