Substituted Diboranes

9 kcal/mol. Other isomers of U2B2H4, which correspond to minima, are derivatives of diborane(6) with both lithium atoms in terminal Li-B(H)[-H-]2B(H)-Li or in bridging H2B[-Li-]BH2 positions. The relative energies of the latter two species are 77.9 kcal/mol and 

SCF calculations using a flexible basis set including double zeta plus polarization, suggest that the species has a planar B-S-B-S ring and exists in two isomers, E (C2h) and Z (C2v), see Fig. 2-50. The C2h structure is the more stable by only 0.38 kcal/mol, and the reaction of H2S and diborane(6), 

The authors suggest that i4-thiabis(diborane(6)) forms in the presence of catalytic amounts of tetrahydrofuran as shown in Fig. 2-53 [6]. 

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Substituted diboranes derived from the hydroboration of 1,3-butadiene l,2-tetramethylenediborane(6), 1 and l,2-bis(tetramethylene)-diborane(6), 2 undergo symmetrical and unsymmetrical cleavage reactions 12). 

The inherent plausibility of metastable bridged carbonium ions as intermediates is supported by two independent types of observation. One is the extensive rearrangements which can occur in allylic systems under conditions in which ionic displacement reactions are possible. A second is the existence of stable bridged compounds, including, in the case of boron compounds, pentavalent atoms. Thus diborane and substituted diboranes have stable bridged structures. 

Since diborane is isoelectronic with C2H, the ethyl carbonium ion, and, in general, substituted diboranes are isoelectronic with the corresponding substituted ethyl carbonium ions, it would seem to argiie very strongly for the analogous bridged structures for the carbonium ions. ... 

Haloboration Reactions. The haloboration of carbon-carbon triple bonds provides another entry point for the synthesis of organoboranes. A wide variety of haloboranes including BBrs, 9-BBN-Br, and 9-BBN-l has been found to react with terminal alkynes to produce (Z)-2-halo-l-aIkenylboranes. The reaction occurs in a stereo-, regio-, and chemoselective fashion specifically with terminal alkynes and has been used to synthesize numerous substituted olefins and related compounds. Diboration reactions of alkynes with B2CI4 are also well known. However, more convenient transition-metal-catalyzed procedures with the less reactive aUcoxy substituted diboranes B2(OR)4 have recently been developed. 

Caution Diborane and substituted diboranes are extremely hazardous materials and should be handled only by persons experienced in nonatmospheric techniques. 

BH2-S(H)-BH2-H-], i-mercaptodiborane(6), is also observed as a product from the reaction between H2S and BgHg. The NMR spectrum exhibits a triplet of doublets at 6 = -22.1 ppm, J(B,Ht)=140 Hz, J(B,H ) = 40 Hz, and is consistent with a bridged substituted diborane(6). SCF calculations as described above indicate that the reaction... 

When a mixture of triphenylborane and diborane is heated to about 80° C in both the presence (65) and absence (66) of ether, 1,2-diphenyl-diborane is the only phenyl-substituted diborane obtained. This is in striking contrast to the above alkyl exchange in which all diborane derivatives except the 1,2-dialkyldiborane are found. 

Only a minimum of mechanistic attention has been paid to the catalytic effect that ethers have on the hydroboration reaction. After determining that the hydroboration of olefins with l,2-bis(3-methyl-2-butyl)diborane is first order in olefin and also first order in the substituted diborane, it was suggested that the role of the solvent is to coordinate with the dialkylborane monomer, the leaving group in this particular reaction 101). However, the room temperature hydroboration of ethylene with borine carbonyl 102) intimates, by analogy, that a rather loosely held adduct may account for acceleration of B—H addition to unsaturated systems in ether solvents. The use of a molybdena-aluminum catalyst has been moderately effective in converting an ethylene-diborane mixture to a mixture of ethyldiboranes at room temperature 103). 

A comparison of relative hydroboration rates of olefins with tetra(3-methyl-2-butyl)diborane indicates the following general order alkynes > terminal olefins > as internal olefins > tram internal olefins 143). Kinetic data establish the hydroboration of olefins with this tetraalkyldiborane to be second order, first order in olefin, and first order in the substituted diborane 101). Presumably, the rate-determining step gives, in addition to the tri-alkylborane product, one molecule of a dialkylborane monomer which may dimerize or react with a second molecule of olefin in a rapid second step. Since the dialkylborane dimer rather than monomer participates in the... 

However, in such cases attachment of boron to the less substituted carbon can be ensured by hydroboration with a substituted diborane, particularly a tetra-alkyldiborane R2BHBHR2 in which the groups R are bulky. Di- ec-isoamylborane ( di-siamylborane ) (Me2CHCHMe)2BH, itself prepared from BiHg and Me2C CHMe,... 

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