This contribution describes catalytic tandem C-N and C-C bond-forming reactions involving the intramolecular hydroamination/bicyclization and intermolecular hydroamination/cyclization of olefins and alkynes using the organolanthanide complexes Cp'2LnCH(SiMe3)2 and Me2SiCp''2LnCH(SiMe3)2 (Cp' = η5-Me5C5; Cp'' = η5-Me4C5; Ln = lanthanide) as precatalysts. In the case of the intramolecular processes, substrates of the structures RC≡C(CH2)(a)NH(CH2)(b)C≡CR, RC≡C(CH2)(c)NH(CH2)(d)CH=CH2, and H2C=CH-(CH2)(e)-NH(CH2)(f)CH=CH2 are regiospecifically bicyclized to the corresponding pyrrolizidine and indolizidine skeletons, with turnover frequencies ranging from 2 to 777 h-1 at 21°C and isolated product yields ranging from 85 to 93%. In the case of e = 3 and f = 1 mediated by Cp'2Sm-, the kinetic rate law is zero-order in substrate concentration and first-order in lanthanide concentration. In the case of R = Ph, c = 3, and d = 1, the Cp'2Ln-catalyzed turnover frequencies fall precipitously with decreasing Ln+3 ionic radius. In the intermolecular processes, substrates of the type HC≡CCH2NHR undergo regiospecific coupling and cyclization to the corresponding pyrroles MeCC(H)=C(CH2NHR)N(R)CH with high turnover frequencies where R and N(t) (h-1) are the following: CH2=CHCH2, 236 (60°C); CH3CH2CH2, 208 (60°C); CH2=CHCH2CH2CH2, 58 (60°C). In addition, hydroamination/cyclization processes after intermolecular insertion can be effected when R = CH2=CHCH2, to afford a 2,7-dimethyldipyrrolo[1,2-a:1',2'-d]pyrazine derivative via two successive intramolecular olefin insertion processes. The mechanism for such tandem C-N and C-C bond formations is postulated to involve turnover-limiting intra- or intermolecular alkene/alkyne insertion into the Ln-N functionality, followed by rapid intramolecular insertion of a pendant C=C/C≡C-containing functionality into the resulting Ln-C bond (prior to protonolysis). Such a scenario is consistent with well-documented, stepwise transformations in organo-f-element-catalyzed insertions of unsaturated carbon-carbon multiple bonds into metal-amide and metal-alkyl functionalities.
ASJC Scopus subject areas
- Colloid and Surface Chemistry