Dipolar s. Cycloaddition

Addition l 4-dipolar s. Cycloaddition, 1,4-dipolar —, double to nitrogen-carbon double bonds of mercaptans 18 641 —, electrophilic s. Electrophilic addition —, free-radical... 

Addition, Diene syntheses, Michael addition -, dipolar s. Cycloaddition, dipolar... 

Concerted + n s] cycloadditions are, in principle, forbidden by orbital symmetry [90]. This restriction is bypassed when these reactions occur via zwitterions or biradicals, or by the symmetry-allowed [A + 2 ] process. Since cycloadditions proceeding through zwitterionic intermediates or dipolar activated complexes should be affected by solvent polarity, the investigation of the solvent effects on rates can be of considerable value when considering potential models for the activated complex and the reaction mechanism [91-93]. The possible solvent effects on one-step and two-step cycloaddition reactions are shown schematically in Fig. 5-6 [92] ... 

In order to form the activated complex required for the formation of product D, rotational changes of the less dipolar anti-form A to the more dipolar s jn-conformer B are necessary, to give an activated complex C with more parallel bond dipoles, which is thus more dipolar and better solvated than the reactant molecule. In agreement with this explanation is the observation that the reverse refro-Diels-Alder reaction exhibits no large solvent effect, since the activated complex C is quite similar to the reactant D [807], A very subtle solvent effect has been observed in the Diels-Alder addition of methyl acrylate to cyclopentadiene [124], The polarity of the solvent determines the ratio of endo to exo product in this kinetically controlled cycloaddition reaction, as shown in Eq. (5-43). The more polar solvents favour endo addition. 

As measured by the criteria of stereospecificity, regioselectivity, kinetic isotope effects, and solvent effects [117-120, 541-543], 1,3-dipolar cycloaddition reactions represent orbital symmetry-allowed [n + n s] cycloadditions, which usually follow concerted pathways Diels-Alder reactions and 1,3-dipolar cycloadditions resemble each other, as demonstrated by the small solvent effects on their bimolecular rate constants. In going from nonpolar to polar solvents, the rate constants of 1,3-dipolar cycloadditions change only by a factor of 2... 10 [120, 131-134]. 

This very ingenious type of process was recently reported by Torii and co-workers, who proposed a mechanism proceeding via a dipolar-type cycloaddition followed by CO insertion, as shown in Scheme 59,[80],[8i] jjjjs jg interesting approach to /S-lactams, as it... 

P. Salehi, M. Dabiri, M. Koohshari, S. K. Movahed, M. Baraijanian, Mol. Divers. 2011, 15, 833-837. One-pot synthesis of 1,2,3-triazole linked dihydropyrimidinones via Huisgen 1,3-dipolar/Biginelli cycloaddition. 

In this transformation first step is 1, 3-dipolar cycloaddition, which is isoelectronic with [tc s+ti s] cycloaddition and is therefore S3rnunetry allowed under thermal conditions. Second step of this reaction is, 1, 3-dipolar cycloreversion involving elimination of CO2 too and the product (B). 

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

The class of 1,3-dipolar cycloadditions embraces a variety of reactions that can accomplish the synthesis of a diverse array of polyfunctional and stereochemically complex five-membered rings.3 The first report of a 1,3-dipolar cycloaddition of a nitrone (a 1,3-dipole) to phenyl isocyanate (a dipolarophile) came from Beckmann s laboratory in 1890,4 and a full 70 years elapsed before several investigators simultaneously reported examples of nitrone-olefin [3+2] cycloadditions.5 The pioneering and brilliant investigations of Huisgen and his coworkers6 have deepened our under-... 

Due to the presence of a heterocumulene unit, sulphines may be considered as a group of compounds which are able to undergo cycloaddition reactions. Reaction of sulphines with enamines and phosphorus ylides reported by Sheppard217 and Trippett218 and their coworkers may be considered formally as an example of [2 + 2] cycloaddition. In fact, Sheppard and Dickman217 obtained a 1 1 adduct from thiofluorenone S-oxide and 1-morpholinocyclohexene to which they assigned the dipolar sulphoxide structure 168. 

The 1,3-dipolar cycloaddition of mesitonitrile oxide 575 to benzo[h]thiophene S-oxides 576 in non-stereoselective and both syn and anti adducts 577 are obtained674,675 (equation 366). 

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