Titanocene dicarbonyl

Recently an ansa-titanocene dicarbonyl complex containing a siloxy bridge has been reported by Curtis et al. (78). The reaction sequence is shown below. 

Hartwig65 reported that dimethyltitanocene is an efficient catalyst for the hydroboration of alkenes and examined the mechanism of titanocene dicarbonyl-catalyzed the hydroboration of alkynes (Scheme 5).66... 

With regard to PK-type reactions, Buchwald studied titanium species as efficient catalysts in the PKR and in PK-like reactions with cyanides. Following preliminary results with [Cp2Ti(PMe3)2] and [Cp2TiCl2] [79-81], they reported a more practical procedure which improved the TON using commercial titanocene dicarbonyl (33) [82,83]. This complex is able to catalyze the... 

The synthesis and spectroscopic features of a number of titanocene dicarbonyl derivatives have been reviewed previously.41 Typically, these complexes are prepared via magnesium reduction of the corresponding dihalide derivative under 1 atm of carbon monoxide. New compounds prepared since COMC(1995) are presented in Scheme 10. Mixed cyclopentadienyl phosphinimide dicarbonyl compounds such as (if-CsMcs)-(Buc3PN)Ti(CO)2 61 have also been isolated.46... 

Substituted-titanocene dicarbonyl complexes, ( ]S-CsRBHs B)2Ti(CO)2, have also been used as isolable precursors to highly reactive titanium sandwiches, ( ]S-CsRBHs B)2Ti. For example, reaction of a,/3-unsaturated ketones with ( -CsHs TFCO 68 results in reductive coupling of the organic substrate to form (77S-CsHs)2Ti(0C(R1)CHCH(R2)CH(R2)CHC(R1)0) 69 with liberation of carbon monoxide (Scheme 12).47 48... 

Another example of a titanium complex is the binuclear titanocene complex 18 which is obtained by the insertion of titanocene dicarbonyl into the S-S bond of a fused ring compound 17, involving elimination of CO (Scheme 5)... 

Recently, it has been shown that the titanocene dicarbonyl (rj -C5H5)2Ti(CO)2 113 is a convenient complex that catalyzes a number of cyclization reactions. Among suitable substrates are enynes that can be easily cyclized into the corresponding vinylmethylenecycloalkanes 114 (Scheme 48) in the presence of this compound. Representative examples are shown in Table 21 [62]. The reaction mechanism is outlined in Scheme 49 and proceeds through the formation of the titanacyclopentene 115, which is followed by j3-hydrogen elimination to 116 and, finally, by reductive elimination to 114. Formally, it resembles cyclization of dienes via metallacycle formation. 

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