Cross with 1,1-disubstituted alkenes

Sequential cross-coupling can be used also for synthesis of 1,1-disubstituted alkenes from 2,3-dibromopropene (Aldrich). The allylic halogen couples directly with a Grignard reagent vinylic coupling is slower and requires a catalyst (equation I). 

Ozonolysis of cyclic olefins in the presence of carbonyl compounds gives the corresponding cross-ozonides.1329 In the ozonation of 1,2,4,5-tetramethyl-1,4-cyclohexadiene, oxidative dehydrogenation (formation of 1,2,4,5-tetramethylben-zene) was found to compete with oxidative cleavage because of steric hindrance.1330 Secondary ozonides (the 76 1,2,4-trioxolanes) are formed in high yields in the gas-phase, low-temperature ozonation of terminal and disubstituted alkenes.1331... 

For cross metathesis of alkenes, it is found that in the preparation of disubstituted alkenes with one or more aUyhc substituents Grubbs-Hoveyda catalysts possessing iV-(o-tolyl) groups in the azolecarbene unit are more efficient than those with the iV-mesityl groups. But for the formation of trisubstituted alkenes the Al-mesityl catalysts are superior due to discrimination between productive and nonproductive reaction pathways. 

Disubstituted cyclopropenes undergo ring-opening cross-metathesis with certain alkenes under the influence of 6 to produce (3S)-1,4-alkadienes. ... 

For a number of 1,1-disubstituted alkenes metathesis catalysts first bring about conversion to isomers which then undergo cross-metathesis with the remaining substrate (Strel chik 1976 Usov 1983 Kawai 1990). 

Cross-metathesis of two different alkenes to give an acyclic alkene is complicated by the possible formation of not only the desired cross-metathesis product, but also self-metathesis products, each as a mixture of alkene isomers. However, some alkenes are amenable to efficient cross-metathesis to give the desired substituted alkene. This is particularly the case with alkenes that are slow to homod-imerize, such as a, -unsaturated carbonyl compounds or alkenes bearing bulky substituents. Hence, cross-metathesis of methyl acrylate with an alkene proceeds efficiently (2.116). The ruthenium catalyst reacts preferentially with the more electron-rich alkene 98, which then undergoes cross-metathesis with the acrylate or self-metathesis with another molecule of the alkene 98. The latter reaction is reversible and hence a high yield of the desired substituted acrylate results over time. The use of 1,1-disubstituted alkenes as partners in cross-metathesis provides a route to trisubstituted alkenes. This chemistry is therefore a useful alternative to conventional syntheses of alkenes, such as by the Wittig reaction. 

An analogous 5-exo-dig carbopalladation, performed on an iodoarene with an ortho-alkynylamino substituent in the presence of hexabutyldistannane, produces an alkenyl-stannane, which undergoes a second cyclization by intramolecular Stille cross-coupling with the second o-iodoaniline moiety in the molecule. The hnal product, though, arises by double bond migration (Scheme 25, Eq. 1). Alternatively, a 1,1-disubstituted alkene... 

Application of RCM to acrylates derived from terpene aldehydes containing a remote double bond yielded the 6-substituted dihydropyran-2-ones <01TL6069> and oxabicyclo-[3.2.1]octane undergoes an efficient ring-opening cross-metathesis with electron-rich alkenes to give unsymmetrically 2,6-disubstituted tetrahydropyran-4-ones <01OL4275>. 

---