Ene-addition mechanism

The reaction between secondary enediamines and propiolic acid esters has been studied in detaiP. In aprotic solvents such as benzene and dioxane, 8 adds to methyl propiolate at ambient temperature to give almost quantitatively product 118 with a tron -configuration of the new double bond, as indicated by a Jhh coupling constant of around 16 Hz in the NMR spectrum. It appears that the reaction proceeds via an ene-addition mechanism. In refluxing ethanol, the C-adducts 118 are converted to the heterocyclic compounds 119. The direct transformation of 8 to 119 is easily achieved in alcoholic solvents. 1,1-Enediamines with jS-nitro, ester and acetyl substituents undergo analogous addition and cyclocondensation reactions to give heterocyclic compounds jj5(39,i2o A -Ethynylcarbonyl imidazole behaves similarly to propiolic acid esters ° (equation 41). 

This reaction is reversible and suitable p-hydroxy alkenes can be cleaved by heat (17-34). There is evidence that the cleavage reaction occurs by a cyclic mechanism (p. 1351), and, by the principle of microscopic reversibility, the addition mechanism should be cyclic too. Note that this reaction is an oxygen analog of the ene... 

Bode and co-workers have extended the synthetic ntility of homoenolates to the formation of enantiomerically enriched IV-protected y-butyrolactams 169 from saccharin-derived cyclic sulfonylimines 167. While racemic products have been prepared from a range of P-alkyl and P-aryl substitnted enals and substitnted imi-nes, only a single example of an asymmetric variant has been shown, affording the lactam prodnct 169 with good levels of enantioselectivity and diastereoselectivity (Scheme 12.36) [71], As noted in the racemic series (see Section 12.2.2), two mechanisms have been proposed for this type of transformation, either by addition of a homoenolate to the imine or via an ene-type mechanism. 

The effect of ring substituents on the rate constants, deuterium kinetic isotope effects and Arrhenius parameters for ene-additions of acetone to 1,1-diphenylsilane have been explained in terms of a mechanism involving fast, reversible formation of a zwitterionic silene-ketone complex, followed by a rate-limiting proton transfer between the a-carbonyl and silenic carbon. A study of the thermal and Lewis acid-catalysed intramolecular ene reactions of allenylsilanes with a variety of... 

In a thorough study on photooxidation of 2,5-dimethyl-2,4-hexadiene (455) it was found that 1,2-dioxene 456, 1,2-dioxetane 457, hydroperoxy dienes 458 and 459 and, when methanol was used as solvent, also hydroperoxy(methoxy)octene 460 are formed (Scheme 124) . Product distribution was found to be highly solvent dependent. These results led investigators to postulate a mechanism involving the intermediacy of perepoxide 461 and zwitterion 462 (Scheme 124). Accordingly, the product of [4-1-21-cycloaddition 456, the product of [2 + 2]-cycloaddition 457, as well as the products 458 and 459 deriving from ene-addition would originate from polar intermediates 461 and... 

The following examples show how open and closed transition states may be invoked by the choice of the reaction type. For instance, aldol-type addition normally proceeds via a closed transition state because the metal ion is shifted from the enolate oxygen to the carbonyl oxygen in an ene-like mechanism ( Zimmerman-Traxler transition state 9). The crucial interactions in the Zimmerman-Traxler transition state 16 are those between the 1,3-diaxially oriented substituents around the chair-like structure. R2 adopts the location shown, thus R3 avoids the 1,3-interaction and assumes an equatorial position. Therefore, the diastereomeric ratio depends mainly on the ( )/(Z) configuration of the enolate. Whereas (Z)-enolates 13 afford syn-config-urated enantiomers, 17 and 18, the corresponding ( )-enolates 14 lead to anti-configurated adducts 19 and 20 10. 

Similarly, activated olefins react with triphenylchromium. Bicyclo [2.2.1] hepta-2 5-diene reacts to produce 2,5-diphenylbicyclo [2.2.1] hept-2-ene. The mechanism of this reaction is not clear, but an initial addition of phenylchromium to the unsaturated system is probably involved (67),... 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

The maleimide group can undergo a variety of chemical reactions. The reactivity of the double bond is a consequence of the electron withdrawing nature of the two adjacent carbonyl groups which create a very electron-deficient double bond, and therefore is susceptible to homo- and copolymerizations. Such polymerizations may be induced by free radicals or anions. Nucleophiles such as primary and secondary amines, phenates, thiophenates, carboxylates, etc. may react via the classical Michael addition mechanism. The maleimide group furthermore is a very reactive dienophile and can therefore be employed in a variety of Diels Alder reactions. Bisdienes such as divinylbenzene, bis(vinylbenzyl) compounds, bis(propenylphenoxy) compounds and bis(benzocyclobutenes) are very attractive Diels Alder comonomers and therefore some are used as constituents for BMI resin formulations. An important chemical reaction of the maleimide group is the ENE reaction with allylphenyl compounds. The most attractive comonomer of this family is DABA particularly when tough bismaleimide resins are desired. 

