Reaction with Tebbe’s reagent

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. 

For a review on the methylenation reaction with Tebbe s reagent, see H. U. Reissig, Org. Synth. Highlights, 1991, 192. 

Parallel to Schrock s efforts were those of Grubbs and co-workers.28 They studied Tebbe s reagent and found that titanacyclobutanes derived from the reagent could be isolated. For example, they found that titanacycle 18 was in direct equilibrium with metal carbene 19 (equation 11,13). This system was not an active metathesis catalyst, but the work showed that both intermediates were important in the metathesis pathway. Further work found that reaction of Tebbe s reagent with norbomene resulted in metallacycle 20, which promoted ROMP of additional equivalents of norbomene and produced a polymer that was relatively monodisperse29 (equation 11.14).30... 

Substitution reactions may be effected on saturated analogues as well as on aromatic species. An interesting reaction of Tebbe s reagent with succinic thioanhydride derivatives 9 rapidly leads to monoolefination and. more slowly, to bisolefination. Acidic isomerization of the bis>adduct leads to 2.5-dimethyl thiophene derivatives (Eq. 3) <94JOC494>. 

The 5,6-unsaturated compound (51), which is also the methylene derivative (52) prepared from the corresponding D-gluconolactone tetraether with Tebbe s reagent, underwent the reactions Illustrated in Scheme 19. A set of furanoid compounds (53) have been found... 

Another interesting feature of Tebbe s reagent is its capability for converting ketones to terminal olefins. For example, reaction with cyclo-... 

A titanabicyclo[2.1.0]pentane system 23 was formed from 3,3-dimethylcyclopropene and the precursor of Tebbe s reagent. Among various reactions with cyclopropane ring opening, release of 1,1,2-trimethylcyclopropane (89%) was reported to occur upon treatment with hydrochloric acid. ... 

In the first synthesis,53 the 2-propenyl ether is made from the corresponding acetate using Tebbe s reagent.54 Reaction with a hydroxyl compound gives a mixed... 

The carbonyl group of an ester function can be converted to the corresponding methylene compound using Tebbe s reagent. The reaction can thus be considered as a general method of enol ether synthesis. An example of this reaction is given in Scheme 5. The same conversion can be achieved by reagents prepared from the reduction of 1,1-dibromoalkanes with zinc and TiCU in the presence of TMEDA. °... 

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