Reduction of titanium trichloride

An 80% yield of tetraphenylfuran is obtained by treatment of benzoyl chloride with active titanium generated by lithium aluminum hydride reduction of titanium trichloride (Scheme 84e) (8UOC2407). The reaction nroceeds via benzil and tetraphenylbut-2-ene-l,4-dione, both of which are minor products of the reaction. 

Reduction of aromatic aldehydes to pinacols using sodium amalgam is quite rare. Equally rare is conversion of aromatic aldehydes to alkenes formed by deoxygenation and coupling and accomplished by treatment of the aldehyde with a reagent obtained by reduction of titanium trichloride with lithium in dimethoxyethane. Benzaldehyde thus afforded /ra/is-stilbene in 97% yield [206, 209]. 

An interesting reaction takes place when diketones with the keto groups in positions 1,4 or more remote are refluxed in dimethoxyethane with titanium dichloride prepared by reduction of titanium trichloride with a zinc-copper couple. By deoxygenation and intramolecular coupling, cycloalkenes with up to 22 members in the ring are obtained in yields of 50-95%. For example, 1-methyl-2-phenylcyclopentene was prepared in 70% yield from 1-phenyl-1,5-hexanedione, and 1,2-dimethylcyclohexadecene in 90% yield from 2,17-octa-decanedione [206, 210]. 

A modified procedure suitable for intramolecular reductive coupling is achieved using low-valence titanium prepared by reduction of titanium trichloride with a zinc-copper couple followed by the extremely slow addition of ketone to the refluxing reaction mixture (0.0003 mol over a 9-hour period by use of a motor-driven syringe pump) [S60. ... 

Pinacolization sometimes occurs when undesired, but it is difficult or unsatisfactory when needed. A few attempts to improve this reaction by sonication (Fig. 18) have met with limited success, and a general solution to this problem remains to be found. Aromatic aldehydes and ketones pinacolize when sonicated with low-valent titanium in DME.57 The reaction is highly sensitive to the nature of the solvent. In apolar media, the reduction of titanium trichloride does not occur, and in THF, the McMurry coupling to olefinsHO is preferred. 

Titanium trichloride is almost always prepared by the reduction of TiCl, most commonly by hydrogen. Other reduciag agents iaclude titanium, aluminum, and 2iac. Reduction begias at temperatures of ca 500°C and under these conditions a-TiCl is formed. The product is cooled quickly to below 450°C to avoid disproportionation to the di- and tetrachlorides. P-TiCl is prepared by the reduction of titanium tetrachloride with aluminum alkyls at low (80°C) temperatures whereas y-TiCl is formed if titanium tetrachloride reacts with aluminum alkyls at 150—200°C. At ca 250°C, the P-form converts to d. d-TiCl is made by prolonged grinding of the d- or y-forms. 

Reduction. Triaryknethane dyes are reduced readily to leuco bases with a variety of reagents, including sodium hydrosulfite, 2inc and acid (hydrochloric, acetic), 2inc dust and ammonia, and titanous chloride in concentrated hydrochloric acid. Reduction with titanium trichloride (Knecht method) is used for rapidly assaying triaryknethane dyes. The TiCl titration is carried out to a colorless end point which is usually very sharp (see Titanium COMPOUNDS, inorganic). 

Solutions of low-valence titanium chloride (titanium dichloride) are prepared in situ by reduction of solutions of titanium trichloride in tetrahydrofuran or 1,2-dimethoxyethane with lithium aluminum hydride [204, 205], with lithium or potassium [206], with magnesium [207, 208] or with a zinc-copper couple [209,210]. Such solutions effect hydrogenolysis of halogens [208], deoxygenation of epoxides [204] and reduction of aldehydes and ketones to alkenes [205,... 

The amount of amorphous polymer, which is generally produced in small percentage (9-16%) contemporaneously with the non-atactic polymer, is independent of reaction time (see Table II). It is on the contrary closely connected with the nature of the catalytic system employed and changes, for instance, when the triethylaluminum is substituted by other metal alkyls (beryllium alkyls, propylaluminum, isobutylaluminum, etc.) 5,28). It also depends on the purity of the a-titanium trichloride, in particular increasing in the presence of other crystalline modifications of titanium trichloride [i.e. -TiCU (27)] and of titanium compounds obtained by reduction of titanium tetrachloride at low temperature with aluminum alkyls. 

Stable selenium sols may be obtained by the reducing action of quadrivalent titanium. If a solution of titanium trichloride (1-5 per cent.) is boiled for some time, hydrolysis and oxidation occur on addition of this solution to one of selenium dioxide (0-2 per cent.) reduction to selenium occurs and any unchanged titanic acid, Ti(OII)4, remains in colloidal solution and exerts a protective action.4... 

