Titanium amides

Titanium amides 411 and alkoxides also produce double insertion products, such as 412. " ... 

Reactions of simple silanediols and disilanols with titanium orthoesters, titanium halides and titanium amides proceed to give cyclic titanasiloxanes [30]. On the other hand, the silanetriols with three functional OH groups would prove appropriate synthons for constructing three-dimensional titanasiloxanes which would in turn serve as model compounds for catalytically useful Ti-doped zeolites [32]. The synthesis of cubic titanasiloxanes has been achieved in two ways. 

The reaction of (EBI)H2 [EBI = ethylene-1,2-bis(indenyl)] and titanium amides did not give the expected ansa-titanocene products. Even with the azetidine complex, Ti(NC3H6)4 reacts with only a single amine elimination to give the mono(indenyl) derivative (C9H6CH2CH2C9H7)Ti(NC3H6)3. By contrast, -zirconocene and hafnocene are easily obtained by the amine elimination process. It is likely that increased steric crowding around the smaller Ti disfavors the second amine elimination.674... 

Scheme 7 Modification of mixed Cp/Cp titanium amidate complexes to probe ligand effects upon alkene polymerization [1 Ob]...

Reductive amination is historically defined as the combination of a Brpnsted acid, a ketone, an amine, and a coexisting reductant. The recent evolution of titanium (IV) alkoxides, as mild Lewis acid replacements for Brpnsted acids, owes its genesis to a clever modification of titanium amide chemistry demonstrated by Mattson et al. in 1990. By prestirring a ketone, amine, and Ti(OiPr)4 (neat), followed by the addition of EtOH and NaBH3CN, the desired reductive amination product was afforded (Scheme 8.8). The same authors suggested that these reductive aminations proceeded through a hemiaminal titanate intermediate, based on IR spectroscopy before addition of the reductant. 

Examination of the enantioselectivities in Table 7.5 indicates a striking difference in selectivity achieved in the reduction of cyclic (entries 1-8) vs. acyclic imines (entries 9-11). The former is very nearly 100% stereoselective. The simple reason for this is that the acyclic imines are mixtures of E and Z stereoisomers, which reduce to enantiomeric amines vide infra). The mechanism proposed for this reduction is shown in Scheme 7.11 [86]. The putative titanium(III) hydride catalyst is formed in situ by sequential treatment of the titanocene BINOL complex with butyllithium and phenylsilane. The latter reagent serves to stabilize the catalyst. Kinetic studies show that the reduction of cyclic imines is first order in hydrogen and first order in titanium but zero order in imine. This (and other evidence) is consistent with a fast 1,2-insertion followed by a slow hydrogenolysis (a-bond metathesis), as indicated [86]. Although P-hydride elimination of the titanium amide intermediate is possible, it appears to be slow relative to the hydrogenolysis. 

Nitrogen gas may serve as an ammonia equivalent by first capturing nitrogen with TiXVLi/TMSCl, which gives a LnTi-N(SiMe3)2 complex 50.93 (DPPF)Pd or (BINAP)Pd catalyze the amination of aryl bromides and triflates with this titanium amide complex in low yields after an acidic workup. Di-arylation is the major byproduct. 

The polymerization mechanism proposed involves a coordination of the isocyanate with the titanium catalyst and its insertion to form a titanium-amidate end group speeies. The titanium end group stays active imtil quenched by the addition of a protonic source. The addition of other monoalkyl isocyanates gives a block copolymer. 

Two probable systems TiCl and alkyl cyanides form complexes of the type (R-C=N)2TiCl4 [6] and titanium amides (R2N) Ti also complex with the -C=N group [5] as below ... 

Arsenic and titanium amides have been used for the amination of 0X0 compds. to gem-diamines and enamines as well as for the amination of acid derivatives . Recently, the preparation of enamines, including sterically hindered compounds, with free amines and titanium tetrachloride has been reported . A Claisen-type rearrangement to form /,(3-unsatd. amines has been used for the preparation of 17-disubst. steroids . 

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