Isomerization alkene/alkyne derivatives

Burger s criss-cross cycloaddition reaction of hexafluoracetone-azine (76S349) is also a synthetic method of the [CNN + CC] class. In turn, the azomethines thus produced, (625) and (626) (79LA133), can react with alkenes and alkynes to yield azapentalene derivatives (627) and (628), or isomerize to A -pyrazolines (629) which subsequently lose HCF3 to afford pyrazoles (630 Scheme 56) (82MI40401). 

Alkynyl-l-chlorocyclopropanes are formed from l-(bromochloromethyl)alkynes, the corresponding bromoalkynyl derivatives are not detected. (Z)-But-2-ene undergoes stereospecific addition of alkynylhalocarbenes which implies they react in a singlet state. None of the adducts resulting from the addition of isomeric (haloethynyl)alkyl(or aryl)carbene to alkenes are found. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Electrophilic attack on alkene or alkyne complexes can be a route to cr-organonickel derivatives. One example is the reaction of a cyclohexyne compound with iodomethane shown in Equation (43). The reaction of 77 -phosphino-alkyne complexes with HGl gives a mixture of two isomeric phosphinovinyl compounds with iy//-stereoselectivity (Equation (44)). The regioselectivity of the protonation depends on the electronic properties of the alkyne substituents, and has been correlated with the chemical shift of the G resonances of the two acetylenic carbon atoms. ... 

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