Ene reactions with olefins

On a first, very broad, approximation singlet oxygen behaves somehow like ethylene. Three types of reactions of 102 are usually observed and have been utilized in organic synthesis 606 608) a) the Diels-Alder like cycloaddition to dienes (6.1) b) the ene reaction with olefins (6.2) and c) cycloaddition to activated double bonds (6.3). 

Triazoline-3,5-dione 184 underwent an ene reaction with olefins 183 to yield trialkylated allylic urazoles 185, which were further elaborated into allylic amines 186 . DBU has been found to be a mild and convenient base for the alkylation of 1,2,4-triazole with various alkyl halides in the high yielding syntheses of 1-substituted-1,2,4-triazoles <00TL1297>. 

A second characteristic reaction of singlet oxygen is the ene reaction with olefins with an allylic hydrogen to give allylic peroxides. Thus 2,3-dimethyl-hutene-2 (3) is converted in 53% yield into the allylic peroxide 2,3-dimethyl-3-liydroperoxybutcnc-l (4) ... 

Calculations of the reactivity of model nitrocarbenes (77a) demonstrated that these species A much more rapidly undergo a rearrangement to give nitrosocar-bonyl intermediates B than are involved in [3+ 2]-cycloaddition with an external trapping agent. Shortly thereafter, this fact was confirmed experimentally (77b) because, at the same time, intermediates B can be detected in ene reactions with certain olefins to give hydroxylamines (25) (Scheme 3.25). 

Reactions of p-Functionalized a-Nitrosoalkenes with the C,C Double Bond The reactivity of a-nitrosoalkenes toward double bonds depends drastically on the structure and configuration of the reagents. The characteristic features of the behavior of p-functionalized a-nitrosoalkenes were studied using readily available methyl p-nitrosoacrylate (530a). It appeared that this derivative is readily involved in the ene reactions with electron-donating and neutral olefins either... 

Allylic terpenic thiols were prepared in 70-80% yield by this procedure. The reversibility under mild conditions of the ene reaction with N-sulfinylsulfonamides RS02N=S=0 had been exploited in allylic deuteration and tritiation of olefins [516]. 

TABLE 8C.1. Asymmetric catalytic glyoxylate-ene reaction with various olefins... 

It is noteworthy that the ene product 227 is obtained exclusively when allylsilane 226 is treated with butynone in the presence of Znl2 catalyst and molecular sieve (ms) 4A(equation 185)331. Methyl vinyl ketone behaves similarly. High enantioselectivity has been observed in product 228 when triphenylallylsilane is coupled with methyl glyoxylate in the presence of (R)-(BINOL)TiCl2 catalyst (equation 186)332. In addition to [2 + 2] cycloaddition, TCNE undergoes a regiospecific ene reaction with y-alkyl substituted allylsilanes to yield the substituted olefins 229 (equation 187)333. 

Allylic amidation Nitrosocarbonylmethane undergoes an ene reaction with various olefins to form allylic N-hydroxy amides, often in useful yields. 

An asymmetric variant of this kind of allylic amination, based on their phenylcyclohexanol-derived chiral N-sulfinyl carbamates, was developed by Whitesell et al. (see also Sect. 3.2) (Scheme 34) [85]. After the asymmetric ene reaction with Z-configured olefins (not shown) had occurred, nearly di-astereomerically pure sulfinamides 127 were obtained which were found to be prone to epimerization. Their rapid conversion via O silylation and [2,3]-a rearrangement dehvered the carbamoylated allyhc amines 128 with around 7 1 diastereoselectivity as crystalline compounds that can be recrystallized to enhance their isomeric purity to 95 5. Obviously the imiform absolute configuration at Cl in the ene products 127 was difficult to transfer completely due to the already mentioned ease of epimerization. Unhke the sulfonamides of Delerit (Scheme 33) [84], the carbonyl moiety can easily be cleaved by base treatment. 

The asymmetric ene reaction with catalyst 98 is restricted to activated aldehydes as is indicated by the data in Table 16. The rates of the reaction are such that it is not applicable to internal olefins. A variety of 1,1-disubstituted alkenes can be used to give good asymmetric induction with the fastest rates observed with phenyl vinyl thioethers. Turnover can be realized with the more reactive aldehydes and/or alkenes but only in the presence of molecular sieves. The reaction of chloral with a-methyl-styrene shows that higher induction can be achieved with lower temperatures although the reaction is slower. The nature of the solvent affects the rate of the reaction. The reaction is much slower in toluene than in dichloromethane. 

Azodicarboxylates are best recognized for their ability to participate as 2w components in normal (HOMOdiene controlled) Diels-Alder reactions with dienes (Chapter 6)5 71 113 and for their effective participation in ene reactions with reactive olefins.113 In addition, electron-rich or reactive olefins that do not contain a reactive allylic hydrogen atom and consequently cannot enter into an ene reaction do possess the ability to participate in competing [2 + 2] and [4 + 2] cycloadditions with azodicarboxylates.5 71 113 The Diels-Alder [4 + 2] cycloaddition of azodicarboxylates with olefins provides 1,3,4-oxadiazines (Scheme 9-VI) and is sensitive to solvent and substituent effects.114-120 In general, the competing [2 + 2] cycloaddition to provide 1,2-diazetidines intervenes and predominates as the olefin nucleophilicity and the reaction solvent polarity are increased [Eqs. (46) and (47)].113... 

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