Titanocene vinylidene complexes

The alternative building scheme C2 + Q was used by Petasis and Hu [89], who reacted various aldehydes and ketones with alkenyltitanocene derivatives 172 to obtain the corresponding allenes 173 in high chemical yields (Scheme 2.54). The reaction probably proceeds via titanocene vinylidene complexes, which can also be trapped with alkynes and isocyanides to afford allenylketene imines [90],... 

Scheme 2.54 Synthesis of allenes with titanocene vinylidene complexes.

Scheme 2.55 Functionalized allenes formed from ketones and titanocene vinylidene complexes.

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

The titanocene vinylidene intermediate Cp2Ti(=C=CH2) formed by ethane or methane elimination from titanacyclobutane (4) or Cp2Ti(CH=CH2)Me, respectively, reacts with isothiocyanates, RNCS (R = CeHn, Ph or r-Bu), by a [2 + 2] cycloaddition, to give the titanathietane complexes (8). In all cases, the regioisomer in which the S atom is bonded to Ti is found as the primary product. Upon heating in the presence of pyridine, a rearrangement of (8) to the regioisomeric titanacyclobutane derivative (9) was observed (equation 12)." ... 

Alienation Another olefin metathesis-carbonyl olefination sequence utilizing titanacyclobutanes 11 is employed for the preparation of tri- or tetrasubstituted allenes [56]. Titanocene-methylidene 4 generated from 11 reacts with 1,1-disubsti-tuted allenes to produce the a-alkylidenetitanacyclobutanes 25 with the hberation of an olefin. Simple treatment of 25 with ketones and aldehydes at room temperature affords substituted allenes. The vinylidene complexes 26 are formed as the active species in these transformations (Scheme 4.20). 

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