Carbonyl reaction with alkenes

Photochemical reactions of carbonyl compounds with alkenes give the oxetanes (Scheme 30). The stereochanical course depends on the substituents of the alkenes [16]. The reactions proceed with the retention of the configuration of the alkenes for the electron accepting substituent, e.g., CN. The stereochemical integrity is lost for the donating group, e.g., OCH. ... 

Ene and Carbonyl-Ene Reactions. Certain double bonds undergo electrophilic addition reactions with alkenes in which an allylic hydrogen is transferred to the reactant. This process is called the ene reaction and the electrophile is known as an enophile A When a carbonyl group serves as the enophile, the reaction is called a carbonyl-ene reaction and leads to [3,-y-unsalurated alcohols. The reaction is also called the Prins reaction. 

Predict the products of the reactions of excited-state carbonyl compounds with alkenes (Paterno-Buchi reaction). 

Carbonyl cyanide reacts readily with ketene and dialkylketenes to give the corresponding dicyano-/3-lactones (equation 109). This reaction seems entirely analogous to the addition of carbonyl cyanide with alkenes to give oxetanes, described in the preceding section (75MI51302). 

The ability of nitrous oxide to forma 1,3-dipole (Section 7.2) seems to be of critical importance for the reaction with alkenes. The oxygen transfer proceeds via the 1,3-dipolar cycloaddition mechanism, assuming intermediate formation of a 1,2,3-oxadiazoline complex, the decomposition of which leads to a carbonyl compound ... 

This means that the nucleophile is pushed away more strongly in the alkene case than in the carbonyl case. At the same time, the attractive in phase overlap between Nu and Q is smaller with n cc than with 7t co, which tends to maintain Nu nearer to the vertical position in the carbonyl case. Both factors contribute to give a larger angle for the reaction with alkenes or alkynes. 

In contrast to photocycloaddition reactions of carbonyl compounds to electron-rich alkenes (which proceed with a low degree of stereoselectivity in the case of triplet excited carbonyls), reactions with electron-deficient alkenes, such as cyanoalkenes, are, although rather inefficient, but highly stereoselective [101]. Kinetic analysis showed that these reactions involve the... 

U nder thermal or Lewis acid catalyzed conditions, reactions of fluoroalkyl carbonyl compounds with alkenes usually provide the ene adduct. " One example of a [2 + 2] cycloaddition between hexafluoroacetone and an alkyne has been reported. ... 

Oppolzer has classified intramolecular ene reactions into type I, II and III depending on whether the tether is attached to carbon 1,2 or 3 of the ene component (see equations 4, S and 6). Type IV reactions, which are a variation of type I in which the tether is attached to the terminal rather thim internal end of the enophile, are occasionally observed (see equation 7). Type I reactions with alkenes or alkynes as enophiles have been extensively studied. Only a few examples of type II, III and IV reactions with alkenes or alkynes as enophiles are known. Numerous examples of intramolecular type I, II and III reactions with carbonyl compounds as enophiles are discussed in Volume 2, Chapter 2.1. 

Type II reactions with carbonyl compounds as enophiles are well-known, synthetically useful reactions.Type II reactions with alkenes as enophiles are very rare because competing type I reactions usually occur more readily. Thermolysis of (149a) at 280 C gives ene adduct (150a) in 40% yield. Thermolysis of (149b) at 230-280 C leads to type I adduct (151) rather than type II adduct (150b). ... 

Reaction of alkenes with carbonyl compounds or carbonyl derivatives in the presence of Lewis acids, the ene reaction, enables the stereoselective preparation of highly functionalized compounds. Copper Lewis acids activate both aldehydes and imines in ene reactions. Evans has reported that Cu(II) Lewis acids catalyze glyoxylates in reactions with alkenes (Sch. 56) [103]. The homoallylic alcohols 257 and 259 are produced in high yield and enantioselectivity. The bis aquo complex 260 is a readily prepared and air-stable catalyst and gave high chemical yield and excellent selectivity in the ene reactions. Another point of note is that catalysts 260 and 261 furnish enantiomeric products even though they differ from each other only by the substituent at the 4-posi-tion of the oxazoline. 

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