Radical chemistry

More appropriately, these four radical-related name reactions belong to the previous volume (Volume 2) on Name Reactions for Functional Group Transformations. They were, regrettably, not included in that volume due to logistical reasons. Because of their importance in organic synthesis, instead they are added to this volume (Volume 3) on Name Reactions for Homologations-1. After all, it is better to be late than never. 

The Baiton-McCombie reaction describes the overall reduction of a 1°, 2°, or 3° alcohol (1) to the parent alkane (3), effectively replacing -OH with -H. Reduction is achieved via pre-activation of the hydroxyl group as its thiocarbonyl derivative (2) followed by reaction with a reducing agent such as tributyltin hydride. The reductive process involves a radical-mediated transformation. 

In their seminal paper, Barton and McCombie describe a method to deoxygenate secondary alcohols through a radical chain mechanism. This process was presented as an alternative to the standard conditions of derivatization of an alcohol to a tosylate/mesylate followed by reduction. Such polar processes are problematic with hindered carbon centers and can lead to rearrangements and/or eliminations if carbocationic intermediates are involved. 

A radical initiator, such as azobisisobutyronitrile (AIBN, 7) decomposes homolytically under reaction conditions to generate 8 which abstracts a hydrogen from tributyltin hydride (9) creating a tin-centered radical 10. The tin radical attacks at the sulphur atom of the thiocarbonyl derivative 2, generating intermediate 11 which collapses leaving tributyltin xanthate 12 (which can further decompose to 0=C=S and 13) and alkyl radical 14. Intermediate 14 can abstract another atom of hydrogen from tributyltin hydride to generate the desired alkane product 3, and replenish the pool of tin radical 10. 

Fragmentation of 11 is known to be the rate determining step and, depending on its lifetime, this radical can be captured intramolecularly if a proximal acceptor group is present.  

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B. Giese Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds (Pergamon Press NY) 1986 Bull. Soc. Chirn. Fr. 1990, 127,675 Tetrahedron 1981, 37, 3073 Tetrahedron 1987, 43, 3541 Advances in Free Radical Chemistry 1990, 1, 121. 

Both solvent-iaduced swelling and oxygen inhibition ate characteristic of all cross-linking negative resists based on free-radical chemistry. 

Sulfochlorination of Paraffins. The sulfonation of paraffins using a mixture of sulfur dioxide and chlorine in the presence of light has been around since the 1930s and is known as the Reed reaction (123). This process is made possible by the use of free-radical chemistry and has had limited use in the United States. Other countries have had active research into process optimization (124,125). 

Entries 4 and 5 point to another important aspect of free-radical reactivity. The data given illustrate that the observed reactivity of the chlorine atom is strongly influenced by the presence of benzene. Evidently, a complex is formed which attenuates the reactivity of the chlorine atom. This is probably a general feature of radical chemistry, but there are relatively few data available on solvent effects on either absolute or relative reactivity of radical intermediates. 

M. Birkhofer, H.-D. Beckhaus, and C. Ruchardt, Substituent Effects in Radical Chemistry, Reidel, Boston, 1986. J. Fossey, D. Lefort, and J. Sorba, Free Radicab in Organic Chemistry, John Wiley Sons, Chichester, U.K., 1995. 

Aliphatic compounds are straight chain or acyclic compounds and are characterized by addition and free-radical chemistry. 

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

The free-radical chemistry of fluoroalkanesulfenyl chlorides with hydrocarbons was also investigated [S, 9], Depending upon the structures of the sulfenyl chloride and the hydrocarbon, these reactions yield as major products up to three of the following four types of organic compounds thiols, disulfides, sulfides, and chlorohydrocarbons (equation 6), Perfluoroisobutanesulfenyl chloride is unique m that the only major products detected are the thiol and chlorohydrocarbon [ ] (equation 6) (Table 3). 

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. 

