Me = Et > i-Pr & t-Bu"]. I-tert-Butoxy-2,7-octadiene, which was not obtained with conventional catalysts, is also formed. Higher telomers are formed in all cases except with t-BuOH. The presence of large amounts of phenoxy-C,2-derivatives may reflect an alteration in the reaction mechanism.The reaction presumably proceeds via condensation of C8-units, since almost only C8,,-products are formed. Indeed, I -methoxy-2,7-octadiene 2 is converted inter alia into the ether 5. The occurrence of two or even three vacant coordination sites may be a prerequisite for the formation of such higher telomers.
ProcedureSolution of l a : A mixture of Pd,(dba)3.CHC1,[41 (130 mg, 0.125 mmol) and [(CH2=C(CH,)CH20)P(NMe2)3]ePF~[s1 (95 mg, 0.25 mmol) is placed in a 50 mL Schlenk tube and degassed by three vacuum-argon atmosphere cycles. Toluene (10 mL) is then added and the reaction mixture stirred under argon for 120 min.Catalytic reaction: The pale yellow solution is filtered through Celite in order to remove traces of metallic palladium and then transferred via a syringe into a glass-lined 100 mL stainless steel autoclave. Methanol (3 mL, 75 mmol) and butadiene (16.2 g, 300 mmol) are then added. The autoclave is stirred magnetically and heated to 80 "C for 20 h ; it is then cooled down and the excess butadiene is vented through a cold trap. The conversion is 94%. The products are analyzed by GC with dodecane and eicosane as internal standards.All reactions are carried out under argon.
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&
The successful exploitation of radicals in syntheses necessitates a detailed knowledge of the stereoselectivity of their reactions. With the vinyl radical 1 as example it can be shown how this is achieved by variation of the radical trapping agents (H donors; c‐C6H11HgH, Bu3SnH, c‐C6H12). Below the isoselective temperature the stereoselectivity increases with decreasing rate of formation of the products. In H‐abstractions the less stable (Z)‐alkenes are formed.
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