Enhanced Catalytic Performance of Indenediide Palladium Pincer Complexes for Cycloisomerization: Efficient Synthesis of Alkylidene Lactams

Abstract

New SCS Pd pincer complexes featuring a noninnocent indenediide backbone show high catalytic activity in cycloisomerization. A variety of alkylidene lactams (five-to seven-membered rings) have been prepared efficiently from N-tosyl alkynylamides, and good results have also been obtained with challenging alkynoic acids.

Efficient Synthesis of Unsaturated δ‐ and ε‐Lactones/Lactams by Catalytic Cycloisomerization: When Pt Outperforms Pd

“…The corresponding alkylidene ε‐lactone 2e was obtained in pure form and very good yield (84%) after distillation 19. This outcome represents a significant improvement of the result we recently reported with the Pd complex V (51% isolated yield after 22 h at 120 °C using 5 mol% of Pd),10b which was at that time the most efficient system to cyclize 1e into 2e 6b,e,7a,d. The Pt complex IIIb also gave excellent results in the cycloisomerization of the N ‐tosylamide 1f .…”

“…The best results were obtained with the platinum dimer IIIb . Complete conversion was achieved within 3 h at 50 °C in chloroform at 1 mol% Pt loading (Table 1), which represents a significant improvement over the related Pd dimer V (10 h at 90 °C and 5 mol% Pd were required in this case) 10b. Complex IIIb was selected for the rest of the study.…”

“…A slight increase of the reaction temperature (from 50 to 90 °C) enabled us to achieve complete conversion within 25 min with IIIb (5 mol% of Pt, entry 5). The formation of the alkylidene lactam 2b is much faster (∼30 times) with IIIb than with the Pd complex V (which required 12 h of reaction under the same conditions, entry 4) 10b. Given the very high activity of IIIb , we sought to reduce the catalytic loading.…”

“…Full conversion was achieved in 22 h at 90 °C using 5 mol% of Pt (entry 2). Previously, 2f was obtained in moderate yield (53%) after 130 h at 90 °C with the Pd complex V 10b. Replacing Pd for Pt, the reaction time is considerably shortened (by ∼6 times) and pure alkylidene ε‐lactam 2f was isolated in excellent yield (93%).…”

The intermolecular hydro-oxycarbonylation of internal alkynes: current state of the art

“…the cycloisomerization of alkynoic acids. [58][59][60][61][62][63] However, we would like to stress that these intramolecular reactions are favoured from a thermodynamic point of view and do not represent a notable synthetic problem. 52,64 Another challenging aspect related with the use of internal alkynes concerns the control of the regio-and stereoselectivity of the addition process since, starting from non-symmetrically substituted substrates R ), up to four isomeric enol esters can be formed (obviously difficult to separate).…”