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

The efficiency of these inhibitors may depend on reaction conditions. For example the reaction of radicals with stable radicals (e.g. nitroxides) may be reversible at elevated temperatures (Section 7.5.3) triphenylmethyl may initiate polymerizations (Section 7.5.2). A further complication is that the products may be capable of undergoing further radical chemistry. In the case of DPPH (22) this is attributed to the fact that the product is an aromatic nitro-compound (Section 5.3.7). Certain adducts may undergo induced decomposition to form a stable radical which can then scavenge further. 

The general features of the penultimate model in what have become known as the explicit and implicit forms are described in Section 7.3.1.2.1. Evidence for remote unit effects coming from small molecule radical chemistry and experiments other than copolymerization is discussed in Section 7.3.1.2.2. In Sections 7.3.1.2.3 and 7.3.1.2.4 specific copolymerizations are discussed. Finally, in Section 7.3.1.2.5, we consider the origin of the penultimate unit effects. A general recommendation is that when trying to decide on the mechanism of a copolymerization, first consider the explicit penultimate model."... 

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization arc many (Chapter 1). However, one of the often-cited "problems" with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of "structure irregularity" that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers for problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

The data from Beckwith s work given in Table 10-9 look very confusing but, as discussed in the four papers by Beckwith s group (see footnote a) in Table 10-9) and in additional comments in Galli s review (1988), they can all be explained on the basis of our present knowledge of aryl radical chemistry, with the exception of the endo cyclization of the 2-(7V-2,-propenylsulfamoyl)-benzenediazonium ion in Scheme 10-81. 

A few words about the pattern of organization of this chapter. The theoretical studies are included in Section II. In Section VII some interesting chain reactions involving sulfonyl radicals are discussed, although often one of the propagation steps is treated earlier in Section IV or V. Finally, some general concepts of free radical chemistry are introduced at appropriate points throughout the review without any reference. 

Although the free-radical chemistry of organocobaloximes is an interesting and useful reaction of some potential in organic synthesis, the validity of the label SH2 and SHi for... 

The first reports of the observation of transient emission and enhanced absorption signals in the H-n.m.r. spectra of solutions in which radical reactions were taking place appeared in 1967. The importance of the phenomenon, named Chemically Induced Dynamic Nuclear Spin Polarization (CIDNP), in radical chemistry was quickly recognized. Since that time, an explosive growth in the number of publications on the subject has occurred and CIDNP has been detected in H, C, N, and P as well as H-n.m.r. spectra. Nevertheless, the number of groups engaged in research in this area is comparatively small. This may be a consequence of the apparent complexity of the subject. It is the purpose of this review to describe in a quahtative way the origin of CIDNP and to survey the published applications of the phenomenon in... 

The strenuous efforts made in the study of CIDNP over the past few years have produced a basic quantitative theory of the effect. This has been extensively tested, chiefly in well-understood areas of radical chemistry, and found to be satisfactory. There still remains room for theoretical development and refinement, however, but the time is now ripe for the exploitation of the phenomenon in chemical studies. The... 

Radical chemistry has undergone something of a renaissance in recent years. The phenomenon of CIDNP has played an important part in this. The growing interest in the role of radical processes in biological systems may stimulate the application of CIDNP in even wider fields in the future. The development of a practical device for radiofrequency amplification by the stimulated emission of radiation (RASER) may well be one such application. 

Cadogan, J. I. G. (1970). In Essays on Free Radical Chemistry , Chemical Society Special Publication, No. 24, p. 71. 

In Section V, a general discussion of how silicon surfaces can be used to obtain monolayers is presented. The functionalization of silicon surfaces using radical chemistry is an area of intense and active investigation because of the potential for a myriad of practical applications.In order to help those readers who are not familiar with silyl radical chemistry, we discuss some general aspects of silyl radicals in Section II, together with some recent findings. 

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

The removal of the hydroxy group is easily achieved by radical chemistry starting from the appropriate thiocarbonyl derivative or selenocarbonate. 

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