Photochemical reactions. 134th Communication. Photochemistry of 5,6-epoxy-5,6-dihydro-3,4-methano-β-ionone: Influence of a cyclopropane ring on the reactivity of an ylide intermediate

On ¹n,π*-excitation(λ > 347 nm), the diastereomeric methanoepoxyenones (E)- 6 undergo isomerization via C,O-cleavage of the oxirane leading to diastereomeric photoproducts ((E)- 5 →(E/Z)- 13 and 14 ; (E)- 6 →(E/Z)- 16 and 17 ). On ¹π,π*-excitation (λ = 254 nm) of either (E)- 5 ) or (E- 6 the photoproducts 9, 10 and 11 are formed. By laser flash photolysis (λ = 265 nm) the ylide intermediate 3 was detected, with a lifetime of 10 μs in MeCN at ambient temperature. Stern-Volmer analysis of the ylide quenching by MeOH disclosed that compounds 9 and 10 , but not 11 , arise from the ylide intermediate e .     

Photochemical Reactions 150th Communication. Wavelength Dependence of the Photochemistry of 7-Methyl-β-ionone

The wavelength dependence of the photolysis of 7-methyl-β-ionone ((E)- 1 ) was investigated. Irradiation of (E)- 1 with light of λ > 347 nm leads primarily to (E/Z)-isomerization followed by transformation to the tricyclic enol ether 3 as the only secondary photoproduct. On photolysis of (E)- 1 with light of shorter wavelength (λ > 280 nm or λ = 254 nm), however, a series of other products was formed (via a) photocyclization of the dienone chromophore (→ 5 ), (b) photo-enolization (→ 8 ), and (c) a 1,5-sigmatropic H-shift (→ (E/Z)- 7 ). For the structure elucidation of the new products, 7-[¹³C]methyl-β-ionone ((E)-[7-methyl-¹³C]- 1 ) was prepared and irradiated furnishing the corresponding ¹³C-labelled photoproducts.     

Photochemische Reaktionen 113. Mitteilung [1]. Zur Photospaltung konjugierter γ, δ-Epoxyenone: UV.-Bestrahlung von 5, 6-Epoxy-3,4-didehydro-5,6-dihydro-β-jonon

The Photoinduced Cleavage of Conjugated γ, δ-Epoxyenones: UV.-Irradiation of 5,6-Epoxy-3, 4-didehydro-5,6-dihydro-β-ionone On ¹n, π*-excitation (λ ≥ 347 nm) in pentane or CClF₂CFCl₂ (E)- 1 is isomerized to the dihydrofurane (E/Z)- 2 as well to the ethers 3 and 5. Besides these products the isomeric cyclopropane derivative (E)- 4 and the acetal 6 are obtained in methanol. The detection of 6 indicates the formation of an intermediate ketoniumylide a which may give 6 by addition of methanol. ? On ¹π, π*-excitation (λ=254 nm) in acetonitrile-d₃, CClF₂CFCl₂ or pentane (E)- 1 yields almost exclusively (E)- 2. In methanol 6 is obtained in addition to (E/Z)- 2 , but no (E)- 4 and 5 is formed.     

Synthesis of Chiral 12-Phenyl(2H)dodecanoic Acids: Useful metabolic probes for the biosynthesis of 1-alkenes from fatty acids

The synthesis of chiral 12-phenyi(²H)dodecanoic acids as metabolic probes for the evaluation of the stereo-chemical course of the biosynthesis of 1-alkerses from fatty acids in plants and insects is described. The diastereoisomeric (2R, 3R)- or (2S, 3S)-12-phenyl(2,3?²H₂)dodecanoic acids 11 are obtained in high chemical and optical yield (>97% e.e.) from the readily available (E)-12-phenyl(2,3-²H₂)dodec-2-enoic acid ( 10 ) or (E)-12-phenyldodec-2-enoic acid ( 10a ) by microbial reduction with wet packed cells of Clostridium tyrobutyricum in either ²H₂O or H₂O buffer. (2R)- and (2S)-12-phenyl(2?²H)dodecanoic acids 9 (>97% e.e.) are accessible from the allylic alcohol 6 via Sharpless epoxidation with (+)-L- or (?)-D-diethyl tartrate, Synthetic routes to the (E)- and (Z)-11-phenyl(1?²H) undec-1-enes 16 and 16a as reference compounds are also included.     

Synthesis of All Four Stereoisomers of (E)-Vitamin KT (Phylloquinone), Analysis of Their Diastereoisomeric and Enantiomeric Purities and Determination of Their Biopotencies

