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

Abstract

Dinuclear rhodium-NHC complexes of formula [Rh(µ-Cl)(NHC)(η 2-coe)]2 react with 2-vinylpyridine to yield the chelate compounds RhCl(NHC)(κ-N,η 2-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){ 2-CH2=CH(C5H4N)}(PEt3), whereas π-ligands such as olefins or alkynes undergo CC coupling to yield 2-(butenyl)pyridine or 2-(butadienyl)pyridine RhCl(NHC){κ-N,ƞ 2-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  2 derivative RhCl(IPr){ 2-CH2=CH(C4H3N2)}(py). Compound RhCl(IMes)(κ-N,η 2-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. Scheme 8. Preparation of Rh I Cl(IPr)(η 2-vinylpyrazine)(py) Complex K were calculated on the basis of the rigid rotor-harmonic oscillator approximation. The solvent effects (toluene) were calculated by the SMD method. 43 ASSOCIATED CONTENT Supporting Information Available. Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org. NMR data for organometallic and organic products (PDF) Optimized coordinates for the computed compounds (XYZ). Accession Codes CCDC 1825029 (2a), 1825032 (4), 1825033 (6a) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

Preparation of Butadienylpyridines by Iridium‐NHC‐Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement

“…In this regard, we have recently described the outstanding catalytic performance of Rh‐NHC catalysts in C−C coupling reactions leading to the formation of 4 H ‐quinolizine derivatives under mild reaction conditions [10c,d] . These bicyclic compounds result from the thermal 6π‐electrocyclization of 1 Z ,3 gem ‐butadienylpyridine derivatives formed in the hydroalkenylation reactions.…”

New molecular architectures containing low-valent cluster centres with di- and trimetalated 2-vinylpyrazine ligands: synthesis and molecular structures of Ru5(CO)15(μ5-C4H2N2CHCH)(μ-H)2 and Ru8(CO)24(μ7-C4H2N2CHC)(μ-H)3

“…between the N(1)-C (21) and N(2)-C(19) bonds. The other two carbon atoms of the ring, C (18) and C (20), can now be considered sp 3 -hybridized which is in accord with the widening of the acute angle involving these carbons [C(17)-C(18)-Ru(5) 69.52 (9) and Ru 1 2) indicating rapid movement of both hydrides about the three ruthenium-ruthenium edges within the Ru 3 unit at room temperature.…”

“…[13][14][15][16][17][18][19] The chemistry of the closely related 2-vinylpyrazine, which contains an additional nitrogen atom within the heterocyclic ring, has been far less studied. [20][21][22] Usually, reactions of these ligands with unsaturated appendages proceed through C-H activation of the vinyl group with concomitant coordination of the adjacent ring nitrogen. However, 2-vinylpyrazine can also coordinate to trimetallic centres through metalation of its aromatic ring utilizing the second ring nitrogen (F, Chart 1), especially if the dominant C-H bond activation pathway does not furnish a kinetically stable product.…”

New Aspects of Recently Developed Rhodium(N‐Heterocyclic Carbene)‐Catalyzed Organic Transformations

“…The IPr complex undergoes stoichiometric hydroalkenylation with ethylene or styrene to give RhCl(IPr)(κ‐N,η 2 ‐butenylpyridine) complexes. In addition, this reaction could be catalytically performed with 5 mol% of the RhCl(IMes)(κ‐N,η 2 ‐vinylpyridine) species (Scheme ) …”