Titanium-nitrogen bonds

Titanous chloride (titanium trichloride) is applied in aqueous solutions, sometimes in the presence of solvents increasing the miscibility of organic compounds with the aqueous phase [199, 200]. Its applications are reduction of nitro compounds [201] and cleavage of nitrogen-nitrogen bonds [202] but it is also an excellent reagent for deoxygenation of sulfoxides [203] and amine oxides [199] (Procedure 38, p. 214). 

Hydrazones treated with alkalis decompose to nitrogen and hydrocarbons [845, 923] Woljf-Kizhner reduction) (p. 34), and p-toluenesulfonylhydra-zones are reduced to hydrocarbons by lithium aluminum hydride [812], sodium borohydride [785] or sodium cyanoborohydride [813]. Titanium trichloride hy-drogenolyzes the nitrogen-nitrogen bond in phenylhydrazones and forms amines and ketimines which are hydrolyzed to the parent ketones. Thus 2,4-dinitrophenylhydrazone of cycloheptanone afforded cycloheptanone in 90% yield [202]. 

If titanium-nitrogen and titanium-oxygen bonds arc available in the same molecule, CO insertion yields carbamates [104]. If carbon dioxide has the capability (0 insert into a metal-carbon or a metal-nitrogen bond, the insertion into M-N is preferred hafnium dialkyl diarnides react with CO at room temperature to yield the carbamato derivatives HfRjfOjCNR ) [105). 

Finally, insertion of acetylenes into titanium- and zirconium-nitrogen bonds affords isolable alkenyl complexes (54). 

Carbon-nitrogen bond formation is an important subject in the organic synthesis [301], and hydroamination is an atom-efficient process for the generation of amines and imines from olefins, allenes, and alkynes. Titanium-mediated hydroamination was among the most useful protocols thus far developed for this reaction. By using unsymmetrical olefins and alkynes, the addition of HNR2 can in principle lead to two isomeric products, where the isomeric ratio is usually dependent on the type of titanium catalyst used. 

In the early 1970s, Carraher and coworkers developed a number of different organometallic polymers containing metal-oxygen, metal-sulfur, and metal-nitrogen bonds in the polymer backbone. For example, a number of polyethers, " thioethers, amines, " esters, and oximes of titanium, zirconium, and hafnium were prepared. Scheme 1 shows the synthesis of haftiium, zirconium, and... 

The driving force for this reaction is the greater strength of the aluminum-oxygen bond relative to the aluminum-nitrogen bond. Trialkyltin amides and tetrakis(dimethylamino)titanium show similar reactivity. 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

Titanium silicate molecular sieves not only catalyze the oxidation of C=C double bonds but can be successfully employed for the oxidative cleavage of carbon-nitrogen double bonds as well. Tosylhydrazones and imines are oxidized to their corresponding carbonyl compounds (243) (Scheme 19). Similarly, oximes can be cleaved to their corresponding carbonyl compounds (165). The conversion of cyclic dienes into hydroxyl ketones or lactones is a novel reaction reported by Kumar et al. (165) (Scheme 20). Thus, when cyclopentadienes, 1,3-cyclohexadiene, or furan is treated with aqueous H202 in acetone at reflux temperatures for 6 h in the presence of TS-1, the corresponding hydroxyl ketone or lactone is obtained in moderate to good yields (208). 

Once the methoxy group has been installed and nucleophilic capture of the intermediate has occurred, the product (132) can be treated with an enol ether (e.g. 133) and titanium tetrachloride to affect C-C bond formation adjacent to nitrogen. This sequence served nicely in syntheses of both indolizidine alkaloids elaeokanine A (135) and C (136). 

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