Zum cis ‐Effekt bei radikalischen Additionen an Alkene

Abstract

Die trans-Alkene 2e -h reagieren rascher mit Cyclohexyl-Radikalen als die cls-Alkene 3e -h (Tab. 1). Dabei zeigen radikalische Additionen an Alkylidenmalonsaureester 1, daR dieser cisEffekt mit der GruppengrBBe der Substituenten steigt (Tab. 1). Der Geschwindigkeitsunterschied zwischen trans-und cis-Alkenen hat jedoch keinen EinfluR auf die Regioselektivitat (Tab. 3). Es wird geschlossen, daR der cis-Effekt auf einer Verdrillung des n-Systems beruht, die sich auf die Reaktivitat der beiden olefinischen C-Atome in gleicher Weise auswirkt. On the &Effect of Radical Addition Reactions at AlkenesThe rruns-alkenes 2e-h react faster with cyclohexyl radicals than the crs-alkenes 3e-h (Table 1). Radical addition reactions at alkylidenemalonic esters 1 show that this cis-effect increases with the bulkyness of the substituents (Table 1). However, the difference in the reactivity between transand cis-alkenes exerts no effect on the regioselectivity (Table 3). It is concluded that the reason for the cis-effect is a twisting of the n-system that influences the reactivity of both vinylic C-atoms to the same extent.Fumarsaure-diethylester reagiert mit Cyclohexyl-Radikalen bei 20 "C 9.2mal rascher als Maleinsaure-diethylester 1). Dieser Reaktiviatsunterschied beruht vermutlich auf der Abstohng zwischen den cis-Substituenten, die einer Einebnung des n-Systems entgegenwirken2). Urn diese Hypothese zu iiberpriifen, haben wir den Effekt von Substituenten Y irn trans-Acrylsaureester 2 mit dem im Alkylidenmalonsaureester 1 bzw. c&-Acrylsaureester 3 bei der Addition von Cyclohexyl-Radikalen verglichen.Die relativen Ceschwindigkeitskonstanten wurden in konkurrenzkinetischen Messungen pseudo-erster Ordnung nach der ,,Quecksilber-Methode" bestimmt 3). Hierbei y\ ,CO&

Formation of Macrocyclic Ethers by Free Radical Cyclization: Effects of Chain Length, Substituents, and Solvents

“…As expected for a mechanism involving irreversible cyclization, the exo/endo ratios of products derived from 2c − e were invariant for reactions conducted at various stannane and substrate concentrations. Except in the case of 2b , the regioselectivities of these intramolecular additions are in good agreement with those previously reported for the intermolecular addition of alkyl radicals to the corresponding unsaturated ethyl or methyl esters. − Cyclization of 2b represents an intramolecular analogue of intermolecular addition to a crotonic ester. For the addition of cyclohexyl radical to methyl crotonate Giese and co-workers 25,26 identified the products resulting from addition at each terminus of the olefinic bond.…”

Iridium-Catalyzed Asymmetric Hydrogenation with Simple Cyclohexane-Based P/S Ligands: In Situ HP-NMR and DFT Calculations for the Characterization of Reaction Intermediates

“…The enantiomeric excess was determined by chiral GC or chiral HPLC, and conversions were determined by 1 H NMR. The enantiomeric excesses of hydrogenated products from S1 , S2 , S3 and S4 , S5 , S6 , S7 – S9 , S10 , S11 , S12 , S13 , S14 – S17 , S18 , S19 , d S20 , S21 , S22 , S23 , S24 , S25 – S31 , S32 , S33 – S37 , S38 , S39 , and S40 were determined using the conditions previously described.…”

Palladium‐Catalyzed Formation of Cyclic Ethers – Regio‐, Stereo‐ and Enantioselectivity of the Reaction

“…Technical solvents were distilled by standard literature methods before use. [50] Dimethyl ethylmalonate, [51] dimethyl benzylmalonate, [52] dimethyl (4-methylbenzyl)malonate, [52] dimethyl (2-naphtylmethyl)malonate, [53] dimethyl [(E)-3-phenylprop-2-en-1-yl]malonate, [54] dimethyl (cyclohexylmethyl)-malonate, [55] (Z)-4-hydroxybut-2-en-1-yl acetate (1), [38] (Z)-4-{[tertbutyl(dimethyl)silyl]oxy}but-2-en-1-yl acetate (2), [39] (E)-5-chloropent-2-en-1-ol, [56] (E)-6-bromohex-2-en-1-ol, [57] and [(4-bromobut-2-yn-1-yl)oxy](tert-butyl)dimethylsilane (12) [40] were prepared by literature methods. Tin-layer chromatography was performed on aluminum-backed plates Merck silica gel 60 F 254 .…”