(dppe)Pt(OTf)2 and (R-BINAP)Pt(OTf)2 Lewis acids catalyze the Diels-Alder reaction by different mechanisms

Nicole M. Brunkan, Michel R Gagne

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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+.

LanguageEnglish (US)
Pages1576-1582
Number of pages7
JournalOrganometallics
Volume21
Issue number8
DOIs
StatePublished - Apr 14 2002

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Lewis Acids
Diels-Alder reactions
Nuclear magnetic resonance
acids
adducts
Ligands
nuclear magnetic resonance
Catalysis
Byproducts
ligands
catalysis
Oxazolidinones
Decomposition
Cyclopentanes
Catalysts
decomposition
catalysts
dienes
Substitution reactions
Temperature

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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(dppe)Pt(OTf)2 and (R-BINAP)Pt(OTf)2 Lewis acids catalyze the Diels-Alder reaction by different mechanisms. / Brunkan, Nicole M.; Gagne, Michel R.

In: Organometallics, Vol. 21, No. 8, 14.04.2002, p. 1576-1582.

Research output: Contribution to journalArticle

@article{9511a69d3d504126a37589d2a3357899,
title = "(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 Br{\o}nsted acid mechanism involving the HCp/HOTf byproduct of 2a decomposition, rather than by [(dppe)-Pt]2+.",
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T1 - (dppe)Pt(OTf)2 and (R-BINAP)Pt(OTf)2 Lewis acids catalyze the Diels-Alder reaction by different mechanisms

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AU - Gagne, Michel R

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N2 - 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+.

AB - 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+.

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