Enones unconjugated

The additional structure C, which cannot be drawn for an unconjugated carbonyl derivative, implies that the carbonyl band in an enone has more single bond character and is therefore weaker. The involvement of a carbonyl group in hydrogen bonding reduces the frequency of the carbonyl stretching vibration by about 10 cm-. This can be rationalised in a manner analogous to that proposed above for free and H-bonded 0-H vibrations. 

Carbon-carbon double bonds in unconjugated alkenes usually exhibit weak to moderate absorptions due to C=C stretehing in the range 1660-1640 em V Disubstituted, trisubstituted and tetrasubstituted alkenes usually absorb near 1670 cm The more polar carbon-carbon double bonds in enol ethers and enones usually absorb strongly between 1600 and 1700 crn k Alkenes conjugated with an aromatic ring absorb strongly near 1625 crn k... 

Mixed chromophores with both C=C and C=0 moieties such as ,/3-unsaturated enones are one of the most investigated chromophores in organic photochemistry. Conjugation of the two chromophoric moieties results in lowering the n —> ji and tt ji energy levels relative to the unconjugated chromophores. UV absorption spectra of selected alkenes, ketones and enones are presented in Table 1. 

These reactions may show considerable selectivity. Corey and Chaykovsky19 give an example with the terpene carvone 80. The ylid 78 is made with NaH and reacts only with the enone and not with the unconjugated alkene. The product is one diastereoisomer 81 as the ylid has added to the opposite side of the ring to the only substituent. It also has retained the stereochemistry of the cis alkene but that is inevitable as 3/6 ring fusion must be cis. 

Isoprenyl derivatives 709 (1 3 mixture of Z/E diastereomers) underwent metallation with t-BuLi to give the dienyllithium 710 which, after trapping with McsMCI (M = Si, Ge, Sn), afforded the corresponding derivatives 7111000 (Scheme 185). The (Ef-isomer seems to be destroyed in the reaction, since it is not isomerized to the (Z)-diene and does not remain in the reaction mixture. However, the hydrolysis of isoprenyl ethers 711 with 0.2 M HC1 in a mixture of acetone and water gave mixtures of the expected conjugated and unconjugated enones. 

The disconnections for the aldol reaction in the a-position on an extended enolate would be something like this first move the alkene from conjugated 145 to unconjugated 146 so that aldol disconnection gives the extended enolate 147 derived from the enone 89. This chemistry demands at least one hydrogen atom in the y-position of the enone 89 so that formation of the extended enolate 147 is possible. That hydrogen is of course replaced when the conjugated product 145 is formed from 146. 

Experiments reported by Pollack and his coworkers allow the conclusion that the dienolate anion intermediate is approximately isoenergetic with the more unstable unconjugated enone substrate/product, as proposed by Knowles and Albery in their theory for understanding optimization of catalytic efficiency [9]. Thus, based on the value of the rate constant for proton abstraction from the unconjugated enone, 1.7 x 10 s Pollack and coworkers calculated that the value of the Glint for proton abstraction from carbon is 10 kcal mol, a modest reduction from that expected ( 13 kcal mol ) for the nonenzymatic reaction. 

By dehydrohalogenation of /-butyl chloride, the nucleophilic isobutene, and by its reaction with carbon monoxide the electrophilic pivalyl chloride, are produced. These two reagents afford the monoacylation products 141 and 142 the unconjugated enone 142 is able to undergo a second acylation (reaction of the type in Section II, C, 1, a) leading to the pyrylium salt 142. 

The unconjugated enone (24) is an example of a ketone which can be made from a nitro compound. Direct Diels-Alder disconnection is impossible, but replacement of carbonyl by NO2 (25) reveals a simple Diels-Alder disconnection. The condensation will again give a trans double bond, but this is irrelevant to the final product (24). 

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