Olefinic bonds, cleavage

In efforts towards a synthesis of eremolactone, the dienolate (158) has been found to react with the cyano-ester (159) to give (160) (probable stereochemistry shown) most likely via a double Michael addition rather than a Diels-Alder reaction. Highly substituted cyclohexanes have been prepared by cycloadditions between cyclohexa-1,3-dienes and nitroso-compounds followed by N—O and olefinic bond cleavage (Scheme 12). ... 

Oxidation of olefins and dienes provides the classic means for syntheses of 1,2- and 1,4-difunctional carbon compounds. The related cleavage of cyclohexene rings to produce 1,6-dioxo compounds has already been discussed in section 1.14. Many regio- and stereoselective oxidations have been developed within the enormously productive field of steroid syntheses. Our examples for regio- and stereoselective C C double bond oxidations as well as the examples for C C double bond cleavages (see p. 87f.) are largely selected from this area. 

Other examples of the successful displacement of tosylates are the preparation of 31 -, 16a-,16j - and27- labeled steroids. This displacement reaction fails, however, with certain C-18 and C-19 alcohol derivatives which give mainly O—S instead of C—O bond cleavage. Unsatisfactory results were also obtained with sterically hindered tosylate esters at C-11, C-12 and C-20, which give considerable amounts of olefinic products in addition to O—S bond cleavage. ... 

Hydrolytic cleavage of single carbon-fluorine bonds generally requires activation by a neighboring group such as a carbonyl, sulfonyloxy, or olefinic bond or a negatively substituted aromatic group. 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

Intramolecular nitrone cycloadditions often require higher temperatures as nitrones react more sluggishly with alkenes than do nitrile oxides and the products contain a substituent on nitrogen which may not be desirable. Conspicuously absent among various nitrones employed earlier have been NH nitrones, which are tautomers of the more stable oximes. However, Grigg et al. [58 a] and Padwa and Norman [58b] have demonstrated that under certain conditions oximes can undergo addition to electron deficient olefins as Michael acceptors, followed by cycloadditions to multiple bonds. We found that intramolecular oxime-olefin cycloaddition (lOOC) can occur thermally via an H-nitrone and lead to stereospecific introduction of two or more stereocenters. This is an excellent procedure for the stereoselective introduction of amino alcohol functionality via N-0 bond cleavage. 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

Treatment of tetrahydroberberine (26) with sodium benzenethiolate (48) or -selenolate (49) in the presence of ruthenium catalyst afforded the C-14—N bond cleavage products 51 or 52 with a phenylthio or phenylseleno group at C-14 (Scheme 12). The latter was converted to the 10-membered amino olefin 53 on treatment with m-chloroperbenzoic acid. 

It was assumed that C—C bond cleavage passes through an elementary step of p-alkyl transfer. The mechanism of hydroisomerization passes also by a p-alkyl transfer step, but in this case the P-H elimination-olefin reinsertion occurs rapidly and a skeletal isomerization also occurs. 

The product ratio of 8K/9K is similar to that of the Lewis acid-mediated reaction of 4a-c. These products of the C2-C3 bond cleavage (8K and 9K) may be formed via alkylideneallyl cation intermediate, which is formed by the oxygen protonation of 4. Thus, the product ratio of 10/(8K + 9K) is controlled by the protonations at the olefinic carbon and at the acetal oxygen of 4. 

During the past two decades, within the series of our studies, we have developed a silylative coupling reaction of olefins with vinylsubstituted siHcon compounds which takes place in the presence of transition-metal complexes (e.g. mthenium and rhodium) that initially contain or generate M—H and M—Si bonds (for reviews, see Refs [5] and [6]). The reaction involves activation of the =C—H bond of olefins and cleavage of the =C—Si bond of vinylsilane. The reaction, which is catalyzed by complexes of the type [ M( x-OSiMe3)(cod) 2] (where M = Rh, Ir) occurs according to Equation 14.12 [71, 72). 

The authors hold the opinion that the thiophenolyc moiety adds to the olefin bond and an electron adds to the nitro group. Hence, the anion-radical [R R C(SPh)CH(R )N02] controls the reaction. The final product is formed as a result of the cleavage of the latter anion-radical with the expnlsion of the nitrite ion and the phenylthiyl radical. The radical normally transforms into diphe-nyldisulfide. The yields of the denitrated olefines are high and reach 80-95%. 

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