Olefins, heterosubstituted

Mono- and diaLkylboranes obtained by controlled hydroboration of hindered olefins and by other methods can serve as valuable hydroborating agents for more reactive olefins. Heterosubstituted boranes are also available and used for this purpose. These borane derivatives show differences in reactivity and selectivity. 

Heterosubstituted cyclopropanes can be synthesized from appropriate olefins and car-benes. Since cyclopropane resembles olefins in its reactivity and is thus an electron-rich car bo-cycle (p. 76ft). it forms complexes with Lewis acids, e.g. TiCL, and is thereby destabilized This effect is even more pronounced in cydopropanone ketals. If one of the alcohols forming the ketal is a silanol, the ketal is stable and distillable. The O—Si-bond is cleaved by TiCl4 and a d3-reagent is formed. This reacts with a -reagents, e.g. aldehydes or ketals. Various 4-substituted carboxylic esters are available from 1-alkoxy-l-siloxycyclopropanes in this way (E. Nakamura, 1977). If one starts with l-bromo-2-methoxycyclopropanes, the bromine can be selectively substituted by lithium. Subsequent treatment of this reagent with carbonyl compounds yields (2-methoxycyclopropyl)methanols, which can be transformed to /7,y-unsaturated aldehydes (E.J. Corey, 1975B). 

Very commonly, however, the sample of interest is not a pure compound, but is a complex mixture such as a coal liquid. As a result, a specific structure determination for each molecular type is not practical, although it is possible to determine an average chemical structure. Features which may be determined include the hydrogen distribution between saturate, benzylic, olefinic, and aromatic sites. The carbon distribution is usually split into saturate, heterosubstituted saturate, aromatic + olefinic, carboxyl, and carbonyl types. More details are possible, but depend greatly on the nature of the sample, and what information is desired. 

Radical cations of heterosubstituted olefins were obtained by polarographic reduction of the corresponding dications 183 according to the scheme in Eq. (239) 550) ... 

One should therefore always keep in mind that /3-heterosubstituted saturated organometallic compounds hardly ever exist. On the contrary, they almost inevitably fragment through M+/Het elimination to an olefin ... 

In the path b process, the addition of a heterosubstituted carbene to an olefin is governed by the reactivity of each component. Bis-thiomethyl- and bis-thiobenzylcarbenes add only to electron-rich olefins, whereas dimethoxycarbene adds primarily to electron-poor systems. Ambiphilic carbenes such as phenoxychlorocarbene and methoxychlorocarbene add to a wide range of double bonds. Table 16 lists the cyclopropanone equivalents which have been prepared in this manner. 

TABLE 16. Cyclopropanone derivatives from the addition of a heterosubstituted carbene to an olefin... 

The chelation between zinc and heteroatoms allows a diastereoselective allyl- or crotylzincation of secondary y-heterosubstituted vinyl metals. However, the intramolecular interaction between the metal center and an olefinic site was also described in the case of di(4-pentenyl)zinc [114]. This weak dipolar chelation is the result of the inherent polarity of the Zn - C " bond and the polarity of the double bond (having a partial negative charge on the terminal carbon). Such interactions were evidenced by NMR investigations and the spiro structure of di(4-pentenyl)zinc was postulated to interpret this phenomenon (Fig. 7-1). The oxygen chelation described in Scheme 7-95 is now replaced by a jr-chelation between a vinyl organozinc and an cu-olefinic residue. 

The diastereoselective MesAI-mediated intramolecular Diels-Alder reaction, a highly ( )-selective Takai olefination and a Suzuki coupling were the key steps in the enantioselective total synthesis of (-)-equisetin by K. Shishido et al. It should be noted that the type of T-U olefination utilized allowed the preparation of functionalized heterosubstituted ( )-alkenes. 

The method involving a-heterosubstituted cyclopropyllithiums and a-selenenylalde-hydes provides31,3S,77) quite exclusively the (E) stereoisomers of 1-heterosubstituted-1-vinylcyclopropanes after treatment of the l-(l-heterosubstituted)-l -hydroxy-2 -selenoalkyl)cyclopropanes with PI3/NEt3 (Schemes 60, 62b, 63 b). This implies that the two steps involved in the transformations are stereoselective (Schemes 60, 62b, 63b). The stereochemical result of the first step can be rationalized123 on the basis of the Cram-Karabatsos-Felkin rules124,125 and it is well-known that the synthesis of olefins from (P-hydroxyalkyl)selenides occurs by a formal anti elimination of the hydroxyl and selenyl moieties 4 9 11,12 123 163 . 

The reaction which uses the phosphorus ylide also produces31 35,77 highly stereo-selectively 1 -heterosubstituted-1 -vinylcyclopropanes, but now the (Z)stereoisomers are the major ones (Schemes 61, 62 a, 63 a). It is well-known that in the Wittig reaction the betaines which lead to the (Z)-disubstituted olefins are formed under kinetically controlled conditions and that any feature which favours the elimination of triphenyl-phosphine oxide from these betaines should increase the amount of the (Z)stereoiso-... 

Several mono- and dialkylboranes obtained by controlled hydroboration of olefins are valuable hydroborating agents. Heterosubstituted borane derivatives are also available, and the search for new reagents, including higher boranes, continues . The most frequently used are shown in Fig. 1 where for convenience monomeric forms are presented. 

Good yields (60—97%) of olefins can be obtained by the action of RSe on /3-heterosubstituted alkyl halides,and a-phenylselenyl enones result from the... 

Vinylcarbonyl compounds are common precursors for the generation of conjugated dienes. Recently, transformations including Wittig or related reactions, Julia or Takai olefination reactions, have been used to access heterosubstituted 1,3-dienes. 

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