Alkenes conjugated with carbonyl groups

To start this chapter, let us take you back to one of the first reactions we introduced nucleophilic addition to carbonyl groups. Here are two examples, both of which give products which you should fully expect. We ve included details of the IR spectra of the products to confirm firstly that the product has no carbonyl group and secondly that the alkene is still there. 

Now let s tweak the conditions we repeat the first reaction at a higher temperature, and we add to the second a small amount of a copper salt. Now the products are different  

Both products A and B have kept their carbonyl group (IR peak at 1710-1715 cm i) but have lost the C=C. Yet A, at least, is unquestionably an addition product because it contains a C=N peak at 2250 cm h 

the identities of A and B are revealed here they are the products of addition, not to the carbonyl group, but to the C=C bond. Here s a mechanism, for both reactions of cyanide firstly the direct addition to C=0 and secondly addition to the C=C bond. 

This type of reaction, where a nucleophile adds to a C=C double bond, is called conjugate addition, and this chapter is about the sorts of alkenes (and arenes) that do this kind of thing. We will also explain how such small differences in reaction conditions (temperature, or the presence of CuCl) manage to change the outcome so dramatically. 

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Alkenes conjugated with carbonyl groups are polarized... 

Alkenes conjugated with carbonyl groups, such as acrylates (derivatives of acrylic acid), are easily polymerized by a variety of mechanisms. Indeed, these compounds are often difficult to store because they polymerize spontaneously when traces of weak nucleophiles (even water) or radicals (even oxygen) are present. Radical polymerization occurs very easily because the intermediate carbon radical is stabilized by conjugation with the carbonyl group. 

Using ammonium cerium(IV) nitrate (CAN) as the oxidant for the azide anion, the azido radicals are trapped by alkenes, to form, ultimately, /i-azido nitrates84 cerium azide species may be considered as intermediates. Alkenes conjugated with carbonyl groups are recovered intact. The stereochemistry of the adducts from acenaphthylene and indene was trans, as shown by H-NMR studies (/AX < 2 Hz for the acenaphthylene adduct), but with (-Eyi-phenyl-l-propene both syn and anti additions were formed. 

To show why alkenes conjugated with carbonyl groups behave differently from unconjugated alkenes, we use curly arrows to indicate delocalization of the n electrons over the four atoms in the conjugated system. Both representations are extremes, and the true structure lies somewhere in between, but the polarized structure indicates why the conjugated C=C bond is electrophilic and why the 3 carbon is attacked by nucleophiles. 

Vinyl substitutions on alkenes not having their double bonds conjugated with carbonyl groups often proceed more rapidly and give better product yields when the reactions are conducted in the presence of an unhindered secondary amine. Conjugated and nonconjugated dienes are usually only minor products in these cases. The major products normally are allylic amines obtained by nucleophilic attack of the secondary amine upon the ir-allylpalladium intermediates. Since allylic amines may be quatemized and subjected to the Hoffmann elimination, this is a two-step alternative to the direct vinyl substitution reaction.90... 

The action of triethylamine may cause base-induced reactions, such as a-epimerization of carbonyl compounds isomerization of alkenes into conjugation with carbonyl groups and, elimination in carbonyl compounds posssessing a good-leaving group at the (3-position... 

Equilibration of alkenes not conjugated with carbonyl groups requires different reagents... 

CjX 100000 conjugation with carbonyl group much more effective than with simple alkene or benzene ring. These a-carbonyl halides are the most reactive of all... 

We end with a summary of the factors controlling the two modes of addition to a, ( -unsaturated carbonyl compounds, and by noting that conjugate addition will be back again—in Chapters 23 (where we consider electrophilic alkenes conjugated with groups other than C=0) and 29 (where the nucleophiles will be of a different class known as enolates). 

The nitrile group is not as reactive towards direct attack by nucleophiles as Its carbonyl cousins but is equally able to stabilize an adjacent negative charge in the style of enoiates. Alkenes conjugated with nitriles are thus activated towards nucleophilic attack without the complications of competing direct addition to the activating group. 

The ability of chiral bis(camphorquinone-a-dioximato)cobalt(Il) complexes (Section 1.2.1.2.4.2.6.3.1.) to catalyze carbene transfer from diazocarbonyl compounds (diazoacetic esters, 2-diazo-l-phenylethan-l-one) to terminal alkenes conjugated with vinyl, aryl, carbonyl, and cyano groups, has already been mentioned. The ee-values are 75-88 /o at best, often lower. The highest values are again obtained with bulky diazoacetic esters. The significance of these catalysts, however, is their ability to promote cyclopropanation of electron-deficient alkenes, such as acrylates and acrylonitriles, in contrast to the rhodium and copper catalysts discussed above. 

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