Radical ring enlargement

Some free-radical ring enlargements are also known, an example being ... 

Ring Enlargement. The ring enlargement of five-membered cycles to six-membered ones is possible over pure metallic sites. This reaction is, however, generally slower than any of the cyclization processes. We believe that it proceeds by some kind of radical type bond shift mechanism (see Section III,A) rather than via ionic intermediates owing to some kind of acidic properties of platinum assumed by Lester (108). 

Though the fragmentation is one of the basic reactions of radical ions, this destructive reaction pathway seems to be synthetically useless at first sight. Nevertheless, the electron-transfer-induced bond cleavage can be specific and, for this reason, synthetically useful (e.g., for ring enlargement reactions). 

Cyclobutanes which are suitably substituted to form carbon-centered radicals in the a-, / - or y-position of the side chain undergo ring enlargement to cyclohexanones on treatment with tributyltin hydride and a radical initiator in refluxing benzene.115-119 A cascade of different ring closures and bond breakages follows the initial radical formation. To assure the necessary low concentration it is important to add the tributyltin hydride slowly over several hours to the refluxing mixture of the other components. 

Free Radical Induced Ring Enlargement of 2-(3-Halopropyl)cyclobutanones and l-(3-Halopropyl)-2-methylenecyclobutanes... 

Very recently, Bailey and Endo have enlarged the scope of the free-radical ring-opening polymerization.13 15> Several examples of their studies are described below. All the polymerizations are represented by the following general pattern of fundamental reaction. It is seen that the free-radical ring-opening by a P-scission mechanism is coupled with the addition of a free radical to the carbon-carbon double bond 13). 

The above examples of free-radical ring-opening polymerization, which have been explored by Bailey and Endo, produce polymers containing ketonic carbonyl and/or ester groups in the main chain. In addition, these cyclic monomers can be copolymerized with vinyl monomers by free-radical mechanism. Thus, the variety of the polymers produced by radical polymerization has been enlarged. 

The use of organic tin compounds leads to another aspect of the same topic. An effective alternative to the ring enlargement following ionic reaction mechanisms are those expansions which are radical chain processes. The stannylation... 

Analogous ring enlargement reactions, including a radical promoted Q incorporation, are known in steroid chemistry Irradiation of the 11/3-nitrite of 4-androstene-ll/3-ol-3,17-dione (VII/59) (Barton reaction) in toluene gave 18-nor-D-homo-4,13(17a)-androstadiene-llj8-ol-3,17-dione (VII/63) [33]. For Q radical rearrangements mediated by cobalamin, see [34] [35] [36] [37]. 

Radical initiated fragmentation reactions were used for the synthesis of ring enlarged lactones (already discussed in VII. 1). Several modifications and applications of this type are reported in the literature [75] [80] [111] [112] [113] [114] [115],... 

The search for other reactions in which a polymer-supported organotin hydride is capable of substituting BusSnH as radical source has led to the application of P—CH2CH2—SnBu2H for the ring enlargement of dichloromethylcyclohexadienones. Two out of twelve examples for the preparation of tropone ring systems are shown in... 

Detailed studies of H-atom abstraction reactions of cyclopropyl diketones U1), 2,2,5,5-tetramethyltetrahydrofuran-3-4-dione 130) cycloalkanediones57) and substituted benzils 121 , among others, have appeared. The cyclopropyl diketones reacted normally via semidione radicals without ring enlargement. 

A single electron transfer- (SET) induced ring enlargement has been found in the conversion of substituted 2-furylmethanols (10) into 6-hydroxytetrahydropyran-3-ones (12) by the action of the binary reagent PhI(0Ac)2-Mg(C104)2 it has been suggested that the reaction occurs via a furan radical cation (11) as illustrated in Scheme 10 [44],... 

Cyclopropanes adjacent to an electron-deficient centre X (carbon or heteroatom) undergo C3 - C4 ring enlargement into four-membered ring derivatives via routes involving cations, radicals or car bene intermediates. Furthermore, when the cyclopropanes also bear on the same carbon an electron-donor substituent Y they undergo a specific C3 - C4 ring expansion, important from the synthetic point of view, into cyclobutanone (or related) derivatives (equation 67). 

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