Thioacetal-titanocene system

Another synthetic application of olefin metathesis using a thioacetal-titanocene(II) system is the ring-closing olefin metathesis (RCM) of carbene complexes possessing an olefin moiety, e. g. 33 (Scheme 14.17). The success of the RCM apparently depends on the substituents at the carbon—carbon double bond (i. e. the substituent(s) on the resulting carbene complex 34). 

Table 14.8. Carbonyl olefination utilizing a thioacetal-titanocene(l I) system.

Scheme 14.27. Intramolecular carbonyl olefination utilizing a thioacetal-titanocene(ll) system.

Carbonyl Olefination Utilizing a Thioacetal-Titanocene(M) System... 

One of the most versatile procedures for the olefination of carbonyl compounds utilizes a thioacetal-titanocene(II) system [40, 92]. The organotitanium species generated by the desulfurization of thioacetals with the triethyl phosphite complex of titanocene(II) 44 react with a variety of multiple bonds, such as those of alkenes [93], alkynes [94], and nitriles [95], as well as carbonyl functions. The reactivities observed in these reactions indicate that the active species involved are titanocene-alkylidene complexes 48 (Scheme 4.40). 

As described in the previous section, a-elimination of dialkyltitanocenes is of limited use for the preparation of titanocene-alkylidenes. Since thioacetals are readily available from carbonyl compounds or through alkylation of bis(phenylthio)methane and related organosulfur compounds, a thioacetal-titanocene(II) system enables the use of different types of carbene complexes. Use of appropriate thioacetals is of crucial importance in this system for the preparation of alkylidene complexes 48, the corresponding diphenyl thioacetals are the starting materials of choice. No carbene complexes are formed from dialkylthio-acetals. To generate vinylcarbene complexes 49, trimethylene thioacetals of aji-unsaturated aldehydes or l,3-bis(phenylthio)propene derivatives are employed (Scheme 4.41). The low-valent titanium species 44 is also easily prepared by the reduction of titanocene dichloride with magnesium in the presence of triethyl phosphite at room temperature. The presence of molecular sieves 4A is essential for the reproducibility of this preparation. In relatively large-scale preparations, care should be taken to control the reaction temperature [92, 93g]. 

Alkylidenation with a thioacetal-titanocene(II) system is useful for the conversion of carboxylic acid derivatives into enol ethers, alkenyl sulfides, and enamines. As with other titanium-based reagents, mixtures of stereoisomers are formed in... 

Carbonyl Olef nation Utilizing a Thioacetal-Titanocene(ll) System 1183 Tab. 4.14. Intramolecular carbonyl olefination of alkyl co,< -bis(plienyltliio)alkanoates. 

A novel route to 2,3-dihydrothiophenes involved a titanocene-promoted carbene formation and subsequenct intramolecular cyclization onto a thiol ester <99SL1029>. Treatment of thioacetal 9 with the low-valent titanium complex 10 gave 2,3-dihydrothiophene 12 by intramolecular olefination of the thiol ester of titanium-carbene intermediate 11. Another metal-mediated cyclization onto the thiophene ring system involved the palladium-catalyzed cyclization of 1,6-diynes <99T485>. For example, treatment of thioether 1,6-diyne 13 with Pdlj in the presence of CO and Oj in methanol followed by treatment with base gave 14. 

---