Every now and then two articles from two separate research groups appear at the same time having almost identical content. The coincidence of two dihalosilylene articles appearing back to back in the Angewandte this week is of course engineered by the Angewandte editors but nevertheless the question is how did this happen? Ghadwal et al. (received 1 April 2009 DOI) and Filippou et al (received 7 MayDOI) both decided to add a halosilane (tetrachlorosilane or tetrabromosilane) to cyclic diamino carbene 1 and reduce the intermediate 2 with potassium graphite to form the silylene dibromosilylene and dichlorosilylene 3. The latter compound is the silicon version of dichlorocarbene and also a family relation of lead dichloride. Key to the stability of the silyl carbene group is the presence of the diaminocarbene and without it SiCl2 polymerizes quickly to (SiCl2)n
Both groups have been paying close attention to a 2008 article by Wang et al. (DOI) in which this reaction is pioneered only with twice the amount of reducing agent and with a surprisingly different reaction product namely the carbene stabilized bis-silylene L-Si=Si-Li 4 with both silicon atoms having formal oxidation state zero. This research group has been using this reaction protocol before when they synthesised a similar looking diborene in 2007 and by now they must be pulling their hair out realizing they missed the dihalosilylene opportunity that has now presented itself.