Daniel Bojer

CH Activation versus Yttrium–Methyl Cation Formation from [Y(AlMe4)3] Induced by Cyclic Polynitrogen Bases: Solvent and Substituent‐Size Effects

The reaction of 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) with [Y(AlMe(4))(3)] resulted in the formation of [(TiPTAC)Y(Me(3)AlCH(2)AlMe(3))(μ-MeAlMe(3))] by C-H activation and methane extrusion. In contrast, the presence of bulkier cyclohexyl groups on the nitrogen atoms in 1,3,5-tricyclohexyl-1,3,5-triazacyclohexane (TCyTAC) led to the formation of the cationic dimethyl complex [(TCyTAC)(2)YMe(2)][AlMe(4)]. The investigations reveal a dependency of the reaction mechanism on the steric bulk of the N-alkyl entity and the solvent employed. In toluene C-H activation was observed in reactions of [Y(AlMe(4))(3)] with 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) and TiPTAC. In THF molecular dimethyl cations, such as [(TCyTAC)(2)YMe(2)][AlMe(4)], [(TMTAC)(2)YMe(2)][AlMe(4)] and [(TiPTAC)(2)YMe(2)][AlMe(4)], could be synthesised by addition of the triazacyclohexane at a later stage. The THF-solvated complex [YMe(2)(thf)(5)][AlMe(4)] could be isolated and represents an intermediate in these reactions. It shows that cationic methyl complexes of the rare-earth metals can be formed by donor-induced cleavage of the rare-earth-metal tetramethylaluminates. The compounds were characterised by single-crystal X-ray diffraction or multinuclear and variable-temperature NMR spectroscopy, as well as elemental analyses. Variable-temperature NMR spectroscopy illustrates the methyl group exchange processes between the cations and anions in solution.

Lewis Base Induced Reductions in Organolanthanide Chemistry

Recent years have seen a remarkable progress in the molecular chemistry of divalent lanthanides, which was previously restricted to just the three ions Eu, Yb, and Sm. Today even compounds of Tm, Dy, 5] Nd, and La [8] are known. Samarium(II) (usually as SmI2) is in widespread use as a powerful reducing agent in synthetic chemistry. It has a relatively strong negative redox potential (Sm/Sm E1/2 = 1.55 V) compared to europium and ytterbium and is not accesible under mild conditions. It should be noted that the potentials for the reduction M/M are generally more negative for organometallic systems than for reductions in aqueous solution and that both solvent and ligand contributions are important in lanthanide redox chemistry. A striking variant of redox reactivity of samarium compounds was found by Evans and Davis when they investigated pentamethylcyclopentadienyl lanthanide compounds. The tris(pentamethylcyclopentadienyl) complex [(C5Me5)3Sm] was previously thought to be non-existent for steric reasons, but it turned out to behave as a strong oneelectron reducing agent analogous to the divalent compound [(C5Me5)2Sm]. The steric demand of the (C5Me5) ligand facilitates reduction to [(C5Me5)2Sm], whereby the third (C5Me5) unit is oxidized and then dimerizes to give (C5Me5)2. The term “sterically induced reduction” (SIR) [14]

Substituent Size Effects in Lewis Base Induced Reductions in Organolanthanide Chemistry

