B---C Bonds

Compounds containing B—C Bonds.—An ab initio MO calculation has been carried out on H3B,CO. 34 

The adduct H3B,CO,NMe3 is only stable at low temperatures, decomposing to CO and H3B,NMe3 on warming.135 I.r. assignments and some UB n.m.r. data have been reported for the adduct, the assignment of y(CO) to a band at 1798 cm-1 being consistent with the postulated 0 — N co-ordination. 

B4H8CO may be prepared in a convenient one-step synthesis from B2H6 and CO in a hot-cold reactor.136 The carbonyl derivative may then react with C2H4 to give (CH2)2B4H8. The structure of this is (22), as shown by 3H 

Thermal decomposition of triborane(7) carbonyl, B3H7CO, gives the new species bis(carbonyl)diborane(4), B2H4(CO)2. This is relatively stable, and preliminary X-ray data suggest that it possesses a 1,2-disubstituted, ethanelike structure137 with B—B 1.78(1), B—C 1.52(1), C—O 1.125(7), B—H(l). 1.14(6), and B—H(2) 1.11(6) A. 

Bis(cyanotrihydroborato)-1,1,4,7,7-pentamethyldiethylenetriamine-copper(n) crystallizes in the space group Pbca. The Cu is five-co-ordinate, forming a distorted square-based pyramid, and the two NCBHJ ligands are distinct, one apical [r(Cu—N) = 2.153(3) A] and one in the square plane [r(Cu—N) - 1.980(4) A].139 

Compounds containing B—C Bonds.—High-resolution i.r. spectra of BDsCO and BD3CO were obtained in the region of the X = 0 sub-bands of the Vj and 2+vg-v8 bands.It was not possible, however, to determine the borine carbonyl structure accurately using only the Bq rotational constants. 

Exchange reactions of HBr+BMes, BMc2Br, or BMeBr2 yield single specific products under the influence of CO2 laser radiation.Thus, reaction (9) is the only one to occur when irradiation is at 970.5 cm  

If 1039.4 cm radiation is used, however, the only process to be activated is reaction (10). 

Me2B(CH2Cl) forms two crystalline modifications, belonging to the space groups Pbcm and Pnma. The chief molecular parameters are shown in (21).  

The Li chemical shift data for Et20, THF, or dimethoxyethane solutions of LiMMe4 (M = B, Al, Ga, or Tl) are consistent with the formation of ion-pairs via the equilibria (11), except when M = B. In this case there is evidence for an interaction of the type (22a) or (22b) 

Compounds containing B—C Bonds.— The B—C stretching vibration of BHsCO has been analysed to give values of B and D which are in agreement with those obtained from microwave spectra. Values of the rotational constants B and Dj for the first-excited vibrational states of and -h Vg 

Levasseur, and F. Fouassier, /. Solid-State Chem., 1973, 6, 179. 

The reaction of diborane in THF with HCN gives a product B2H5CN,THF, which is best formulated as an internal salt in which the BH2 cation is stabilized by co-ordination to the cyano-group of the cyanotrihydroborate anion. This compoimd is unstable, but it can be converted into the more stable pyridine and NMeg analogues. I.r. and n.m.r. data confirm the for-+ — 

Lithium triethylborohydride LiEtgBH possesses enormous nucleophilic power (10 times that of LiBH ).  

Controlled pyrolysis of a BBr3-CH4 H2 mixture over a BN surface at 1550—1650 C produces a rhombohedral boron carbide B13C2. This is believed to contain icosahedral B12 units and linear CBC chains. 

Compounds containing B—C Bonds.—Detailed high-resolution measurements have been made on the i.r. spectrum of BH3CO in the region of Vj. Spectroscopic constants were calculated for this band.  

calculations have been carried out on BH3CO, using single-determinant approximation methods, and a number of one-electron properties of the molecule have been calculated.  

Ab initio SCF and Cl calculations have been performed on BH3CO, using a Gaussian basis set of double-zeta quality. The gas-phase heat of reaction for the reaction (8) was calculated as -10.98 kcal moF (SCF), or -14.56 kcal mol (Cl 

BX3 (where X = F, Cl, or Br) adducts have been characterized for tetra-methylurea and l,l-bis(dimethylamino)ethylene (15). A large shift to high field for the CH2 proton resonance of the latter on complexation with BF3 indicates the loss of olefinic structure. Thus the CHj carbon is the donor site.  

He (I) photoelectron spectra of Mc3 BX (X = F, Cl, or Br n = l or 2) have been analysed by analogy with those of BX3 and BMe3, on the basis of ab initio M.O. calculations. Some correlations were found between orbital and ionization energies, Tr-charge densities, and B chemical shifts.  

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Methods of producing B —C bonds include hydroboration, nucleophilic displacement at a boron atom in BX., (X = halogens or B(0R>3) by e.g. a Grignard reagent, and a psewiio-Friedel-Crafts reaction with an aromatic hydrocarbon, BX3, and AICI3. 

B-0 and B-C bonds are shorter and stronger than the corresponding Li-O abd Li-C bonds (more covalent character)- therefore tighter more organized transition state. 

Structure Bond lengths (A) a b c Bond angles (°) ca ab be CRSE (kJ mol ) Section number/ref. 

This reasoning was set forth by Johnston and Rapp [1961] and developed by Ovchinnikova [1979], Miller [1975b], Truhlar and Kupperman [1971], Babamov and Marcus [1981], and Babamov et al. [1983] for reactions of hydrogen transfer in the gas phase. A similar model was put forth in order to explain the transfer of light impurities in metals [Flynn and Stoneham 1970 Kagan and Klinger 1974]. Simple analytical expressions were found for an illustrative model [Benderskii et al. 1980] in which the A-B and B-C bonds were assumed to be represented by parabolic terms. 

