Neophyl radical, rearrangement

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

The first indication of something unusual in the reactivity of this species was that the EPR signal of 2 was found to decay via a first-order process to produce a new radical. The product was shown to be the neophyl radical 3, whose EPR spectrum was identical with an independently prepared authentic sample. Over the temperature range -30 to -90°C, Arrhenius plots indicated an unusually low preexponential log A (s ) value of only 5.3, and a strikingly large k /ko ratio of ca. 50 observed at -30°C (based on comparison of reaction rates of 2 versus the in-A)-tert-b xiy analog) for the rearrangement. 

Thus the average lifetime of the neophyll radical depends on the probability of encountering an aldehyde molecule and hence on the aldehyde concentration. At high aldehyde concentrations the radical is removed by reaction (3) before it has an opportunity to rearrange. This would not be the case if rearrangement were simultaneous with formation of the radical. 

Table II. Most of the data was obtained from radical clock studies. The neophyl radical rearrangement24 [Eq. (2)] was used for the majority of the kinetic data in Table II, but the ring expansion rearrangement reactions25-27 of radicals 7 and 8, cyclizations of 5-hexenyl type radicals,...

An interesting neophyl-type radical rearrangement process has been established for the synthesis of azabicycles, which are not readily accessible by other means. Barton McCombie deoxygenation of xanthate 70 under slow addition of (TMS)3SiH and AIBN in refluxing toluene furnished the 2-azabenzonorbor-nane derivative in good yield (Reaction 7.72) [82]. 

Carbon-bromine cleavage from upper triplet states has also been observed by two-color photolysis of 55 and for the ketones 83-85 [69]. In the latter cases cleavage is followed by a neophyl-type rearrangement and decarboxylation yielding the arylmethyl radical (Scheme 4). 

Neophyl(tri- -butylphosphine)copper decomposes at temperatures between 30° and 120°C mainly, if not totally, by a free-radical mechanism. The usual products resulting from rearrangement of the neophyl radical were isolated [Eq. (37)], and no evidence was obtained to indicate the formation of an intermediate phenyl-substituted tert-hu. y -copper compound, C6H5CH2(CH3)aCCu. 

In the former instance, the reaction gave rise to a 1 1 mixture of hydrocarbons 63 and 64, which indicated that the primary neophyl radical 56 had partially rearranged in solution to the tertiary radical 57. Similarly, thermal decomposition of perester 65 yielded... 

Neopentyl and Neophyl Iodides. The halides mentioned above formed the corresponding organocobalt complexes on reaction with pentacyanocobaltate(II). While neopentyl radical is stable to rearrangement, the neophyl radical undergoes aryl migration (56) as follows ... 

Hence, the rate of rearrangement of the neophyl radical, if such is indeed produced, is considerably slower than its reaction with pentacyanoco-baltate(II). Similarly, neophyl magnesium chloride may be prepared in the usual manner (58), although the Grignard reaction is considered to involve a radical intermediate (36, 52, 62). 

In recent years, radical aryl migrations have received increased attention within the synthetic organic chemistry community. Yet, these reactions are also found as key steps in complex natural product synthesis [78]. For example, the neophyl rearrangement-which is the 1,2-phenyl migration of the neophyl radical 42 to form the tertiary radical 44 (probably via spirocyclohexadienyl radical 43)-was discovered by Urry and Kharasch more than 60 years ago (Scheme 13.9) [79], since which time numerous reports on neophyl-type rearrangements have been presented [80]. However, despite these efforts the postulated intermediate 43 has not yet been identified [81], The slow neophyl rearrangement (k = 762 s at 25 °C, [82]) can be used as a radical clock [83], The 1,2-aryl migration can also occur from C- to... 

Analysis of the products found in the reaction mixture led the authors to the conclusion that their results per exclusionem speak for the occurrence of neophyl radicals and their rearrangement. 

In some cases, aryl radical migration appeared as a side reaction [16a, 17], with the most extensively investigated one being the 1,2-aryl migration in p-aryl carbon-centered radicals, called the neophyl-type rearrangement (Eq. 9.5), where the neophyl radical 30 is converted to tertiary radical... 

Neophyl chloride in 13—75% current yield forms f-butylbenzene (94%) and isobutylbenzene (6%) as side product by rearrangement of the intermediate neO phyl radical 487>. Reduction of monobromomaleic acid at pH 0—4 yields up to 50% of the dimer 163 (Eq. (228)). With increasing pH the amount of dimer... 

Evidence for a surface-bound radical was also provided by work of Ruchardt et al. [59]. Reaction of neophyl chloride 55 with magnesium led, after carbonylation, to acid 61 in 65% yield, without any contamination by acid 60, which could have arisen by rearrangement of radical 56 to radical 57 (Scheme 24). 

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