Lone pairs enones

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

The enone is a fully conjugated cyclic system with six electrons in it - in fact, it is aromatic. This is clearer when we delocalize a lone pair from the ether oxygen atom into the carbonyl group. 

AcOH, H2O, 89% yield. A factor of 400 in the relative rate of hydrolysis is attributed to a conformational effect where the lone pair on oxygen in the silyl ketals does not overlap with the incipient cation during hydrolysis. Hydrolysis of the second ketal is retarded by the enone, which destabilizes the intermediate carbenium ion. 

This is a strange compound. The exocyclic (arrowed) alkene is an enol at one end but an enamine and a vinyl sulfide at the other. All three atoms, O, N, and S, have lone pairs making this a very electron rich alkene. The nitrogen atom dominates so that conjugate addition 192 to enones works well. The product 194 loses the catalyst 189 to give a 1,4-diketone 195. The reaction is simplicity itself the aldehyde, enone, catalyst and weak base (usually amine or sodium acetate) are heated in ethanol. 

Nitrogen can be incorporated as an oxime into a different kind of ene reaction that has been explored by Grigg and his group. The ene component now bears no resemblance to a diene one pair of electrons comes from the lone pair on nitrogen and the other from the OH bond of the oxime 140. The enophile is a more conventional enone and the initial product is a nitrone 141. No nitrogen heterocycle is formed in this step, but, if the enophile contains a second alkene, a 1,3-dipolar cycloaddition gives a bicyclic structure. The simplest reagent for this job is the rather unstable divinyl ketone (penta-l,4-dien-3-one, 143). Fortunately this can be released from the dichloroketone 142 with base and distilled with the solvent THF into the reaction mixture.24... 

Despite this, bifunctional thiourea-tertiary amine catalysts have also emerged as useful and very convenient compounds for the activation of enones in Michael reactions. The mentioned problems associated to the single position available for H-bonding interactions and to the lower Bronsted basicity of the carbonyl moiety are circumvented by the formation of a double H-bonding network in which both lone pairs at the oxygen atom participate with two... 

In the absence of divalent metals, the enone sulfoxide was thought to exist mainly in the conformation shown in Figure 5.3, having the sulfoxide and carbonyl dipoles oriented in opposite directions. Conjugate addition then occurs at the face not shielded by the p-tolyl group, i.e. that containing the sulfur lone pair (si-face). The zinc bromide-mediated addition of vinylmagnesium bromide to (98a) has led... 

The acylium ion thus generated is a very reactive electrophile, capable of reacting with the relatively weakly nucleophilic lone pairs on the carbonyl oxygen of the cholesta-4-en-3-one. The resulting oxonium ion renders the 7-protons of the enone quite acidic, and thus they are readily removed by a very weak base, such as chloride ion, to generate the product. 

U V spectra are also important in other compounds containing double bonds, particularly ketones. For a simple ketone, the it to it transition is at 187 nm although the e value is high, typically 10,000, it is out of range of normal observation. The n to k transition (from the lone pair of electrons on oxygen to the Jt orbital of the double bond) lies between 270 and 300 nm, but the e value is much lower, typically 20. More useful spectra are obtained from enones such as 5.5. The it to Jt transition is now in range at 219 nm and is a strong band (e 7100). The to Jt transition is 324 nm but is still weak (e 27). 

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