7- endo-type radical addition

Acyl selenides serve as acyl radical precursors when treated with tin radicals [43], In the following reaction (Scheme 6.25), a cydoheptanone fused to an indanol skeleton is prepared via a three-component reaction [44], The sequential addition of an acyl radical to two molecules of methyl acrylate followed by a 7-endo-type radical addition account for the annulation. 

In bicyclohexenyl radicals additionally substituted at C-6, an endo - exoisomerization was found to proceed with a rate comparable to the walk rearrangement (120). If one starts with endo-78-3-d (R = CH2OH), the reaction product consist of a mixture of exo-78-2- and -3-d. Both reactions, walk rearrangement and endo - exo isomerization, have been assumed to involve an intermediate cyclopentadienylmethyl radical of type 79. In view of MO considera-... 

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

Stereospecificity manifests itself in the dimerization of a diene with a diene. The double bond that remains free may deviate from the ring formed (exo configuration) or may approach it (endo configuration). Endo condensation is the predominant pathway in the case of ion radical reactions (Scheme 6-17). As seen, the charge distribution in the reactants dictates the head-to-tail pathway of the reaction. For the cation radical, the position selectivity at the C(l) atom is 100%, regioselectivity being 0%, whereas at the C(4) atom the position selectivity is 0% and regioselectivity is 100%. In other words, only the addition of the D+ -C(l) + D°-C(4) type is observed (symbol D° refers to a neutral diene and D+ to a diene in cation radical form). 

Another important factor to deduce the involvement of radicals in a transition metal-catalyzed process is the integrity of stereocenters. In oxidative addition or Sr 2-type processes the stereochemical information - retention or inversion, respectively - is preserved for optically active substrates like sec-butyl bromide (Sect. 2.2), while racemic products are observed when radical intermediates are generated. On the other hand, stereochemical convergence is observed for strongly biased diastereomeric substrates, such as exo- and endo-norbomyl substrates 25 (Fig 9) The reactions occur almost exclusively at the exo-face of the norbomyl... 

Radicals of the norbornyl (bicyclo[2.2. l]hcptyl) type also show high facial selectivity in addition reactions to alkenes 34,54. In solvomercuration/rcductivc alkylation reactions of norbornene34 or 2-norbornanone hydrazones54 it is found that 2-norbornyl radicals substituted at the radical center- 4 or in the / -position34 yield only e.vo-addition products. The addition of alkenes, therefore, proceeds more selectively than hydrogen, or other atom abstraction reactions, in which small amounts of endo attack are also found (see Section D.2.2.). 

Recently, Endo and coworkers have developed novel sulfonium type initiators that can initiate polymerization either upon irradiation or upon thermal treatment. In addition, these photoinitiators are shown to be functional for both cationic and radical polymerizations. This dual activity is particularly important in hybrid curing systems for coatings and adhesions [28]. Scheme 11.6 contains the general steps for the synthesis of these photoinitiators from sulfides. 

Cyclobutanes from the addition of 1-chloro-l-fluoroethylene to chlorotrifluoro- or tetrafluoro-ethylene, of 2-fluoropropene to tetrafluoroethylene, and of 1-chloro-2,2-difluoroethylene to vinyl fluoride or propene have been claimed to be anaesthetics. Diels-Alder addition of cyclopentadiene to acids of the type, trans-RpCHiCH COaH (Rf = CHaF, CHFa, CFs, C2F5, or n-CsFv) and a number of related esters and other derivatives, at 25 °C, results in predominant formation of adduct witii an endo fluoroalkyl group. The addition of cyclopentadiene to the 1,1-difluoroethylenes, CFalCFa, CFa CF CFs, CFa CCla, CFa C(CF3)2, CFaiCCl-CFaCl, and CFa.CFH has been recorded and the fiee-radical bromina-tion of the resulting norbomenes studied. Buta-1,3-diene and trifluorovinyl-sulphur pentafluoride yield essentially a mbcture of cis- and rmns-cyclobutanes (116), with little or no cyclohexene formation the olefin resembles perfluoropropene which yields ca. 5% of cyclohexene, in this respect. Perfluoroindene, which adds... 

The addition of thiyl radicals to alkenes and alkynes can also occur in an intramolecular fashion to lead to sulfur heterocycles. Surzur and co-workers [33, 34] have shown that the pent-4-enylthiyl radical yields both five- and six-mem-bered rings. The six-membered rings were formed in much higher yield than the five-membered rings (ca. 10 1). Since these cyclizations are reversible, the product distribution reflects a thermodynamic control (Scheme 5). The same type of reactions can be initiated by photolysis of disulfides, as demonstrated by Maki and Sako in the synthesis of cepham and penam derivatives [35]. The photoreaction of (5) is concentration dependent at low concentrations cepham derivatives are formed almost exclusively via 6-endo cyclization, whereas at high concentrations penam derivatives predominate. 

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