B-C bonds, 3, 97 B-N bonds, 3, 97 B-O bonds, 3,94 B-P bonds, 3, 97 B-Si bonds, 3, 97 oxo acid anion complexes, 3, 96 Borates, alkoxo-, 3, 94 Borates, amidotrihydro-, 3,92 Borates, aryloxo-, 3, 94 Borates, carboxylato-, 3,96 Borates, catechol, 3,95 Borates, chlorosulfato-, 3,97 Borates, dicarboxylato-, 3,96 Borates, dipyrazol-l-yl-, 3, 92 Borates, halogeno-, 3,92 Borates, halogenohydro-, 3,90 Borates, hydro-, 3,90 Borates, hydrohydroxo-, 3,90 Borates, hydropyrazol-l-yl-, 3, 92 Borates, hydroxo-, 3,94 Borates, hydroxycarboxylato-, 3,96 Borates, inositol, 3, 95 Borates, monoalkyl-, 3, 92 Borates, monophosphido-, 3, 92 Borates, peroxohydroxo-, 3, 94 Borates, polyol, 3, 95 Borates, pyrrol-l-yl-, 3, 92 Borates, sulfato-, 3, 97 Borates, tetrabromo-, 3, 92 Borates, tetrachloro-, 3, 92 Borates, tetrafluoro-, 3, 92 minerals, 6, 847 Borates, tetrahalogeno-mixed, 3, 93 nB NMR, 3, 92 Borates, tetraiodo-, 3, 92 Borates, tetranitrato-, 3, 96 Borates, tetraperchlorato-, 3, 97 Borates, tripyrazol-l-yl-, 3, 92 Borax, 3,101 Borazines... 

Rotational Constants (A, B, C), Bond Lengths (r, A) and Bond Angles (a) for Small Radicals... 

Even with the diborylacetylenes illustrated by structures 39a-c, XRD data again show a shortening of the B—C bond as more electronegative groups are affixed to the boron centers but only a modest contraction of the C=C bond36 ... 

These changes are more consistent with an /-effect, rather than an R-effect, on the B—C bond. That such diborylacetylenes have a diminished electron density at boron relative to alkyl- and vinylboranes is evident from their enhanced Lewis acidity bis(diethylboryl)acetylene (40) forms a bis(tetrahydrofuran) complex, as is evident from the1 B NMR spectrum37 (Eq. 11). Alkyl- and vinylboranes exhibit no such shifts in their11B signals in THF solution. 

What is of immediate interest are the various XRD determinations of B—C bond lengths in classical methyleneboranes, exemplified by 55 and 56, which are 1.391 and 1.631 A, respectively.60 Furthermore, the C=B—C linkage is essentially linear (179.6(3) A). The observed B—C bond length is only somewhat longer than the C=C bond in a trisubstituted ethene (1.35 A). Thus there is no doubt that some boraethenes have a genuine B=C bond. 

A derivative of 62 has been prepared, as shown in Scheme 2, and an XRD analysis of lithium salt 63 has been carried out.63 The mesityl B,—C bond length is 1.339 A and the mesityl B2—C bond is 1.492 A, while the B —C—B2 linkage is almost linear (1.765(5) A). These findings support the conclusions that the B,—C bond has considerable triple bond character, since it is 0.10 A shorter than that in the Mes2B=CH2 anion (54) and the B2—C bond has significant double bond charracter, being about 0.09 A shorter than a B—C bond. 

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

In view of the highly strained o- B—C bonds of 74-75, there is experimental evidence that such ir-bonding is actually responsible for the stability of the ring to cleavage. For example, addition of pyridine to colorless 74d generates an intense yellow color and many new resonance signals in the l3C-NMR spectrum consistent with isomeric zwitterions 76a,b. Addition of r-BuOH (which does not itself react with 74d) yields borinate ester 77 (Scheme 3). These results are consistent with the interpretation... 

However, elongation of only two B-C bonds of starting 1-boraadamantane takes place (Figure 7). 

In a molecule of 2-phenyl-l-boraadamantane there are two markedly different types of B-C bonds two of them are boron-alkyl and one is boron-benzyl. On treatment of THF complex 34 with 8-hydroxyquinoline at 20 °C, mpture of the 1-boraadamantane core takes place, resulting in a mixture of boron chelates 52-54 (Scheme 16). When trimethylamine adduct 16 is used as the starting compound, reaction takes place only in boiling toluene. Interestingly, all the products result from the protolysis of B-CH2 bonds only <2006UP1>. 

Reaction of alkenes with boryl and silyl groups c/r-located at the C=C bond with alcohols is known to proceed in two different ways <1973S309, 1986JOM(310)151> by protolytic cleavage of the B-C bond and by complexation... 

To decrease the strength of the B-C bond, an electron is removed from the BC molecule, forming the BC+ ion. One of the electrons from the bonding orbital, n2Pz is removed. 

Triorganoboron complexes of Af,0-heterocyclic carbenes are also known (36). Thus, triphenylboron-carbene complex 48 can be isolated from the reaction of methanol with an isocyanide adduct of BPh3 (47) in the presence of a catalytic amount of KF. The X-ray crystal structure of 48 demonstrated that the boron atom is tetrahedrally coordinated and that all four B-C bond distances are equal within experimental error. 

Each of the 2c— lc donor-acceptor interactions in Figs. 3.97-3.100 exemplifies what are also termed agostic interactions. Such interactions are generally characterized by T-shaped (near-perpendicular) attack of an electrophile A on a filled B—C bond (obc or 7tBc) ... 

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