Borirenes

Over the last decade we have seen a variety of successful synthetic routes to the boron analog of the cyclopropenium ion, the boracyclopro-pene, or borirene nucleus (74). Illustrated examples are presented in Eqs. (22-25).76-80... 

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. 

Electronically related to such borirenes are the salts of the dianion of 1,2-di-tert-butyl-3-[bis(trimethylsilyl)]methyl]-l, 2-diborirane.85 An XRD analysis of the dipotassium salt (86a,b) uncovered shortened B—B (1.58 A) and B—C (1.50 A) bonds, as suggested by resonance contributions 86a and 86b. Salt 86 can be considered to have largely a 2- r-electron Hiickel aromatic ring. 

Boron. Neutral boron-containing systems with a //-electron doublet (heterocyclic counterparts of the cyclopropenyl cation55 are called borirenes (64—66, Scheme 32) and azadiboriridines (67, Scheme 33)80 81 Scheme 32. Borirenes... 

The (ButC)2(BMe)2 derivative of 28A was reported to be formed from Bu 2C2, MeBBr2 and CsK. However, the product could not be separated from the borirene byproduct [38], and attempts for reproducing these results were not successful [39]. Dehalogenations of 29b and 31a yield directly 28b [40] and 28c [41], respectively (Scheme 3.2-15). Their X-ray structure analyses confirmed the results of ah initio computations [10]. 

The title compounds may be classified into two groups, viz., the hetero-substituted annulenes containing a type Y heteroatom, for example, pyridine and azete, and the derivatives of monocyclic conjugated ions with heteroatoms of type X (two 7r-electrons) or Z (zero 7r-electrons) (Table VI), such as pyrrole (42)-borole (98) or borirene (99)-2-azirine (100). 

The UV-irradiation of dimesityl(mesitylethynyl)borane in donor solvents induces isomerization to the crystalline trimesitylborirene (87-JA2526) Bulky substituents prevent subsequent dimerization into derivatives of l,4-diboracyclohexa-2,5-diene, which was the chief obstacle to obtaining borirene. According to ab initio calculations (86JA3960), the... 

The disproportionation of borirene to acetylene and diboretene (58) is slightly endothermic (by 5 kcal/mol), whereas the activation barrier of this BH transfer reaction equals 14.6 kcal/mol (86JA3960) The estimation of the aromatic stabilization energy for isodesmic reaction (59) yields values of 47.5 (6-31G //6-31G ) (86JA3960), 47.1 kcal/mol (6-31G //STO-3G), [81JA( 103)2589] which makes about 70% of the stabilization energy... 

A compared to the same bond in cyclopropene. These features of the geometry of the borirene ring may be regarded, in accordance with the structural criteria, as evidence for the aromaticity of borirene. 

The saturated analog of borirene, borirane (135), is less stable by... 

G) (86JOM1), than vinylborane. Borirene is, according to ab initio calculations (85MI4), more stable than ethynylborane and other open-chain isomers (136)-(139) [in parentheses, 6-31G //4-31G-calculated energies are given (in kcal/mol) relative to borirene]. 

Electropositive substituents stabilize the three-membered ring of borirene, whereas ir-donors (NH2, OH) coplanar with the ring lead to destabilization (85MI4). 

Unlike aromatic borirene, only thiirene of the antiaromatic three-membered ring species (100), (123), and (127) (see Table XII) could be experimentally detected by means of matrix isolation at low temperature [81JA(103)486]. The IR spectrum of thiirene is consistent with the data of ab initio calculation [80JA2507 also see 83JA( 105)396]. 

Borirene has also been subjected to INDO calculations and compared with other small ring systems. The molecule was found to be planar and a substantial electron delocalization was present the boron p-orbital was occupied by 0.527 electron and the carbon p-orbital by 0.732 electron. The B—C bond length was 1.46 and the C—C bond 1.38 A (74JOC373). 

Some effort has been devoted to the preparation of the lH-borirene (9) system. No such compound has been isolated but triphenylborirene has been suggested as an intermediate... 

Photorearrangement of a,/3-unsaturated organoboranes in synthesis of borirenes 91PAC365. 

Borirane, calculations, 56, 371 Borirenes, syntheses, 56, 370 Borirene, calculations, resonance energies 56, 364... 

In almost all metallacarbaboranes, the total number of B and C atoms in the polyhedral cluster exceeds the number of metal atoms. A rare example of a metal-rich metaUacarbaborane is c/oio-Fe3(CO)9BHCHCMe (32) the complex has also been described in terms of an -borirene ligand coordinating to an Fc3 unit. ... 

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