Complex, Titanium-methylene

The polymerization proceeds without termination or chain transfer to give poly-norbomene with a narrow molecular weight distribution [35], After an induction period, the rate of monomer consumption (rate of polymerization) becomes constant, indicating a zero-order dependence on the monomer concentration. The induction period is caused by part of titanacycle 7 undergoing non-productive, but rapidly reversible, cleavage to norbornene and the titanium methylene complex, Eq. (17 a). 

A plausible intermediate of this olefination is the titanium-methylene sjtecies 4, which is formed from 1 by removal of AlMe2Cl with a Lewis base, from 2 by fragmentation with elimination of isobutene, and from 3 by a-elimination and release of methane. However, none of these three routes to titanium-carbene complexes of type 4 proved to be generally applicable. Consequently, the use of these reagents in synthesis is essentially limited to the transfer of a methylene unit 18]. From a synthetic viewpoint, a general and easy route to substituted titanium-alkylidene species and their use in carbonyl olefinations would be more desirable. 

METHYLENATION Alkyidimesitylboranes. p,-Chlorobis(cyclopentadienyl)(dimethyl-aluminum)-p.-methylene-titanium. Methylene bromide-Zinc-Titanium(IV) chloride. Organomolybdenum reagents. Titanocene methylene-Zn halide complex. N,N,P-Tri-methyl-P-phenylphosphinothioic amide. Trimethylstannylmethyllithium. 

It was found that titanacyclobutanes could be prepared by removing the aluminum from 3 with a strong Lewis base in the presence of an olefin, as shown in Eq. 9. These metallacycles were stable with respect to rearrangement to an olefin via a 3 hydride process, and showed a tendency to generate intermediate alkylidene complexes in situ. They ultimately proved to be useful as catalysts for the ring opening metathesis polymerization of a variety of strained olefins. [47,48] It should be noted that the masked titanium methylene complex 3 (Eq. 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

Citreoviridin and aurovertin B synthesis (mycotoxins) [109]. Intermediates for the synthesis of C-glycosides and oxygen heterocycles (in connection with the synthesis of these mycotoxins), are available through titanium methylenation of a lactone with a titanium carbene complex. 

Spec A = 665 nm of the titanium-methylene blue-ascorbic acid redox reaction Spec A= 420 nm of the yellow Tiron derivative (pH = 5.2 to 5.6) Spectrophotometric determination of tungsten with thiocyanate after both tungsten(VI) and molybdenum(VI) are extracted into chloroform as benzoin a-oxime complexes... 

Interestingly, a similar process is observed for the thermal decomposition of dimethylbis(pentamethylcyclopentadienyl)titanium(IV) (65). The decomposition takes place at elevated temperatures around about 110 °C and results in the formation of fulvene complex 68 and methane. Extensive labeling experiments and kinetic measurements resulted in two reaction pathways compatible with the experimental observations. Either one involves a titanium methylene species 66 as the key intermediate. The next step is an intramolecular C,H-activation leading to fulvene complex 67, similar to the formation of 63. Einally, a hydrogen shift accounts for the final product, fulvene complex 68 (Scheme 10.23) [71]. 

A titanium-methylene complex was readily generated from CH2CI2 by treatment with TiCLt and magnesium metal. This generation of active titanium-methylene complex 445 was useful for cyclopropane formation with enamines 446 or amides 447 to give aminocyclopropanes 448 and 449, respectively, in good yields (Scheme 1.210) [294]. TiCU worked catalyticaUy in the reaction with enamines 446, while substoichiometric amounts of TiClq were necessary for the reaction with amides 447. 

A titanium-methylene complex was also generated from Cp2Ti[P(OEt)3]2 and dithioacetal 450 (Scheme 1.211) [295]. Deuterium was delivered at the trowi-position of the cyclopropane 451 when the reaction was quenched by deuterium... 

Very recently, Eisch and co-workers have developed new alkylidene-group IV metal complexes such as methylidene titanium dichloride 67, readily accessible from titanium(iv) chloride and an excess of methyllithium at low temperature (Scheme 24).53 The new methylenating agent 67 can easily convert benzophenone at low temperature into 1,1-diphenylethylene in quantitative yield. 

Titanium enolates.1 This Fischer carbene converts epoxides into titanium enolates. In the case of cyclohexene oxide, the product is a titanium enolate of cyclohexanone. But the enolates formed by reaction with 1,2-epoxybutane (equation I) or 2,3-epoxy butane differ from those formed from 2-butanone (Equation II). Apparently the reaction with epoxides does not involve rearrangement to the ketone but complexation of the epoxide oxygen to the metal and transfer of hydrogen from the substrate to the methylene group. 

Detailed study [18] of the system isobutene-titanium tetrachloride-water-methylene dichloride showed it to be highly complex, but the kinetics and the temperature-dependence of rate and DP could be explained, at least qualitatively, on the hypotheses that the chain-carriers are ions, that paired and free cations have appreciably different reactivities, and that the degree of dissociation of the ion-pairs increases with decreasing temperature. 

A thioformaldehyde complex of titanium (55) was similarly obtained from the titanocene methylene complex 54 and either elemental sulfur or sulfur sources such as propene sulfide, cyclohexene sulfide, styrene sulfide, or Ph3P = S. Best yields were achieved with propene sulfide [Eq. (II)].67... 

Another allyl compound which reacts stoichiometrically with carbon dioxide is (fj5-C5H5)2Ti(l-methylallyl) (120). The titanium acetate complex which is formed is interesting in that the carbon dioxide carbon atom is attached to the substituted end of the allyl. It seems unlikely, then, that the product is the result of C02 insertion into the -methylallyltitanium bond in view of the fact that methyl-substituted allyls tend to form fj -complexes in which the metal is bonded to the least substituted end of the allyl. One possible explanation offered by the authors is that the allyl is bonded to titanium at the methylene carbon, but that rearrangement occurs subsequent to adduct formation [Eq. (49)]. 

Complex 4 can be regarded as titanium carbene Cp2Ti = CH2 coordinated by Me2AlCl. This is the first example of a well-defined metal carbene that catalyzes olefin metathesis and is re-isolated in high yield at the end of the reaction, Eq. (10). In a comparatively slow degenerative metathesis reaction (near equilibrium after 47 h at 51 °C). The 13C label of isobutylene was shown to grow into the methylene group of methylene cyclohexane [29]. 

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