Cyclic trapping, reactive

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

Accordingly, cyclic nitronates can be a useful synthetic equivalent of functionalized nitrile oxides, while reaction examples are quite limited. Thus, 2-isoxazoline N-oxide and 5,6-dihydro-4H-l,2-oxazine N-oxide, as five- and six-membered cyclic nitronates, were generated in-situ by dehydroiodination of 3-iodo-l-nitropropane and 4-iodo-l-nitrobutane with triethylamine and trapped with monosubstituted alkenes to give 5-substituted 3-(2-hydroxyethyl)isoxazolines and 2-phenylperhydro-l,2-oxazino[2,3-fe]isoxazole, respectively (Scheme 7.26) [72b]. Upon treatment with a catalytic amount of trifluoroacetic acid, the perhydro-l,2-oxazino[2,3-fe]isoxazole was quantitatively converted into the corresponding 2-isoxazoline. Since a method for catalyzed enantioselective nitrone cycloadditions was established and cyclic nitronates should behave like cyclic nitrones in reactivity, there would be a good chance to attain catalyzed enantioselective formation of 2-isoxazolines via nitronate cycloadditions. 

Nitroalkenes are also reactive Michael acceptors under Lewis acid-catalyzed conditions. Titanium tetrachloride or stannic tetrachloride can induce addition of silyl enol ethers. The initial adduct is trapped in a cyclic form by trimethylsilylation.316 Hydrolysis of this intermediate regenerates the carbonyl group and also converts the ad-nitro group to a carbonyl.317... 

Reaction of the carbonyl complex 26 with the mercury diazomethane 27 gives the highly reactive 17e intermediate carbyne complex 28 which dimerizes to form the / -biscarbyne complex 30. In this case, the intermediate terminal carbyne complex 28 has been trapped by reaction with the mercury diazomethane 29 to form the cyclic vinylidene complex 31. 31 was also characterized by a single crystal X-ray structure analysis. 

Photochemical transformations of cyclic and short chain polysilane oligomers have been intensively investigated (39). Irradiation of these materials in the presence of trapping reagents, such as silanes or alcohols, has suggested that substituted silylenes and silyl radicals are primary reactive intermediates. The former have been... 

The desUylation strategy has been used for the cycloaddition of the parent thiocarbonyl yhde la with aldehydes and reactive ketones. The product obtained using A-methyl-3-oxoindolinone as the trapping agent corresponds to the spiro-cyclic compound 125 (168). Thioketene (5)-methylide (127) was reported to react with aromatic aldehydes and some ketones to furnish 2-methylene-substituted 1,3-oxathiolanes (128) (51) (Scheme 5.42). 

Rate constants for quenching of 1-7 by methanol and acetic acid in hexane solution from fluorescence quenching and quantum yield data are 10 M l-s-l-. Limiting quantum yields for adduct formation are 0.1. The observation of reactions of protic solvent with 1-7 but not 1-t may reflect the longer lifetime and/or enhanced reactivity of the cyclic molecule. While photo-induced nucleophilic addition is a common reaction of aryl olefins, it is normally observed to occur only under conditions of electron-transfer sensitization (139). Under these conditions, it is the aryl olefin cation radical which undergoes nucleophilic attack. The reaction of 1-7 with protic solvents appears to be the only reported example of nucleophilic trapping of an aryl olefin it,it singlet state (140). 

The cyclization of allyl silyl amine 697 by hydrosilylation led to silaazetidine 698, which was subjected to flash vacuum thermolysis at 700-900°C at 10-4 hPa313. The silanimines 699 and 700 themselves were too reactive to be observed by high resolution mass spectrometry of the reaction mixture, but their cyclic dimers, the cyclodisilazane 701 and 702 and a trapping product with t-BuOH 703, were definitely confirmed... 

The conversion of enol ether 80 to cyclic ketal 83 in water in 12% yield exemplifies the chemoselectivity possible with 14D9.79 Although 83 is the normal product of the acid-catalyzed hydrolysis of 80 in organic solvents, it is never observed in water because the highly reactive oxocarbenium intermediate is rapidly trapped by the solvent to give ketone 82 (via hemiacetal 81) as the sole product. The ability of the antibody to protect the reactive oxonium ion intermediate from hydrolysis and partition it toward a product that is not typically observed under these conditions (i.e., 83) mimics the capabilities of rather sophisticated enzymes. Extension to other reactions involving reactive, water-incompatible intermediates can be easily imagined. 

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