Intramolecular atom transfer

Viswanadhan and Matticc carried out calculations aimed at rationalizing the relative frequency of backbiting in these and other polymerizations in terms of the ease of adopting the required conformation for intramolecular abstraction (see 

More recent theoretical studies generally support these conclusions and provide more quantitative estimates of the Arrhenius parameters for the 

Cases of addition-abstraction polymerization have also been reported where propagation occurs by a mechanism involving sequential addition and intramolecular 1,5-hydrogen atom transfer steps (Section 4.4.3.4). 

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It has been known for some time that intramolecular atom transfer, or backbiting, complicates polymerizations of E (Scheme 4.40 - Section 4.4.3.1), VAc and VC (see 4.4.3.2). Recent work has shown that backbiting is also prevalent in polymerization of acrylate esters (Section 4.4.3.3) and probably occurs to some extent during polymerizations of most monosubstituted... 

The radical intermediates from Cr(II) reduction of alkyl halides can in principle be used synthetically, but have only seen limited attention to this point. co-Haloalkynes (bromides, iodides), in the presence of excess Cr(C104)2, undergo cyclization reactions to form exo-alkylidene cycloalkanes (equation 176)347. These reactions proceed by the radical cyclization of intermediate 42 onto the alkyne unit, which undergoes subsequent reduction by Cr(II) to give a hydrolytically unstable vinylchromium(III). Rings of four, five and six members can be formed. Alternatively, a-iodo esters undergo intramolecular atom transfer radical cyclizations onto alkynes or alkenes with catalytic or stoichiometric amounts of... 

If both the radical source and the radical trap are in the same molecule, then an intramolecular atom-transfer radical cyclisation reaction occurs. In 2001, Yang et al. reported the oxazoline 39d-magnesium-catalysed bromine atom-transfer radical cyclisation reaction of unsaturated p-keto ester 49 for... 

The primary alkyl radical, H , is anticipated to be more reactive and may show different specificity to the secondary or tertiary radical, T. In VAc and vinyl chloride (VC) polymerizations, the radical H appears more prone to tmdertake intermolecular (Sections 3.04.3.2.1(1) and 3.04.3.2.1(ii)) or intramolecular atom transfer reactions. 

In 2014, an intramolecular atom transfer radical cyclisation (ATRC) between a trichloroacetamide and an anisole or enol acetate moiety was applied to the synthesis of highly functionalized 2-azaspiro[4.5]decanes or morphan compounds, respectively, using the second-generation Grubbs catalyst 20. The procedure was further employed to construct the azatricyclic framework of the immunosuppressant FR901483 by the elaboration of its azatricyclic core [eqn (7.15)]. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

Intramolecular chain transfer due to the detachment of a hydrogen atom in the macroradical proper and movement of its valence to the end of the chain followed by the breaking of C—C bond. 

There are no proven examples of 1,2-hydrogen atom shifts this can be understood in terms of the stereoelectronie requirements on the process. The same limitations are not imposed on heavier atoms (e.g. chlorine). The postulate309 that ethyl branches in reduced PVC are all derived from cbloroethyl branches formed by sequential 1,5-intramolecular hydrogen atom transfers as described for PE (Section 4,4.3.1) has been questioned.,6,6 It has been shown that many of these ethyl branches are derived from dichloroethyl groups. The latter are formed by sequential 1,2-chlorine atom shifts which follow a head addition (Section 4.3.1.2). 

The vinylsilanes (e.g. 40, 41) do not readily homopolymerize. Forsyth et al.Mj explored the mechanism of grafting these monomers using dodecane as a model for PE. Their work suggests that multiple monomer units are attached through a sequence of addition and intramolecular hydrogen atom transfer steps by a mechanism analogous to that shown in Scheme 7.33 on page 394. 

Margerum et al (Ref 9) photolyzed solns of aromatic nitrocompds in 95% ale using an unfiltered 400 watt mercury lamp. No compd was found to be phototropic which did not have a nitrogroup ortho to a benzyl hydrogen. They hypothesized that an intramolecular process involving an H-atom transfer was operative ... 

Wettermark (Refs 10, 11 12) studied o-nitrotoluene and dinitrotoluene, flash photolyzed in w and ale. He observed transients, and noted that the absorption spectra was a function of pH. He concluded that intramolecular H-atom transfer was involved... 

A true intramolecular proton transfer in the second step of an azo coupling reaction was found by Snyckers and Zollinger (1970a, 1970b) in the reaction of the 8-(2 -pyridyl)-2-naphthoxide ion (with the transition state 12.151). This compound shows neither a kinetic deuterium isotope effect nor general base catalysis, in contrast to the sterically similar 8-phenyl-2-naphthoxide ion. Obviously the heterocyclic nitrogen atom is the proton acceptor. 

This oxidation of DMSO is catalyzed by Ag+ cations. Kinetic and infrared spec-trometric evidence fits a mechanism where DMSO coordinates rapidly with Ag+ through its oxygen atom. The oxidation of this complex by Ce4 + then constitutes the slow step. The Ag2+ adduct would then undergo an intramolecular electron transfer in a fast step resulting in the oxidation of DMSO. 

The predominating type of nonlinearity in polyethylene appears to consist of short chain branches three or four chain atoms in length formed by intramolecular chain transfer as follows ... 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

The success of such reactions depends on the intramolecular hydrogen transfer being faster than hydrogen atom abstraction from the stannane reagent. In the example shown, hydrogen transfer is favored by the thermodynamic driving force of radical stabilization, by the intramolecular nature of the hydrogen transfer, and by the steric effects of the central quaternary carbon. This substitution pattern often favors intramolecular reactions as a result of conformational effects. 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

Lukeman, M. Wan, P. A new type of excited-state intramolecular proton transfer proton transfer from phenol OH to a carbon atom of an aromatic ring observed for 2-phenyl-phenol. J. Am. Chem. Soc. 2002, 124, 9458-9464. 

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