In the isomerisation of the tetra-substituted olefin 3,4-dimethylhex-3-ene over palladium—alumina [146], it has been shown that double bond migration is a necessary precursor to cis—trans isomerisation. This has been interpreted as showing that the mechanism involves a series of elementary steps, each of which is stereospecific, although no definite conclusions were drawn as to whether an addition—abstraction or an abstraction-addition mechanism was involved. 

Mikami has carried out a number of investigations aimed at elucidating mechanistic aspects of this Si-atom transfer process. In particular, when the aldol addition reaction was conducted with a 1 1 mixture of enoxysilanes 60 and 62, differentiated by the nature of the 0-alkyl and 0-silyl moieties, only the adducts of intramolecular silyl-group transfer 63 and 64 are obtained (Scheme 8B2.6). This observation in addition to results obtained with substituted enol silanes have led Mikami to postulate a silatropic ene-like mechanism involving a cyclic, closed transition-state structure organized around the silyl group (Scheme 8B2.6). 

Product 19 in equation 39 is formed by trapping extruded Me2Sit, but 20, 21 and 22 are rearrangement products of silacyclopent-3-ene 23. Their formation is in accord with stepwise retroaddition with silylene extrusion from a vinylsilirane that is an intermediate in a reversible addition mechanism. As expected from such a mechanism, 20, 21 and 22 are coproducts with 23 in the addition of Me2Si to the piperylenes20. 

The absolute rate constants for ene-addition of acetone to the substituted 1,1-diphenyl-silenes 19a-e at 23 °C (affording the silyl enol ethers 53 equation 46) correlate with Hammett substituent parameters, leading to p-values of +1.5 and +1.1 in hexane and acetonitrile solution, respectively41. Table 8 lists the absolute rate constants reported for the reactions in isooctane solution, along with k /k -, values calculated as the ratio of the rate constants for reaction of acetone and acctonc-rff,. In acetonitrile the kinetic isotope effects range in magnitude from k /k y = 3.1 (i.e. 1.21 per deuterium) for the least reactive member of the series (19b) to A hA D = 1.3 (i.e. 1.04 per deuterium) for the most reactive (19e)41. Arrhenius plots for the reactions of 19a and 19e with acetone in the two solvents are shown in Figure 9, and were analysed in terms of the mechanism of equation 46. 

Absolute rate constants for quenching of 19a have been reported for other ketones besides acetone, although product studies were not carried out42. These data are included in Table 3. The rate constant for quenching by pinacolone [54 kq = (4.0 0.2) x 108 M s-1] is similar to that by acetone under similar conditions, as would be expected for ene-addition by the mechanism of equation 46. The factor of ca 20 reduction in the rate constant for quenching by 1,1,1-trifluoroacetone [55 kq = (1.6 0.1) x 107 M 1 s 1]... 

The addition of singlet oxygen to 2-methyl-2-pentene oeeurs via a concerted ene-type mechanism as shown in Eq. (5-46). This is entirely consistent with the small solvent effect observed for this reaction [138], When the solvent is changed from methanol to carbon disulfide, the rate changes by a factor of seven. Thus, it would appear that the activated complex does not involve much charge separation [138],... 

A palladium-catalyzed 1,4-dialkoxylation of conjugated dienes was obtained when the 1,4-oxidation was performed in an alcohol as the solvent [105]. In this case it is necessary to run the reaction in the presence of a catalytic amount of a strong acid such as methanesulfonic acid or perchloric acid. Cyclic dienes gave a highly stereoselective 1,4-ds addition of the two alkoxy groups [Eq.(48)j. Also, the reaction with acyclic conjugated dienes proceeded in a, 4-syn addition. Thus, ( , )-hexa-2,4-diene gave E)- 2R, 5R ) dimethoxyhex-3-ene. The mechanism is depicted in Scheme 8-27. 

Scheme 30.2 The free radical chain reaction mechanism between a thiol and an unactivated carbon-carbon double bond to form the anti-Markovnikov thiol-ene addition product. Adapted from Ref [22].

Ethoxycarbonylcarbene addition to 3-bromocyclohexene gives a complex mixture of products, including the bromonorcarane (96) and a-bromo-ester (97). It is not clear whether the latter is formed by a simple carbene insertion into the C—Br bond, or by an ene type mechanism (Scheme 10). 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multi-functional monomers [37-44]. The thiol-ene polymerisation proceeds by a step growth addition mechanism which is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV-irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

However, thiol-ene addition forming yS-products usually predominates. Hence only this addition will be considered in further discussions on TEC reactions. The TEC reaction ocurs by either radical or nucleophilic mechanisms (see Fig. 12.4), depending on the reaction conditions and the imsaturated substrates. 

An inverse electron demand Alder-Ene reaction has been reported in which a nitrilium ion acts as the ene and an arene as enophile. The reaction occurred at room temperature and was proposed to be a double Alder-Ene reaction. The first step provided compound 133, which could undergo a stepwise addition of the vinyl sulphide to the azonia-allene followed by elimination or alternatively, a second concerted Alder-Ene type mechanism was suggested. The heterobicycle 134 was isolated in 90% yield. 

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