Spiro boron atom

The complex anion (XXXIII) is the first known example of a boron heterocyclic having a spiro boron atom (43, 44). 2,2 -Dilithiobiphenyl and BF3-0(C2Hj)2 give (XXXIII) in 44 % yield (43). 

Two other well defined carboranes 55g and 76 (vide infra, Section 3.2.7) are present in the reaction mixture containing 64c (vide infra, Scheme 3.2-35). Although pure 55g could not be isolated, its solution-state structure follows from a complete set of NMR spectra [71b], This unusual carborane is a spiro derivative without precedence and belongs to the family of 2,3,5-tricarba-mdo-hexaboranes(7). Ab initio MO calculations and NMR data indicate that the boron atom linked to the C3B3-framework by an endohedral C-B bond takes part in the cluster bonding [71b] (similar to an endohedral C-H bond in the other carboranes 55). 

There is but one brief report on the formation of boron spiro-cations in which three or more boron atoms are bridged by pyrazolyl groups. The first such species was obtained on interaction of the [B(pz)4] ion with two molar equivalents of diethylboryl tosylate, (C2H5)2B(0S02C6H4-4-CH3)... 

Closely related to the spiro-cations described above are a few examples of ions containing three bridging pyrazolyl groups between the same two boron atoms. Such species were first obtained on interaction of pyrazolide anion with ethylboryl ditosylate, C2H5B(0S02C6H4-4-CH3)2... 

The final product 63 retains the Ph3P+ group from the first vinyl phosphonium salt 59 and in the example below, using a boron enolate, this was hydrolysed to the stable phosphine oxide11 66. Note the creation of a quaternary carbon atom at a spiro centre. 

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