Cascade Reactions: Sequential Homobimetallic Catalysis Leading to Benzofurans and β,γ‐Unsaturated Esters

Abstract

Abstract:The concatenation between a Pd(0)-promoted deallylation catalytic cycle and a Pd(II)-promoted heterocyclization catalytic cycle (an example of what we have named "sequential homobimetallic catalysis") has been shown to occur starting from 1-(2-allyloxyphenyl)-2-yn-1-ols 1 to afford 2-benzofuran-2-ylacetic esters 2 and b,g-unsaturated esters in high yields under carbonylative conditions. In view of the conceptual as well as the synthetic importance of the process, the mechanistic aspects and the synthetic scope of the reaction have been investigated in detail. All the experimental evidence is in agreement with the sequential homobimetallic mechanism, and the reaction has proven to be of general synthetic applicability.

PdI2-Based Catalysis for Carbonylation Reactions: A Personal Account

“…In order to achieve the allyl deprotection in situ, we carried out the carbonylation experiments in the presence of two different kinds of catalysts: Pd(PPh 3 ) 4 (able to promote substrate deallylation) and PdI 2 /KI (able to promote the oxidative carbonylation-reduction sequence). The use of these two catalysts simultaneously caused the selective formation of the desired product deriving from an ordered deallylation-oxidative carbonylation-reduction sequence, as shown in Scheme 40 [103,104]. For a process like this, consisting of two cycles catalyzed by the same metal, but in two different oxidation states, we coined the term "sequential homobimetallic catalysis" [103][104][105].…”

“…The use of these two catalysts simultaneously caused the selective formation of the desired product deriving from an ordered deallylation-oxidative carbonylation-reduction sequence, as shown in Scheme 40 [103,104]. For a process like this, consisting of two cycles catalyzed by the same metal, but in two different oxidation states, we coined the term "sequential homobimetallic catalysis" [103][104][105]. In more detail, the first cycle, catalyzed by Pd(0), relates to substrate deallylation, while the second cycle, catalyzed by Pd(II), corresponds to 5-exo-dig heterocyclization followed by oxidative alkoxycarbonylation and reduction of the HO-C-C=C-CO 2 Me moiety (Scheme 40) [103][104][105].…”

“…For a process like this, consisting of two cycles catalyzed by the same metal, but in two different oxidation states, we coined the term "sequential homobimetallic catalysis" [103][104][105]. In more detail, the first cycle, catalyzed by Pd(0), relates to substrate deallylation, while the second cycle, catalyzed by Pd(II), corresponds to 5-exo-dig heterocyclization followed by oxidative alkoxycarbonylation and reduction of the HO-C-C=C-CO 2 Me moiety (Scheme 40) [103][104][105]. No cyclocarbonylation product was observed, because the intramolecular triple bond insertion into a phenoxycarbonylpalladium intermediate was much less favored with an internal triple bond with respect to a terminal one.…”

“…Interestingly, the PPh3-stabilized Pd(0) complex could also be formed starting from PdI2+PPh3+H2O (Scheme 41) [103][104][105], since it is known that PdI2 may be reduced to Pd(0) by the action of water and CO, through decarboxylation of the I-Pd-CO2H intermediate formed by the reaction between PdI2, CO, and H2O (Scheme 42) [33,108,109].…”

Recent Advances in Palladium‐Catalyzed Cascade Cyclizations

“…Gabriele et al. communicated a novel sequential homobimetallic palladium‐catalyzed cascade also yielding benzofurans ( 177 , Scheme ),78 which they studied more thoroughly later 79. Sequential homobimetallic catalysis is a catalysis involving two different active catalysts, both of the same transition metal but with different oxidation states.…”