Trapping reagent

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

Bis(tnfluoromethyl)-substituted nitrile ylides undergo dimerization reactions in the absence of trapping reagents [143, 168, 170] (equation 38)... 

Also, reactive silylene complexes of iron and chromium can be generated at low temperatures and subsequently derivatized by trapping reagents. In THF as solvent, first labile THF adducts are formed, which are converted to the more stable HMPA adducts. The THF complexes dimerize above —40 °C with loss of THF. The silylene complexes can be utilized for reactions if they are generated in the presence of reagents like dimethylcarbonate. The resulting reaction products... 

In related work, the reactions of hydrogen peroxide with iron(II) complexes, including Feu(edta), were examined.3 Some experiments were carried out with added 5.5"-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent fa so-called spin trap) for HO. These experiments were done to learn whether HO was truly as free as it is when generated photochemically. The hydroxyl radical adduct was indeed detected. but for some (not all) iron complexes evidence was obtained for an additional oxidizing intermediate, presumably an oxo-iron complex. 

If the two trapping reagents give a common product, then other considerations come into play. Imagine that the intermediate reacts as follows ... 

SsO may also function as a precursor for S2O units. However, it should be noticed that free S2O has never been detected directly in liquid solutions and that the isolated products most probably arise from a reaction of the S2O precursor with the trapping reagent since the reaction temperature is always 20 °C (e.g., by a transition-metal-induced retro-Diels-Alder reaction) [52, 53]. An exception may be the thermal decomposition of the substituted tetrathiolane-2,3-dioxide shown in Scheme 2 this compound evidently... 

Sulphides are known to be effective trapping reagents for nitrenes... 

References to the silenes so prepared since 1985 are listed in Table I. None of these silenes was stable, most undergoing head-to-tail dimerization to give 1,3-disilacyclobutanes in the absence of trapping reagents. Some interesting spontaneous silene-to-silene rearrangements were observed,52 which will be described in Section IV.E. 

Some unusual behaviour was displayed by the benzodisilacyclobutane 84 as described by Ishikawa et al.95 When thermolyzed, it appeared to form the quinodimethane bis-silene species 85 shown in Scheme 13, as confirmed by trapping reactions with f-butyl alcohol, alkynes, or aldehydes, all of which added in a 1,4-manner (see Scheme 13). In the absence of a trapping reagent, 85 decomposed, but not to 86 as claimed earlier.95 ... 

In addition to undergoing cycloaddition reactions with alkenes and al-kynes, silenes readily undergo cycloaddition reactions with heteroatom multiple bonds such as C=0 and C=N, most commonly when the trapping reagent for the silene is either an aldehyde, ketone, or imine. In many... 

The stability of molecules depends in the first place on limiting conditions. Small, mostly triatomic silylenes and germylenes have been synthesized successfully at high temperatures and low pressures, 718). Their reactions can be studied by warming up the frozen cocondensates with an appropriate reactant, whereas their structures are determined by matrix techniques 17,18). In addition, reactions in the gas phase or electron diffraction are valuable tools for elucidating the structures and properties of these compounds. In synthetic chemistry, adequate precursors are often used to produce intermediates which spontaneously react with trapping reagents 7). The analysis of the products is then utilized to define more accurately the structure of the intermediate. 

Typical carbene reactions, such as norcaradiene formation with solvent (20) must be accepted, as must the occasional cyclopropanation of the trapping reagents. 

A somewhat different result is obtained on reaction of methyleneoxophosphorane 9 (generated thermally from 7) with a,p-unsaturated ketones. Owing to the unusual thermal stability of 713,24> the reaction is carried out in a melt of the trapping reagents at 125 °C. Not surprisingly, the same product spectrum as for photolysis is initially observed, i.e. formation of oxaphosphetanes 22b-d (14-26%) and of... 

For example, in the case of intramolecularly base-stabilized silanone 4 it was trapped by excess hexamethylcyclotrisiloxane to give the corresponding insertion product 5 but isolated as a trimer 6 in the absence of such a trapping reagent (Scheme 2).17... 

Butadiene telomerization using nitroethane as a trapping reagent is applied to the total synthesis of the natural product, recifeiolide, where the secondary nitro group is converted into the ketogroup by the Nef reaction, and the terminal double bond is converted into the iodide via hydro alumination (Scheme 5.6).71... 

Extension to carbocyclization of butadiene telomerization using nitromethane as a trapping reagent is reported (Eq. 5.48).72 Palladium-catalyzed carbo-annulation of 1,3-dienes by aryl halides is also reported (Eq. 5.49).73 The nitro group is removed by radical denitration (see Section 7.2), or the nitroalkyl group is transformed into the carbonyl group via the Nef reaction (see Section 6.1). 

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