Radicals addition to olefins

Commercial phosphine derivatives are produced either by the acid-cataly2ed addition of phosphine to an aldehyde or by free-radical addition to olefins, particulady a-olefins. The reactions usually take place in an autoclave under moderate pressures (<4 MPa (580 psi)) and at temperatures between 60 and 100°C. 

For a long time, this finding was correlated with the observation that substituents at a radical center tend to enhance its stability (Section 1.1.2). This in turn led to the belief that the degree of stabilization conferred on the product radical by the substituents was the prime factor determining the orientation and rate of radical addition to olefins. That steric, polar, or other factors might favor the same outcome was either considered to be of secondary importance or simply ignored. ... 

Enantioselective cydizations by radical additions to olefins have been reported and a few of them have already been discussed in Sect. 2.2.1. Cycliza-tions were performed by Nishida et al. using chiral aluminum Lewis acid... 

Viewed in this way, reactions yielding cyclic ethers should be thought of as two-step processes, epoxides probably arising from peroxy radical additions to olefins followed by 1,3-displacements, and larger rings arising via intramolecular hydrogen abstraction by peroxy radicals followed by 1,4- or 1,5-displacements. In all such reactions it is probably the first step which is slow and which determines the yield of product observed. 

C. Walling and E. S. Huyser, Free-Radical Additions to Olefins to Form Carbon-Carbon Bonds," Organic Reactions 3, 91 (1963). 

Table 5. Regiochemistry of trifluoromethyl radical additions to olefins [85,92,93]...

As we just argued, for reactions of electrophiles and nucleophiles/increases as the nucleophile becomes more delocalized. Thus, the series of delocalized nucleophiles, in Fig. 6.10, is more selective to changes (of any kind) that affect the gap, G, compared with the series of localized nucleophiles. This would be general for other processes as well delocalization of the single electrons in the R states of the diagram results in higher/values, and vice versa. Such trends abound in electrophile—nucleophile combinations they were analyzed also for radical addition to olefins (40), and are likely to be a general feature of reactivity. 

C. Walling and E. S. Fluyser, Free radical addition to olefins to form carbon-carbon bonds, Org. React. 1963,13, 92-149. 

Following the pioneer work of Kharasch [60], methods involving radical transfer of halides have been developed. The atom transfer method has emerged in the 1980s as one of the best method for conducting intra- and intermolecular radical additions to olefins [61]. This approach is particularly appealing from an atom economy point of view since all atoms remains in the final product. The non-reductive nature of these reactions is also particularly important for the preparation of functionalized molecules. Halides transfers and more particularly iodine atom transfers have found nice applications for cyclizations, annula-tions and cascade reactions [62]. These reactions are based on exothermic radical steps, such as the addition of an alkyl radical to an olefin, followed by an... 

O. R. XIII-3, C. Walling, and E. S. Huyser, Free Radical Additions to Olefins and Acetylenes to Form Carbon-Carbon Bonds XI11-1, G. Zweifel and H. C. Brown, Hydration of Olefins, Dienes, and Acetylenes via Hydroboration XIII-2, W. E. Parham and E. E. Schweizer, Halocyclopropanes from Halocarbenes. ... 

Radical addition to olefins has been studied extensively by R. N. Haszeldine for a summary with references, see Walling. ... 

Table 1.38 Regioselectivity in trifluoromethyl radical addition to olefins...

Phenyl selenide transfer radical addition reactions can be limited by the lack of reactivity observed in some precursors. Simple alkyl phenyl selenides do not undergo inter- or intramolecular radical additions to olefins. Phenylselenotrichloromethane will add to olefins upon photolysis, and the products formed can be elaborated into a, ff-unsaturated carboxylic acids (Scheme 20) [57], Benzyl phenylselenides have been observed to undergo atom transfer cyclization (Scheme 20) [58]. 

Sulfonyl thiocyanates undergo free radical addition to olefins or acetylenes to give p-thiocyanate sulfones in fair to good yields (equation 111)97. p-Toluenesulfonyl thiocyanate (224) is prepared from sodium p-toluenesulfinate and thiocyanogen in benzene... 

The correct assignment of values of Aa is the main problem. In this regard it is instructive to examine the results from studies of alkyl radical addition to olefins in the gas phase [93]. Although data are limited, addition of CH3, Et, and i-Pr give progressively smaller values of Aa in adding to ethylene for both Me and Et radicals values of Aa decrease as steric bulk around the olefinic center increases. Electronic factors are reflected both in Ea and Aa but stabilization of the adduct radical is reflected mostly in Ea. If, to a first approximation, we can consider R02 similar to an Et, the steric bulk of R can be disregarded and we may then... 

---