The reaction of the tripodal 1,3,5-trialkyl-1,3,5-triazacyclohexanes (L=cyclo-[N(R)CH(2)](3) , R=Et, iPr, tBu), with [Sm(AlMe(4))(3)] resulted in the formation of divalent samarium complexes of the constitution [{L(n)Sm(AlMe(4))(2)}(m)] (n, m=1,2) under ethane extrusion. These compounds were characterised by single-crystal X-ray diffraction and elemental analyses. Simultaneous occurrence of Lewis base induced reduction and C--activation reactions is observed. The ratio of products depends on the bulkiness of the N-alkyl substituent R. The reaction of [Sm(AlMe(4))(3)] with 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) in benzene afforded the inversion-symmetric dimer [{(TiPTAC)(η(3)-AlMe(4))Sm}(2)(μ(2)-AlMe(4))(2)], whereas in toluene the pseudo-samarocene [(TiPTAC)(2)Sm(η(1)-AlMe(4))(2)] was obtained. The trisaluminate [(TiPTAC)Sm{(μ(2)-Me)(Me(2) l)}(2)(μ(3)-CH(2))(2)AlMe(2))] was found to be the C--activation product. In the case of the particular bulky 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane (TtBuTAC), the reaction led to the formation of the dimeric [{(TtBuTAC)(η(3)-AlMe(4))Sm}(2)(μ(2)-AlMe(4))(2)] even in toluene in comparably high yields. The decrease of the steric demand to ethyl groups in 1,3,5-triethyl-1,3,5-triazacyclohexane (TETAC) afforded the samarocene-like [(TETAC)(2) Sm(η(1)-AlMe(4))(2)] in lower yields. The resulting divalent samarium compounds are found to be stable with respect to reagents like dinitrogen, conjugated olefins and polycyclic aromatic systems.

Variations in the Mechanisms of Direct Metallation of Cyclic and Acyclic Aminals

The development of preparative protocols for direct ametallation of amines is a challenging but worthwhile task, due to the importance of such reagents for synthesis. Because of the electron-density accumulation on the adjacent nitrogen atom and carbanionic function, these systems are regarded as non-stabilised or even destabilised carbanions. Alternative multi-step approaches to a-metallated amines via transmetallation, reactions of iminium salts with lowvalent metal halides, C S or C Te bond cleavage or activation of amines by BF3 adduct formation [5] involve enormous preparative effort and are inefficient due to loss of substance. Despite substantial progress in the few last years, the number of amine systems directly accessible to deprotonation is still easy to overlook. n-Butyllithium mono-lithiates N,N ,N -trimethyl-1,4,7-triazacyclononane at one methyl group; N-methylpiperidine is deprotonated at the CH3 group using Schlosser s base; the deprotonation of Me2NACHTUNGTRENNUNG(CH2)2NMe2 (TMEDA) was described by two groups, but never occurs in large yield; MeNACHTUNGTRENNUNG[(CH2)2NMe2]2 (PMDTA) can be lithiated, but again in limited yield; and the chiral diamine ((R,R)-tetramethyl-1,2-diaminocyclohexane ((R,R)TMCDA) was found to undergo direct metallation—again at its terminal CH3 group. [10] The latter three examples are important due to the fact that the substrates are widely applied as auxiliary bases in alkyllithium chemistry, without the intention of metallating these. Generating a carbanion adjacent to two nitrogen atoms seemed even more difficult. However, we recently found that aminal units incorporated in saturated six-membered rings are selectively lithiated at the endocyclic NCH2N position. The reaction of 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) with nBuLi and even better with tBuLi in hexane proceeds smoothly giving a product consisting of a dimer of lithiated TMTAC linked into chains alternating with TMTAC, [{MeN ACHTUNGTRENNUNG[CH2N(Me)]2CHLi}2·TMTAC]1.[11] Later we found that conducting the reaction for a longer time at low temperatures leads to the completely lithiated compound. A similar result was found for 1,3-dimethyl-1,3-diazacyclohexane (DMDAC), which is also lithiated at the NCH2N position by tBuLi. These compounds have been shown to be applicable as Corey–Seebach lithiated dithiane analogous reagents, but with the advantage of heavy-metal free workup procedures. Even a system that has two diazacyclohexane units joined via a methylene group H2CACHTUNGTRENNUNG[NCH2N(Me)ACHTUNGTRENNUNG(CH2)3]2 can be doubly deprotonated in high yields to give a dimer of H2C ACHTUNGTRENNUNG[NCH(Li)N(Me) ACHTUNGTRENNUNG(CH2)3]2.[13] These results are in contrast to reports by Karsch, in which open-chain aminals of the type RMeNCH2NMeR can be simultaneously lithiated at both methyl groups by treatment with tBuLi (Scheme 1). Obviously there are different mechanisms involved.