Tebbe Complex

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

The corresponding metallacyclobutane can be isolated when the Tebbe complex reacts with one equivalent of norbornene (Fig. 4.19) [62]. This metallacycle continues to react with excess norbornene to generate poly (norbornene), and then carbene transfer to acetone can be used to remove the propagating metal species and end-cap the polymer chain. Subsequent studies estabhshed that these systems are living polymerizations. The experimental proof for living character includes the observations that (i) the propagating species can be observed for extended periods, (ii) the propagating species remains active for extended periods, (iii)... 

Subsequently, other Ti-cyclobutane species were shown to be the reaction products of the Tebbe complex with olefins the analogous reaction with acetylenes gives metallacyclobutenes." Utilization of Ti-metaUacycles as initiators in metathesis provides the first example of a living metathesis polymerization system. Clear evidence of the intervention of metaUacarbenes and metallacyclobutanes in olefin metathesis was later furnished by Kress et al. through minute nuclear magnetic resonance (NMR) studies on norbomene polymerization with tungsten alkylidenes. 

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

Carboxylic esters undergo the conversion C=0— C=CHR (R = primary or secondary alkyl) when treated with RCHBr2, Zn, and TiCl4 in the presence of A,A,A, iV -tetramethylethylenediamine. Metal carbene complexes R2C=ML (L = ligand), where M is a transition metal such as Zr, W, or Ta, have also been used to convert the C=0 of carboxylic esters and lactones to CR2. It is likely that the complex Cp2Ti=CH2 is an intermediate in the reaction with Tebbe s reagent. 

Both the Tebbe and Petasis reagents, Cp2CH2ClAlMe2 and Cp2TiMe2, effect the direct conversion of alkenic esters to dihydropyrans. This olefin metathesis has been successfully applied to the synthesis of complex polyether frameworks <96JA1565,96JA10335>. 

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

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],... 

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

Most experimental data suggest that the actual methylenating agent derived from the Tebbe reagent upon treatment with a weak base, is the highly reactive carbene complex Cp2Ti=CH2 [709]. This complex is a typical Schrock-type carbene, because it is high-valent [Ti(IV)], electron-deficient (16 valence electrons) and nucleophilic at carbon. 

Apart from the tandem metathesis/carbonyl o[efination reaction mediated by the Tebbe reagent (Section 3.2.4.2), few examples of the use of stoichiometric amounts of Schrock-type carbene complexes have been reported. A stoichiometric variant of cross metathesis has been described by Takeda in 1998 [634]. Titanium carbene complexes, generated in situ from dithioacetals, Cp2TiCl2, magnesium, and triethylphosphite (see Experimental Procedures 3.2.2 and 3.2.6), were found to undergo stoichiometric cross-metathesis reactions with allylsilanes [634]. The scope of this reaction remains to be explored. 

Reaction between carboxylic esters and Tebbe s reagent or metal car-bene complexes... 

Most of the sustained work on the different aspects of /x-methylene complexes has been performed in our own laboratories at Regensburg (59-61, 296 -299), and by the innovative groups of Knox (Bristol) (62-67), Levisalles and Rudler (Paris) (68-71), Pettit (Austin) (72-74), Shapley (Urbana, Illinois) (75-83), and Stone (Bristol) (84-94). Other significant contributions originate from the research of Wilkinson (London) (95-97), Bergman (Berkeley) (98), Tebbe (Du Pont) (99-101), Puddephatt (Liverpool) (102. 103), and Ziegler (Heidelberg) (104). 

Another broad class of compounds are the bridged carbene complexes. These compounds contain two identical or two different metal centers with the carbene centers bonded to both of the metal atoms in a bridging relationship. However, these binuclear complexes generally do not show classical carbene reactivity and will therefore not be discussed further, except to mention briefly the special case of the titanium-aluminum complex (3) developed by Tebbe and Grubbs and their coworkers.101 This, and related complexes, has proven to be particularly useful in organic synthesis, although its principal importance is in reactions other than cyclopropanations. 

The Tebbe reagent functions as a nucleophilic carbenoid in its reactions with carbonyl groups. The carbenoid is activated in the presence of a Lewis base which presumably complexes with the aluminum atom. Tetrahydrofuran is the Lewis base in the reactions described above. If the reaction is performed in the absence of added tetrahydrofuran, the carbonyl oxygen atom can function as a weak Lewis base, although the methylenation process is considerably slower. 

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