Reactions trapping

In general bromination of 20-ketones is directed to the introduction of functionality at C-21. However, on occasion 17-bromo compounds are required for dehydrobromination to A -20-ketones, although these are generally obtained in other ways. Kinetic enolization of a 20-ketone gives the A °-enol, whereas the thermodynamic product is the A kjsomer. An interesting enolate trapping reaction has been used recently to prepare 16-methyl-A -20-ketones ... 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

Polymer formation during the Kharasch reaction or ATRA can occur if trapping of the radical (123), by halocarbon or metal complex respectively, is sufficiently slow such that multiple monomer additions can occur. Efficient polymer synthesis additionally requires that the trapping reaction is reversible and that both the activation and deactivation steps are facile. 

The incorporation of the trapping reaction changes the stoichiometry of the net reaction. The result is an induced reaction. For example, the oxidation of vanadium(IV)... 

In this section it is intended to discuss the role of the solvent, the base electrolyte and the other reagents which are themselves not electroactive but which are added to vary the pH of the medium, to trap reaction intermediates or to vary the activity of the substrate, an intermediate or the product. It would seem correct, however, to discuss the various... 

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

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 ... 

The existence of 6 was proved by trapping reactions, using the corresponding lithium(fluorosilyl)phosphanylide as precursor. Thus, thermolysis of the latter at 80-120°C in the presence of /Bu Si—N3, butadiene, isobutene, and lithium-tri-ferf-butylsilylphosphanide led to the products 8-11 (Scheme 2). 

Scheme 2. Trapping reactions of the transient phosphasilene /Bu2Si=P(Si/Bu3) (6), leading to 8, 9,10, and 11.

Scheme 7).97,98 These doubly bonded species can be identified from their trapping reactions. 

Stannene 1 was found to be an extremely air-sensitive compound, which was identified only by its trapping reactions.99 Replacement of the bis(tri-methylsilyl)methyl groups with the bulky aromatic group 2,4,6-triisopropylphenyl led to the isolation of the stable stannene 2 as its diethyl ether... 

In extreme cases where all electron transfer steps are reversible and the water trapping reactions are very slow, the charge is distributed over the guanines according to the thermodynamic stabilization. From these experiments one cannot deduce the influence of the sequence on the hole transfer rate. Therefore, using a chemical assay of this type leads to results that have to be discussed with great care. 

The G doublet and triplet effectively functions as a thermodynamic sink in DNA-mediated hole transport. However, the rate determining step of hole trapping at guanine clusters and the rate of hole trapping are not well understood. Furthermore, hole transfer between the donor and acceptor should compete with the hole trapping reaction the relative rate of hole trapping versus the hole transfer rate determines the overall efficiency for the hole transfer. 

Until now, none of these reactions has ever lead to stable monomolecular stannylenes. Nevertheless, trapping reactions have confirmed the presence of intermediate Sn(II) species 7,80,81). 

In the course of the tempestuous development of organophosphorus chemistry, interest has only recently been focused on compounds of formally quinquevalent phosphorus having coordination number 3, such as 1, 2, or 3, although one of the other species of this kind has long been postulated as reactive intermediate of solvolysis of phosphorylation reactions. Definite evidence of even proof of the existence of such coordinatively unsaturated phosphorus compounds, however, has been obtained only recently in mechanistic studies, by trapping reactions with suitable cycloaddends, or actually by direct isolation. 

In the case of 22b-e, the butadienes 24b-d and the hexatriene 24 e, respectively, are also obtained on trapping with cinnamaldehyde, the 1,3-diene 24a is even the sole reaction product. It is quite obvious that the olefins 24 are secondary products of the trapping reaction of 9 arising by photofragmentation of 22. The other product is phenyldioxophosphorane (23) which also numbers among the short-lived compounds of quinquevalent phosphorus with coordination number 3 (see Sect. 3.1). 

Trapping reactions of benzoylmethyleneoxophosphorane 39 a with carbonyl compounds dispel any remaining doubts as to the existence of acylated phosphenes. Unlike the diphenylmethyleneoxophosphorane 9, whose P/C double bond participates in cycloadditions, compound 39 a acts as a hetero-1,3-diene and undergoes [4 + 2]-cycloaddition with aldehydes and ketones 10 I7,35> it may again be assumed that the reaction is a two-step process involving 55 as intermediate. 

The chemical reactions of sulphonyl nitrenes include hydrogen abstraction, insertion into aliphatic C—H bonds, aromatic substitution , addition to olefinic double bonds, trapping reactions with suitable nucleophiles, and Wolff-type rearrangement. Hydrogen-abstraction from saturated carbon atoms is usually considered to be a reaction typical of triplet... 

The formation of silaneselone 57 was evidenced by the trapping reaction with mesitonitrile oxide leading to the corresponding cycloadduct 58 and was also supported by the observation of a remarkably downfield 29Si chemical shift (8Si = 174) indicative of the Si=Se double bond of 57. Although this direct selenation of silylene 55 with an equimolar amount of selenium was not reproducible, the use of excess amount of elemental selenium resulted in the formation of a new cyclic diselenide, diselenasilirane 59, as a stable compound (8Si = -44 and... 

Under suitable conditions, this can be a useful preparative method for cyclopropanes another preparative trapping reaction of CC12 is its electrophilic attack on phenols in the Reimer-Tiemann reaction (p. 290). 

By now, a lot of different reaction routes to the novel silicon-it-compounds have been explored. Because of the high reactivity of these compounds an enormous variety of products was found by trapping reactions with suitable partners. In addition, many theoretical papers on this topic have contributed to construct a well equipped building on a solid foundation. 

Much cleaner and synthetically useful are, however, the rapid trapping reactions with dienes and heterodienes [4], carried out by the copyrolysis technique. In this way a variety of five membered unsaturated silicon heterocycles may be produced. 

Scheme 13 Trapping reaction of an intermediate arylphosphole oxide...

An investigation of the wound reaction of Caulerpa spp. showed that the aldehyde 55 found as minor component in algal extracts is indeed an intermediate in a wound-activated transformation of 54. This sesquiterpene is degraded within seconds after tissue disruption to form the reactive aldehydes 55 and 59-64 that were characterized after trapping reactions (Scheme 17) [121]. 

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