Pentacoordinate boron compounds

Boron and carbon are consecutive first-row elements. Consequently, pentacoordi-nate monopositively charged carbonium ions are isoelectronic with the corresponding neutral pentacoordinate boron compounds that is, the BH5 molecule is isoelectronic with CH5+. Within this context, it is interesting to refer to the five-coordinate... 

A DFT method has been used to calculate the energy of several derivatives of pentacoordinated boron compound 35 <2005JA4354>. 

There has been an interest in the existence of hypervalent pentacoordinate boron compounds for some time. In part this interest stems from the possibility that these could be akin to transition states in SN2-type reactions at boron atoms, for example the reaction of the complex BHs O with NMcs. The synthesis has recently been reported of a tridentate anthracene Hgand, l,8-dimethoxy-9-bro-moanthracene, which has been used in the formation of l,8-dimethoxy-9-borylanthracene, whose X-ray structure is reported. This represents the first fully characterised hypervalent pentacoordinate boron compound. 

Pentacoordinated (lO-B-5) and hexacoordinated (12-B-6) boron compounds are available from the reaction of BCI3 with the dilithio salt of the bis addition product of HFA and pyridine (265). These hypervalent compounds show signals in the B NMR spectrum at very high field (173a). 

The cross-coupling reaction of (36) and (37) (Scheme 15) with organomagnesium compounds occurs in a similar way, and differs from those above, only in that m these compounds the iodine atom is bonded to the pentacoordinated boron atom in the pentagonal plane. Cross-coupling reactions of organomagnesium compounds with polyhedral B-iodo-substituted compounds containing pentacoordinated boron atoms were previously unknown. 

The l,8-bis(methoxy)-9-boraanthracenes 95-100 are a truly unusual group of compounds. Compound 95 shows a fairly ordinary B-0 coordination bond, which is very similar to that shown by the corresponding bis(dimethylamino) compound 78. However, the structures of 96-100 show that the boron atom is pentacoordinated with approximately equivalent bonds to both methoxy groups. These B-O bonds are very long. 

The anionic pentacoordinated and even hexacoordinated boron species can be stabilized in a heterocyclic system by complexation with appropriate ligands. Compounds of this type (structures 2 and 3, Figure 3) were first reported by Lee and Martin in 1984 (1984JA5745). H, and NMR spectra of products 2 and 3 are consistent with the symmetrical structures incorporating pentacoordinated and hexacoordinated central boron atom however, X-ray data were not available in this publication. 

More recendy, Akiba and coworkers have synthesized pentacoordinated hypervalent boron heterocyclic species 4—6 by employing a sterically rigid anthracene skeleton (Scheme 1) (2000ACE4055,2005JA4354). Compounds 6—8 were prepared by lithiation of bromoanthracene 4 followed by the addition of E-chlorocatecholateborane derivatives 5. Products 6—8 were... 

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