Cooperative 1,1-addition reactions of vicinal phosphane/borane frustrated Lewis pairs

The unsaturated vicinal phosphane/borane frustrated Lewis pairs (P/B FLPs) Ph2PC(p-tolyl) = C(R)B(C6F5)2 (R = C6F5 or CH3) react with tert-butyl isocyanide to give an equilibrium mixture of the starting materials, the borane/isonitrile adduct and the P/B addition product to the isonitrile carbon atom. The cooperative 1,1-adducts were exclusively formed upon treatment of these FLPs with n-butyl isocyanide. The saturated vicinal FLP Mes2PCH2CH2B(C6F5)2 reacts analogously with carbon monoxide yielding the respective five-membered zwitterionic carbonyl heterocycle. Similarly cooperative CO addition was observed starting from a norbornane based vicinal P/B FLP. These saturated vicinal P/B FLPs (and several related systems) undergo analogous cooperative 1,1-addition reactions to nitric oxide (NO) to give the corresponding persistent five-membered heterocyclic FLP-NO nitroxide radicals. They were characterized spectroscopically and by X-ray diffraction and shown to undergo typical N-oxyl radical reactions. Some vicinal P/B FLPs also undergo 1,1-addition reactions to mesityl azide. Anomalous Staudinger reactions were observed to take place upon subsequent thermolysis or (in one case) photolysis.

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