Wurtz reaction cyclization

A classical method using Na- or Li-liquid ammonia (Birch reduction conditions) is effective for reductive dehalogenations of aryl and vinylic halides, but it is not always successfully applied to alkyl halides, although cyclopropyl halides and bridgehead halogens are exceptions.Under such conditions, the reactions are often accompanied by side reactions, such as elimination, the Wurtz coupling reaction, cyclization and reduction of carbonyl compounds. An example, a synthesis of pentaprismane (1), is shown in Scheme 4. ... 

Wurtz reaction as the key cyclization step. The cyclization was catalyzed by naphthalene. 

Boekelheide el al. 2 found this modified Wurtz reaction serviceable in the synthesis of [2.2]metacyclophanes, for example, 4,12-dimethyl[2.2]mctacyclophane (2). In the case of the dibromide corresponding to (1) but lacking the two methyl groups, cyclization is accomplished in yield of 77%. 

In the Wurtz coupling reaction of dichlorosilanes with sodium to poly silanes [Eq. (42)], polysilanyl anions are considered to be crucial reaction intermediates in the propagation step and in the cyclization via back-biting (Scheme 23) 88). 

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