1.3- Dipolar adducts

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

Tnfluoroacetonitrile oxide also reacts with stabilized enolate ions, such as that derived from 2,4-pentanedione, to give good yields of 1,3-dipolar adducts [38] (equation 38). 

Thermolysis of indenone oxides (151) is an equally useful route to the betaines (150). Dimethyl acetylenedicarboxylate and compound 151 (R = R = Ph) at 175°C give adduct 153, and cyclohexanone at 150 C gives adduct 154. - Similarly, 1,3-dipolar adducts (e.g., 155) have been obtained using a wide variety of olefins—including cis- and [rans-, 2-dichloroethylene, dimethyl maleate, dimethyl fumarate, maleic anhydride, cis- and tran -stilbene, fran -dibenzoylethylene, tra .y-l,2-dicyanoethylene, A -phenylmaleimide, vinylene carbonate, acenaphthylene, and norbor-nadiene. With cis olefins the endo adduct (155) is usually the predominant isomer. Diphenylcyclopropenone gives compound 156 by spontaneous elimination of carbon monoxide from the initial adduct (157). Adduct 156... 

Like isoconjugate mesomeric betaines, which have been discussed in preceding sections, phthalazinium-l-olates (196) give 1,3-dipolar adducts with acetylenes. Thermally induced isomerization reactions of these adducts (255) make them of special interest. The A-methyl derivative and diphenyl-... 

Benzisoquinolines (303) have not been isolated, but they have been generated in situ and trapped as 1,3-dipolar adducts. Treatment of the... 

Cycloaddition reactions of phenyl and tosyl azides to the strained double bond in cyclopropenes have been investigated.170il The reaction products from 3,3-dimethylcyclopropene indicate that the initially formed intermediate is a normal 1,3-dipolar adduct. Tetrachlorocyclopropene yields the primary adducts with several aryl azides. 70b However, cyclopropenedicarboxyl ester gives only unstable triazolines with phenyl and methyl azides.170 ... 

The addition of ZnBi 2 to die tandem 1,3-azaprotio cyclotransfer-cycloaddition of a ketoxime with divinyl ketone results in rate enhancement and the exclusive formation of l-aza-7-oxabicyclo[3.2.1]octan-3-ones.79 The 1,3-dipolar cycloaddition of 1-aza-l-cyclooctene 1-oxide with alkenes produces the corresponding isoxazolidines in high yields with a minimum of polymeric material.80 The cycloaddition of thiophene-2 -carbaldehyde oxime with acetonitrile and methyl acrylate produces the 1,3-dipolar adduct, substituted isoxazolidines, and not the previously reported 4 + 2-adducts.81 Density functional theory and semi-empirical methods have been used to investigate the 3 + 2-cycloaddition of azoxides with alkenes to produce 1,2,3-oxadiazolidines.82 The 3 + 2-cycloaddition of a-nitrosostyrenes (62) with 1,3-diazabuta-l,3-dienes (63) and imines produces functionalized cyclic nitrones (64) regioselectively (Scheme 22).83... 

Nitrones. Secondary amines can be oxidized directly to nitrones with HjO, (30%) catalyzed by Na2W04-2H20. Oxidation in the presence of an activated alkene gives the 1,3-dipolar adduct. The reaction can also be used to effect nucleophilic a-substitution of the amine. 

The mesoionic pyrrolo[l,2-c]oxazolium oxide (223), postulated as intermediate in the reaction of an A -substituted proline and acetic anhydride, reacts with DM AD to give a 1,3-dipolar adduct, which by extrusion of carbon dioxide afforded the diester (224) (Scheme 39) <85TL1295>. 

A mechanophore (blue in Fig. 2a) is a strategically designed chemical entity which responds to mechanical force in a predictable and useful manner (Fig. 2d-f). The polymer strand here acts as an actuator to transmit macroscopic force to the target. For a fully extended polymer chain, the maximum tension force is at the middle point of the chain contour. So the mechanophore should be incorporated into the middle of the chain with its active bond along the chain contotu (Fig. 2a) [15, 29, 32]. Examples of mechanochemical reactions include homolytic scission of weak bonds (diazo [33]), electrocyclic ring-opening (benzocyclobutenes [29], spiropyrans [32, 34 5], gem-dichlorocyclopropanes [46-49], ge/n-difluorocyclo-propanes [30, 50], and epoxide [51]), cycloreversion reactions (cyclobutane derivatives [52-56], Diels-Alder adducts [57, 58], 1,3-dipolar adducts [59, 60], and 1,2-dioxetanes [61]), dative bond scission [62-64], and flex-activated reactions [34, 65, 66], as recently reviewed by Bielawski [67]. 

The diazomethane-keten reaction gave no 1,3-dipolar adducts, such as those obtained readily enough from the corresponding thioketen (see Vol. 1, p. 44), even at low temperatures. The 2 1 adduct obtained (165) cannot arise by prior dimerization of the keten, whose jS-lactone dimer (166) reacts to give a y-lactone (167) by methylene insertion at the C—O bond, so that quite different hydrolysis products are obtained (see Scheme 48). ... 

From Oxathiazolones (Type B).—3-Phenyl-l,2,4-oxathiazol-5-one (17) is decomposed thermally to benzonitrile and sulphur, but condenses with activated dipolarophiles to yield 1,3-dipolar adducts that may arise from the intermediate benzonitrile sulphide (18). Thus, interaction of the oxathiazolone (17) and dimethyl acetylenedicarboxylate in chlorobenzene at 130 °C produces dimethyl 3-phenylisothiazole-4,5-dicarboxylate (19) in excellent yield. The use of ethyl propiolate (HC=CCOaEt) similarly affords the expected isomeric esters (20) and (21), each in 35% yield. The... 

Several additional examples in the literature demonstrate the generality of the 1,1 trapping of nitrile ylides. Thus treatment of o-allyl-substituted imidoyl chloride 194 with triethylamine gave benzobicyclo[3.1.0]hex-2-ene 196. Irradiation of the closely related methyl-substituted azirine system 197 produced a mixture of endo- and ex o-benzobicyclohexenes 199 in quantitative yield.No detectable quantities of the isomeric 1,3-dipolar adduct were observed in either system. In the case of 197, the major product (exo-199) is the thermodynamically more favored exo isomer. 

Dioxolans. Some of the most interesting work in this series has come from Hoffmann s group in London. The debromination of aa -dibromo-ketones, e.g. (202), with a zinc-copper couple in dimethylformamide or dimethylacetamide gives 2-dimethylamino-4-methylene-l, 3-dioxolans (203). These compounds can be regarded formally as 1,3-dipolar adducts of dimethyl amides with oxyallyl (204), or as 1-amino-acetals. Their great... 

Trimethylsilyl-stahilized Azomethine Ylides. Some studies were aimed at using A-[bis(trimethylsilyl)methyl] amides and thioamides as precursors for azomethine ylides (eq 18). Amides and thioamides were treated with methyl triflate to form the corresponding methyl imidate or thioimidate. Fluoride mediated monodesilylation generated the corresponding azomethine ylides, which were trapped with methyl acrylate. The 1,3-dipolar adducts were not isolated but were oxidized by 2,3-dichloro-5,6-dicyano-/7-benzoquinone (DDQ) to furnish the p3rrole derivatives. With the amide (19, X = O), (20) and (21) were obtained in 25% and 13% yield, respectively. Only pyrrole (20) was obtained, in 41% yield, in the case of the thioamide (19, X = S). 

---