Dithioacetals carbonyl olefination

Scheme 6. Carbonyl olefination with titanium-alkylidene species 12 prepared from dithioacetals according to Takeda et al.

Breit, B. Dithioacetals as an entry to titanium-alkylidene chemistry a new and efficient carbonyl olefination. Angew. Chem., Int. Ed. Engl. 1998, 37, 453-456. 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

It is difficult to obtain cross-coupling products of two different carbonyl compounds by an intermolecular version of the McMurry reaction. Examples that use excess amounts of one carbonyl component are few. "" When one carbonyl component is replaced by a 1,1-dihalo compound or dithioacetal and the alternative is reduced with a low-valent metal such as low-valent titanium or chromium(ii), cross-coupling products, that is, Wittig-type olefins, are produced in high yields. Because the alternative approach is described elsewhere, we concentrate on only its important features here. 

Interestingly, the subsequent reactions of the titanium-alkylidene species 12 obtained from dithioacetals are not limited to carbonyl olefina-tions. When the carbene complex is prepared in the presence of olefins, the latter are smoothly cyclopropanated (Scheme 8 13) [14]. Furthermore, the reaction of symmetrically disubstituted acetylenes with dithioacetals containing a methylene unit provides the corresponding trisubsti-tuted 1,3-dienes 14 in a stereoselective fashion 115]. 

Dithiolanes, also named five-membered 1,3-dithioacetals or A,3 -acetals, find wide applications in organic synthesis, particularly in protection of carbonyl functions and their reductive conversion to hydrocarbons or olefins. Due to the stability of 1,3-dithiolanes toward various reagents and reaction conditions, they have attained an important position in this area despite the fact that dedithioacetalization to the corresponding carbonyl compounds is sometimes not an easy process. There are three general strategies that can be used for deprotection of 1,3-dithiolanes involving... 

Promoted by TiCU, a-oxoketene dithioacetals 281, as a special kind of activated olefin bearing two a,(3-dialkylthio substituents, reacts with aryl aldehydes to afford polyfunctionalized 1,4-pentadienes 283, rather than normal MBH adduct 282 (Scheme 2.155). In addition, a series of a-(l,3-dithiolan-2-yhdene)-p-amino carbonyl derivatives 284, the aza-MBH adducts, were obtained in good to excellent yields from novel one-pot, three-component reaction of a-oxo... 

The formation of a dithioacetal as an intermediate in organic synthesis is not new to most chemists. However, in recent years there has been a continuing improvement in the methods of preparation as well as the subsequent reactions. The early use of the dithioacetal group as a means to reduce carbonyl functions with Raney nickel has been expanded to extensive use as a protecting group, methylene blocking group and as an intermediate in the preparation of complex hydrocarbons, olefins, aldehydes and ketones. 

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