Abstraction-recombination path

Fig. 1. Interstellar formation scheme illustrating the CH, CH, C H and higher hydrocarbon cycle. The left side of the reaction cycle pertains to tenous clouds (Uj, 100 cm ), whereas the right hand side is more appropriate to areas where is present, i.e. dense molecular clouds (n 10 -10 cm" ). The thick arrows indicate assumed preferential reaction paths leading to the higher order hydrocarbons. The following processes are involved (v, e) photoionization (v, H) photodissociation (e, v) radiative recombination (H) (Hj, v) radiative association (e, H), (e, Hj) dissociative electron recombination. (Hj, H) hydrogen abstraction reaction (C, H) charge exchange (M, M ) metal charge exchange metal = Mg, Fe, Ca, Na,...

The fact that irradiation of disilanes la - 3a did not produce detectable products other than the corresponding silyl radicals indicates that the radicals produced photochemically undergo clean recombination to produce the starting silane, as shown in Scheme 5 path (b). An exception is the photochemical reaction of silyl mercurial compounds 4c, which produce the hydrosilane 6 [Scheme 5, path (c)], while the corresponding dimer 4a [Scheme 5, path (b)] is not detected. Apparently, for radical 4b, hydrogen abstraction to produce silane 6 occurs significantly faster than its dimerization to 4a. 

Figure 9 drastically simplifies the major reaction paths of alkyl-naphthalene components. Via H-abstraction and successive decomposition reactions, they can easily form, either naphthalenes with unsaturated side chains (vinyl, allyl or alkenyl side chains) or RSR and smaller decomposition products. The preferential radical attack on the alkyl side chain is in the benzyl position due to the weak hydrogen bond. This makes it easy to justify either the formation of RSR or the successive / -decomposition reaction to form vinylnaphthalene. The net result of the successive recombination and condensation reactions of these aromatic species is the formation of PAH of increasing molecular weight with a progressively lower hydrogen to carbon ratio. 

The reaction presumably involves the C—H functionalization via a three-step sequence consisting of (i) oxidation of the Fe(II) catalyst to an Fe(lll) imido radical by the alkyl azide substrate, (ii) intra-molecular H-atom abstraction causing the formation of an alkyl radical and an Fe(in) amide (Path la), and finally (iii) radical recombination to form the observed V-heterocycUc product. An alternative direct C—H bond insertion (Path 11) by the Fe(III) imido radical cannot be ruled out. Both hypothesized mechanisms necessitate the alignment of substrate C—H bond to be in close proximity to the reactive Fe-imido radical. A conformation requiring the C—H bond substrate to approach the imido radical opposite the chloride ligand is expected to enable rapid C—H bond functionalization. With this potentially promising synthetic strategy, azetidines, pyrrolidines, and piperidines can be prepared. 

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