Thermodynamic and Kinetic Relationships between Lewis Acid−Lewis Base Complexes Relevant to the P2Pt(OTf)2-Catalyzed Diels−Alder Reaction
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Abstract
The Pt(II) Lewis acids P2Pt(OTf)2 (2; P2 = dppe (a), R-BINAP (b)), which catalyze the Diels−Alder reaction of acryloyl-N-oxazolidinone (4) with cyclopentadiene (HCp), were generated in situ by activation of P2Pt(S-BINOL) precursors 1 with 2 equiv of triflic acid (HOTf). Catalysts 2 and catalytically relevant Lewis acid−Lewis base complexes [P2Pt(L2)]2+[OTf]-2 (L2 = 2 H2O (3), dienophile 4 (6), Diels−Alder adduct 5 (8)) were characterized by 1H and 31P NMR spectroscopy at 195 K. 2a was also characterized by X-ray crystallography. The thermodynamic relationships between 2, 3, 6, 8, and a BF4- analogue of 2 were determined through competitive binding experiments monitored by 31P NMR at 195 K, to assess the effects of competitive inhibition of counterions, additives, and product on substrate coordination to the catalyst. These experiments demonstrated that the Lewis bases bind to [P2Pt]2+ with relative strengths BF4- ≪ OTf- < 4 < 5 ≪ H2O. The rates of two ligand exchange processes (dienophile/dienophile (k1) and dienophile/OTf- (k2)) were measured at 270 K by simulation of exchange-broadened 31P NMR spectra of 2/6 equilibrium mixtures, revealing that ligand substitution occurs more rapidly in the dppe system than the R-BINAP system (180 times faster for k2). Finally, the reaction of HCp with 6 to give 8 was monitored by 31P and 1H NMR at 195 K; attack of HCp on Pt-coordinated 4 occurred much faster than ligand exchange, indicating that ligand exchange is the turnover-limiting step of the catalytic cycle.
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