(dppe)Pt(OTf)2 and (R-BINAP)Pt(OTf)2 Lewis Acids Catalyze the Diels−Alder Reaction by Different Mechanisms

Abstract

In situ observation, by 31P and 1H NMR at 195−240 K, of acryloyl-N-oxazolidinone (4)/cyclopentadiene (HCp) Diels−Alder reactions catalyzed by the Pt(II) Lewis acids P2Pt(OTf)2 (2; P2 = dppe (a), R-BINAP (b)) revealed that the ditriflate catalysts are consumed by HCp at different rates to produce the catalytically inactive 18-electron complexes [P2Pt(η5-Cp)]+[OTf]- (9). Mechanistic studies point to formal loss of HOTf from the proposed intermediate diene complex [P2Pt(HCp)]2+[OTf]-2, facilitated by a necessary second equivalent of HCp; ill-defined HCp/HOTf byproducts, whose 1H NMR resonances include a broad singlet at 12.7 ppm, are also formed. Both catalyst decomposition and Diels−Alder catalysis proceeded faster in reactions involving dppe Lewis acid 2a than in those involving R-BINAP Lewis acid 2b. In the 2b reaction at 195 K, Pt-bound dienophile 4 was immediately converted to the Pt-bound Diels−Alder adduct 2S-5 on addition of HCp; however, no turnover occurred and no η5-Cp complex was detected until the temperature was raised to 225 K. At this temperature, no low-field resonances were observed by 1H NMR, a highly enantiopure Diels−Alder adduct was obtained, and TOFs less than the rates of ligand exchange were observed. These data are consistent with catalysis by a chiral [P2Pt]2+ Lewis acid, in which ligand substitution is the turnover-limiting step of the catalytic cycle. In contrast, 9a and Diels−Alder adduct rac-5 formed rapidly at 195 K in the 2a reaction. The observation of low-field HCp/HOTf resonances (1H NMR) and TOFs greater than the rates of ligand exchange together suggest that, in this case, the Diels−Alder reaction is catalyzed primarily by a Brønsted acid mechanism involving the HCp/HOTf byproduct of 2a decomposition, rather than by [(dppe)Pt]2+.