Tebbe nitriles

Alternative precursors of titanocene-methylidene 4 are titanacydobutanes 11, known as Grubbs reagents, which are prepared by reaction of the Tebbe reagent 3 with appropriate olefins. The carbene complex 4 is highly reactive towards multiple bonds, such as those of alkenes, alkynes, and nitriles, as well as carbonyl functions [11, 40]. The reaction of 3 with a variety of olefins in the presence of a Lewis base such as pyridine affords four-membered metallacycles 11 [41]. Since thermolysis of these compounds effects the reverse reaction, the titanacydes 11 are employed as precursors of titanocene-methylidene 4 (Scheme 4.10). The mechanism of thermal cleavage of 11 to give a titanocene-methylidene has been extensively studied [42], and the temperature required for the thermal degradation of compounds 11 was found to be dependent on the substituents present. The crystalline metallacycles 11 can be handled in air for a reasonable period of time and are employed under various conditions (e.g. R, R = Me 5 °C, R = Me, R = Bu 5 °C, R = H,... 

As has been shown previously, the choice of the linker is of crucial importance in solid phase synthesis. Barrett et al. [142] showed that immobilized carboxylic esters 69 undergo Tebbe olefination resulting in polymer-bound vinyl ethers 70, that can be used in different chemical transformations such as [3+2] cycloadditions of nitrile oxides (Scheme 56). This reaction is known to be troublesome in solution however, when performed on sohd phase, the resulting products 71 are obtained in satisfactory yields and excellent purities. 

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