Titanacyclobutanes formation

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

Scheme 14.5. Formation of titanocene-methylidene from titanacyclobutanes and dimethyltitano-...

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

Scheme 14.21. Formation of cyclopropanes by the oxidative cleavage of titanacyclobutanes. A Me xMe A Me qvie...

Scheme 14.24. Formation of titanium enolates by the reaction of titanacyclobutanes with acyl chlorides or acid anhyd rides.

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

In the formation of block copolymers by sequential addition of monomers it generally does not matter which monomer is polymerized first, and diblock or multiblock copolymers of narrow MWD and of any desired sequence length are readily prepared. Termination is usually effected by reaction of the living ends with aldehydes ketones can be used for terminating titanacyclobutane ends, while unsaturated ethers are used for terminating ruthenium carbene complexes. 

Titanacycle 10 ring-opens photochemically at temperatures well below 0°C, Eq. (27) [41]. The unstable titanium carbene 20 can be trapped by exo-trans-exodiene 21, derived from (2 + 2) cycloaddition of norbornadiene, to produce a mixture of two isomeric bis(titanacyclobutanes), 22a, b [42]. Bis(titanacycle) 22a, b could not be synthesized cleanly by thermolysis of 10 because formation of 22a, b competes with ring-opening and the addition of a second equivalent of diolefin. 

A series of aryl-substituted titanocene diazoalkane complexes, ( 5-CsMes)2Ti(N2CHAr), has also been synthesized 183.111 Dinitrogen loss affords the transient alkylidene complexes 184 that can be trapped with excess styrene to yield the titanacyclobutane complexes, (if -C5Me5)2Ti(CHArCHAr C112) 185. Measuring the rate of metallacycle formation as a function of /wra-substituent on the diazoalkane ligand has produced little effect. This observation has... 

Titanathietane complexes are obtained as brown crystals of high thermal stability when the vinylidene intermediate species reacts with isothiocyanates RNGS by a [2 + 2]-cycloaddition process (Scheme 529). Complexes with the sulfur atom in cr-position bonded to titanium are formed exclusively. The structure for the cyclohexyl derivative has been confirmed by X-ray diffraction. Heating of these titanathietane complexes in the presence of pyridine at 80 °C for 20 min results in isomerization, with the formation of a new titanacyclobutane compound (Scheme 529). The regioselectivity of these reactions was discussed on the basis of Hartree-Fock ab initio calculations.1365... 

Complexes Cp2TiMe2, Cp2TiClMe, CpTiMe3, and Cp2Ti(CH2CMe3)2 catalyze the ROMP of norbornene. The initiation of this process by thermally generated alkylidene titanium species and subsequent formation of titanacyclobutane complexes is proposed. The presence of THF as the solvent can inhibit the ROMP activity.1409 Developments of practical methods to carry out olefination reactions of aldehydes, ketones, and... 

Electrochemical oxidation of stereospecifically deuterated titanacyclobutanes 17 or reaction with tetrakis(trifluoromethyl)cyclopentadienone (TTFC), 2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone (DDQ) or ferricinium ion leads to stereospecific formation of cyclopropanes with a high degree of retention of configuration. ... 

Cleavage of bonds 1—4 and 2—3, followed by rotation of the olefin in the titanium-carbene-olefin complex and re-formation of the titanacyclobutane complex, would have led only to cis-ll-A-d, while cleavage of bonds 1—2 and 3—4, followed by rotation of the olefin, would have led only to trans-l - i-d. In fact both cis and trans isomers of 21 and 22 are formed, as well as the cis isomer of 20 (Ikariya 1985). 

Treatment of titanacyclobutanes with proton sources such as HX results in various reactions ranging from clean cleavage of one M-C bond to metallacycle decomposition. The reaction of metallacycles below their decomposition temperature with anhydrous HCl gives mainly CpaTiClz and the corresponding alkane. Whatever the decomposition pathway, the direct formation of a titanocene alkylchloride by reacting HCl with almost all di or tri substituted titanacyclobutanes seems to be precluded. However softer protic acids like alcohols have been shown to be able to protonate titanium-carbon bonds of some disubstituted titanacyclobutanes under mild conditions, obtaining the corresponding alkoxides. ... 

Scheme 4.10. Formation of titanacyclobutanes and regeneration of titanocene-methylidene.

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