Cycloadditions step-wise

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

Butadiene has also been reported to form a [4 + 2] cycloaddition-like product at the Si(l 11)—7 x 7 surface. Theoretically, the reaction has been predicted to occur between a rest atom-adatom pair via a step-wise, diradical path pathway that is unactivated [19,249]. Recent STM studies of butadiene adsorbed on Si(l 11)—7 x 7 have observed the formation of either a [4+ 2]-like or a [2+ 2]-like product with the Si adatom-rest atom pair, but were not able to definitely assign the product as one or the other [250]. Interestingly, the STM images also reveal that about a third of the cycloaddition product forms between two Si adatoms. Because both adatoms are positively charged, the stepwise reaction does not seem likely, and the authors suggest that this reaction occurs in a concerted fashion [250]. 

For recent reviews on step-wise [3 + 3] formal cycloadditions leading to bridged carbocycles, pyridines or pyridones using enamines, enaminones, enol ethers, or (J-... 

Cyclobutanones are obtained from cycloaddition of ketenes to enamines (see Section IV.A and Chapter 18)266. Such cycloadditions may be concerted or step-wise depending upon the amine moiety and the experimental conditions. Vinylketenes are especially interesting since these may undergo [2 + 2] or [4 + 2] cycloadditions to give after oxidation a vinylcyclobutanone or a cyclohexenone, respectively267,268. 

Now are there such reactions as [2 + 2] cycloadditions Can, say, two molecules of ethylene combine to form cyclobutane The answer is yes, but not easily under thermal conditions. Under vigorous conditions cycloaddition may occur, but step-wise—via diradicals—and not in a concerted fashion. Photochemical [2 + 2] cycloadditions, on the other hand, are very common. (Although some of these, too, may be stepwise reactions, many are clearly concerted.)... 

Another chemoselective ligation reaction is the [2 + 3] cycloaddition between an azide and an alkyne. This reaction has been discovered by Huisgen and was lately named click-reaction by Sharpless and Meldal [180, 181]. Whereas the Huisgen 1,3-dipolar cycloaddition leads to two isomeric triazole products at high temperature, click chemistry is performed under the catalysis of Cu(I), thus changing the reaction mechanism from a concerted to a step-wise route and resulting in the formation of the 1,4-substituted triazole as the only product, usually isolated in high yields [174, 182-186],... 

Cerfontain and coworkers150,325-327 have shown that both sultone and carbyl sulphate formation are stereospecific reactions. Roberts and coworkers324 have indicated that the reaction is a [2 + 2] cycloaddition process however, some earlier workers have argued that the reaction is a step-wise process321,329. 

The enone (92) is reactive in its triplet state and when irradiated in methylene chloride solution is converted into the tetracyclic compound (93). The reaction involves a step-wise process in which the biradical (94) is involved. This process is reminiscent of (2-i-2)-cycloaddition reactions where bonding occurs at the P-atom of the enone, and rather than completing the cyclization, hydrogen (or deuterium) abstraction occurs. A detailed stereochemical analysis of the system was carried out and proof of the stereochemistry of the final product (93) has been presented. 

Cycloaddition. The use of the highly reactive compound chlorosul-phonyl isocyanate (CSI) has provided an opportunity to study a wide range of cycloaddition reactions, and it now seems generally accepted that at least the majority of these reactions involve a step-wise addition via an acyclic zwitter-ion. Paquette has made an extensive study of the addition of CSI to cis-bicyclo[6,l,0]nonatrienes, and the structure and stereochemistry of the... 

The PES s of the [2 + 2]-cycloaddition were also computed for the following reactions formaldehyde + formaldehyde and ethylene + singlet oxygen [10]. The results of these computations indicate that for a [2 + 2]-cycloaddition only step-wise reaction paths exist involving diradicaloid transition states and intermediate gauche- and trans-forms. 

Kinetics of the reactions of pyridinium ylides (210) and their isoquinolinium and quinolinium congeners with arylidene malonates (211) and related electrophiles, such as diarylcarbenium ions and quinone methides, have been studied in DMSO by UV-vis spectroscopy. The second-order rate constants thus obtained were used to derive the nucleophile-specific parameters Nand % for these ylides. Pyridinium substitution turned out to have a similar effect as that of alkoxycarbonyl substituents on the reactivity of carbanionic reaction centres. Agreement between the experimental rate constants and those calculated from E, N, and % shows that this correlation can also be employed for predicting the absolute rate constants of step-wise or highly unsymmetrical concerted cycloadditions. On the other hand, deviation by a factor of 10 would indicate a change of reaction mechanism. ... 

Step-Wise Double [2-1-2] Cycloaddition Reactions in Ladder Structures... 

The Matlin group has shown that irradiation of cyclooctadienone 36 in the presence of vinyl ethers produces 7-norbornanones 41 and oxa-triquinanes 42 (Table 81.5). The reaction is envisioned as proceeding via the step-wise cycloaddition to oxyaUyl 37. The electron-rich vinyl ether stabilizes the incipient cationic center in 43, which then undergoes internal O- and C-alkylation of the enolate moiety to give 41 and 42. The product distribution and yields did not show any significant variation with the polarity of the solvent (hexanes, benzene, CHjClj or neat vinyl ether). It is interesting to note that no enone-alkene [2-1-2]-adduct was observed, even when ethyl vinyl ether was used as solvent. This is in contrast to the behavior of pyran-4-one, where irradiation in neat furan produced [2-1-2]-adducts. The photo-excited state lifetime of 36 is limited by rapid cis,trans-isomerization to 40, which does not permit intermolecular capture to be competitive. 

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