Group 13 Boron, Aluminium, Gallium, Indium and Thallium

1 General. - There are many papers in which organoboron compounds are used as reagents in organic synthesis. These are beyond the scope of this review and they are not included here although mention has been made of one or two interesting examples. 

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. 

The compound phenalenyl(dipropyl)borane has been studied by NMR spectroscopy. The interest in this work is that two different [1,3]-B shifts are observed, implying fluxionality in the compound. The behaviour of this species is at odds with the behaviour of the trialkyl-silyl and -stannyl analogues. The first 

Organometallic Chemistry, Volume 30 The Royal Society of Chemistry, 2002 

New zwitterionic heterocycles have been derived from l-chloro(dimethyl)silyl-1-borylalkenes and 2-lithio-l-methylimidazole or 1-lithio-indazole. The zwitterionic products are obtained amongst other minor products. A DFT theoretical study has been made of borepin and its analogues the aminoborepins, methylated borepins, dibenzoborepins and tribenzoborepin. The interest here is to determine which of these compounds are planar and which are non-planar. Some of these compounds are, of course, known and here measured and B NMR chemical shifts agree well with the calculations. The calculations predict that tribenzoborepin and two of the dibenzoborepins are non-planar. 

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The elements in group 13 - boron, aluminium, gallium, indium and thallium - show a wide variation in properties B is a non-metal, A1 is a metal but exhibits many chemical similarities to B, and the later elements essentially behave as metals. The diagonal relationship between aluminium and beryllium was discussed in Section 11.10. Although the M(III) oxidation state is characteristic for elements in group 13, the M(I) state occurs for all elements except B, and for T1 this is the more stable oxidation state. Thallium shows similarities to elements outside those in group 13, and can be compared to the alkali metals, Ag, Hg and Pb, an observation that led Dumas to describe it as the duckbill platypus among elements . 

Group 13 Boron, Aluminium, Gallium, Indium and Thallium... 

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