In search of alkene-diazene cross metathesis

Abstract

Aside from alkene cross metathesis, involving two different alkene units, cross metathesis reactions of two functionally different unsaturated units - heterofunctional cross metathesis - are virtually unknown. We report here on our efforts to realize alkene-diazene cross metathesis to produce imines. The proven alkene-metathesis catalyst (Cy3P)2Cl2Ru = CHPh (1) reacts with aryl diazenes at room temperature to produce imines derived from coupling of the alkylidene unit of 1 with the nitrene fragment of the diazene. Attempts to induce catalytic alkene-diazene cross metathesis by 1 or (NHC)(PCy3)Cl2Ru = CHR (NHC = N-heterocyclic carbene, 2) result only in the transfer of the alkylidene unit from 1 without reaction of the alkene. 1H- and 31P NMR monitoring and other characterizational probes are consistent with the formation of an unreactive imido- or aza-metallacycle intermediate. DFT computational modeling of a potential catalytic pathway for diazene-alkene metathesis with (PMe3)2Cl2RuCH2 (4) has revealed viable intermediates and transition states, involving metal-carbene, metal-nitrene (imido) and derived azametallacyclobutanes. The turnover-limiting steps are calculated to occur during combination of the intermediate imido-complex (PMe3)2Cl2Ru = NMe (4) with alkene via a transition state for retrocyclization of an aza-ruthenacyclobutane (J) with a large energetic span of 47.8 kcal/mol. Modeling of the same reaction step with the experimental complex 1 reacting with PhN = NPh and PhCH = CHPh finds an even larger energetic span, accounting for the experimentally observed stoichiometric production of imine PhCH = NPh. In contrast, the corresponding energy profile for alkene-diazene metathesis promoted by phosphite-carbene complex [(MeO)3P]2Cl2RuCH2 (5) finds a much lower energetic span of 26.4 kcal/mol for conversion of imido complex G′ to imine via the aza-ruthenacyclobutane (J′) and that the turnover-limiting step would involve retrocyclization of the first stage diaza-metallacyclobutane C′ with an energetic span of 29.7 kcal/mol.