C=C Bond forming Reactions: Part II

C=C bond forming reactions

4. Alkene Metathesis

Alkene metathesis or olefin metathesis is a C=C forming reaction in which the components of two alkenes are redistributed to form a new C=C bond. This fascinating reaction led to the award of Nobel Prize to Chauvin, Grubbs, and Schrock collectively in 2005. They have worked enormously on the reaction mechanism and kinetics of alkene metathesis.

The reaction is catalyzed by Ru or Mo catalysts and are known after their discoverers as Grubb’s catalyst and Schrock catalysts respectively.

The reaction may involve opening or closing of rings, thus the reactions named as ring opening metathesis (ROM) and ring closing metathesis (RCM), respectively. The reaction has certain limitations regarding selectivity in ring formations and in preparation of highly substituted alkenes.

The alkyl group attached to the metal in the catalyst (1) is lost during the catalytic cycle along with one of the carbon from the alkene after opening of the metallocyclobutane intermediate (2). A second cyclic intermediate (4) is formed with second molecule of alkene and the rearranged catalyst (3), which upon opening gives a substituted alkene (5). One of the carbon atoms from the second alkene is released with the metal, regenerating the catalyst (6) having a different alkyl group attached to it.  

5.     Petasis-Tebbe Olefination

This is a one carbon homologation reaction of carbonyl compounds for synthesis of alkenes. F. N. Tebbe reacted titanocene dichloride with AlMe₃ to produce a methylene bridged complex. This complex provides a carbenoid which upon reaction with carbonyl compounds produces alkenes.

The reaction is applicable to variety of carbonyl compounds. The high nucleophilicity and less basic nature of the titanocene makes it a better reagent than the one used in Wittig reaction. The limitation of this reaction is that only terminal alkenes can be prepared, as only a methylene can be transferred to the carbonyl compounds. The driving force for the reaction is the formation of Titanocene oxide owing to the formation strong Ti-O bond.

6.     Julia Olefination

Julia olefination or Julia-Lythgoe olefination is the preparation of alkenes from carbonyl compounds using sulfones. E-alkenes are selectively produced through this reaction. Sulfone first reacts with carbonyl to form alkoxy sulfone, which is acylated followed by reduction to yield the corresponding alkene.