1 -Methyl-1,2-diphenylcyclopropane

Further attempts to trap the chiral l-methyl-2,2-diphenylcyclopropyl radical, before inversion, by using excellent radical scavengers as solvents were also abortive. Decomposition of the diacyl peroxide (48) in thiophenol and reduction of (-)-(/ )- -bromo-l-methyl-2,2-diphenylcyclopropane (51) with tri-n-butyltin hydride as solvent resulted in essentially racemic hydrocarbon (49) ... 

The thermal decomposition of ( —)-(R)-methyl-2,2-diphenylcyclopropanoyl peroxide, ( + )-(R)-48, in pure carbon tetrachloride yielded, besides the expected ( )-l-chloro-l-methyl-2,2-diphenylcyclopropane (52), a 2% yield of ( + )-(S)-l-methyl-2,2-... 

Finally the most definitive evidence for a cage reaction was the observation that when an equimolar mixture of the peroxide (48) and the peroxide-dio (48-Djo) were decomposed, no crossover products were obtained only equal amounts of fully protonated hydrocarbon (49) and the hydrocarbon-d (49-0 ) were formed. The optical purity of the isolated ( + )-(5)-1-methyl-2,2-diphenylcyclopropane (49) was found to be 31-37 % with a net retention of configuration. Thus, when the lifetime of the rapidly inverting g radical is sufficiently great to permit diffusion out of the solvent cage the product formed by the... 

In 1961 it was reported that the reaction of chiral l-bromo-l-methyl-2,2-diphenylcyclopropane (51) with magnesium metal produced a partially optically active Grignard reagent . It was suggested that the racemization observed occurred in the Grignard formation step. In 1964 it was also established that the racemization occurred... 

Annino and his coworkers have postulated a mechanism for the reaction at the zinc surface patterned after the one proposed by Walborsky and coworkers for Grignard formation The organozinc intermediate formed is rapidly hydrolyzed by the protonic solvent. Note also that the reaction of zinc, in ethanol-10 % KOH, with chiral l-bromo-l-methyl-2,2-diphenylcyclopropane (51) yielded l-methyl-2,2-diphenylcyclopropane (49) with 21 % retention of configuration a result comparable to the 15% retention that is found in Grignard formation. 

It is well established that lithium and sodium derivatives of l-methyl-2,2-diphenylcyclopropane are capable of retaining their optical activity and configuration It has also been shown that when the corresponding radical is generated in solution the resulting product is racemic . Only in a solvent cage and on metal surfaces can this rapidly inverting o cyclopropyl radical be intercepted. 

Because of these observations a study of optically active l-halo-l-methyl-2,2-diphenylcyclopropanes with solutions of sodium in liquid ammonia was undertaken As will be seen, the stereochemical results observed were shown to be dependent on the concentration of sodium in ammonia, the nature of the halogen and a heterogeneity factor. 

An entirely different result has been reported by Jacobus and Pensak. They found that the reduction of the optically active 1-bromo-l-methyl-2,2-diphenylcyclopropane (51) with sodium naphthalenide (NaN) in DME (0.5 m) yields the corresponding hydrocarbon 49 of 29% optical purity with net retention of configuration. This observation was interpreted to mean that the l-methyl-2,2-diphenylcyclopropyl (t radical was being captured by a second SET from sodium naphthalenide to give the sodium derivative (which transforms in DME to 49) at a rate faster than its inversion frequency (Scheme 14). 

SCHEME 16. Proposal for the reduction of l-methyl-2,2-diphenylcyclopropane 49 and other phenyl-substituted cyclopropanes with sodium in... 

That intermediates such as 139 , not unexpectedly, would cause loss of optical activity was shown by using ( —)-(R)-l-n-pentyl-l-methyl-2,2-diphenylcyclopropane (143) as the starting material. The resulting 144 was completely racemic ... 

Other reversible ET-catalyzed stereoisomerizations of cyclopropanes have been observed with cjs-l-methyl-2-phenyl-, r-1-phenyl-l-methyl-c-2-methyl- and optically active l-methyl-2,2-diphenylcyclopropane (190, 191 and ( + )-(/ )-49, respectively) . Experimental evidence for the existence of intermediate cyclopropane radical anions like CIS- or trans-llH" (Scheme 18) has not been found in the course of these investigations. 

