Vinyl compounds alkyl substituted

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

Once regiocontrol was achieved, donor-substituted allyl metallics found two synthetically important applications. The hetero-substituted vinyl compounds, obtained by alkylation of a substituted allyl carbanion in the 3-position ( ), can be hydrolyzed to carbonyl compounds substituted on the /1-carbon atom. Thus, the substituted allyl carbanion was used as an equivalent of the homoenolate synthon18 19. 

Gallium(III) triflate catalysed the rearrangement of 2-substituted vinyl epoxides into /3,y-unsaturated carbonyl compounds with high regio- and chemo-selectivity with low catalyst loading. Alkyl-substituted trimethylsilylvinyl epoxides gave /i,y-unsaluralcd ketones, but aryl-substituted vinyl epoxides gave the aldehydes.80... 

Similar reactions were performed with dioxolanes 400 bearing abstractable hydrogen atoms in the 2-position (Fig. 93) [436]. Radical addition to acceptors 401 and oxygenation gave protected 2-hydroxy-4-oxo carbonyl compounds 402 in 16-84% yield. For these substrates a catalyst system of 0.1 mol% Co(OAc)2 and 5 mol% 398 proved to be sufficient to achieve good yields. Low yields were only obtained with (i-alkyl-substituted substrates 401. Methyl vinyl ketone undergoes a fragmentation under the reaction conditions. 

Phenyl, vinyl or carbonyl substituted cyclopropanes are more easily hydrogenolyzed than are the alkyl substituted species. Such compounds are commonly cleaved over palladium catalysts at room temperature and atmospheric pressure. Hydrogenolyses run over platinum, rhodium or nickel catalysts frequently result in the saturation of the double bond, the benzene ring or the carbonyl group with the cyclopropane ring remaining intact or cleaved to only a slight extent. 20 As illustrated in Fig. 20.22 the bond broken in the... 

Ganglion c Blocking Agents - A number of cyclic hindered amines substituted on the a-carbon with ethynyl, vinyl and alkyl groups have been synthesized and their antihypertensive properties compared with a corresponding series of open chain compounds. It was found that the open chain compounds were... 

Marks and co-workers (12) have studied the alkyl substituted compounds 7-16. Assuming that INDO/2 molecular orbital calculations on alkyl radicals can reasonably predict experimental electron-nuclear hyperfine coupling constants, a, they have calculated the a values for each of the alkyl substituents. Taking the ratio of the contact shifts of the ortho positions in 7 and vinylic position in 16 as equal to the ratio of calculated a values and the ratio of the geometry factors as equal to the ratio of pseudocontact shifts, Marks and co-workers could solve for the contact and pseudocontact shifts in 7 and 16. Factoring the... 

Alkoxybenzenes give vinyl ethers (33) by Birch reduction. These compounds are latent ketones as acid hydrolysis gives a ketone. Epoxide ( ) can first be disconnected, by removal of the epoxide and the acetal, to ketone (39). The position of the double bond, out of conjugation with the ketone but at the alkyl substituted position, is the clue for a Birch reduction. The rest is simple aromatic chemistry. 

A radical tpso-type substitution on indoles and benzofurans has resulted in other dearomatized spirocycles [83]. Aryl, vinyl, and alkyl radicals added to the C-2 position of the indole to give novel compounds, 128,130, and 132, in moderate to excellent yields. In the case of the addition of alkyl radicals, significant amounts of the reduced starting material were also isolated. 

The stereochemistry of addition is usualy anti for alkyl-substituted alkynes, whereas the addition to aryl-substituted compounds is not stereospecific. This suggests a termolecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation intermediate in the aryl case. Aryl-substituted alkynes can be shifted... 

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