Über die stereoelektronischen Bedingungen der 1,2‐Umlagerungen bei Vinylkationen

Abstract

Die Triflate 8,9, 12,17, 19 und 23 wurden synthetisiert, um die Produkte ihrer Solvolyse in absol. 2,2,2-Trifluorethanol (TFE) zu untersuchen (Tab. 1). Auch die Kinetik der Solvolysen in 50proz. Ethanol wurde bestimmt (Tab. 2). Die Solvolysegeschwindigkeiten ergeben eine Korrelationsgerade bei einer vereinfachten Taflschen Darstellung, woraus abgeleitet wird, da8 die Solvolysen nach einem k,-Mechanismus erfolgen; eine Ausnahme ist 12, das durch S -0-Spaltung solvolysiert.Wir schlieRen daraus, daR p* von sterischen Effekten unabhangig ist. Es werden Beziehungen zwischen Struktur und Umlagerung bei Vinylkationen diskutiert und Mechanismen fur die Bildung der Reaktionsprodukte vorgeschlagen. On the Stereoelectronic Requirements of 1,2-Rearrangements of Vinyl CationsThe triflates 8,9, 12, 17, 19, and 23 have been synthesized to investigate their solvolysis products in absol. 2,2,2-trifluoroethanoI (TFE) ( Table 1). The kinetics of the solvolysis in 50% ethanol have been measured as well. The rate constants give a straight correlation line when we apply an approximate Taft's equation, from which we deduce that the solvolyses take place by a k, mechanism; one exception is 12, as it reacts with S -0 bond cleavage. We also deduce that the value of p* does not depend on steric factors. Relations between structure and rearrangement of vinyl cations are discussed. Mechanisms for the formation of reaction products are proposed.1,2-Umlagerungs-Produkte entstehen haufig bei Umsetzungen uber Carbokationen. Da Carbokationen-Zwischenstufen sehr haufig sind, ware es interessant voraussagen zu konnen, welche Bedingungen Substrat und Reaktionsmedium erfiillen mussen, damit eine bestimmte Umlagerung auftritt. Die Umlagerungen von trisubstituierten, gesattigten Carbokationen sind bereits gut untersucht I), die durch Topologie und Stabilitat stark abweichenden Vinylkationen nur verhaltnismaBig wenig2). Bei den sp-hybridisierten Vinylkationen (3) sind Umlagerungen in Richtung auf die Doppelbindung (a) und uber die Doppelbindung hinweg (b) moglich; in der vorliegenden Arbeit werden wir uns nur mit der ersten Moglichkeit beschaftigen. 0 Verlag Chemie GmbH, D

Diastereoselection in the Formation of Spirocyclic Oxindoles by the Intramolecular Heck Reaction

“…The TBDMS group was then exchanged for triflate by a reaction with CsF, Cs 2 CO 3 , and PhNTf 2 to provide 31 in 66% overall yield. Octalin 33 , and its double-bond isomer 34 , were prepared in 56% combined yield by a similar two-step sequence from the known mixture of alkene isomers of triflate 32 and were separated by preparative HPLC.

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“…Lithiation of propiolic acid anilide 24a ,19 followed by quenching with CO 2 provided the corresponding acid. Subsequent coupling of this acid with N-benzyl-2-(tert-butyldimethylsiloxy)aniline (25) promoted by 1ethyl-3-(3'-dimethylaminopropyl)carbodiimide hydrochloride (EDCI) resulted in the formation of dianilide 26a. 20 Attempts to carry out the cycloaddition of 26a and 1,3-butadiene thermally were not successful.…”

Mechanism of hydrogenolysis. Part 1. Catalytic hydrogenation of vinyl and aryltrifluoromethanesulphonates

“…The ether phase was washed successively with saturated aqueous sodium hydrogen carbonate and water, and dried (MgSO,). Removal of the solvent and chromatography of the residue on basic alumina using pentane as the eluant afforded the product (10) 1-Methoxycyclohexene (14).-This was prepared from cyclohexanone and trimethyl orthoformate adapting the same procedure as that described for compound (10) General Procedure for Hydrogenation.-To a solution of the substrate (5 mmol) in absolute ethanol (50 ml) contained in a hydrogenation flask, was added the catalyst (70 mg). After the flask had been evacuated and filled with hydrogen three times, the pressure was adjusted to 1 bar and the contents of the flask were hydrogenated at room temperature, with shaking.…”

“…However, the likewise hindered vinyl triflate (9) undergoes hydrogenolysis under the same conditions giving a good yield of the hydrocarbon product cis/trans-(18) indicating that the reduction is not taking place according to the Nishimura m e c h a n i~m .~ The Nishimura mechanism explains the fact that the yield of the hydrogenolysis products from the aryl and vinyl triflates (7) and (12) are higher than the aryl and vinyl ethers (8) and (14) owing to the easy elimination of trifluoromethanesulphonic acid as compared to ethanol/methanol in the latter cases. However, the likewise hindered vinyl triflate (9) undergoes hydrogenolysis under the same conditions giving a good yield of the hydrocarbon product cis/trans-(18) indicating that the reduction is not taking place according to the Nishimura m e c h a n i~m .~ The Nishimura mechanism explains the fact that the yield of the hydrogenolysis products from the aryl and vinyl triflates (7) and (12) are higher than the aryl and vinyl ethers (8) and (14) owing to the easy elimination of trifluoromethanesulphonic acid as compared to ethanol/methanol in the latter cases.…”

“…data, the product contained 1-Methoxycyclohexene (14).-This was prepared from cyclohexanone and trimethyl orthoformate adapting the same procedure as that described for compound (10) General Procedure for Hydrogenation.-To a solution of the substrate (5 mmol) in absolute ethanol (50 ml) contained in a hydrogenation flask, was added the catalyst (70 mg). and 'H n.m.r.…”