Titanacyclobutane complexes

The first documented example of the living ROMP of a cycloolefin was the polymerization of norbornene using titanacyclobutane complexes such as (207) 510-512 Subsequent studies described the synthesis of di- and tri-block copolymers of norbornenes and dicyclopentadiene.513 However, functionalized monomers are generally incompatible with the highly electrophilic d° metal center. 

With a titanacyclobutane complex as initiator the rate of polymerization is independent of [M] and therefore governed by the rate of opening of the titanacyclobutane chain carrier432. The comparative stability of the metallacyclobutane bearing an endo substituent is a feature which is also found in the ROMP of other substituted norbomenes see Section Vm.C.12. In contrast, the rate of ROMP of 235 induced by ReCl5/Me4Sn (1/1.5) is first-order in both catalyst and monomer and here the addition of monomer to the metal carbene complex must be rate-determining562. 

Similarly, tethered titanacyclobutane complex 74 also undergoes protonolysis with 1 and 2 equiv of HC1 in toluene to give alkyltitanium chloride 75 and titanocene dichloride 76, respectively, in excellent yield (Scheme 10). Titanacyclobutane complex 74 can undergo also intramolecular proton transfer from one of the ligand methyl groups, presumably via a cr-bond metathesis, to give the 76-tetramethylfulvene alkyl complex 77 <19950M5481>. 

A mechanistically obscure transformation occurs upon treatment of the tetramethylfulvene titanium complex 161 with methallyl Grignard, producing bridged titanacyclobutane complex 162. This reaction is proposed to proceed by intramolecular alkylation at the central carbon of an 7]4-fulvene, 7]3-methallyl intermediate, but with due consideration... 

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... 

The unsaturated cationic titanium allyl compound [Gp 2Ti(allyl)]+BPh4- is prepared by oxidation of the titanium(m) allyl complex Cp 2Ti(allyl) with a ferrocenium cation. Nucleophilic addition to the cationic unit proceeds regioselectively to the central allylic position. Reaction with benzyl Grignard or the enolate of propiophe-none affords the corresponding titanacyclobutane complexes (Scheme 513).1305... 

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... 

Titanacyclobutane complexes have been explored as catalysts for the metathesis of alk-l-enes (non-productive reaction) (Lee, J.B. 1982), alk-2-enes and alkadienes (poor yields) (Straus 1985). The titanacyclobutane 19, formed by reaction of 17 with norbomene at 20°C, eqn. (17), initiates the ROMP of fiirther norbomene when the temperature is raised to 65°C. The polymer formed has a narrow molecular... 

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). 

With TiCl4//-Bu3Al (2/1) as catalyst system the polymer formed shows little unsaturation, but a 1/2 catalyst/cocatalyst ratio gives ring-opened polymer. With Et3Al as cocatalyst the cationic side reactions can be suppressed by the inclusion of a tertiary amine in the reaction mixture (Saegusa 1964 Tsujino 1964, 1965 Winstein 1977). More recently, a number of titanacyclobutane complexes have... 

Conversion of Titanacyclobutane Complexes for Ring Opening Metathesis Polymerization into Ziegler-Natta Catalysts... 

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