Dimerization reactions 3 + 2 cycloaddition

Mechanistic studies were performed for simple systems such as 1-thiabutadiene and ethylene481, although dimerization and cycloaddition of thiochalcones have received more attention, including several calculations482-484. Nevertheless, more synthetic utility possesses the uses of Lewis acids as catalysts in hetero-Diels-Alder reactions (equation 142)485,486 involving 1-thiabutadienes. 

The amount of 1,2,4,5-hexatetraene in solution was estimated by the submitters from H NMR spectra and GC analyses. The major product of the dimerization reaction is l,2-hexadien-5-yne. The submitters have shown that this hydrocarbon does not interfere with the cycloaddition in part B. 

Methylenecyclopropanes undergo a variety of [2 + 3] cycloadditions (1,3-dipolar cycloadditions) across the double bond. Examples of the addition of diradicals are some of the dimerization reactions of methylenecyclopropane vide infra). Small quantities (< 5%) of an addition product of a 1,3-diradical to methylenecyclopropane with preservation of the three-membered ring were isolated from the thermal reaction of dispiro[2.2.2.2]deca-4,9-diene with methylenecyclopropane. ... 

A number of cycloaddition and dimerization reactions of norbornadiene catalyzed by transition metal complexes has been described. The mechanism of these reactions was discussed from an empirical and phenomenological viewpoint. Several of the reactions have previously... 

Direct irradiation of the enone (112) results in the selective formation of the anti-head-to-head dimer (113). Cycloaddition reactions between the enone (112) and alkenes were also studied and, for example, afforded the products (114) and (115) with 2,3-... 

Little is known about the reactivity of silanethiones. The reactions that have been invoked to explain the results observed so far and listed above are (i) nucleophilic attack on Si=S leading to an addition, in particular addition of Si-O and Si-S bonds, and (ii) nucleophilic attack on Si=S leading to cycloaddition, in particular 2 4- 2 dimerization and cycloaddition to a silene. 

Condensed and other carbocyclic systems An interesting dimerization reaction was performed with cyclohexylidenemalononitrile 188. The reaction was performed in methanolic sodium methanolate and yielded the spiro compound 189. Acidic intramolecular cycloaddition of alkylidenemalononitrile 190 (with concentrated sulfuric acid) gave the naphthalene 191. 1 -Tetraloneanil 192 reacted with 179 to yield phenanthrene 193. ... 

Acyl sulfenes, like all sulfenes, prefer to participate as 2ir components of [2 -I- 2] or [4 + 2] cycloadditions (Chapter 5). Nonetheless, a range of [4 -I- 2] cycloaddition reactions of acyl sulfenes have been described - (Scheme 8-XII), including their 47t participation in dimerization reactions and reactions with imines, carbodiimides, ketenimines, 1-azirines, vinyl ethers,and ketenes. The reactions often provide mixtures of [4 + 2] and [2 + 2] cycloadducts, and the observed course of the reaction usually depends on the reaction conditions. Consequently, many of the observed [4 + 2] cycloadditions of acyl sulfenes proceed by a stepwise, polar addition-cyclization reaction. 

---