No abstract

In recent years, N-heterocyclic carbenes (NHC) have proved to be a versatile class of spectator ligands in homogeneous catalysis. Being robust anchoring functions for late transition metals, their ligand donor capacity and their molecular shape is readily modified by variation of the substituents at the N-atoms and the structure of the cyclic backbone. After the first attempts to use chiral NHC ligands in asymmetric catalysis in the late 1990's, which initially met with limited success, several novel structural concepts have emerged during the past two years which have led literally to an explosion of the field. With a significant number of highly selective chiral catalysts based on chiral NHCs having been reported very recently, several general trends in the design of new NHC-containing molecular catalysts for stereoselective transformations in organic synthesis emerge.

No abstract

A tactical approach: C3‐chiral ancillary ligands such as tris(oxazolinyl)ethane allow stereocontrol in olefin polymerization (see scheme). An alkyl ScIII catalyst polymerizes 1‐hexene with activities ranging from 2000 to 36 000 kg mol−1 h−1 (at −30 to +21 °C) and good tacticity control at low temperatures.

The present contribution describes the synthesis and structural characterization of a novel family of robust titanium complexes, supported by a tridentate pincer ligand of the type bis-phenolate-N-heterocyclic carbene [ tBu (OCO) 2-]. For the most part, these complexes were found to be accessible in high yields via an alcohol elimination route involving the reaction of the imidazolinium salt [ tBu (OCO)H 3 ]Cl (1) with ClTi(O i Pr) 3 or Ti(O i Pr) 4 to afford the corresponding NHC-Ti complexes [ tBu (OCO)]TiCl 2 (2) and [ tBu (OCO)]TiCl(O i Pr) (3), respectively, when the reaction is carried out in noncoordinative solvents such CH 2 Cl 2 and toluene. When these reactions are performed in THF, the corresponding Ti-THF adducts [ tBu (OCO)]TiCl 2 (THF) (2-THF) and [ tBu (OCO)]TiCl(O i Pr)(THF) (3-THF) are isolated in quantitative yields. The molecular structures of complexes 2, 2-THF, and 3-THF were determined by X-ray crystallographic studies, establishing the effective coordination of the tBu (OCO) 2pincer to Ti. While the alcohol elimination pathway appears to be most suited to access Ti complexes of the type [ tBu (OCO)]TiX 2 (X = halide, alkoxide), the amine elimination was also found to be effective, albeit in lower yield. Thus, the reaction of salt species 1 with Ti(NMe 2 ) 4 in THF afforded the corresponding NHC-Ti amido complex [ tBu (OCO)]TiCl(NMe 2 )(THF) (5) in a modest yield. The direct reaction of the salt species 1 with 0.5 or 1 equiv of TiCl 4 (THF) 2 in the presence of NEt 3 afforded the homoleptic bis-adduct Ti complex [ tBu (OCO)] 2 Ti (6), whose molecular structure was confirmed by X-ray crystallographic analysis. As for the potential of such complexes in catalysis, the Ti isopropoxide chloro complex 3-THF was found to readily initiate the ring-opening polymerization of rac-lactide in a controlled manner and, interestingly, without apparent involvement of the NHC moiety in the catalytic process. The tridentate nature of the tBu (OCO) 2ligand as well as some level of steric protection provided by the t Bu groups may rationalize the excellent stability of the Ti-NHC bond in the present systems.

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium-catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L(n)Rh(I)]+ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh-silyl intermediates [L(n)Rh(III)(H)SiHMe2]+. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the Rh-Si bond occurs, as previously proposed by Ojima et al., or into the Si-H bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the Si-H bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This "silylene mechanism" was found to have the lowest activation barrier for the rate-determining step, the migration of a rhodium-bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R2SiH2 compared to R3SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, C-D versus M-D.

No abstract

We describe the synthesis, structure, and reactivity of low-coordinate Al-alkyl and -alkoxide cationic complexes incorporating the sterically bulky aminophenolate bidentate ligand 6-(CH(2)NMe(2))-2-CPh(3)-4-Me-C(6)H(2)O- (N,O). These complexes are derived from the ionization of neutral dialkyl Al complexes (N,O)Al2) (1 a, R=Me; 1 b, R=iBu), readily obtained by alkane elimination between AlR3 and the corresponding aminophenol ligand, with the alkyl abstracting reagents B(C(6)F(5))3 and [Ph(3)C][B(C(6)F(5))4]. The reactions of 1 a,b with B(C(6)F(5))3 yield complicated mixtures or decomposition products, however the ionization of the Al-diisobutyl derivative 1 b with [Ph(3)C][B(C(6)F(5))4] affords a stable four-coordinate Al-PhBr cationic adduct [(N,O)Al(iBu)(PhBr)]+ (3+), as deduced from elemental analysis data. Complex 3+ readily coordinates Lewis bases such as THF to form the corresponding adduct [(N,O)Al(iBu)(thf)]+ (4+), and also rapidly chain-transfers with 1-hexene to yield the three-coordinate Al-hexyl cation [(N,O)Al-hexyl]+ (5+). Both cations 3+ and 5+ slowly dimerize to form unprecedented organoaluminum dications [(N,O)AlR+]2 (3'++, R=iBu; 5'++, R=hexyl) as deduced from X-ray crystallographic analysis. Cation 3+ reacts quickly with iPrOH to form a stable Lewis acid/base adduct [(N,O)Al(iBu)(HOiPr)]+ (6+), which constitutes the first X-ray characterized adduct between an Al-alkyl complex and a simple ROH. The Al-ROH proton in 6+ is readily abstracted by NMe(2)Ph to form the neutral isopropoxide Al complex [(N,O)Al(iBu)(OiPr)] (7). Upon reaction with THF, cation 6+ undergoes an intramolecular proton transfer to yield the ammonium Al-THF complex [(eta1-HN,O)Al(iBu)(OiPr)(thf)] (8 b+), in which the aminophenolate is eta1-coordinated to the Al center. Cation 8 b+ can then be converted to the desired Al-alkoxide derivative [(N,O)Al(OiPr)(thf)](+) (10+), by an intramolecular protonolysis reaction, as confirmed by X-ray crystallography. The synthesized Al-alkyl cations form robust four-coordinate adducts in the presence of cyclic esters such as epsilon-caprolactone and (D,L)-lactide, but no insertion chemistry occurs, illustrating the poor ability of the Al-R+ moiety to ring-open. In contrast, the Al-alkoxide cation 10+ polymerizes epsilon-caprolactone in a controlled manner with excellent activity, but is inactive in the polymerization of (D,L)-lactide and L-lactide. Control experiments with L-lactide show that cation 10+ ring-opens L-lactide to yield a robust five-coordinated Al--lactate cation [(N,O)Al(eta2-L-lactate-OiPr)(thf)]+ (13+), derived from a monoinsertion of L-lactide into the Al--OiPr bond of 10+, that does not further react. Cation 13+ may be regarded as a structurally characterized close mimic of the initial intermediate in the ring opening polymerization (ROP), of lactides by [{LX}M(OR)(L)] (where LX-=bidentate monoanionic ligand and L=labile ligand) metal complex initiators.