Catalytic
transformations involving metal carbenes are considered
one of the most important aspects of homogeneous transition metal
catalysis. Recently, gold-catalyzed generation of gold carbenes from
readily available alkynes represents a significant advance in metal
carbene chemistry. This Review summarizes the advances in the gold-catalyzed
nitrene-transfer reactions of alkynes with nitrogen-transfer reagents,
such as azides, nitrogen ylides, isoxazoles, and anthranils, and gold-catalyzed
carbene-transfer reactions, involving oxygen atom-transfer reactions
of alkynes with nitro compounds, nitrones, sulfoxides, and pyridine N-oxides, through the presumable α-imino gold carbene
and α-oxo gold carbene intermediates, respectively. Gold-catalyzed
processes are reviewed by highlighting their product diversity, selectivity,
and applicability, and the mechanistic rationale is presented where
possible.
Ynamides are special
alkynes bearing an electron-withdrawing group
on the nitrogen atom, and they have been extensively studied over
the past decade. However, the addition of functional groups across
ynamides in these transformations typically occurs at the α-position
of the ynamide because of the strong polarization of the alkynyl moiety.
Studies of umpolung transformations in ynamide chemistry may not only
discover organic reactions but also lead to divergent organic syntheses,
thus significantly enriching ynamide chemistry. This review summarizes
four main strategies utilized to achieve reversal of the regioselectivity,
including the ring strain factor, metal–carbonyl (or sulfonyl)
chelation, and base-mediated and radical-initiated addition.
Platinum-catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom-economical synthesis of valuable 1,3-oxazepines and 2,5-dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α-imino platinum carbene intermediates.
An efficient yttrium-catalyzed intramolecular hydroalkoxylation/Claisen rearrangement sequence has been achieved, thus enabling facile access to a diverse array of valuable medium-sized lactams. Furthermore, a mechanistic rationale for this novel cascade reaction is well supported by a variety of control experiments.
The generation of metal-containing
1,3-dipoles from metal carbenes represents a significant advance in
1,3-dipolar cycloaddition reactions. However, these transformations
have so far been limited to reactions based on diazo compounds or
triazoles as precursors. Herein, we disclose a copper-catalyzed enantioselective
reaction of alkenyl N-propargyl ynamides with styrene
derivatives by formal [3 + 2] cycloaddition via Cu-containing all-carbon
1,3-dipoles, which constitutes a novel way for the generation of metal-containing
1,3-dipoles via metal carbenes. This protocol allows the practical
and atom-economical synthesis of valuable chiral pyrrole-fused bridged
[2.2.1] skeletons in moderate to good yields (up to 90% yield) with
excellent diastereoselectivities (dr > 50/1) and generally excellent
enantioselectivities (up to >99% ee).
Here, we report a copper-catalyzed asymmetric cascade cyclization/[1,2]-Stevens-type rearrangement via a non-diazo approach, leading to the practical and atomeconomic assembly of various valuable chiral chromeno [3,4c]pyrroles bearing a quaternary carbon stereocenter in generally moderate to good yields with wide substrate scope and excellent enantioselectivities (up to 99 % ee). Importantly, this protocol not only represents the first example of catalytic asymmetric [1,2]-Stevens-type rearrangement based on alkynes but also constitutes the first asymmetric formal carbene insertion into the SiÀ O bond.
A novel zinc-catalyzed reaction of isoxazoles with thioynol ethers involving an unprecedented 1,2-sulfur migration has been developed, which represents the first example of a non-noble metal-catalyzed reaction between isoxazoles and alkynes. This method allows the facile and atom-economical synthesis of a range of valuable β-keto enamides. Moreover, the computational study provides further evidence for the feasibility of the proposed reaction mechanism.
A zinc-catalyzed formal [3 + 2] annulation of isoxazoles with ynol ethers has been developed, leading to the atom-economical and divergent synthesis of 2-alkoxyl 1H-pyrroles and 3H-pyrroles, respectively.
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