Rhodium–N‑Heterocyclic Carbene Catalyzed Hydroalkenylation Reactions with 2‑Vinylpyridine and 2‑Vinylpyrazine: Preparation of Nitrogen-Bridgehead Heterocycles

Dinuclear rhodium–NHC complexes of formula [Rh­(μ-Cl)­(NHC)­(η2-coe)]2 react with 2-vinylpyridine to yield the chelate compounds RhCl­(NHC)­(κ-N,η2-CH2​CH­C5H4N) {NHC = IPr, 1,3-bis­(2,6-diiso­propyl­phenyl)­imidazolin-2-carbene; IMes, 1,3-bis­(2,4,6-trimethyl­phenyl)­imidazolin-2-carbene}. The strained metallacycle can be opened by substitution of the pyridine ring by the small electron-rich PEt3 to give RhCl­(IPr)­{η2-CH2CH­(C5H4N)}­(PEt3), whereas π ligands such as olefins or alkynes undergo C–C coupling to yield 2-(butenyl)­pyridine or 2-(butadienyl)­pyridine RhCl­(NHC)­{κ-N,η2-CH­(R)​CH­(C5H4N)} complexes by formal hydroalkenylation of the unsaturated bond by vinylpyridine. Reaction of the dinuclear precursors with 2-vinylpyrazine in the presence of pyridine affords the η2 derivative RhCl­(IPr)­{η2-CH2​CH­(C4H3N2)}­(py). Compound RhCl­(IMes)­(κ-N,η2-CH2​CHC5H4N) is an efficient catalyst for the hydroalkenylation of a range of alkenes and alkynes with 2-vinylpyridine and 2-vinylpyrazine. The butadienyl-heterocycle derivatives resulting from coupling of 2-vinylazines with alkynes undergo a thermal 6π-electrocyclization to yield 4H-quinolizines or 6H-pyrido­[1,2-a]­pyrazines depending on the nature and position of the substituent of the butadienyl fragment. DFT calculations reveal that fused N-bridgehead heterocycles are more stable than opened butadienylazine derivatives, in spite of the dearomatization of the azine moiety.