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.Financial support from the Spanish Ministerio de Economía y Competitividad (MINECO/FEDER) under the Projects CTQ2013-42532-P and CTQ2016-75884-P, the CSIC under the Project Proyectos Intramurales Especiales (201680I011), and the Diputación General de Aragón (DGA/FSE-E07) are gratefully acknowledged. A.D.G. thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the postdoctoral grant Juan de la Cierva - Incorporación 2015 (IJCI-2015-27029).Peer Reviewe