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circumvent this issue, variants such as the metallo-ene reaction have been developed. In this
case, the transfer of hydrogen is replaced by a metal (lithium, magnesium, zinc) and requires
milder conditions. 68 More recently, transition metals have been used to effect ene-type reactions.
In this regard, 1,n-enynes have served as a standard substrate class for development of new
methodologies. 37-39 Because an alkyne is used as an enophile component, the product of the
rearrangement is generally a 1,4- or 1,3-diene (81 or 82) (Scheme 3.2). Transition metal
catalyzed ene-reactions do not occur via the classical pericyclic mechanism, and they are often
referred to as formal ene reactions or ene-type cycloisomerizations. A recent review by
Malacria 39 has categorized the three mechanisms by which enyne cycloisomerization can occur:
path a involves simultaneous complexation of the metal to the olefin and the alkyne, leading to
oxidative addition and formation of a metallocycle 78. This intermediate then undergoes a β-
hydride elimination from one allylic position giving the product; path b occurs via a π-allyl
complex 79 formed by insertion of the metal into the allylic position; and path c involves a
carbo- or hydrometalation of the alkyne leading to vinyl-metal species 80, which then undergoes
carbometallation of the olefin.
Scheme 3.2 Transition metal-catalyzed ene-type cycloisomerization of enynes.
77
M
a
b
c
78
79
M
80
path a: via metallocycle.
path b: via a π-allyl complex.
path c: via alkyne metallation
M
M
R
31
81
or
82