Esters Tebbe reaction

The Tebbe reaction, which converts the oxo groups of esters and lactones to methylene groups to give enol ethers, is described in section 2.1.2. 

A reaction that appears to be mechanistically similar to the Tebbe reaction was developed by Oshima in 1978. Diiodomethane or dibromomethane in the presence of zinc is treated with a Lewis acid to form, presumably, a divalent complex (72), which reacts with aldehydes and ketones to produce the corresponding methylene derivative (73 Scheme 18). This reagent complements the reactivity of the Tebbe reagent, in that the zinc methylenation is not reactive towards esters or lactones. Because it is an electrophilic reagent, it is suitable for the methylenation of enolizable ketones and aldehydes. 

The presumed mechanism of action of the Tebbe reagent is very simple after dissociation of Me2AlCl, [2 + 2] cycloaddition of Cp2Ti=CH2 and R2C=0 is followed by [2 + 2] retro-cycloaddition to give the product. Unlike the conventional Wittig reaction, the Tebbe reaction works well with esters. 

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. 

Reaction between carboxylic esters and Tebbe s reagent or metal... 

Recently, Nicolaou and coworkers have devised a novel, one-pot strategy for the direct transformation of acyclic olefinic esters to cyclic enol ethers [34]. Unlike the molybdenum alkylidene 1 (see Sect. 3.2), initial reaction between the Tebbe reagent 93 and an olefinic ester results in rapid carbonyl olefination to afford a diene intermediate. Subsequent heating initiates RCM to afford the desired cyclic product (Scheme 17). 

Preliminary investigations in this area involved treatment of olefinic ester 125 with a large excess (4 equiv) of the Tebbe reagent 93 (Scheme 18) [34a]. After 20 min at 25°C, the mixture was heated at reflux for 5 h. This resulted in the formation of tricyclic enol ether 127 in 71% overall yield. If only 1.3 equiv of Tebbe reagent 93 was employed and the reaction stopped after 20 min at 25°C,the olefinic enol ether 126 could be isolated in 77% yield. The proposed intermediacy of diene 126 in the initial tandem sequence was validated by its subsequent conversion into the cyclic enol ether 127 under the original reaction conditions [34a],... 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

---