Selective [2+2] Cycloaddition Reactions of Isocyanates and Thioisocyanates across the M=C Bond in a Ruthenium Carbene Complex

The reactivity of the (carbene)ruthenium complex [(η6‐p‐cymene)Ru=CRR′] with the unsymmetrical methandiide ligand [CRR′=C(Ph2PS)(SO2Ph)] towards different unsaturated organic substrates was examined. While no transformation was observed with a series of different ketones (no Wittig‐type reactivity), treatment with isocyanates as well as thioisocyanates resulted in selective [2+2] cycloaddition reactions across the Ru=C double bond. The products were isolated in high yields and characterized in solution as well as in the solid state. The addition reactions were found to proceed selectively according to the HSAB principle with the soft ruthenium center preferring the softer donor atoms. Hence, isocyanate addition occurred with the C=N bond, while the thioisocyanates added to the metal–carbon bond with the C=S bond. Density functional theory studies on the reaction mechanism confirmed the observed selectivity and the formed complexes as kinetically as well as thermodynamically favoured products. Selective [2+2] cycloaddition reactions of heteroallenes across the M=C bond in a ruthenium carbene complex were achieved. The reactions occurred selectively according to the HSAB principle with the soft ruthenium center preferring the softer donor atom of the heteroallene.