The procedure for tellurium is similar to that used for the previous elements except that there is no time delay in the preliminary reduction step, the collection time is 15 sec, and only 6 ml of titanium trichloride solution is used to optimize the determination. 

Triarylmethane dyes are reduced readily to leuco bases with a variety of reagents. Reduction with titanium trichloride (Knecht method) is used for rapidly assaying triarylmethane dyes. 

When the boiling of the tetrachloride has reached a steady state, the filament is turned on and brought to a bright red heat. Reduction begins with the formation of a dark purple smoke of titanium trichloride which collects on the walls of the reaction vessel. Most of these particles are washed down to the bottom by the refluxing tetrachloride. 

The use of titanium trichloride for the reduction of sulfoxide groups allows a combined approach to the analysis of urine for metabolites derived from hydrolysis and the p-lyase pathway, as shown in Figure 7 (l0 211. Urine is treated with titanium trichloride, divided into two aliquots, and analyzed for TOG and the reduced P-lyase product (10) as described above. 

The reactions described in Equation (20) are, however, only known for heterogeneous catalysts. For example, formaldehyde was formed selectively by a four-electron reduction of CO2 by an aluminum amalgam in the presence of titanium trichloride, even at room temperature and atmospheric pressure. The reaction is catalytic wiih respect to titanium [117,118]. 

Thiophene derivatives were also prepared from the respective furans 2 [37]. Thus, the 2,5-dihydroxymethyl thiophenes were obtained from 2 by reduction, protection of the hydroxyl groups, oxidation with m-chloroperbenzoic acid to the respective cis-hexenedione whose reduction with titanium trichloride and subsequent reaction with Lawesson reagent gave the protected thiophene derivative. 

The reduction of titanium tetrachloride by an equimolar quantity of alkylaluminum gives /S-TiCls, one of several crystalline modifications of the trichloride (Natta et al., 1961a). For example, Natta et al. (1959c) found that (C2H5)3A1TiCU at Al/Ti = 1.0-1.3 produced the form almost quantitatively. This catalyst yielded a polybutadiene composed of about 50% cis-1, 4-polybutadiene, 40% imns-l,4-poly butadiene, and 10% 1,2-... 

The catalyst itself is apparently a form of titanium trichloride (T1CI3, commonly octahedral), which is formed on reduction of the tetrachloride by the alkyl aluminum. Some surface sties of the solid catalyst bear an alkyl group and a vacant site (Scheme 6.34). The polymer that forms is an unbranched linking of ethylene units. 

Titanium dichloride, TiCl2, is produced from titanium trichloride by thermal decomposition, or by the reduction of titanium tetrachloride with sodium amalgam. 

Reduction. Just as aromatic amine oxides are resistant to the foregoing decomposition reactions, they are more resistant than ahphatic amine oxides to reduction. Ahphatic amine oxides are readily reduced to tertiary amines by sulfurous acid at room temperature in contrast, few aromatic amine oxides can be reduced under these conditions. The ahphatic amine oxides can also be reduced by catalytic hydrogenation (27), with 2inc in acid, or with staimous chloride (28). For the aromatic amine oxides, catalytic hydrogenation with Raney nickel is a fairly general means of deoxygenation (29). Iron in acetic acid (30), phosphoms trichloride (31), and titanium trichloride (32) are also widely used systems for deoxygenation of aromatic amine oxides. 

Cyclopentadienyltitanium Compounds with Other Carbon Titanium Links. Cyclopentadienyltitanium trichloride and, particularly, CpgTiClg react with RLi or with RAl compounds to form one or more R—Ti bonds. As noted, the Cp groups stabilize the Ti—R bond considerably against thermal decomposition, although the sensitivity to air and moisture remains. Depending on the temperature, mole ratio, and stmcture of R, reduction of Ti(IV) may be a serious side reaction, which often has preparative value for Cp Ti(Ill) compounds (268,274,275). 

In order to overcome the poor electrophilicity ofimines, nitrones arc used as partners for reaction with iron acyl enolates 428. Benzaldehyde phenylnitrone (5) reacts rapidly with the aluminum-based enolate at —78 C to give a crude /J-hydroxyamino iron acyl 6 (68% yield). Treatment with aqueous titanium trichloride in tetrahydrofuran at room temperature causes a selective reduction of the N—O bond and affords the /1-amino iron acyl 7 with inverse configuration compared to the addition ofimines (99% yield d.r. 11 23). 

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