Hydrogen Abstraction and Substitution

Alkyl hypochlorites and hypobromites are relatively stable molecules alkyl hypoiodites can only be prepared in situ, usually by the reaction of alcohols with metal acetates or oxides and iodine or by the reaction of alcohols with a hypervalent iodine compound and iodine.1319 Alkyl hypochlorites and hypoiodites can be utilized in reactions that parallel those of the organic nitrites (Barton reaction Section 6.4.2). For example, the photochemistry of the steroidal hypoiodite 497, prepared by the reaction of the corresponding alcohol with iodine oxide (I2O) generated from mercury(II) oxide and molecular iodine in situ, affords a new five-membered ring via an O I bond fission, 1,5-hydrogen abstraction and substitution (Scheme 6.244).1353... 

The photoreactions of iV-(trimethylsilyl)methyl- or iV-(tributylstannyl)methyl-substituted a-ketoamides resulted into the formation of complex mixtures including azetidin-2-ones and oxazolidinones with or without the trimethylsilyl or tributylstannyl moiety <2004JOC1215>. It was observed that the reaction of A-(trimethylsilyl)methyl-substituted a-ketoamides proceeded by competitive hydrogen abstraction and sequential single electron transfer (SET)-desilyla-tion pathways, whereas the reaction of iV-(tributylstannyl)methyl-substituted a-ketoamides preferred the sequential SET-destannylation pathway. 

In the past three decades, more sophisticated electrolysis conditions suited for the Kolbe dimerization, for hydrogen abstraction, for substitution, for rearrangement, and for other radical- or cation-induced reactions have accumulated for a number of carboxylic acids and have been used for many synthetic purposes. Most recently, in the field of the... 

Abstraction and substitution reactions also take place in alkenes. No definitive answer on intramolecular distributions of tritium, i.e. CT= versus —CHT— in substitution reactions in the gas phase has been obtained as yet but the indications are that there is no great selectivity for substitution of one or the other types of hydrogen (Lee et al., 1960a). Isomerization about the double bond following substitution has been discussed. 

Many side reactions compete efficiently with the HAS process. Under non-oxidizing conditions, the cyclohexadienyl radical 1 is rather long lived and can dimerize to 5 (radical-radical coupling) or disproportionate to cyclohexadiene 6 (H-atom abstraction) and substitution product 3 (Scheme 9.2). Long-lived radical 1 may also couple with radicals derived from the radical initiator [8]. These reactions can be considered as termination steps [9]. In addition, radical R can be reduced by hydrogen atom abstraction from the solvent to yield R-H, before the attack on the benzene core preventing the HAS reaction. 

The trichloromethyl radical is known to attack the rings of polycyclic aromatics (7). In order to correct for this, it was assumed that any enhanced reactivity of the methylarene relative to the parent arene reflected only hydrogen abstractions. A competition between these two species can thus be used to determine (allowing for statistical correction) the amounts of hydrogen abstraction and ring substitution for each arylmethane. Such an approach presumes that the methyl substituent does not electronically influence attack in the aromatic unit. At the time, however, both experimental (24) and theoretical (25)... 

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