Radical Ion Probes. 8. Direct and Indirect Electrochemistry of 5,7-Di-<i>tert</i>-butylspiro[2.5]octa-4,7-dien-6-one and Derivatives

Phillips, J. Paige; Gillmore, Jason G.; Schwartz, Phillip A.; Brammer, Larry E.; Berger, and Daniel J.; Tanko, James M.

doi:10.1021/ja972795o

Cited by 20 publications

(31 citation statements)

References 37 publications

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“…The obtained value is well consistent with the data in Ref. 21, where the DET reactions of spiro[2.5]octa 4,7 dien 6 one accompanied by the C-C bond cleavage were studied.…”

Section: Methodssupporting

confidence: 92%

Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

2007

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Processes of direct and electrocatalytic (in the presence of electrochemically reduced Cp 2 TiCl 2 ) reduction of three α bromoketones containing the C(sp 3 )-Br or C(sp 2 )-Br bond, viz., 2 bromo and 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 ones and α bromo acetophenone, were studied by cyclic voltammetry and preparative electrolysis. In all cases, the dissociative electron transfer proceeds via the concerted mechanism. Preparative electrolysis of these α bromoketones in the presence of Cp 2 TiCl 2 affords the reductive debromination prod ucts in 40-80% yield at low cathodic potentials (-0.85 V vs. Ag/AgCl/KCl). In the case of 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 one in the potentiostatic regime, only one bromine atom can be eliminated selectively.In most cases, the direct electrochemical reduction of alkyl and vinyl halides occurs at rather high negative po tentials 1 and, therefore, the dehalogenation of compli cated polyfunctional compounds is usually nonselective. The use of non direct electrocatalytic reduction (media tor redox catalysis) makes it possible to decrease consid erably the potential of the reaction, which can be substan tial for selectivity of the process.It is known 2 that in the nature many dehalogenation processes proceed via the reduction mechanism involving the catalysts containing the compounds of Co, 3,4 Ni, 5-7 Cu, 8 and other metals. We have previously shown 9,10 that the electrochemically reduced titanocene dichloride (Cp 2 TiCl 2 ) can serve as a catalyst for the reductive dehalogenation of benzylic halides, whereas aryl halides usually undergo no dehalogenation. The present work con tinues these studies and is devoted to the elucidation of the applicability of this approach to cleavage of the C-Hal bond of other types. α Bromoketones containing the C(sp 3 )-Br or C(sp 2 )-Br bond, which is activated by the adjacent carbonyl group, namely, α bromoaceto phenone (1), 2,6 dibromo (2) and 2 bromo 4,4 di methylcyclohexa 2,5 dien 1 one (3), were chosen as ob jects for the study. The latter two compounds are of spe cial interest, because they represent the structural frag ment of discorhabdine molecules C and E (4 and 5, re spectively), which are present in the Lafrunculia sea sponge. It is known 11 that the antimicrobial properties of natural discorhabdines are due to the ability of the C-Hal bond to cleave. This cleavage proceeds presumably via the reduction mechanism. Therefore, it was interesting to reveal whether the electrocatalytic dehalogenation of simi lar compounds is possible in the presence of the electro chemically reduced titanocene dichloride and whether this process is a model of reactions involving discorhabdines in the nature or not. ExperimentalCyclic voltammograms (CV curves) were recorded using an IPC Win potentiostat at the stationary Pt electrode (d = 5 mm) against Bu 4 NBF 4 in anhydrous organic solvents (THF, MeCN) at 20 °С in a special electrochemical cell switched on according

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“…The obtained value is well consistent with the data in Ref. 21, where the DET reactions of spiro[2.5]octa 4,7 dien 6 one accompanied by the C-C bond cleavage were studied.…”

Section: Methodssupporting

confidence: 92%

Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

2007

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“…The rate constant for electron transfer (k 1 ) is obtained by fitting of the data in Figure 1 to the appropriate working curve as described previously. 20,21 The reduction potential of the substrate is estimated by using the reduction potential of the perylene and assuming k -1 ) 5 × 10 9 M -1 s -1 , the diffusion-controlled rate constant in DMF solvent.…”

Section: Trans-1-benzoyl-2-phenylcyclopropane (1)mentioning

confidence: 99%

Activation/Driving Force Relationships for Cyclopropylcarbinyl → Homoallyl-Type Rearrangements of Radical Anions

Chahma

Phillips

et al. 2005

J. Phys. Chem. A

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By using direct and indirect electrochemical methods, rate constants (ko) for cyclopropane ring opening of radical anions derived from the one-electron reduction of trans-1-benzoyl-2-phenylcyclopropane, trans-1-benzoyl-2-vinylcyclopropane, 2-methylenecyclopropyl phenyl ketone, spiro[anthracene-9,1'-cyclopropan-10-one], 3-cyclopropylcyclohex-2-en-1-one, and 3-(1-methylcyclopropyl)cyclohex-2-en-1-one were determined. Qualitatively, rate constants for ring opening of these (and other cyclopropyl- and cyclobutyl-containing radical anions) can be rationalized on the basis of the thermodynamic stability of the radical anion, the ability of substituents on the cyclopropyl group to stabilize the radical portion of the distonic radical anion, and the stability of the enolate portion of the distonic radical anion. On the basis of this notion, a thermochemical cycle for estimating deltaG(o) for ring opening was presented. For simple cyclopropyl-containing ketyl anions, a reasonable correlation between log(ko) and deltaG(o) was found, and stepwise dissociative electron transfer theory was applied to rationalize the results. Activation energies calculated with density functional theory (UB3LYP/6-31+G*) correlate reasonably well with measured log(ko). The derived log(ko) and deltaG(o) and log(ko) vs E(a) plots provide the basis for a "calibration curve" to predict rate constants for ring opening of radical anions derived from carbonyl compounds, in general.

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“…Assuming k d = 1.0 × 10 10 M -1 s -1 (the diffusion-controlled rate constant in DMF), K d = 0.16 M -1 , and the frequency factor Z = 6 × 10 11 s -1 , the rate constants were fit to eq 2 via nonlinear regression analysis, with E ° 1/1 •− and the reorganization energy λ as the only adjustable parameters. For a more detailed description of this treatment, see ref .…”

Section: Referencesmentioning

confidence: 99%

“…Procedures for determining whether k 1 or k 2 is rate-limiting and for extracting the pertinent rate constants have been described by Savéant; the specific procedures utilized in this study are similar to those described by us in an earlier publication . For 1a and 1b , electron transfer was found to be rate-limiting for all mediators examined, and the pertinent kinetic information is summarized in the Supporting Information.…”

mentioning

confidence: 93%