Trans- , stepwise addition

The formation of the tra/w-fused 4/ ,5a-ethylene adduct (262) has analogies in the addition of olefins to simple a,/ -unsaturated ketones. The possibility is that this compound derives in a concerted antara-antarcfi cial process from an excited state of the dienone (260) in which the a,j8-bond is nonplanar. Alternatively, it has been argued that upon a stepwise addition of the olefin to the excited (di)enone system a diradical is formed (e.g., through the formation of a single bond to the /2-carbon in the first step), and that part of the excess energy of this intermediate is absorbed in the formation of the strained trans-fused system. A two-step reaction is consistent with the result that the addition of either cis- or traws-dichloroethylene to enone (267) affords the same 1 1 mixture of 16a,17a-adducts [(269) and (270)]. 

Figure 21-17 Schematic representation of the energy of the stepwise addition of CH2=CF2 to fra/7s,frans-2,4-hexadiene. The lower diagram is a schematic cross section" that shows everything in two dimensions. If 48 and 49 dissociate to CF2=CF2 and diene, 48 will return the trans.trans isomer, but 49 will go to the cis,trans diene through the transition state represented by the dashed line. In some [2 + 2] cycloadditions of this type, trans.trans to cis.cis isomerizations are observed in competition with cycloaddition, as expected for breaking apart the intermediate corresponding to 49.

When nonstereospecific addition of dimesityl- and bis(2,4,6-triisopropylphenyl)silylene to trans-2-butene, and to a lesser extent to d.v-2-butene, was reported, a stepwise addition mechanism was suspected131. However, the apparent nonstereospecificity was caused by a d.s-butene impurity in the frans-butene and by photoisomerization of the cis- and trans-siliranes132. 

Photochemical addition of cis- or trans-dichloroethylene to the A -20-keto-steroid (62) afforded two a-face adducts (63a) and (64) (see Part II, Chapter 1, ref. 310). The chlorine configurations as shown were assigned on the basis of n.m.r. studies and an AT-ray analysis of the derivative (635)." ° The fact that both cis- and trans-dichloroethylene provide the same product composition is in agreement with the stepwise addition mechanism, forming a diradical intermediate which can undergo free rotation at the 16-17 bond before ring closure. 

Remarkably, with 3,3-dimethoxycyclopropene, the stereochemical outcome of the [2 + 1] cycloaddition reaction with dialkyl fumarate and maleate is different to that observed for the other 3,3-disubstituted cyclopropenes (Table 3). With the 3,3-dimethoxy derivative only the thermodynamically more stable 1,2-trans-dialkoxycarbonylcyclopropanes are formed. This can be taken as an indication for a stepwise addition process with an intermediate of highly zwit-terionic character (structure 29B or 29C) that can undergo rotational isomerization to form the thermodynamically most stable product, i.e. the frans-configurated diester.690... 

Benzyne, when added to the geometric isomers of propenyl methyl ether (in dichloromethane at 40°C) gives mixtures of 1,2-adducts, approaching within 65% of the equilibrium composition . Non-stereospecific 1,2-cycloadditions of benzyne to cis and trans isomers of 1,2-dichloroethylene and propenyl ethyl ether have also been reported in both cases the products are far from completely equilibrated, indicating a stepwise addition with a relatively fast ring closure . 

There have been few mechanistic studies of Lewis acid-catalyzed cycloaddition reactions with carbonyl compounds. Danishefsky et ah, for example, concluded that the reaction of benzaldehyde 1 with trans-l-methoxy-3-(trimethylsilyloxy)-l,3-di-methyl-1,3-butadiene (Danishefsky s diene) 2 in the presence of BF3 as the catalyst proceeds via a stepwise mechanism, whereas a concerted reaction occurs when ZnCl2 or lanthanides are used as catalysts (Scheme 4.3) [7]. The evidence of a change in the diastereochemistry of the reaction is that trans-3 is the major cycloaddition product in the Bp3-catalyzed reaction, whereas cis-3 is the major product in, for example, the ZnCl2-catalyzed reaction - the latter resulting from exo addition (Scheme 4.3). 

A concerted four-center cis addition leads to (52) and a trans adduct a trans addition, possibly via protonium species, leads to (53) and a cis adduct a stepwise cationic addition leads to (54) and a mixture of cis and trans adducts. Recent studies by Marshall and Wurth strongly indicate that intermediate (54) is correct. Irradiation of octalin (55) in aqueous /-butyl alcohol (DaO)-xylene results in formation of the equatorially deuterated alcohols (56) and (57) and the equatorially deuterated exocyclic olefin (58) ... 

Phenylthioacetylene has been prepared by elimination of thiophenol and dehydrobromination of cis-1,2-bis(phenylthio)ethylene5 and cis-1-bromo-2-phenylthioethylene,2 7 respectively. The latter was obtained by addition of thiophenol to propiolic acid in ethanol and subsequent one-pot bromine addition, decarboxylative dehalogenation, and careful distillation to remove the trans isomer.2.7 on the other hand, cis-1,2-bis(phenylthio)ethylene was prepared by double addition of thiophenol to cis-1,2-dichloroethylene.5a d Although these procedures can provide useful amounts of phenylthioacetylene, they were found to be somewhat less satisfactory in our hands as far as operation and/or overall yields are concerned. Furthermore, we have encountered problems with regard to the reproducibility of one-pot dehydrobrominations of phenylthio-1,2-dibromoethane.6 However, the stepwise execution of the double dehydrobromination, as described in the modified procedure reported here, provides preparatively useful quantities of phenylthioacetylene in a practical manner. 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

Treatment of the intermediate silylenolefher, obtained by reaction of 106 with CgQ, with SiOj and triethylamine leaves the methoxy group and yields a mixture of the two isomers 107 and 108 (Scheme 4.18) [99, 100], This reaction can be carried out either photochemically or thermally. Because the trans-product 107 is the major product under both thermal and photochemical conditions, the mechanism of this addition is concluded to proceed stepwise. The first step is probably an electron transfer from the Danishefsky diene to CgQ. At least for the photochemical pathway this electron-transfer step could be proven [100]. 

Only very few examples of alkene-]2-i-2] cycloadditions are known ]345, 347, 348]. By using a large excess of the moderate electron-rich alkene p-propenyl-anisol ]348] or even less electron-rich alkyl-subshtuted 1,3-butadienes [347] no thermal [2-1-2] cycloaddition occurs, but a photochemical cycloaddition can be enforced. The mechanism is proven to be stepwise via a biradical or dipolar intermediate ]347-351], comparable to the addition of the alkynes. During the addihon of cis- and trons-alkenes the existence of this relahvely long lived intermediate leads to a loss of stereochemical integrity. Addihon of ds-4-propenylanisol or trans-4-propenylanisol results in both cases exclusively in the trans-adduct (Scheme 4.61). 

In some cases, a stepwise mechanism is indicated by randomization of the dienophile stereochemistry. For example, addition of cw-anethole radical cation (100 +) to cyclopentadiene produces comparable yields of four possible diastereoi-someric adducts (102) clearly supporting a distonic radical cation intermediate (lOl ). Only products supporting the stepwise mechanim, that is, trans,endo-and trans,exo-lQ2, are shown. " ... 

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