Heteroatom-substituted olefins

B. Cyclization Reaction of Heteroatom-substituted Olefinic Alkyllithiums. 334... 

Depending on the reaction conditions, the (E)- or (Z)-isomers of heteroatom-substituted olefins are formed from a, -epoxysilanes (Eq. 106). ... 

Scheme 17. Phenyl selenide transfer addition to heteroatom-substituted olefins...

Additions to Heteroatom Substituted Olefins by Organopalladium Reagents ... 

Heteroatom-Substituted Olefins At low temperature and with most rhodium catalysts, vinyl ethers and vinyl acetates are hydroformylated preferentially in the a-position (see Section 4.2.2.2) and represent therefore ideal candidates for AHF. 

A few years later, Tebbe and co-workers found that the methylene-bridged metallacycle 3, which has become known as the Tebbe reagent, is useful for the methylenation of ketones and aldehydes [5]. Titanocene-methylidene 4, the active species of this olefination, also transforms carboxylic acid derivatives into heteroatom-substituted olefins. Because the carbene complex 4 is much less basic than conventional olefination reagents such as phosphorus ylides, it can be employed for the olefination of carbonyl compounds possessing highly acidic a-protons or of highly hindered ketones, and has become an indispensable tool in organic synthesis. Various methods for the preparation of titaniumcarbonyl olefination. This chapter focuses on the use of metal-carbene complexes and some related species in carbonyl olefination (Scheme 4.2). 

Similar methylenation of amides, imides, and thiol esters produces the corresponding heteroatom-substituted olefins (Table 4.4). Since the olefination of imides is greatly affected by steric hindrance, an unsymmetrical imide is monomethyl-enated at the sterically less hindered carbonyl group when treated with 3 (entry 3). The low basicity of 3 allows the transformation of a thiol ester to the corresponding alkenyl sulfide without racemization (entry 4). 

Methylenation of carboxylic acid derivatives A variety of carboxylic acid derivatives can be transformed into heteroatom-substituted olefins (Table 4.9). The C-glycoside congeners are easily prepared by the direct methylenation of aldono-lactones with 30 (entry 2) [67]. Since the generation of titanocene-methylidene 4 from 30 only produces methane as a by-product, 30 can be employed for the olefination of acid-sensitive substrates such as silyl esters and acylsilanes (entries 4 and 6). The tolerance toward acid-sensitive functional groups also allowed the application of dimethyltitanocene 30 in the conversion of a lactone bearing an acetal moiety into the corresponding cyclic vinyl ether, which was further transformed into the spiroketal ketone upon oxidation and rearrangement (Scheme 4.24) [69]. 

By the use of di- or trithioorthoformates, heteroatom-substituted olefins are obtained. The active organotitanium species formed in these preparations are assumed to be the corresponding methoxy- and phenylthio-methylidene carbene complexes, SO and 51, respectively (Scheme 4.44). The success of olefinations using these species indicates that the a-heteroatom substituent does not affect the nucleophilic nature of the titanium carbene complex [103]. 

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