Ring opening reactions on (Spacer-linked) Hetero Biaryls

General methodology:

The resulting thiophenesilanes are converted after addition of BuLi via metal-halogen-exchange to 3-lithio-intermediates, which upon warming to ambient temperature suffer ring opening reaction. The resulting dien-diyne thiolate is quenched with CH3I as an electrophile yielding the target thioethers:



Scheme 13

By applying this method to the above discussed spacer-thiophenes (2a), (2c) and (2d) we were able to synthesize a new group of fully conjugated thiomethyl-oligo-enynes. The detected by-products - mono debrominated substrates and mono ring opening derivatives - are mainly resulting from partially converted, mono halogen-metal exchanged starting material.

In spite of all these influences and circumstances the first results of these double ring-opening reactions are very promising and led to the target compounds (3a), (3c) and (3d) with very reasonble, not yet optimized yields:



Scheme 14