Walborsky and Rachon have also reported on the use of Rieke magnesium to obtain reaction with (S)-(+)-l-bromo-l-methyl-2,2-diphenylcyclopropane at —65°C to afford a chiral Grignard reagent that is 33-43% optically pure [73]. In 1961, Young and Walborsky [74] demonstrated that the reaction of chiral (S)-(+ )-l-bromo-l-methyl-2,2-diphenyl-cyclopropane (Table 5 1) with magnesium powder resulted in the formation of a chiral Grignard reagent as evidenced by the fact that on carbonation of the reaction mixture an optically active acid was obtained with an optical purity of approximately 12-18%. 

Thermal decomposition of the diacyl peroxide of (/ )-(-I- )-methyl-2,2-diphenylcyclopropane carboxylic acid 11 without or in the presence of a good radical trap, such as iodine or thiophenol [18] (Scheme 2). 

Reduction of (R)-(-f)-l-bromo-l-methyl-2,2-diphenylcyclopropane 18 with tri-n-butyltin hydride as solvent [18, 21] (Scheme 4). 

The first chiral halides used by Walborsky and Young [43,44] were optically active 1 -bromo-1 -methyl-2,2-diphenylcyclopropane 18, 1 -chloro-1 -methyl-2,2-diphenylcyclopropane 12, and I-iodo-l-methyl-2.2-diphenylcyclopropane 14, the absolute configurations and optical purities of which were established [40,49] as were those of their derivatives, l-methyl-2,2-diphenylcyclopropane 13 and l-methyl-2,2-diphenylcyclopropanecarboxylic acid 17. 

When (S)-( 4-)-l-bromo-l-methyl-2,2-diphenylcyclopropane 18 was treated with a powdered magnesium-magnesium bromide mixture in refluxing tetrahydrofuran and the reaction mixture carbonated (Scheme 11), the resulting products l-methyl-2,2-diphenylcyclopropanecarboxylic acid 17 and l-methyl-2,2-diphenylcyclopropane 13 were optically active, with predominant retention of configuration. This was the first... 

The hydrocarbon fraction was composed of 23% l-methyl-2,2-diphenylcyclopropane 13,9% 2,2-diphenylmethylenecyclopropane 34, 3% l-methyl-2,2-diphenylcyclopropene 35, along with 4% of a higher molecular weight material that turned out to be the dimer l,l-disubstituted-2,2-diphenylcyclopropane 36. All attempts to detect the presence of other possible dimeric material such as 24 and 37 failed. 

It is unreasonable to believe that a model that assumes that the radicals diffuse freely in solution could account for such results. On the other hand, a surface-bound radical is fully in accord with them. Compound (S)-(-l-)-18 behaves similarly when exposed to metal surfaces of alkali metals in hydroxylic solvents [94]. As in the Grignard reagent formation, when a solution of (S)-(-f)-l-bromo-l-methyl-2,2-diphenylcyclopropane 18 in methanol, isopropyl alcohol, or r-butenol was exposed to an alkali metal, such as lithium, sodium, or potassium, the resulting hydrocarbon (K)-( —)-l-methyl-2,2-diphenylcyclopropane 13 was shown to be optically active and with retained configuration, as shown in Table 19 and Scheme 40. 

With one exception, that of 1-fluoro-l-methyl-2,2-diphenylcyclopropane 160, which exhibited no reduction before confomitant reduction of either the solvent or the supporting electrolyte, a single irreversible cathodic wave was always observed. The values of the transfer coefficients were derived from the cyclic voltammetric peaks widths, assuming that the Butler-Volmer kinetics apply a= 1.85(rf/F)(Ep 2 — p) [132g,h]. Of interest was... 

Racemization. Optically active 1-bromo-l-methyl-2,2-diphenylcy-clopropane, l-iodo-l-methyl-2,2-diphenylcyclopropane, and l-bromo-2,2-diphenylcyclopropane carboxylic acid were prepared to study the mechanism of alkane formation by hydrido complex. While the first two substrates could not be reduced, the a-bromo acid absorbed 87 mole % of hydrogen, being converted into optically inactive acid (Reaction 18). A sample of the optically active acid retained its configuration under reaction conditions, indicating that a symmetrical intermediate was formed at some stage of the reduction. 

The reduction of 1-methyl-2,2-diphenylcyclopropane 42 and one of its enantiomers, (+)-(R) 2, with Na/NHj to give 1,1-diphenylbutane 43 and l,l-diphenyl-2-methyl-propane 44 in a — 5.5 1 ratio over a wide concentration range has been studied by Walborsky and Pierce M>). 

---