ConspectusCycloaddition reactions, by involving the formation of at least
two bonds and one cycle in a single operation, represent one of the
more practical ways to assemble carbo- and heterocyclic structures
from simple acyclic precursors. Especially appealing are formal cycloadditions
promoted by transition metals, owing to the ability of these reagents
to open mechanisms that are not accessible using classical chemistry.
Therefore, along the years, a great variety of annulations based on
first-, and particularly second-row transition metals have been discovered.
Most of these reactions involve inner sphere mechanisms, with the
metal participating via standard oxidative addition or reductive elimination
processes. Curiously, metals of the third row like platinum and, especially,
gold remained largely unexplored, likely because of the belief that
they were inert and expensive. However, from the beginning of this
century, many groups realized that these metals can open very interesting
mechanistic scenarios and promote novel types of transformations.
In particular, the π-acidic, carbophilic behavior of gold(I)
complexes, together with the possibility of tuning their reactivity
using designed ligands, has triggered important activity in the field.
Many gold-catalyzed transformations involved addition or cycloisomerization
processes, but during recent years, there have been also important
advances in the development of formal cycloaddition reactions. While
many of these reactions rely on the activation of alkynes, there has
been an increasing number of reports that exploit the peculiar reactivities
of allenes and derivatives.In this Account, we present recent
efforts on the development of
platinum- and gold-catalyzed formal cycloadditions of allenes. For
the sake of simplicity, we only include annulations initiated by a
direct metal-promoted activation of the allene moiety. Thus, alternative
Pt- or Au-catalyzed reactions wherein the allene does not interact
with the metal catalyst are not covered. Upon activation by the metals,
allenes generate allyl-cation alkenylmetal species that can behave
as 1,2- or 1,3-carbon dipoles in cycloaddition processes. Especially
relevant is the reactivity of allenamides. The presence of the amide
substituent provides for the generation of gold intermediates with
a good balance of reactivity and stability, which can therefore react
with the corresponding partners in a controlled manner. Moreover,
despite the difficulties associated with the transfer of stereochemical
information from chiral linear gold(I) complexes, a variety of enantioselective
gold-catalyzed annulations have been discovered.This Account
is organized considering the number of atoms engaged
in the annulation process, and when possible, we present the results
in a chronological order.