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Steric Effects in Free Radical Chemistry
Christoph Riichardt* Chemisches Laboratorium der Universit~itFreiburg, Albertstr. 21, D-7800 Freiburg i. Br., Federal Republic of Germany Dedicated to Professor 1t. Pommer on the occasion o f his 60th birthday.
Table of Contents I II
Introduction .
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Decomposition Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 3 5 12
HI Steric Effects in Aliphatic Substitution Reactions
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IV Steric Effects in Free Radical Addition Reactions
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Sterie Effects in Homolytic 1 Ring Size Effects . . 2 Group Size Effects . . 3 Further Steric Effects .
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Steric Effects in Dimerization and Disproportionation Reactions
VI References
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* This review is an extended version of an article in ,,Zeitschrift der Sowjetischen Chemischen Mendelejew-Gesellschaft", April 1979.
C. Riichatdt I Introduction Steric effects have been discussed in free radical chemistry ever since the discovery of the first free radical, triphenylmethyl I by M. Gomberg in 19001). To what extent is the dissociation of its dimer, which was believed to be hexaphenylethane 23) till 1968 z), determined by electronic stabilization of triphenylmethyl 14) or by sterie strain in its dimer?
9. C--C6H 5 : ; ~ CsI-Is--~ C,I-Is C~s 2
2C~Hs--C-
C61-1s 1
.,,-.--
C:~
)=C
C6Hs
C61-1s 3
The opinion that stabilization of 1 by resonance was decisive, predominated for a long time and mastered the discussion of the relationship between structure and reactivity in free radical chemistry till quite recentlyS): Accordingly selectivity in free radical reactions was assumed to be mainly due to differences in the thermodynamic stability of the radicals taking part in a reaction or a potential competing reaction. The recognition2) that the c~, p-dimer 3 is formed in equilibrium with 1 and not the a,a-dimer 2 was interpreted as a result of the smaller steric strain in 3 than in23). Also the known strong influence of p-substituents on the equilibrium constants between substituted trityl radicals and their dimers 6) found an obvious explanation in this way. The earlier observation that not only those phenoxy radicals 4 carrying three conjugating phenyl substituents 4 (R = C6Hs)7a) are persistent 8) but also their R
t-butylated counterparts 4 (R = t-C4Hg) 7b) pointed to the predominating influence of steric effects. Similar results have been obtained in other classes of persistent radicalsTC, 8). The most convincing evidence for the prime importance of steric effects for the persistence of radicals was provided by the observation of a large series of crowded alkyl radicals like 5 - 7 over longer periods of time by esr. They do not dimerize for energetic reasons 9' 10) R (CH3)3C- .C-C(CH3)3 .5
R--H
6 R=
C(CH3) 3
CH(CH3)2 (CH3)2CH-C.-CH(CH3) 2
Steric Effects in Free Radical Chemistry Since these developments became known the importance of steric effects on the reactivity of free radical reactions has also been more clearly recognized and more thoroughly investigated I 1). Some more important and more recent results along these lines are