Synthesis radical reactions

Miscellaneous Reactions. Dy(OTf)3 has also be used for the synthesis of (V-substituted pyrroles,the synthesis of 2,3-unsaturated glycopyranosides, for heterocycle synthesis, radical reactions, and polymer synthesis. 

Frcc-Radical Reactions. Eree-radical reactions of maleic anhydride are important in polymeri2ations and monomer synthesis. Nucleophilic radicals such as the one from cyclohexane [110-82-7] serve as hydrogen donors that add to maleic anhydride at the double bond to form cyclohexylsuccinic anhydride [5962-96-9] (20) (63). 

Synthesis of heterocycles by forming C—X bonds by radical reactions is not a generally applicable method, and seems not to be useful for making small rings. However, the attack of thiol radicals on double bonds can be a practical synthetic route, such as in the conversion of 1-hexene-7-thiol to thiepane (Section 5.17.3.3.1). 

Pyrido[3,4-d]pyrimidine-2,4-dione synthesis, 3, 215 Pyridopyrimidines, 3, 201 iV-alkylations, 3, 206 biological activity, 3, 260-261 1-electron reductions, 3, 207 IR spectra, 3, 204 mass spectra, 3, 204 MO calculations, 3, 204 NMR, 3, 202, 203 nucleophilic substitution, 3, 213 8-nucleosides synthesis, 3, 206 physical properties, 3, 201-205 protonation, 3, 206 radical reactions, 3, 215 reactions with water, 3, 207 reduced... 

HOFMANN - LOEFFLER FREYTAG Pyrrolidine Synthesis Synthesis of pyrrolidines or piperidines from N-haloamlnes (free radical reaction). 

Free-radical reactions in the synthesis of diketopiperazines and other cyclic derivatives of a-aminoacids 97CRV53. 

Application of radical reactions to organic synthesis has recently received much atrendon, and various important reacdons have been discovered in this field. Alkyl halides, sulfides, seleniJes, and thiocarbonyl compounds have been used as precursors to alkyl radicals. Some examples are illustrated in Scheme 7.18. ... 

A number of techniques for the preparation of block copolymers have been developed. Living polymerization is an elegant method for the controlled synthesis of block copolymers. However, this technique requires extraordinarily high purity and is limited to ionically polymerizable monomers. The synthesis of block copolymers by a radical reaction is less sensitive toward impurities present in the reaction mixture and is applicable to a great number of monomers. 

We had a brief introduction to radical reactions in Section 5.3 and said at that time that radicals can add to alkene double bonds, taking one electron from the double bond and leaving one behind to yield a new radical. Let s now look at the process in more detail, focusing on the industrial synthesis of alkene polymers. 

On the basis of the examples addressed thus far, it is clear that radical reactions can accomplish manifold transformations in organic synthesis. One of the outstanding achievements of synthetic radical chemistry is the development of synthetic strategies based on controlled, tandem radical cyclizations. The efficiency of such strategies is exemplified in the substantial and elegant synthetic work of D. P. Curran and his group.54 The remainder of this chapter will address the concise total syntheses of ( )-hirsutene [( )-1]55 and ( )-A9(12)-capnellene [( )-2]56 by the Curran group. 

A most attractive feature of radical reactions that recommends their use in the synthesis of complex molecules is that steric crowding, particularly on the radical center, is tolerated in many instances. Indeed, radical reactions are ideally suited for the con-... 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The majority of sequential radical reactions deal with cyclizations as the key steps. The constructions of carbocycles, oxygen, and nitrogen heterocycles using (TMSlsSiH as a mediator are many and represents the expansion and importance of these synthetic approaches. For example, Nicolaou and coworkers found that (TMSlsSiH serves as a superior reagent in the radical-based approach toward the synthesis of azadirachtin, an antifeedant agent currently used as an insecticide, and in other related systems. ° ° Here below we collected a number of reactions mostly from the recent work in the area of intramolecular reactions. 

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

A thorough consideration of mechanisms of formation of the organometallic products led to the conclusion " that the j5-decay itself must be the cause of the molecule formation. Neither purely mechanical collisiona substitution, nor thermal chemical reactions, nor radical reactions, nor radiation-induced reactions seem to be responsible for the synthesis reactions. 

A general synthesis of spiro-piperidinyl heterocycles 116 was obtained from intermediate 115 via an efficient radical reaction <96TL(37)5233>. 

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