Reactions with Electrophilic Alkenes

The highly 7r-deficient character of the 1,2,4-triazine ring increases the nucle-ophilicity of the methyl group in methyl-1,2,4-triazine A-oxides in reactions with electrophilic alkenes and aldehydes. Thus treatment of the 6-methyl-3-phenyl-1,2,4-triazine 4-oxide 113 with l-(dimethylamino)-l-ethoxyethylene leads to the... 

Reactions with electrophilic alkenes, ketones and ketenes 709... 

The chemistry of pyrrol-1-ylbenzylidene pentacarbonyl chromium, molybdenum and tungsten complexes was investigated. Reaction with electrophilic alkenes gives l-(phenylcyclo-propyl)pyrroles Under photolytic decarbonylation conditions 2 + 2 cycloaddition products were obtained with nucleophilic alkenes, cyclic dienes and imines. <950M2522>... 

As pointed out by Stork and coworkers in their definitive 1963 paper3, the reaction with electrophilic alkenes is especially successful since reaction at nitrogen is reversible. Reaction at the /2-carbon is (usually) rendered irreversible by, in the case of cyclohexanone enamines, internal proton transfer of the axial C-/2 proton to the anionic centre of the initially formed zwitterionic intermediate (34), under conditions of stereoelectronic control (Scheme 22). When this intramolecular proton transfer cannot occur in aprotic solvents, or when the product produced in protic solvents is a stronger carbon acid than adduct 35 (i.e. when Z = COR, N02), then carbon alkylation is also reversible and surprising changes in the regioselectivity of reaction may be observed (vide infra see also Section VI.D and Chapter 26). Cyclobutanes (36) and, in the case of a,/ -unsaturated... 

C-Alkylations of hindered aldehyde enamines can be effected with a variety of alkylating agents, but only allylic and benzylic reagents are useful for alkylations of unhindered systems.Acyclic, homoan-nular and heteroannular dienamines undergo alkylation primarily at the a-positions. ° As discussed above, reactions of enamines with electrophiles containing sp -hybridized carbon atoms have numerous limitations. On the other hand, enamine reactions with electrophilic alkenes are highly useful and have received wide coverage in the literature. 

Subsequent reaction with a nucleophile affords a metal-vinylidene complex. This subject has been reviewed by Bruce. Reactions with electrophilic alkenes initially lead to a cyclobutenyl complex in a two-step process via a paramagnetic intermediate. Subsequently, the ring opens in a concerted fashion to a butadiene derivative. 

The iodomethyl group of 91 can be dehydrohalogenated generating methyleneindolines. These compounds readily undergo ene reactions with electrophilic alkenes and alkynes and other electrophiles. Iminium ions give rise to tryptamine derivatives. <94JA11797>... 

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