Page 893 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Scheme 10.1. (Continued) 869

a. J. A. Marshall, N. Cohen, and A. R. Hochstetler, J. Am. Chem. Soc., 88, 3408 (1966). SECTION 10.1
b. W. S. Johnson and T. K. Schaaf, J. Chem. Soc., Chem. Commun., 611 (1969).
c. B. E. McCarry, R. L. Markezich, and W. S. Johnson, J. Am. Chem. Soc., 95, 4416 (1973). Reactions and
d. E. E. van Tamelen, R. A. Holton, R. E. Hopla, and W. E. Konz, J. Am. Chem. Soc., 94, 8228 (1972). Rearrangement
e. S. P. Tanis, Y.-H. Chuang, and D. B. Head, J. Org. Chem., 53, 4929 (1988). Involving Carbocation
f. E. J. Corey, G. Luo, and L. S. Lin, Angew. Chem. Int. Ed. Engl., 37, 1126 (1998). Intermediates
g. W. S. Johnson, M. S. Plummer, S. P. Reddy, and W. R. Bartlett, J. Am. Chem. Soc., 115, 515 (1993).
h. E. J. Corey and J. Lee, J. Am. Chem. Soc., 115, 8873 (1993).

10.1.1.3. Ene and Carbonyl-Ene Reactions. Certain double bonds undergo electro-
philic addition reactions with alkenes in which an allylic hydrogen is transferred to
the reactant. This process is called the ene reaction and the electrophile is known as
an enophile. 18 When a carbonyl group serves as the enophile, the reaction is called
a carbonyl-ene reaction and leads to , -unsaturated alcohols. The reaction is also
called the Prins reaction.
R R
H X
H
X Y
Y

A variety of double bonds give reactions corresponding to the pattern of the ene
reaction. Those that have been studied from a mechanistic and synthetic perspective
include alkenes, aldehydes and ketones, imines and iminium ions, triazoline-2,5-diones,
1
nitroso compounds, and singlet oxygen, O=O. After a mechanistic overview of the
reaction, we concentrate on the carbon-carbon bond-forming reactions. The important
1
and well-studied reaction with O=O is discussed in Section 12.3.2.
The concerted mechanism shown above is allowed by the Woodward-Hoffmann
rules. The TS involves the electrons of the alkene and enophile and the electrons
of the allylic C−H bond. The reaction is classified as a [ 2+ 2+ 2] and either an
FMO or basis set orbital array indicates an allowed concerted process.

LUMO

H
six electrons,
HOMO zero nodes
Basis set orbital
FMO orbitals for
ene reactions array for ene reactions

Because the enophiles are normally the electrophilic reagent, their reactivity
increases with addition of EWG substituents. Ene reactions between unsubstituted
alkenes have high-energy barriers, but compounds such as acrylate or propynoate esters

18
For reviews of the ene reaction, see H. M. R. Hoffmann, Angew. Chem. Int. Ed. Engl., 8, 556 (1969);
W. Oppolzer, Pure Appl. Chem., 53, 1181 (1981); K. Mikami and M. Shimizu, Chem. Rev., 92, 1020
(1992).