Who is anti-Markovnikov?

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Edited by GEORGE WlGER

textbook forum

California State University Carson. CA So747

Who is Anti-Markovnikov? J. M. Tedder University of St. Andrews, Scotland, KY16 9ST A common reaction in organic chemistry which is of wide commercial and academic interest is the addition of free radicals to olefins. Naturally all organic chemistry textbooks describe these reactions, and the majority discuss the factors which control the rate and orientation of addition of radicals to unsymmetric olefins. Unfortunately the great majority of textbooks give intermetations which are derived from explanationsif the rate and orientation of heterolytic addition to similar olefins. Such analoeies are extremelv. nrecarious. . and thcy ha\,e unduubtedly led to much rnisundersranding. The farrors which rmtrol frcc radic31 additiot~rtwtimd are complex, but this cannot be used as a justification for perpetratine incorrect ideas. ~ h & i s t ; ;arc usually fairly quick to accept new ideas and are willing to adiust the strurture of thc iu11iei:t to arcommodate new knowledge. Occasionally, however, new factors are ignored if they appear to challenge accepted belief, in the hope that further work will prove the "new facts" wrong. This phenomenon occurs either when a really major breakthrough is achieved which involves a substantial change in existing outlook, or when the point in question affects a fairly small area of knowledge and the existing incorrect interpretations give predictions which are qualitatively correct in many common cases. It is the latter type problem we are concerned with in this article.' The auestion we are .. eoine.. to consider is: "What are the factors which wntrul the rate and urit:ntntiun of free radical addition K, alkenes?" The addition ofalksl radicals toolefins is usually an exothermic reaction, irreversihle a t normal temperatures. Addition of atoms (e.g., halogens) or some hetero-radicals (e.g., HS-, CFaS., etc.) is often reversible. This article is based primarily on results involving the behavior of alkyl radicals. The arguments to he developed, however, undoubtedly apply to the addition of atoms and hetero-radicals to olefins, though experimental verification is difficult because of the kinetic complications associated with reversible processes. The problem was first confronted when it was established that the orientation of addition of hydrogen bromide to unsymmetric olefins differed according to the experimental conditions. In the presence of ultraviolet light or in the presence of peroxides the normal orientation of addition was reversed. Markovnikov dmk, solution phase / RCHBrCHz + HBr RCH-H, UV light or peroxides RCH,CH,Br Anti-Markovnikov I t was suggested almost simultaneously by Kharasch in America and independently by Hey and Waters in Britain that this reversal of addition was due to a change in mechanism, the normal Markovnikov addition involving ions and the anti-Markovnikov addition involving atoms and radicals. ~

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The oresent article is based on conclusions derived from fifteen )ears research aidea by numerous collaborators of whose contribuilons that of J. C Walfon was predominant. The importance of sleric effects in Free Radical reactions was emphasized by Ruchardt ten years ago ( 1) ~F~~~

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RCH-H,

, RCHB~CH* 5-R ~ H C H L

HBr

R ~ H C H ~ B ~ RCH,CH,Br (+Br-) This occurrcd at the end of the 1930's. thnt is, a t a time whcn the electronic theory was having an enormous impact on organic chemistrv. .. and i t was natural to invoke similar ideas to explain hoth ionir and radicnl mtxhnnisrns.Thus mnic addition ~ . a helir\.efl $ to vield the intermtdiate rarhmium ion in which the charge w i t h e most delocalized, so similarly radical addition was assumed to yield the initial addnct radical in which the unpaired spin was the most delocalized. This was lone before the contribution steric com~ressionmakes to bond strengths was fully app~eciated.~ ~ t t h a time t the relative weakness of a tertiary carbon-hydrogen bond was attributed exclusively to resonance stahiI&ati& through hyperconjugation in the incipient tertiary radical. However, even at that time the "resonance explanation" of anti-Markovnikov addition to vinyl chloride was unsatisfactory.

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:CL-~?H--CH, "

-: E I = C H ~ H ,

CI--CH=CH2,

T h r resonance stabilization i,f thr intermediate ci~rllonium ion is rrasunable: we hwe d e n t s ofwidmce that chlorine ran act as a donor when attached to; site of high electron demand. In contrast, the corresponding resonance structure for the 2-hromo-1-chloro-ethyl radical is unimportant; it requires the donation of an electron from electroneaative chlorine to neutral carbon, and it results in the formation of a dipole. The contribution such a structure can make to the ground state of the 2-hromo-1-chloro-ethyl radical is small. Finally since radical addition is usually a very fast exothermic reaction it would be expected to have an "early transition state," and therefore resonance stabilization of the adduct radical would be unlikely to he a major controlling factor. The importance of other factors, expecially steric effects was clearly discussed by Mayo and Walling in their seminal review ( 2 ) .However, the analogy with ionic addition proved too strong and emphasis on "resonance stabilization of the addnct radical" continued to he the popular textbook explanation of the orientation of radical addition. More than 15 years ago it was discovered that trichloromethvl radicals nreferentiallv attacked the unsuhstituted end I~ at the substituted end was of vinyl tluorid(. I I I ~ C ~ I I I Sattack s b e ~ ~ n e s t a l ~ l tu d cLit d uuite retarded (:{I. Since then t h ~ has generally true and similar data is available for vinyl chloride, propene, 3,3,3-trifluoropropene, acrylonitrile, methylvinyl ketone, etc. (4). (At 164'C the rate of addition of triflnoromethyl radicals to the unsubstituted ends of these olefins varies by little more than a factor of two although the rate of addition to the substituted ends varies by several powers of end of muno-sub.;titen.) Radical;; attack the ~lns~fhitiruted f u ~ c dJefins preferentially, l r w n ~ i attuck r at [he suhstitured end is retarded by the substituent. One cannot emphasize too Volume 61

Number 3

March 1984

237