Borirene derivatives

A third synthetic route that has been successfully exploited in the formation of five- and six-membered heterocycles has been studied in an attempted synthesis of a borirene derivative. Reaction of phenylboron dibromide with tolan gives a mixture of products resulting from both haloboration and phenylboration 109). A Wurtz coupling reaction of the former derivative could generate a borirene derivative... 

The (ButC)2(BMe)2 derivative of 28A was reported to be formed from Bu 2C2, MeBBr2 and CsK. However, the product could not be separated from the borirene byproduct [38], and attempts for reproducing these results were not successful [39]. Dehalogenations of 29b and 31a yield directly 28b [40] and 28c [41], respectively (Scheme 3.2-15). Their X-ray structure analyses confirmed the results of ah initio computations [10]. 

The title compounds may be classified into two groups, viz., the hetero-substituted annulenes containing a type Y heteroatom, for example, pyridine and azete, and the derivatives of monocyclic conjugated ions with heteroatoms of type X (two 7r-electrons) or Z (zero 7r-electrons) (Table VI), such as pyrrole (42)-borole (98) or borirene (99)-2-azirine (100). 

The UV-irradiation of dimesityl(mesitylethynyl)borane in donor solvents induces isomerization to the crystalline trimesitylborirene (87-JA2526) Bulky substituents prevent subsequent dimerization into derivatives of l,4-diboracyclohexa-2,5-diene, which was the chief obstacle to obtaining borirene. According to ab initio calculations (86JA3960), the... 

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