Organolanthanide-catalyzed intra- and intermolecular tandem C-N and C-C bond-forming processes of aminodialkenes, aminodialkynes, aminoalkeneynes, and aminoalkynes. New regiospecific approaches to pyrrolizidine, indolizidine, pyrrole, and pyrazine skeletons

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.