Catalysis of the debromination of 4-alkyl-4-bromo-2,5-cyclohexadienones in aqueous solution by .alpha.-cyclodextrin

Abstract

Cyclodextrin (CD) has little or no effect on the rates of enolization of transient 4-bromo-2,5-cyclohexadienones (2), formed during the aqueous bromination of alkylphenols. In contrast, saturation kinetics and large catalytic effects are observed for the debromination of the title dienones (4), formed by ipso bromine attack on 4-alkylphenols (alkyl = Me, Et, z'-Pr, n-Pr, Z-Bu, 3,4-diMe). With the exception of the -propyl case, the extent of the catalysis (kz/ku = 23-78) and the dissociation constants of the CD-dienone complexes (ATd = 2.32-4.83 mM) show surprisingly little variation for the different alkyl groups. The simplest interpretation of the results is that the CD-catalyzed debromination reaction involves attack by free bromide ion on the CD-dienone complex. However, the kinetically equivalent pathway, reaction between the free dienone 4 and the CD complex of bromide ion, is much more consistent with the low sensitivity of the catalysis to the length and size of the different alkyl grpups. For this mechanism the rate enhancements are much larger (2400-4600) and almost constant. They imply that Br" in its CD complex is a stronger nucleophile than bromide ion that is completely solvated by water. The preferred mechanism is the microscopic reverse of that postulated for the CD-catalyzed bromination of phenols. The common transition state for the ipso bromination of 5 (R = Me) and the debromination of 4 (R = Me) is strongly bound by CD (Afd ~4.5 X 10"5 M).

Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives

“…[136][137][138] nation of some bromocyclohexadienones (the proposed mechanism is shown in Figure 7), the reverse of one of the steps in the bromination of aromatic rings. 147,148 R-CD also modestly catalyzes the reaction of formic acid with bromine. 149 The Diels-Alder reaction is an example of an important chemical process for which enzyme catalysts are not available.…”

“…Also, there are many examples in which the point of reaction of an external reagent is influenced when a substrate binds into a cyclodextrin cavity − ,− but so far with no evidence that the cyclodextrin catalyzes the process as it does in the HOCl chlorinations. The bromination of anisole and some phenols shows at most a slight acceleration by α-CD and is often retarded because tribromide ion binds into the cyclodextrin cavity. , However, α-CD catalyzes the debromination of some bromocyclohexadienones (the proposed mechanism is shown in Figure ), the reverse of one of the steps in the bromination of aromatic rings. , α-CD also modestly catalyzes the reaction of formic acid with bromine 7 Cyclodextrin binding promotes the alkylation of substrate 8 140 and the debromination of a bromocyclohexadienone …”

Effect of cyclodextrin complexation in bromine addition to unsymmetrical olefins: evidence for participation of cyclodextrin hydroxyl groups

“…This result also provides evidence for the absence of any inclusion of bromine into the CD cavity under the present experimental conditions. It is relevant to note here that in ring bromination of phenols catalyzed by CDs, bromine complexed in CD is proposed 21 to account for the observed catalytic activity.…”

Proteïnes com a microreactors en fotoquímica supramolecular

“…Les ciclodextrines són oligosacàrids cíclics formats usualment per 6, 7 o 8 unitats de D-(+)-glucopiranosa i en general es produeixen per hidròlisi enzimàtica [30] . Són unes macromolècules molt atractives ja que, d"una banda, tenen una cavitat hidrofòbica on poden unir-se certs hostes i, de l"altra, posseeixen grups hidroxil que poden bé permetre A la bibliografia es troben nombrosos exemples de com diverses ciclodextrines i ciclodextrines modificades són capaces de catalitzar reaccions simples del tipus DielsAlder [31,32] , alquilacions [33] , halogenacions [34] , deshalogenacions [35] , fosforilacions [36] , etc. I també reaccions més complexes mimetitzant enzims tals com estearases [37] , glioxalases [38] , transaminases [39] o epoxidases [40] .…”

“…. Aquests resultats estaven en concordança amb el ja conegut efecte de protecció que la HSA presenta front a l"autodesactivació (en anglés selfquenching) i front a l"atac de l"oxigen [35][36][37] . Així doncs, l"HSA estabilitza els estats excitats triplets de MeAB, BrAB i PrAB.…”