The design and mechanism of palladium catalysts for synthesis of methylene lactones by cyclocarbonylation of acetylenic alcohols

Abstract

The Pd(II)-catalyzed cyclocarbonylation of acetylenic alcohols to methylene lactones proceeds through carboalkoxypalladium intermediates, followed by intramolecular cis addition to the triple bond. Such intermediates have been independently synthesized, isolated, and found to undergo appropriate interconversions. A PdI2/Bu3P/CH3CN catalyst system gives rates first order in CO pressure, with the rate-determining step evidently being the uptake of CO by Pd. The use of an SnCl2 cocatalyst (PdCl2/2Ph3P/SnCI2/CH3CN) labilizes the coordination sphere of the palladium, dissociating Cland forming a cation which is a much faster cyclocarbonylation catalyst. The rate is now independent of CO pressure and first order in Pd and in substrate. The rate-determining step is coordination of the substrate, followed by rapid uptake of CO and completion of the cyclocarbonylation reaction. As the carboalkoxy intermediates also react intermolecularly with terminal acetylenes, yields in the catalytic cyclocarbonylation reaction improve substantially when it is run at concentrations below 0.3 M.

119Sn‐ and 31P‐NMR. Spectroscopy of the 5‐coordinate complexes [Rh (SnClnBr(3−n)) (norbornadiene) (tertiary phosphine)2]

“…-It is well known that transition metal complexes combine with tin (11) chloride to produce effective homogeneous hydrogenation and hydroformylation catalysts [ 1-41. Specifically, excellent results have been achieved using phosphine complexes of platinum [5] and palladium [6]. The chemistry of these tin complexes is not always straightforward but may be qualitaiively expressed by reactions 1-3.…”

Heterocycles Synthesis at Carbonylation of Acetylenic Compounds

“…At that not only α-methylene γ-lactones, but δ-lactones of various structure in cis-and trans-fused rings can be made with rather high yields, if the substrate concentration is kept sufficiently low (0.1-1М), to direct the reaction into intramolecular cyclization way. Study of cyclocarbonylation mechanism shows at first carboalkoxy intermediate species is formed from Pd(II), CO and acetylene alcohol, followed by intramolecular cis addition to the triple bond [37,38] (Scheme 21).…”

Synthesis of heterocycles by carbonylation of acetylenic compounds

“…In this system, not only a-methylene-g-lactones, but also -d-lactones, of various structure in cis-and transfused rings can be synthesised in rather high yields, if the substrate concentration is kept sufficiently low (0. Pd(II), CO and the acetylene alcohol, followed by intramolecular cis addition to the triple bond 77,78 (Scheme 60).…”

“…As in PdI 2 /Bu 3 P/MeCN catalyst system, the rate-determining step is evidently the uptake of CO by Pd. Similarly, in the second catalyst system (PdCl 2 /2Ph 3 P/SnCl 2 /MeCN), the rate-determining step is coordination of the substrate, followed by rapid uptake of CO and completion of the cyclocarbonylation reaction 78 by simple distillation, which enabled the re-use of the ionic catalyst solution. 79 The carbonylative lactonisation of 5-hydroxy-1-pentyne 147 to produce a-methylene-d-lactone 148 has been found to proceed efficiently by using catalytic Pt ( (2) regioselective acylplatination leading to species C, followed by reductive elimination to produce the a,b-unsaturated thioester D and Pt(0); and (3) intramolecular cyclisation of D to provide a-methylene-d-lactone 148 and ArSH, the latter adding oxidatively to Pt 0 and regenerating the catalyst A.…”