Organolanthanide complexes of the type Cp′2LnCH(SiMe 3)2 (Cp′ = η-Me5C5; Ln = La, Nd, Sm, Lu) and Me2SiCp″2LnCH(SiMe 3)2 (Cp″ = η5-Me4C 5; Ln = Nd, Sm, Lu) serve as efficient precatalysts for the regioselective intermolecular hydroamination of alkynes R′C≡CMe (R′ = SiMe3, C6H5, Me), alkenes RCH=CH2 (R = SiMe3, CH3CH2CH 2), butadiene, vinylarenes ArCH=CH2 (Ar = phenyl, 4-methylbenzene, naphthyl, 4-fluorobenzene, 4-(trifluoromethyl)benzene, 4-methoxybenzene, 4-(dimethylamino)benzene, 4-(methylthio)benzene), di- and trivinylarenes, and methylenecyclopropanes with primary amines R″NH 2 (R″ = n-propyl, n-butyl, isobutyl, phenyl, 4-methylphenyl, 4-(dimethylamino)phenyl) to yield the corresponding amines and imines. For R = SiMe3, R = CH2=CH lanthanide-mediated intermolecular hydroamination regioselectively generates the anti-Markovnikov addition products (Me3SiCH2CH2NHR″, (E)-CH 3CH=CHCH2NHR″). However, for R = CH 3CH2CH2, the Markovnikov addition product is observed (CH3CH2CH2CHNHR″CH3). For internal alkynes, it appears that these regioselective transformations occur under significant stereoelectronic control, and for R′ = SiMe 3, rearrangement of the product enamines occurs via tautomerization to imines, followed by a 1,3-trimethylsilyl group shift to stable N-SiMe 3-bonded CH2=CMeN(SiMe3)R″ structures. For vinylarenes, intermolecular hydroamination with n-propylamine affords the anti-Markovnikov addition product β-phenylethylamine. In addition, hydroamination of divinylarenes provides a concise synthesis of tetrahydroisoquinoline structures via coupled intermolecular hydroamination/subsequent intramolecular cyclohydroamination sequences. Intermolecular hydroamination of methylenecyclopropane proceeds via highly regioselective exo-methylene C=C insertion into Ln-N bonds, followed by regioselective cyclopropane ring opening to afford the corresponding imine. For the Me2SiCp″2Nd-catalyzed reaction of Me 3SiC≡CMe and H2NCH2CH2CH 2CH3, ΔH‡ = 17.2 (1.1) kcal mol-1 and ΔS‡ = -25.9 (9.7) eu, while the reaction kinetics are zero-order in [amine] and first-order in both [catalyst] and [alkyne]. For the same substrate pair, catalytic turnover frequencies under identical conditions decrease in the order Me2SiCp″2NdCH(SiMe 3)2 > Me2SiCp″ 2SmCH(SiMe3)2 > Me2SiCp″ 2LuCH(SiMe3)2 > Cp′ 2SmCH(SiMe3)2, in accord with documented steric requirements for the insertion of olefinic functionalities into lanthanide-alkyl and -heteroatom σ-bonds. Kinetic and mechanistic evidence argues that the turnover-limiting step is intermolecular C=C/C≡C bond insertion into the Ln-N bond followed by rapid protonolysis of the resulting Ln-C bond.
ASJC Scopus subject areas
- Colloid and Surface Chemistry