Radical addition 1 effects

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

A clear demonstration of the relative importance of steric and resonance factors in radical additions to carbon-carbon double bonds can be found by considering the effect of (non-polar) substituents on the rate of attack of (nonpolar) radicals. Substituents on the double bond strongly retard addition at the substituted carbon while leaving the rate of addition to the other end essentially unaffected (for example, Table 1.3). This is in keeping with expectation if steric factors determine the regiospeeificity of addition, but contrary to expectation if resonance factors are dominant. 

Figure 1.2 Effect of polar factors on regiospecificity of radical addition.

The above argument is also at odds with the conventional wisdom that the well-known tendency for monomer alternation in copolymerization can primarily be attributed to polar factors. It was suggested9 that, in most cases, radical stabilization could provide an alternate explanation. A discussion on the relative importance of steric polar and radical stabilization effects on radical addition appears in Section 2.3. 

Given the important role that stcric and polar factors play in determining the rate and regiospecifieity of radical additions (see 2.3), it might be anticipated that reagents which coordinate with the propagating radical and/or the monomer and thereby modify the effective size, polarity, or inherent stability of that species, could alter the outcome of propagation. 

The trend in relative effectiveness of RAFT agents with varying Z is rationalized in terms of interaction of Z with the C=S double bond to activate or deactivate that group towards free radical addition. Substituents that facilitate addition generally retard fragmentation. O-Alkyl xanthates (Z=0-alkyl, Table... 

CoTMP initiator 484 diihiocarbumalc pholoiniferlets 465 methyl ct-chloroacrylaie, temperature effect on alky) radical addition 25 methyl crolonate... 

The (TMS)3Si radical addition to terminal alkenes or alkynes, followed by radical cyclization to oxime ethers, were also studied (Reaction 50). The radical reactions proceeded effectively by the use of triethylborane as a radical initiator to provide the functionalized pyrrolidines via a carbon-carbon bond-forming process. Yields of 79 and 63% are obtained for oxime ethers connected with an olefin or propargyl group, respectively. 

The mechanism of free-radical addition follows the pattern discussed in Chapter 14 (pp. 894-895). The method of principal component analysis has been used to analyze polar and enthalpic effect in radical addition reactions. A radical is generated by... 

In free-radical addition, the main effect seems to be steric. All substrates CH2=CHX are preferentially attacked at the CH2, regardless of the identity of X or of the attacking radical. With a reagent such as HBr, this means that the addition is anti-Markovnikov ... 

The magnitude of the electrical effect is somewhat less in the case of radical addition than it was in the case of electrophilic and nucleophilic addition. 

That it is not entirely negligible, however, is indicated by the fact (demonstrated by structure studies see Chap. VI) that radical addition occurs preferentially—presumably at the unsubstituted carbon. Addition at the substituted carbon would yield a radical on the unsubstituted carbon hence the substituent would be without effect. 

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