Neophyl

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. 

Hexaneopentyldilead and hexaneophyldilead are converted into (neopentyl)3PbI and (neophyl)3PbI, respectively159, by iodination. 

Finally, Franz and coworkers171 measured the rate constants and primary hydrogen-deuterium kinetic isotope effects for the radical reactions between tributyltin hydride and the neophyl and the 2-allylbenzyl radical in diphenyl ether. The isotope effect in the first reaction was 1.64 at 192.5 °C and that in the second reaction was 1.91 at 236 °C. These values compare well with those predicted from Kozuka and Lewis s primary... 

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,...

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. 

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]. 

NEOPHYL CHLORIDE [Benzene (chloro-tert-butyl)-]... 

The reaction mixture is then transferred to a 1-1. separatory funnel, and the sulfuric acid layer is removed. The remaining benzene solution is then washed with four 200-ml. portions of distilled water (Note 3). In this step the amber color disappears and the liquid becomes colorless. The benzene solution is dried with anhydrous sodium sulfate and transferred to a 1-1. distilling flask. The benzene is removed by distillation under a pressure of about 45 mm. The liquid residue is poured into a 500-ml. flask and distilled through a 40-cm. Vigreux column under reduced pressure. The yield of neophyl chloride boiling at 97-98°/10 mm. is 262-275 g. (70-73%) (Notes 4 and 5) ... 

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