Diboron tetrahalides

Reductive methods form B—B bonds from B—X bonds. For B2X4 (X = Cl, Br, I) from BXj, an electric discharge is supplemented by the presence of a metal, or metal atoms, as halide scavenger. The passage of BX3 at low pressure through a rf discharge in the presence of Hg produces the diboron tetrahalides B2X4 at 300 mg h ... 

Diboron tetrahalides, B2X4, are also known. These may be prepared in a variety of ways, among them the reaction of BC13 with mercury. 

Addition of a boron-boron bond across a carbon-carbon triple bond is known for some 40 years since the finding that diboron tetrahalides add to alkenes and alkynes in the absence of catalysts.36 Although the reaction seemed to be potentially attractive, the instability of diboron tetrahalides was the critical drawback for the practical use in synthesis. In 1993, much more stable pinacol ester derivative of diboron was found to add to alkynes in the presence of platinum catalysts such as Pt(PPh3)4, Pt(CH2=CH2)(PPh3)2, and Pt(CO)2(PPh3)2 (Figure 1, Scheme 2).37,38 Other... 

Diboron tetrahalides undergo complex pyrolysis reactions resulting in interesting higher molecular weight subhalides... 

Diborane(4) compounds contain unsupported two-centre, two-electron B-B bonds examples of which, such as the diboron tetrahalides, have been known for many years.1 However, due in large part to the recent work of many groups in the field of transition metal catalysed diboration reactions,2 there has been a resurgence of interest in their synthesis and reactivity. Herein, we describe some of our own work in this area. 

III. Reactions of Diboron Tetrahalides with Unsaturated Compounds 251... 

Structural data refer to the diboron tetrahalides. X-Ray diffraction studies of B2CI4 (5) and B2F4 100) indicate a planar, centrosymmetric structure (Djji) i the solid. Electron diffraction (47) and infrared and Raman studies (65) suggest that the B2CI4 molecule has a skewed (Z>2d) structure in the gas and liquid. The infrared spectrum of B2F4 (37,42) indicates it to be skewed or undergoing essentially free rotation in the gaseous state. 

REACTIONS OF DIBORON TETRAHALIDES WITH UNSATURATED COMPOUNDS... 

One of the most interesting and characteristic reactions of the diboron tetrahalides is their ready addition across carbon-carbon double and triple bonds. This process, which has been called diboration 101) by analogy to the well-known hydroboration reaction, provides a convenient route to vicinal bis(dihaloboryl)organic derivatives ... 

The presence of halogen substituents on an unsaturated carbon atom greatly reduces the reactivity of the carbon-carbon double bond toward addition of a diboron tetrahalide. Schlesinger and his co-workers reported that they observed no indication of adduct formation by B2CI4 with vinyl chloride, vinyl fluoride, 1,1-difluoroethylene, 1,2-dichloroethylene, or tetrafluoroethylene. Insofar as investigated, these haloolefins were equally inert toward B2F4 23). 

While it is clear that many haloolefins inhibit the disproportionation of B2CI4, early workers noted the formation of BCI3 in mixtures of haloethylenes with B2CI4 even though no color developed in these systems 92). This observation may be understood in the light of recent studies 88) which have shown that haloolefins are in fact not unreactive toward diboron tetrahalides. 

Relatively little is known of the reactions of diboron tetrahalides with aromatics apart from a study by Fox and Wartik (39) of the interaction of B2CI4 with benzene and naphthalene. With the former, dichlorophenyl-borane was obtained in high yield after long reaction times. With naphthalene, immediate formation of a loose complex was suggested by the formation of a bright yellow color. The reagents could be recovered from this complex by distillation, but after about 2 weeks at room temperature a product formulated as l,2,3,4-tetrakis(dichloroboryl)tetralin (IX) was obtained. 

There is no clear evidence for formation of stable bis(boryl) derivatives from the interaction of diboron tetrahalides with molecules containing multiple bonds other than C=C or Cr C. Such reactions as occur may in some cases involve initial diboration, but as they are of limited interest from the organometallic viewpoint, they will be mentioned only briefly. 

The addition of diboron tetrahalides to olefins provides a convenient route to vicinal bis(dihaloboryl)alkanes, which in general show most of the reactions to be expected of organoboron dihalides. The simplest member of this class of compounds, l,2-bis(dichloroboryl)ethane is also the most extensively studied. 

The 1 1 addition products obtained from vinylmetallic halides and diboron tetrahalides 28) are of structural interest by virtue of possessing not only vicinal bis(dihaloboryl) groupings but also a geminal arrangement of dihaloboryl and halometal groupings. 

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