Singlet oxygen alkenes

Another interesting cycloaddition, the detailed mechanism of which is still under investigation, is the addition of singlet oxygen to alkenes producing 1,2-dioxetanes (Section 5.15.3.3.2). 

Autoxidation reaction of alkenes with singlet oxygen... 

The interaction between the HOMO of alkenes and the LUMO of singlet oxygen O (Scheme 4) is the most favored in the perepoxide structure (Scheme 13). This suggests... 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

The HOMO of alkenes is an out-of-phase combination of the n and 0, orbitals. The amplitude is larger on n. The LUMO of singlet oxygen is %. The frontier orbital interaction occurs most effectively when the alkenes and the singlet oxygen... 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

A variety of double bonds give reactions corresponding to the pattern of the ene reaction. Those that have been studied from a mechanistic and synthetic perspective include alkenes, aldehydes and ketones, imines and iminium ions, triazoline-2,5-diones, nitroso compounds, and singlet oxygen, 10=0. After a mechanistic overview of the reaction, we concentrate on the carbon-carbon bond-forming reactions. The important and well-studied reaction with 10=0 is discussed in Section 12.3.2. 

Among the oxidants that add oxygen at carbon-carbon double bonds is singlet oxygen.161 For most alkenes this reaction proceeds with the removal of an allylic... 

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165... 

Singlet oxygen reacts with electron rich or highly strained alkenes to form 1,2-dioxetanes. These four-membered ring peroxides decompose on warming to two carbonyl compounds (or moieties), usually with appearance of light emission (chemiluminescence). The macrocyclic bis-lactone in (6.17)608>, a musk fragrance, has been synthesized via such a sequence. 

Fig. 16 Comparison of intrazeolite and solution singlet oxygen ene reactions of electron poor alkenes.

Robbins, R.J. and Ramamurthy, V. (1997). Generation and reactivity of singlet oxygen within zeolites remarkable control of hydroperoxidation of alkenes. J. Chem. Soc. Chem. Commun. 1071-1072... 

The reaction of cis- and frans-stilbene oxides with phenylphosphonothioic dichloride in the presence of magnesium gives cis- and fra/ts-stilbene and (7).13 Phenylphosphinidene sulphide is postulated as being an intermediate. The zwitterion (8) bears a remarkable similarity to the controversial perepoxides which are thought to be intermediates in the reaction of singlet oxygen with alkenes. 

These intrazeolite singlet oxygen ene reactions have synthetic potential because the cis effect observed in solution is suppressed in the zeolite [13]. Consequently, allylic hydroperoxides which are inaccessible by other routes may be available via this new technology. For example, photo-oxidations of aryl-substituted alkenes, 7, in sensitizer-doped NaY react to generate the allylic hydroperoxides as the major or exclusive product [17]. In contrast, in solution, the hydroperoxides are formed in only 5-20% yields, with 2-1-2 and 4-1-2 adducts dominating the reaction mixtures. In the case of 2-methyl-5-phenyl-2-hexene, 8, the regio-selectivity for 8b and 8c improved from 47% to 94% and the diastereoselectivity from 10% to 44% as the reaction is moved from solution into the zeolite [18] ... 

Scheme 12.16. Oxidation of Alkenes with Singlet Oxygen... 

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