Titanium cleavage

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

Titanium.—Cleavage of the titanium-carbon bond in TiClsCHs in the presence of AlEt3 in toluene solution shows a first-order dependence on the concentration of [TiClgMe], the pseudo-first-order rate constants being roughly proportional to the... 

If no aqueous workup, but anhydrous cleavage of the reaction product 11 is performed with iodotrimethylsilane, the titanium iodide 12 can be recovered or used again in the same flask36. 

In the Mukaiyama variation of the aldol reaction, 3-benzoyloxy-2-trimethylsiloxy-l-butene adds to 2-methylpropanal in a stereoselective manner. Best results are obtained in the presence of titanium(IV) chloride, giving the adducts 9/10 in a diastereomeric ratio of 92 8. Hydrolysis of the benzoyl group and subsequent oxidative cleavage of the 1,2-diol moiety liberates / -hy-droxycarboxylic acids593. 

For preparative purposes, titanium metal can be used in place of sodium or lithium in liquid ammonia for both the vinyl phosphate231 and aryl phosphate232 cleavages. The titanium metal is generated in situ from TiCl3 by reduction with potassium metal in tetrahydrofuran. 

Cp2Ti(PMe3)2 catalyzes the reductive cyclization of the enones 44 to the cyclopentanols 46 via the metallacyclic intermediates 45 (Scheme 27) [64-66]. The cleavage of the titanium-oxygen bond in the metallacycles 45 by a hydrosilane provides a route to the generation of the active catalyst. The net transformation resembles the above-mentioned complementary radical pathway, which affords the opposite isomer. 

The added primary amine may facilitate the cleavage of the Ti-N bond of the key intermediate 107 through the coordination to the titanium center followed by u-bond metathesis. Such an intramolecular exchange process is expected to be facile. The amine exchange product is 106, which can then be rapidly converted to 105 and the corresponding silylated amine to complete the catalytic cycle. 

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). 

In the envisaged titanium oxo complex, the Ti atom is side-bound to the peroxy moiety (02H), consistent with all the spectroscopic results mentioned in Section III in Scheme 27, between the two O atoms that are side-bound to Ti4+, the O atom attached to both the Ti and H atoms is expected to be more electrophilic than the O atom attached to only the Ti atom and is likely to be the site of nucleophilic attack by the alkene double bond. The formation of the Ti-OH group (and not the titanyl, Ti=0, as proposed by Khouw et al. (221)) after the epoxidation and its subsequent condensation with Si-OH to regenerate the Ti-O-Si links had been observed (Section III.B) by FTIR spectroscopy by Lin and Frei (133). Because this is a concerted heterolytic cleavage of the 0-0 bond, high epoxide selectivity and retention of stereochemistry may be expected, as indeed has been observed experimentally (204). 

Four equivalents of complex 1 react with tBuC C—C=C C=C C CtBu to give complex 110 with an intact C4 chain. Under the same conditions, the corresponding zirconocene complex 2a forms complex 111 through twofold C—C single-bond cleavage. If two butadiyne moieties are separated by a para-phenylene unit, as in RC=C C=C C6H4 C=C—C=CR, the titanium complex 112 is obtained. Depending on the stoichiometry, the two zirconocene complexes 113 and 114 can also be obtained [56 b],... 

In the case of the epoxide openings, Me3SiCl is not an appropriate reagent for the cleavage of a titanium alkoxide because of the high oxophilicity of silicon. This results in the... 

Despite the successful reactions mentioned above, olefin metathesis utilizing titanocene-methylidene is not necessarily regarded as a useful synthetic tool. Indeed, the steric interaction between the substituent at the carbon a to titanium and the bulky cyclopentadienyl ligand disfavors the formation of the titanocene-alkylidene 15. Hence, cleavage of the titanacycle affords only titanocene-methylidene and the starting olefin (Scheme 14.9). 

In the presence of allyltrimethylsilane, titanium tetrachloride catalyzed the ring cleavage of the tetrahydro-l,5-dioxa-3a-aza-pentalen-4-ones 79 giving the oxazolidinones 80 (Equation 33) <2001T5393>. 

Looking for a more efficient catalyst to carry out this reaction thus became the most important issue. To achieve this, a large number of common Lewis acids were screened, including the halides of aluminum, iron, zinc, titanium, zirconium, nickel, copper, tin and lead. A number of these compounds did show activities as ether cleavage catalysts. The most effective catalysts were the halides... 

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