Titanacyclobutanes

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. 

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Ring-Opening Metathesis Polymerization. Several new titanacyclobutanes have been shown to initiate living ring-opening metathesis polymerization (ROMP) systems. These have been used to make diblock and triblock copolymers of norbomene [498-66-8] (N) and its derivatives (eg, dicyclopentadiene [77-73-6] (D)) (Fig. 2) (41). 

Aluminum-free titanocene-methylidene can be generated by thermolysis of titana-cyclobutanes 6, which are prepared by reaction of the Tebbe reagent with appropriate olefins in the presence of pyridine bases [9]. Alternatively, the titanacyclobutanes are accessible from titanocene dichloride and bis-Grignard reagents [10] or from 71-allyl titanocene precursors [11]. The a-elimination of methane from dimethyltitanocene 7 provides a convenient means of preparing titanocene-methylidene under almost neutral conditions [12] (Scheme 14.5). 

Tebbe and co-workers reported the first olefin metatheses between titanocene-methyli-dene and simple terminal olefins [13]. They showed cross-metathesis between isotopically labeled isobutene and methylenecyclohexane to be catalyzed by titanocene-methylidene. This process is referred to as degenerate olefm metathesis as it does not yield any new olefin (Scheme 14.6). The intermediate titanacyclobutane has been isolated and characterized [14], and its stability [15] and mechanism of rearrangement [16] have been investigated. 

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

One of the synthetically useful titanium-based olefin metatheses is the reaction of titanocene-methylidene with terminal allenes 8. Productive olefin metathesis occurs when titanacyclobutanes are treated with 8 (Scheme 14.7) [17] and the resulting a-alkyli-... 

The alkyl-substituted titanium carbene complex 18 reacts with norbornene 24 to form a new titanacycle 25, which can be employed for the ROMP of 24 (Scheme 14.13). The titanacycle generated by the reaction of the Tebbe reagent with 24 is also used as an initiator for the same polymerization [23]. These preformed titanacyclobutanes also initiate ROMP of various other strained olefin monomers [24],... 

The organotitanium compounds produced by desulfurization of the diphenyl thioacetals of aldehydes 28 with the titanocene(II) species Cp2Ti[P(OEt)3]2 29 react with carbon—carbon double bonds to form the olefin metathesis-type products. Thioacetals 28 may be transformed into terminal olefins by desulfurization with 29 under an ethene atmosphere (Scheme 14.15) [27]. This reaction is believed to proceed through a titanacyclobutane intermediate, formed by cycloaddition of the titanocene-alkylidene with ethene. 

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]. 

Titanacyclobutanes also serve as useful synthetic intermediates the titanacycle 43, prepared by the intramolecular reaction of the alkenylidene complex 44, affords the a-dike-tone 45 and the other functionalized cyclic compounds by further transformations (Scheme 14.20) [35]. 

Scheme 14.20. Transformation of titanacyclobutanes into functionalized carbocycles.

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...

An unusual reductive elimination can ensue from titanacyclobutanes possessing an alkenyl group at the carbon a to the titanium atom. Thus, alkenylcarbene complexes 48, prepared by the desulfurization of (fy-unsaturated thioacetals 49 or l,3-bis(phe-nylthio)propene derivatives 50 with a titanocene(II) reagent, react with terminal olefins to produce alkenylcyclopropanes 51 (Scheme 14.22, Table 14.4) [37]. This facile reductive... 

Reactions of titanocene-methylidene generated from titanacyclobutanes with acyl chlorides 55 [46] or acid anhydrides 56 [47] lead initially to the titanium enolates 57 (Scheme 14.24), which then afford aldols upon treatment with the carbonyl compounds. On the other hand, five-membered cyclic anhydrides are methylenated with dimethyltitanocene (Table 14.5, entry 7) [45]. 

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]. 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

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. 

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. 

A titanacyclobutane carrier for the living ROMP of norbomene can be converted, by reaction with methanol, into an alkyl titanocene methoxide complex which can then be used in conjunction with EtAlCl2 to propagate the ZNP of ethene, so forming a block copolymer of norbomene and ethene620. 

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... 

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