Titanium complexes amides

Dianionic bis(amide) ligands bearing additional donor atoms have been described by several researchers. High activities for ethylene polymerization are observed for pyridyldiamido zirconium complexes such as (42) (1,500gmmol-1 bar-1 h-1),145 although the corresponding titanium complex is much less active.146... 

Snapper and Hoveyda reported a catalytic enantioselective Strecker reaction of aldimines using peptide-based chiral titanium complex [Eq. (13.11)]. Rapid and combinatorial tuning of the catalyst structure is possible in their approach. Based on kinetic studies, bifunctional transition state model 24 was proposed, in which titanium acts as a Lewis acid to activate an imine and an amide carbonyl oxygen acts as a Bronsted base to deprotonate HCN. Related catalyst is also effective in an enantioselective epoxide opening by cyanide "... 

The metathesis of symmetrical aliphatic carbodiimides is also catalyzed by tungsten imido complexes above 140 Other carbodiimide metathesis catalysts include Cr(ll)/Si02/ iminophosphoranes, imido circonocenes, guanidine supported titanium complexes and group 14 amide complexes. ... 

A more recent and in-depth polymerization study of a tight bite angle A,0-chelated early transition metal being utilized for alkene polymerization was presented by Sun s group [10b] in 2010. A set of bis(chloro)mono(cyclopentadienyl)mono(amidate) titanium complexes, in a half-sandwich configuration, were synthesized. Electronic... 

Figure 14 Tethered bis(amidate) titanium complex for preliminary studies as initiators for ethylene polymerization. Ar=4-tert-butylphenyl, X=NMe or Cl [10a]...

In 2013, Schafer s group [22b] reported titanium bis(amidate) and bis(pyridonate) complexes for the homopolymerization of rac-lactide and e-caprolactone, and also the formation of a random copolymer of the two. These complexes form pseudo-octahedral six-coordinate species, which were characterized in the solid state. Complexes were synthesized by first installing 2 equiv. of the ligand on homoleptic TifNMe ) followed by protonolysis of dimethylamido ligands with 2 equiv. of alcohol (Figure 19). 

The chiral titanium complex 41 promotes the c.vo-selective and enantioselective 1,3-dipolar cycloaddition of the unsaturated acylsuccinimide 42 to the nitrones 43 (R, R2 = Ph, / -Tol or Bn) to yield the isoxazolidines 44. The latter were cleaved to the coiresponding amides by aqueous hydrazine <97JOC2471>. 

Aminocyclopropane derivatives were similarly obtained from tertiary amines or nitriles. Whereas the reaction starting from amides requires the use of stoichiometric amount of titanium complexes, the reaction is catalytic with cyanoesters, giving l-azaspirocompounds. In all cases, the enantioselectivity remains too low to be synthetically useful, and efforts are thus needed to improve the enantioselectivity and/or to reduce the amount of catalyst. 

Schafer found that the bulky bis(amidate) complex is an effective catalyst for intermolecular hydroamination of terminal alkyl alkynes with alkylamines, giving exclusively the anti-Markovnikov aldimine product [309]. The same titanium complexes can also be utilized in the hydroamination of substituted allenes in good yields (Scheme 14.132). Under the catalysis of an imidotitanium complex, the highly strained methylenecyclopropane can undergo hydroamination reaction with either aromatic or aliphatic amines, to give ring-opened imine products in good to excellent yields and chemoselectivities [310]. 

Linked half-titanocenes (so-called constrained geometry-type titanium complexes) like [Me2Si(C5Me4)(N Bu)]TiCl2 have also shown to be efficient catalyst precursors [2,16-19], although they have shown extremely low activity for syndiospecific styrene polymerization [16a,19a]. It has also been reported that styrene incorporation by linked Cp-amide Ti catalyst (constrained geometry... 

Group 4 (IV B) dithiocarbamate chemistry is constrained to the 4-4 oxidation state. The first reported example was the eight-coordinate tetrakis(dithiocarba-mate) titanium complex, [Ti(S2CNBz2)4], prepared by Dermer and Femelius in 1934 (608), while the heavier zirconium and hafnium analogues were first prepared by Bradley and Gitlitz (193) from the reaction of metal amides, [M(NR2)4] (M = Ti, Zr, Hf), with carbon disulfide. 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

The increased Markovnikov selectivity in the hydroamination of aliphatic terminal alkynes with aniline derivatives seems to be universal for a number of titanium-based hydroamination catalysts, such as Ind2TiMe2 (49) [184], the di-(pyrrolyl) amine complex 50 [186, 187], and the di(pyrrolyl)methane complex 51 [188]. The bis(amidate) titanium complex 43 exhibited enhanced catalytic activity compared to titanocene catalysts, thus combining high a ri-Markovnikov selectivity with high catalytic activity [191]. 

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