Regiochemistry Control by Bipyridine Substituents in the Deprotonation of Re^I and Mo^IIN‐Alkylimidazole Complexes

Abstract

Compounds containing N‐alkylimidazoles (N‐RIm) and 4,4′‐disubstituted 2,2′‐bipyridines (4,4′‐R′2bipy) coordinated to cationic {Mo(η3‐C4H7)(CO)2} and {Re(CO)3} fragments undergo deprotonation of the C2‐H group of the N‐RIm ligands in their reactions with KN(SiMe3)2. The resulting internal nucleophile adds either to one pyridyl ring, which becomes dearomatized and can undergo ring opening in the subsequent reaction with excess MeOTf, or to the metal center, yielding imidazol‐2‐yl complexes, which in turn add HOTf or MeOTf, affording N‐heterocyclic carbene complexes. Which pathway is followed is dictated by the metal and the nature of the imidazole (R) and bipyridine (R′) substituents. For ReI compounds, addition to pyridine is found with R′=tBu and OMe, whereas for R=Me and R′=NMe2, imidazolyl formation is preferred. Coordination of 4,7‐Cl2‐1,10‐phenanthroline to MoII favors C−C coupling, in contrast to the analogous parent bipy or phenanthroline complexes, for which formation of the imidazol‐2‐yl complexes had been found. DFT calculations showed the theoretically expected products in each case, and following their predictions new types of products were obtained experimentally.

Visible-Light-Induced Amination of Quinoline at the C8 Position via a Postcoordinated Interligand-Coupling Strategy under Mild Conditions

“…4 Furthermore, coordination to a metal ion also preorganizes the substrates via a geometric control, facilitating their subsequent coupling in regio-and stereoselectivity. 5 In addition, highly activated intermediates such as N and S radicals can be stabilized by coordination to a metal ion via electron delocalization. 6 Indeed, the oxidation of metal−thiolate complexes into metal−thiyl radical intermediates, affording disulfides via S−S radical coupling, is well-documented (see Scheme 1a).…”

Building C(sp3) Molecular Complexity on 2,2′‐Bipyridine and 1,10‐Phenanthroline in Rhenium Tricarbonyl Complexes

“…Rhenium complexes containing dearomatized pyridyl rings from intramolecular nucleophilic addition have been protonated or methylated at the amido nitrogen [14a, b] . Examples of protonation at a carbon atom have been found in complexes containing dearomatized 4,4′‐X 2 ‐2,2′‐bipyridine (X=OMe, [14a] NMe 2 ), [14e] for which the preference for carbon (over the amido nitrogen) protonation was attributed to the presence of the resonance electron‐donating groups, which increased the nucleophilicity of the C3 and C5 carbon atoms of the dearomatized pyridyl ring. The tert ‐butyl substituent in the chelates of 4M‐C4 and 4 m‐C4 could be playing a similar role, leading to functionalization of the 3 position upon both protonation and methylation.…”

“…In previous studies, we showed that bipy and phen coordinated to cationic fac ‐rhenium(I) tricarbonyl [14] or cis ‐molybdenum(II) dicarbonyl [14a, e] fragments are not chemically inert ligands [15] and can undergo, under very mild conditions, the addition of internal nucleophiles generated, in most cases, by deprotonation of appropriately positioned co‐ligands (Scheme 1 a). As a result of the additions being intramolecular, the C(sp 3 ) centers were built on the chelate positions closest to the metal fragment—C2 upon deprotonation of SMe 2 [16] and C6 upon deprotonation of N ‐alkylimidazoles, [14a, b, e] PMe 3 , [17] or α‐methyl‐ N ‐heterocycles [18] . Outer‐sphere MeLi addition to coordinated bipy and phen ligands of fac ‐[Re(N‐N)(CO) 3 (PMe 3 )]OTf (N‐N=bipy, phen, Scheme 1 b) complexes [17b] suggests that chelate identity plays a role in the site‐selectivity of the addition.…”

Elucidation of the Pyridine Ring-Opening Mechanism of 2,2′-Bipyridine or 1,10-Phenanthroline Ligands at Re(I) Carbonyl Complexes

“…We have shown that 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen) coordinated to cationic fac -rhenium­(I) tricarbonyl fragments are not chemically inert ligands and can undergo, under mild conditions, the addition of internal nucleophiles generated, in most cases, by deprotonation of appropriately positioned coligands. − In the particular case of the deprotonation of 1-methylimidazole (N-MeIm) coordinated to fac -{Re­(bipy)­(CO) 3 }, followed by the action of excess MeOTf, the pyridine ring contraction concomitant with C–N bond scission was obtained ([ A ] in Scheme ). As we have demonstrated more recently, this type of pyridine ring-opening process is favored by more electron-rich cis -rhenium­(I) dicarbonyl complexes, obtained from fac -{Re­(CO) 3 } species by carbonyl substitution by a more σ-donor trimethylphosphane ligand.…”