Page 288 - Mechanism and Theory in Organic Chemistry

1,2-Shifts in Carbenium Ions 275

give the same products, 11 and 12, in the same ratio (9 : 1) when they undergo the
pinacol rearrangement with BF,-ether complex, as shown in Scheme 1 .I7 (Note
that the hydroxy group lost here, as usual, is the one that gives the most stable
carbocation.ls) If the migrating group provided anchimeric assistance, it would
have to come in from the back side of the departing -+OH2. In Compound 9 the
group that can come in from the back side is the hydride, and Compound 11 should
be the principal product. Conversely, in Compound 10 it is the alkyl chain of the
ring that can come in from the back side, and the chief product should be
Compound 12. The fact that the products are formed in a constant ratio indicates
that a common intermediate-presumably the planar carbocation-must be
formed from both starting materials.lS More direct stereochemical evidence has
been provided by Kirmse and co-workers. Chiral (S)-2-methylbutan-l,2-diol (13)
rearranges to racemic 2-methylbutanal (14) as shown in Equation 6.13.20

(See also pp. 281-284.)
Aryl Participation-The Phenonium Ion Controversy21
A question that has aroused considerable controversy in the past 25 years is
whether aryl groups can provide anchimeric assistance and if so under what
conditions. The controversy began in 1949 when Cram solvolyzed the L-threo
and L-erythro isomers of 3-phenyl-2-butyl tosylate in acetic acid. L-Threo-tosylate
gave 96 percent racemic threo-acetate (plus olefins), whereas the L-erythro isomer
gave 98 percent L-erythro-acetate.22 To explain the experimental facts, Cram
postulated that neighboring phenyl begins a backside migration to C, as the
tosylate departs. At the first energy maximum both the tosylate and the phenyl

P. L. Barili, G. Berti, B. Macchia, F. Macchia, and L. Monti, J. Chem. Soc. C, 1168 (1970).
(a) See note l(b), p. 268, for examples and exceptions; for another exception see (b) W. M.
Schubert and P. H. LeFevre, J. Amer. Chcm. SOC., 94, 1639 (1972).
lo Departure of the leaving group is apparently rate-determining when the first-formed carbocation
is not particularly stabilized. This is shown by the fact that the rate of rearrangement of alkyl
glycols is dependent on the concentration of
R R
I I
R--C-R
I I
+OH2 OH
U. F. Duncan and K. R. Lynn, J. Chm. Soc., 3512, 3519 (1956); J. B. Ley and C. A. Vernon, Chcm.
Ind. (London), 146 (1956).] That the rate-determining step can be the migration when the first-
formed carbocation is particularly stable has been shown by Schubert and LeFevre [note 18(b)].
These workers subjected 1,l-diphenyl-2-methyl-1,2-propanediol to the pinacol rearranggment and
found that deuterium substitution in the migrating methyls caused the reaction to slow down.
20 W. Kirmse, H. Arold, and B. Kornrumpf, Chem. Ber., 104, 1783 (1971).
21 C. J. Lancelot, D. J. Cram, and P. v. R. Schleyer, in Carbonium Zonr, G. A. Olah and P. v. R.
Schleyer, Eds., Wiley-Interscience, New York, 1972, Vol. 111.
22 D. J. Cram, J. Amer. Chcm. Soc., 71, 3863 (1949).