The
metal-catalyzed “cascade C–H activation/annulation”
is a highly promising approach to the construction of aromatic and
heteroaromatic compounds. In this context, Pd-complexes play a crucial
role whereby molecules bearing a directing group can be activated
with high catalytic efficiency. These reactions will expand the synthetic
potential of Pd-catalyzed C–H activation and subsequent aromatic
and aliphatic C–C bond formation. Described herein is an outline
of the recent advances in this research subject.
A variety of heteroatom-chelated ruthenium alkylidenes have been developed as metathesis-active catalysts. Alkenechelated ruthenium alkylidenes, however, have not been considered as a viable alternative because alkene coordination is a necessary step in the catalytic cycle. Relying on common design principles with varying steric and electronic factors, a series of structurally diverse alkene-chelated ruthenium alkylidene complexes were prepared by trapping the intermediates of enyne ring-closing metathesis (RCM) of 1,n-enynes and diynes with a stoichiometric amount of an initiator ruthenium complex. One of the crucial structural elements that promotes the formation of 1,5-alkenechelates is the exo-Thorpe−Ingold effect, exerted by a gem-dialkyl moiety. These alkene-chelated complexes show a trans relationship between the N-heterocyclic carbene (NHC) ligand and the chelated alkene. On the other hand, η 3 -vinyl alkylidene complexes were generated from the RCM of ynamide-tethered 1,n-enynes. The presence of an ynamide moiety with a right connectivity is essential for the formation of these rare η 3 -vinyl alkylidene complexes with a cis relationship between the N-heterocyclic carbene (NHC) ligand and the chelated alkene. The stability and reactivity of these alkene-chelated ruthenium alkylidenes could be finely tuned to show characteristic behaviors in RCM, cross-metathesis (CM), and ring-opening metathesis polymerization (ROMP) reactions.
A new approach for the synthesis of 4-pyrones with broader substrate scope and functional group tolerance is described. The reaction proceeds via an initial 1,4-addition by piperidine, followed by nitrogen-assisted 6-endo-dig cyclization and hydrolysis. 1,3-Diynones with nonenolizable electron-withdrawing ketones and nonpropargylic H provide relatively high yields. For substrates with particular R 2 substituents, 1,4-or 1,6-adducts were isolated, suggesting that steric and electronic factors of the R 2 substituent should have a strong impact on the 6-endo-dig cyclization.
The hydrogen bonding-directed sequential 1,6/1,4-additions developed herein allow for creating unique heterocycles with structural diversity under mild conditions without using metal catalysts or organometallic reagents.
We describe Pt(II)- and Fe(III)-catalyzed iminocarboxylations of oxime esters conjugated with 1,3-enyne and an ortho-alkynylarene moiety, followed by a spontaneous O→N acyl migration of the enol carboxylate intermediate to generate N-acyl pyrroles and isoindoles. The reaction scope for pyrrole synthesis is general, whereas the formation of isoindoles has a relatively narrow scope because of their instability.
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