Sterically hindered free radicals.  14.  Substituent-dependent stabilization of para-substituted triphenylmethyl radicals

Neumann, Wilhelm P.; Uzick, Wolfram; Zarkadis, Antonios K.

doi:10.1021/ja00273a034

Cited by 90 publications

(51 citation statements)

References 1 publication

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…are in the vicinity of 90 kJ mol −1 . They are thus much smaller than those for an ordinary C−C bond but are still approximately three times larger than those for Jacobson–Nauta‐type dimers . For bulky Fc Me5 OMe .…”

Section: Resultsmentioning

confidence: 99%

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl‐Substituted Tritylium Dyes

Oßwald

Casper

Anders

et al. 2018

Chemistry A European J

View full text Add to dashboard Cite

Four monoferrocenyl tritylium derivatives with donor-substituted (OMe, NMe ) aryl rings are reported, along with their spectroscopic and electrochemical properties. All the complexes show a one-electron reduction and a quasi-reversible ferrocene oxidation at a very positive potential. Small quadrupole splittings, ΔE , in Mößbauer spectra agree with highly electron-deficient ferrocenes. Comparison of the experimental half-wave potentials for ferrocene oxidation, E (Fc/Fc ), with those estimated from established correlations of E (Fc/Fc ) with ΔE indicates that the E values of the anisyl-substituted congeners FcOMe and FcMeOMe are affected by Coulombic repulsion between the positive charges at the Fe ion and the neighboring methylium site. Electronic spectra are recorded and interpreted with the aid of quantum chemical calculations. UV/Vis spectroelectrochemical measurements as well as chemical reduction provide insight into the redox-induced color changes upon ferrocene oxidation or upon reduction to the neutral trityl radicals. The neutral radicals reversibly form EPR-silent dimers. This process is studied by temperature-dependent EPR spectroscopy, and thermodynamic data for their dimerization are determined. Experimental and quantum chemical data suggest that the dimers assume classical hexaarylethane structures as opposed to normal or "offset" Jacobson-Nauta-type structures.

show abstract

Section: Resultsmentioning

confidence: 99%

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl‐Substituted Tritylium Dyes

Oßwald

Casper

Anders

et al. 2018

Chemistry A European J

View full text Add to dashboard Cite

show abstract

“…However, it was demonstrated that the symmetric pinacol coupling of an aromatic ketone PhCOR is impeded by the presence of bulky R groups and/or coordination of sterically demanding ligands on the metal complex. [39] The dissymetric dimerization of the ketyl radical through para phenyl/carbonyl carbon coupling, reminiscent of coupling of trityl radicals, [40] was then found to occur.…”

Section: Resultsmentioning

confidence: 99%

Reactions of Aliphatic Ketones R2CO (R=Me, Et,iPr, andtBu) with the MCl4/Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions

Villiers

Ephritikhine

2001

Chem. Eur. J.

View full text Add to dashboard Cite

Analysis of the products of the reactions of ketones R2CO (R = Me, Et, iPr, tBu) with the MCl4/Li(Hg) system (M = U, Ti) at 20 degrees C revealed significant differences. For R = Me, the reaction proceeded exclusively (M = U) or preferentially (M = Ti) via a metallopinacol intermediate resulting from dimerization of ketyl radicals. Pinacol was liberated by hydrolysis, and tetramethylethylene was obtained after further reduction at 65 degrees C. For R=iPr, formation of iPr2C=CiPr2 as the only coupling product, the nonproduction of this alkene by reduction of the uranium pinacolate [U]-OCR2CR2O-[U] (R= iPr) at 20 degrees C, and the instability of the corresponding titanium pinacolate towards rupture of the pinacolic C-C bond indicated that reductive coupling of iPr2CO did not proceed by dimerization of ketyl radicals. Formation of 2,4-dimethyl-2-pentene was in favor of a carbenoid intermediate resulting from deoxygenative reduction of the ketyl. These results revealed that for sterically hindered ketones, McMurry reactions can be viewed as Wittig-like olefination reactions. For R=tBu, no coupling product was obtained and the alkane tBu2CH2 was the major product. The involvement of the carbenoid species [M]=CtBu2 was confirmed by its trapping with H2O, leading to tBu2CH2, and with the aldehydes RCHO, giving the cross-coupling products tBu2C=C(R)H (R = Me, tBu). Therefore, in the case of severely congested ketones, McMurry reactions present strong similarities to the Clemmensen reduction of ketones, owing to the involvement in both reactions of carbenoid species which exhibit similar reactivity.

show abstract

“…The dissociation enthalpy of t Bu- 1 2 is 2.8 kcal mol –1 smaller than the 10.7(2) kcal mol –1 Δ H 298d of H- 2 , 32 while the entropy contribution in t Bu- 1 2 is significantly larger ( T Δ S 298d = 6.0 kcal mol –1 ). This is not surprising because covalent t Bu- 1 2 requires much more ordering of the t Bu-groups for optimizing the LD interactions, leading to a large, counteracting Δ S contribution.…”

mentioning

confidence: 82%

Sizing the role of London dispersion in the dissociation of all-meta tert-butyl hexaphenylethane

2017

View full text Add to dashboard Cite

show abstract

Sterically hindered free radicals. 14. Substituent-dependent stabilization of para-substituted triphenylmethyl radicals

Cited by 90 publications

References 1 publication

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl‐Substituted Tritylium Dyes

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl‐Substituted Tritylium Dyes

Reactions of Aliphatic Ketones R2CO (R=Me, Et,iPr, andtBu) with the MCl4/Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions

Sizing the role of London dispersion in the dissociation of all-meta tert-butyl hexaphenylethane

Contact Info

Product

Resources

About