Hypercarbon intermediates

In Chapter 6, we shall draw attention to the wide range of reactions now seen to involve hypercarbon intermediates. We give a few examples here to illustrate their scope. 

Figure 1.22. Involvement of hypercarbon intermediates in the hydroboration, dehydroboration and alkylborane rearrangement (brachiation) reactions.

These and other reactions believed to involve hypercarbon intermediates are discussed in Chapter 6. 

In this chapter, we successively review reactions of electrophiles, coordinatively unsaturated metal compounds, carbenes, nitrenes, and heavy analogs of carbenes (silylenes, germylenes, stannylenes) with C-H and C-C bonds. Discussion of some electrophilic reactions of ti-donor systems is also included, along with Sn2 reactions. Tlie emphasis in all these discussions is centered on the involvement of hypercarbon intermediates (or transition states) of the reactions. 

Olah and coworkers also prepared the parent dodecahedryl cation 29 under superacid conditions [Eq. (6.20)]. Upon standing in the superacid medium at -50°C, it slowly and irreversibly transformed into 1,16-dodecahedryl dication 31. The formation of dication 31 can be rationalized by protolytic ionization of the C-H bond at C(16) farthest from the cation center presumably involving hypercarbon intermediate 30. 

The first clear example of cyclometallation of sp C-H bonds was shown by Chart and Davidson" and Cotton and coworkers in the case of the ruthenium complex [Ru(dmpe)2] [dmpe = l,2-bis-(dimethylphosphino)ethane]. This complex was found to spontaneously cyclometallate the C-H a bond of the methyl group on the phosphorous through a pentacoordinate hypercarbon intermediate to provide a dimer"" [Eq. (6.62)]. Tlie same complex also reacts with the aromatic C-H bond of naphthalene... 

A number of heteroatom-substituted dilakylaluminum compounds (R2AICH2-X) can undergo apparent a-, aP-, or ay-eliminations. The apparent a-elimination, when halomethylaluminum compounds cyclopropanate alkenes, is actually a combination of carboalumination and elimination [Eq. (6.87)]. Such eliminations involve hypercarbon intermediates or transition states. 

In the case of linear alkanes such as n-hexane, the he ion inserts into the central C-C bond followed by P-methyl transfer [Eq. (6.104)]. Again, these reactions presumably involve hypercarbon intermediates. 

---