Titanocene-bis

Sneddon has applied these results to the development of a process to form monoalkyldecarboranes.67 Preliminary mechanistic investigations on the catalytic hydroboration by Cp2TiMe2 led to the discovery of the titanocene bis(borane) complex Cp2Ti(HBcat)2.68... 

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

The variety of C2-bridged PBs was further extended by Muhoro via hydroboration of diphenyl(vinyl)phosphine with catechol- and pinacol-boranes (Scheme 29).56 To compensate for the low Lewis acidity of these boronates, the hydroboration reactions were carried out in the presence of 5 mol% of titanocene bis(catecholborane) as catalyst. The desired products 40g and 40h were obtained with complete anti-Markovnikov selectivity. The spectroscopic data and the crystallographic study performed on 40h showed the expected monomeric open structure. 

A review covering the use of titanium carbene complexes in organic synthesis has appeared.8 Special emphasis is placed on titanium carbene generated from titanocene bis(triethylphosphate) by action on thioacetals or gem-dichlorides. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

Titanocene bis(ti Alkenes. 1 titanocene dichio gem-dichlorides derived from enal analogous reactic dithioacetal and t... 

Alkenes. Titanocene bis(triethyl phosphite), which is prepared in situ from titanocene dichloride, triethylphosphite, and Mg, promotes carbonyl olefmation with gem-dichlorides and dithioacetals [e.g., l,l-bis(phenylthio)cyclobutane ] including those derived from enals (to give 1,3-dienes). Enol ethers-and alkenyl sulfides are obtained in the analogous reaction with dithioorthoformates and trithioorthoformates. Cross-coupling of dithioacetal and thiolesters furnishes predominantly (Z)-alkenyl sulfides. ... 

Polyene synthesis. In the presence of titanocene bis(triethyl phosphite), activated alkynes react with (Z)-alkenyl sulfones to give conjugated dienes in a highly legioselective and stereoselective fashion. Mixed unsaturated sulfones react in an analogous manner, and when the titanated coupling adducts are quenched with carbonyl compounds it results in 1,2,4-trienes. ... 

Desulfurative alkylation and acylation. Titanocene bis(triethyl phosphite) promotes desulfurative alkylation of allylic sulfides (dithioacetals) with f-alkyl halides. ... 

Reactions catalyzed by titanocene and lanthanocene complexes occur by different mechanisms (Schemes 16.13 and 16.14). The mechanism of the hydroboration of vinylarenes catalyzed by titanium complexes is shown in Scheme 16.13. In this mechanism, a titanocene bis-borane complex dissociates borane to generate a 16-electron complex that coordinates the alkene or alkyne. Coupling of one carbon of the resulting metallacycle with the boron of the coordinated borane forms the final product. The mechanism of the lanthanocene-catalyzed reactions, shown in Scheme 16.14, relates to the mechanism of... 

Hydroboration of carbonyl compounds by pinacolborane is chemoselectively catalysed by titanocene bis(catecholborane) (A). Aryl aldehydes and ketones produce alkoxypinacolboronate esters (B) in moderate to high yields. The facile hydrolysis of B over silica affords alcohols in good yields. The catalytic hydroboration of electron-poor acetophenones is faster than that for electron-rich acetophenones. Computational studies with benzophenone and benzaldehyde indicated that hydroboration is spontaneous and probably proceeds via Ti metallacycle intermediates whose structures... 

Hayashi, Y. Osawa, M., and Wakatsuki, Y. (1997). Reductive Coupling Reaction Induced by Remote-Site Oxidation in Titanocene Bis(metallocenylacetylide), where Metallocenyl = Ferroeenyl or Ruthenoeenyl a Novel Route to C n = 4, 6, and 8) Wire with the Metallocenyl Groups at Both Terminals. J. Organomet. Chem., 542, 241-246. 

C-C bond cleavage reactions of titanacyclopentanes and titanacyclohexane were independently studied by Grubbs [10] and Whitesides [11]. In the case of titanacyclopentane, ethylene was eliminated via a titanocene-bis(ethylene) complex as shown in Eq. 8. According to the deuterium labeling reaction, the real mechanism is more complicated, since scrambling of D was observed (Eq.9)[10]. 

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