Borole, Diels-Alder dimers

Using a procedure similar to that described here, or using isoiated zirconium metaiiacycies as reagents, we have been able to prepare not only phospholes, but also arsoles, stiboles, bismoles, siloles, germoles, stannoles, galloles, thiophenes, selenophenes, and borole Diels-Alder dimers. Since a number of other titanium and zirconium metaiiacycies are readily available, these reagents should be useful in the preparation of a variety of heterocycles. 

A third approach to the borole system bearing alkyl substituents (99) has recently been developed93 but because these substituents are smaller, the Diels-Alder dimer (100) is the actual product (Eq. 32). 

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