Radical chain reaction intramolecular

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

In conclusion, it appears that allyl or polyenyl radicals are much less reactive than alkyl radicals, which restricts the use of polyenes in intermolecular radical chain reactions to simple two-step processes. Allyl radicals are, however, reactive enough to partake in a variety of intramolecular reactions. 

Reviewing now the last four sections, it is obvious that the major problem in radical chain reactions involving dienes or polyenes is the low reactivity of the diene (or polyene) adduct radicals. This allows for the occurrence of allyl radicals in intramolecular reactions but poses a major problem in intermolecular radical chain reactions. The obvious solution to this problem is to use methods in which radicals are produced stoichiometrically and not... 

Curran2 has reviewed recent applications of the tin hydride method for initiation of radical chain reactions in organic synthesis (191 references). The review covers intermolecular additions of radicals to alkenes (Giese reaction) as well as intramolecular radical cyclizations, including use of vinyl radical cyclization. 

The free-radical construction of C—C bonds either inter- or intramolecularly using a hydride as mediator is of great importance in chemical synthesis. The propagation steps for the intermolecular version are shown in Scheme 2. For a successful outcome, it is important (i) that the R sSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene and (ii) that the alkyl radical reacts faster with alkene (to form the adduct radical) than with the silane. In other words, for a synthetically useful radical chain reaction, the intermediates must be disciplined. Therefore, in a synthetic plan one is faced with the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and, often, the concentration of silane provides the variable by which the products distribution can be influenced. 

Until now, the chemistry of radicals on solid supports has been investigated mostly in respect to intramolecular radical cyclizations and radical chain reactions. One reason for refraining from free radical transformations is the chemical nature of the polystyrene with its abundance of benzylic positions that are prone to H-radical abstraction and oxidation. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

The disproportionation reaction of the free radical chain can generate the monomer as a successive process. There are, however, some other issues regarding the propagation for free radical chain reactions. In addition to the "regular" propagation step, different reactions may occur in a so-called transfer step. In this step, the free radical chain reacts with another molecule and generates a different radical chain and a new polymeric molecule. There are two possible types of transfer reactions. The transfer step can be an intermolecular chain transfer or an intramolecular chain transfer. An example of an intermolecular chain transfer is... 

Intramolecular cyclization of unsaturated acyl chlorides. Tributyltin hydride reduces acyl chlorides to aldehydes by a free-radical chain reaction involving an acyl radical (I, 1193). Cekovic10 now finds that treatment of acyl chlorides with a double bond in the S- or 6-position with tributyltin hydride (azobisisobutyronitrile initiation) gives cyclohexanone derivatives. Thus 5-hexenoyl chloride (1) is converted into cyclohexanone (2), and citronelloyl chloride (3) is converted into menthone (4). 

The observed first-order rate constant for decomposition is independent of the initial concentration and experiments to distinguish an intramolecular mechanism from a free-radical chain reaction rule out the former. It is rather more difficult to establish whether the a-H atom is transferred to Ta initially or directly to the more basic axial alkyl group (Scheme 1). 

An interesting aspect of the radiolysis of longer-chain alkyl bromides and chlorides is the high yield of isomerization (G(isomer) > 6 pmol e.g., 1-chloroalkane isomerizes to 2-chloroalkane). These reactions are attributed to free-radical chain reactions, which involve intramolecular shift of a halogen atom. 

The main feature of the pyrolysis spectra of polyethylene, polypropylene and isoprene can be interpreted on the basis of simultaneous breakage of carbon-carbon bonds in the main polymer chain. For a more detailed interpretation of the pyrolysis spectrum it is necessary to assume a series of radical-chain reactions in which intramolecular hydrogen abstraction plays an important role. 

One proposed mechanism involved an intramolecular rearrangement, while a second involved a free radical chain mechanism composed of the following sequence of elementary reactions ... 

---