11558 J. Am. Chem. Soc., Vol. 123, No. 47, 2001
Olah et al.
Scheme 6
Figure 1. Isomeric minimum and transition-state structures calculated
for the protonated pivalaldehyde cation (number of found imaginary
frequencies in parentheses).
calculations have shown that the protonated formyl dication
HCOH2 and protonated acetyl dication CH3COH are stable
minima. Our studies on acid-catalyzed organic transformations
in superacidic trifluoromethanesulfonic acid also support su-
+
2+
15-17
perelectrophilic activation.
Protosolvation of Pivalaldehyde. Balaban and Nenitzescu18
reported the formation of methyl isopropyl ketone (2) in the
reaction of pivaloyl chloride with a large excess of aluminum
chloride in the presence of isobutane as a hydride donor. When
SnCl4 was used instead of AlCl3, no ketone was formed. A
1
2
possible mechanism, based on our theoretical calculations,
involves hydride abstraction by the O-complexed aprotic
superacidic pivaloyl cation-AlCl3 complex (9), followed by
further complexation to pivalaldehyde-AlCl3 complex (11),
which in turn undergoes fast rearrangement to methyl isopropyl
ketone (2) (Scheme 6).
Formation of pivalaldehyde dication in superacid media
(
trifluoromethanesulfonic acid or HF-BF3) could also be
explained in a similar manner. Theoretical studies at the B3LYP/
-31G* support the mechanism involving a diprotonated piv-
Figure 2. Isomeric minimum and transition-state structures calculated
6
for the diprotonated pivalaldehyde cation and the parent pivalaldehyde
alaldehyde dication intermediate.
(number of found imaginary frequencies in parentheses).
Scheme 7 shows the mechanistic pathways for the reaction,
based on the mono- or diprotonated species. To rationalize the
experimental results, density functional theory (DFT) calcula-
tions were performed, at the B3LYP/6-31G* level with the
presented only one imaginary frequency, whose normal modes
are associated with the expected reaction pathway. Figures 1
and 2 contain the optimized geometries for the studied species.
The number in parentheses is the number of imaginary frequen-
cies.
Table 2 shows the relative enthalpies for monoprotonated
pivalaldehyde and diprotonated pivalaldehyde. Figures 3 and 4
show a description of the potential energy surface (PES) of the
rearrangement of the mono- and diprotonated pivalaldehyde,
respectively.
19
Gaussian 98 program. Structures for the monoprotonated and
diprotonated intermediates and transition states shown in Scheme
6
were fully optimized. The final geometries were subjected to
further vibrational analysis that afford the zero-point energy
ZPE) and thermal corrections (298.15 K and 1 atm) for each
(
structure. If not stated otherwise, all energies given refer to
enthalpies at 298.15 K and 1 atm. Minima were characterized
by the absence of imaginary frequencies. All transition states
Analysis of the PES of the monoprotonated pivalaldehyde
(
Figure 3) shows a barrier for methyl shift of 9.2 kcal/mol,
(
10) Arnett, E. M. Quantitative Comparisons of Weak Organic Bases.
Prog. Phys. Org. Chem. 1963, 1, 223.
affording the intermediate 20 in an endothermic process (4.2
kcal/mol). This intermediate undergoes easy rearrangement
(
(
11) Campbell, H. J.; Edward, J. T. Can. J. Chem. 1960, 38, 2109.
12) Hartz, N.; Rasul, G.; Olah, G. A. J. Am. Chem. Soc. 1993, 115,
(activation barrier of 2.3 kcal/mol) to the protonated methyl
1
277.
13) Olah, G. A.; Burrichter, A.; Rasul, G.; Prakash, G. K. S.; Hachoumy,
M.; Sommer, J. J. Am. Chem. Soc. 1996, 118, 10423.
14) (a) Vol’pin, M.; Akhrem, I.; Orlinkov, A. New J. Chem. 1989, 13,
71. (b) Akhrem, I. S.; Orlinkov, A. V.; Afanaseva, L. V.; Vol’pin, M. E.
Dokl. Acad. Nauk. SSSR 1988, 298 (1), 107.
15) (a) Olah, G. A.; Rasul, G.; York, C.; Prakash, G. K. S. J. Am. Chem.
(
isopropyl ketone (3). The total enthalpy for the rearrangement
is exothermic by 15.5 kcal/mol. Thus, the slow or rate-
(
7
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98,
Revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998.
(
Soc. 1995, 117, 11211. (b) Olah, G. A.; Klumpp, D. A.; Neyer, G.; Wang,
Q. Synthesis 1996, 321. (c) Klumpp, D. A.; Beak, D. N.; Prakash, G. K.
S.; Olah, G. A. J. Org. Chem. 1997, 62, 6666.
(
16) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767.
(17) Hwang, J. P.; Prakash, G. K. S.; Olah, G. A. Tetrahedron 2000,
5
6, 7199.
(18) (a) Balaban, A. T.; Nenitzescu, C. D. Justus Liebigs Ann. Chem.
1
959, 625, 66. (b) Nenitzescu, C. D.; Balaban, A. T. In Friedel-Crafts
and related reactions; Olah, G. A., Ed.; Wiley: New York, 1964; Vol. III,
No. 1, pp 1033-1152.