Stereochemistry of the pentadienyl-cyclopentenyl cation rearrangement

Campbell, P. H.; Chiu, N. W. K.; Deugau, K. V.; Miller, Ian; Sorensen, T. S.

doi:10.1021/ja01051a038

Cited by 51 publications

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“…spectra with that found for 3a and 3b (9). For example, the C-4 and -5 methyl groups in 3a (cis) appear at a higher field than in 3b (trans) and a corresponding observation is consistent with the assignments for 8a and 8b.…”

Section: (supporting

confidence: 83%

“…It can be described as an electrocyclic transformation and is predicted to proceed in a conrotatory manner.' In our previous paper relating to the stereochemistry of this transformation (9), we reported the thermal acid cyclization of the isomeric dienols la-c (via the presumed intermediacy of the dienylic cation 2) to the cyclopentenyl cation 3a or 3b. The cyclization is nearly stereospecific and indeed proceeds in a conrotatory manner as predicted by the Woodward-Hoffmann rules (8).…”

Section: Introductionmentioning

confidence: 99%

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The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

Chiu¹,

Sorensen²

1973

Can. J. Chem.

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Two isomeric 1,2,4,5-tetramethylpentadienyl cations ( 4 1 and 4b) have been observed in FSO3H-SOzCIF at -125". They undergo stereospecific thermal cyclizations to cyclopentenyl cations in accord with the Woodward-Hoffmann rules.Les deux cations isomtres ( 4 1 et 5 ) du tetramethyl-1.2,4,5 cyclopentadienyle ont CtC observes dans un melange de FSOsH-SOzCIF a -125". Ils subissent des cyclisations thermiques sterCospCcifiques qui les transforment en cations cyclopenttnyles suivant les rkgles de Woodward-Hoffmann.[Traduit par le journal]

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Section: (supporting

confidence: 83%

Section: Introductionmentioning

confidence: 99%

The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

Chiu¹,

Sorensen²

1973

Can. J. Chem.

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“…This reaction proceeds via the intermediacy of a diallylic carbocation, which undergoes a conrotatory electrocyclic ring closure in accordance with the Woodward-Hoffman rules. 10 This reaction is widely utilized in synthetic organic chemistry, as well as in organometallic chemistry where it has provided a route to substituted polymethylcyclopentadienyl ligands. 11 As proof of principle, 10 equivalents of 3,4,5-trimethylhepta-2,5-dien-4-ol 2 (obtained as a mixture of the three possible olefin stereoisomers, Scheme 2b), were added to a solution of 1 in H 2 O (buffered to pH 11.0), and heated at 50 °C.…”

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confidence: 99%

Enzymelike Catalysis of the Nazarov Cyclization by Supramolecular Encapsulation

et al. 2010

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A primary goal in the design and synthesis of molecular hosts has been the selective recognition and binding of a variety of guests using non-covalent interactions. 1 Supramolecular catalysis, which is the application of such hosts towards catalysis, 2 has much in common with many enzymatic reactions, 3 chiefly the use of both spatially appropriate binding pockets and precisely oriented functional groups to recognize and activate specific substrate molecules. Although there are now many examples which demonstrate how selective encapsulation in a host cavity can enhance the reactivity of a bound guest, all have failed to reach the degree of increased reactivity typical of enzymes. 4 We now report the catalysis of the Nazarov cyclization by a self-assembled coordination cage, a carbon-carbon bond-forming reaction which proceeds under mild, aqueous conditions. The acceleration in this system is over a million-fold, and represents the first example of supramolecular catalysis that achieves the level of rate enhancement comparable to that observed in several enzymes. We explain the unprecedented degree of rate increase as due to the combination of (a) preorganization of the encapsulated substrate molecule, (b) stabilization of the transition state of the cyclization by constrictive binding, and (c) increase in the basicity of the complexed alcohol functionality.Raymond and coworkers have designed a supramolecular host (1) with Ga 4 L 6 stoichiometry (L = N,N ′ -bis(2,3-dihydroxybenzoyl)-1,5-diaminonaphthalene, Figure 1) that exploits reversible metal-ligand interactions to spontaneously self-assemble. 5 In these assemblies the biscatecholate ligands span the edges of a tetrahedron in which the vertices are occupied by metal ions. Polyanion 1 is soluble in water and other polar solvents, while the host interior is panelled by the naphthalene rings of the ligand, creating a hydrophobic inner environment.A wide variety of cationic guests are encapsulated in 1, from quaternary ammonium and phosphonium cations to organometallic complexes. In aqueous solution, neutral molecules, such as hydrocarbons, are bound by 1 due to the hydrophobic effect. 6 Our approach towards supramolecular catalysis using 1 exploits the polyanionic host's preference for encapsulating monocationic guests. Encapsulation in 1 can perturb acid-base equilibria to favor the protonation of a wide range of amines and phosphines, even at strongly basic pH. 7 These investigations led to the development of proton-catalyzed hydrolysis reactions inside of 1, in which a protonated transition-state is stabilized in the host interior. 8 The stabilization of even transient protonated species produces a several thousandfold rate acceleration of orthoformate and acetal hydrolysis under basic conditions. However, even this substantial acceleration does not reach that typically seen in enzyme-catalyzed reactions.

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“…The analytical procedure, using n.m.r. spectroscopy, for determining the relative amounts of 1 and 2 in mixtures, has been published (4). The cis ion 1 -trans ion 2 equilibrium (211) favors the trans isomer (K = 3.9 k 0.3) and the most accurate kinetic data are obtained naturally by measuring the cis + trans conversion.…”

Section: Resultsmentioning

confidence: 99%

“…The purification of the acid solvents, the method of adding the organic compound to the acids and the method of analyzing for 1 and 2 in mixtures have been reported previously (4). Particular care was taken to prepare the ion solutions under anhydrous conditions and in each case the n.m.r, tubes were sealed with tape.…”

Section: Methodsmentioning

confidence: 99%

Mechanism of a base-catalyzed carbonium ion rearrangement

Sorensen¹,

Miller²,

Urness³

1970

Can. J. Chem.

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The rearrangement of cis-1,2,3,4,5-pentamethylcyclopentenyl cation to the trans isomer in aqueous sulfuric acids has been found to follow Ho with unit slope (d log k,,,,,. = dHo). This rearrangement involves the intermediacy of a neutral compound, 1,2,3,4,5-pentamethylcyclopentadiene, which on reprotonation is only partly efficient in leading to the trans ion product. Under both steady-state intermediate and direct protonation conditions, this diene protonates to give about 55 % of the cis ion and only 45 % of the thermodynamically more stable trans isomer.

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Stereochemistry of the pentadienyl-cyclopentenyl cation rearrangement

Cited by 51 publications

References 0 publications

The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

Enzymelike Catalysis of the Nazarov Cyclization by Supramolecular Encapsulation

Mechanism of a base-catalyzed carbonium ion rearrangement

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