All four stereoisomers of (E)-vitamin Kb i. e. (2¹E, ⁷R, 11¹R)-l (= 1a), (2¹E, 7¹ R, l1¹S)-1 (= 1b), (2¹E, 7¹ S, 11¹S) 1 ( = 1c), and (2¹E, 7¹S, 11¹R)-l ( = Id), have been synthesized in a state of high chemical and stereoisomeric purity. The synthesis of stereoisomers lb-d relied on the use of the optically active Cf¹* and C*¹⁰-building blocks (R)- or (S)-4-(benzyloxy)-3-methylbutanal ((R)- or (S)-2) and (R)- or (S)-citronellal ((R)- or (S)- 3 ) which had been secured by the Rh¹-catalyzed allylamine-to-enamine isomerization technology. For the synthesis of the natural (E)-vitamin-K1 stereoisomer 1a , a new route starting from natural phylol was developed, based on an O-alkylation/rearrangement procedure. A HPLC method was developed which separates with remarkable efficiency all four stereoisomers of (E)- as well as three out of the four stereoisomers of (Z)-vitamin K¹ on optically active poly(trityl methacrylate) as the chiral stationary phase supported on Nucleosil. By this method, the stereoisomeric content of the stereoisomers 1b-d synthesized was shown to be in the range of 96-98 %, while the natural isomer 1a was configurationally uniform. The biological activity of the four (E)-vitamin-K₁ stereoisomers was determined by means of the curative prothrombin time test with vitamin-K-depleted chicks. A high precision of the results was obtained with the recently introduced up-and-down organization of the test and the statistical evaluation according to an estimation procedure. With the natural (E)-vitamin-K₁ stereoisomer 1a as standard (set at 1. 0), activities of 0. 93, 1. 19, and 0. 99 were found for stereoisomers 1b, 1c , and 1d , respectively. Within the confidence limits, these activity ratios can be regarded as identical, A very similar efficacy was obtained by comparison of (E, all-rac )-vitamin K₁ ((2¹E, RS, 11′ RS)- 1 ; equimolar mixture of the four stereoisomers 1a-d) with the natural (E)-vitamin-K₁ stereoisomer 1a ). A synergistic effect was not detectable, as was the case with the eight α-tocopheryl-acetate stereoisomers.     

Potential cerebral perfusion agents. Synthesis and evaluation of a 1,4-disubstituted dihydropyridine analogue

The synthesis of a 1,4-disubstituted dihydropyridine, 1-(E-1[¹²⁵I]iodo-1-penten-5-yl)-4-(β-N-acetylaminoethyl)-1,4-dihydropyridine ([¹²⁵I] 10 ), is described. Acetylation of 4-(β-aminoethylpyridine) with acetic anhydride followed by condensation with E-1-borono-5-iodo-1-pentene ( 7 ) gave 1-(E-1-borono-1-penten-5-yl)-4-(β-N-acetylaminoethyl)pyridinium iodide ( 8 ). Chloramine-T and sodium iodide iodination of 8 gave the corresponding E-1-iodo compound 9 which was reduced with sodium borohydride to furnish 1-(E-1-iodo-1-penten-5-yl)-4-(β-N-acetylaminoethyl)-1,4-dihydropyridine ( 10 ). The corresponding radioiodinated compound was prepared similarly using Na[¹²⁵I]. The tissue distribution studies in rats indicate that [¹²⁵I] 10 crosses the blood brain barrier (0.49% dose/g in the brain) but gradually washes out from the brain.     

Vapor–liquid equilibria of 2-butanol + aromatic hydrocarbons at 308.15 k

Vapor–liquid equilibria (VLE) data of 2-butanol?+?benzene or toluene or o- or m- or p-xylene measured by static method at 308.15?±?0.01?K over the entire composition range are reported. The excess molar Gibbs free energies of mixing (G ^E) for these binary systems have been calculated from total vapor pressure data using Barker's method. The G ^E for these binary systems are also analyzed in terms of the Mecke–Kempter type of association model with a Flory contribution term using two interaction parameters and it has been found that this model describes well the G ^E values of binary systems benzene or toluene.     

Experimente zum kompetitiven Einbau der Stereoisomeren des Farnesols in Cantharidin. 6. Mitteilung zur Biosynthese des Cantharidins

Experiments on the competitive incorporation of farnesol-stereoisomers into cantharidin Farnesol ( 2 ) has been demonstrated to be an efficient precursor for cantharidin ( 1 ), into which it is transformed by elimination of C(1), C(5), C(6), C(7) and C(7′) [1]. The following incorporation experiments with doubly labelled (³H and ¹⁴C) stereoisomers of farnesol present strong evidence that (E,E)- farnesol ((E,E)- 2 ) in fact is the precursor for cantharidin, whereas (2E, 6Z)- 2 and (Z,Z)- 2 are not utilized for the biosynthesis of cantharidin. A possible mechanism for the incorporation of (2Z,6E)-farnesol ((2Z,6E)- 2 ) to an extent of 56,8% relative to (E,E)- 2 is discussed.     

Reaction of Methylmalonyl Dichloride with Unsaturated Carboxamides

Methylmalonyl dichloride reacts with (2E)-3-phenylacrylamides and (2E)-3-(2-furyl)acrylamide to give the corresponding E-isomeric 2-substituted 4-hydroxy-5-methyl-6H-1,3-oxazin-6-ones. The reaction of methylmalonyl dichloride with acrylamide afforded N-(3-chloropropionyl)-2-methylmalonamic acid. The structure of the products was confirmed by the ¹H and ¹³C NMR, IR, and UV spectra.     

Reduktion von 1,2-bis[(Z)-(2-nitrophenyl)-NNO-azoxy]benzol: Synthese von Cyclotrisazobenzol ( = (5E,6aZ,11E,12aZ,17E,18aZ)-5,6,11,12,17,18-Hexaazatribenzo[aei][1,3,5,7,9,11]cyclododeca-hexaen)

Reduction of 1,2-Bis[(Z)-(2-nitrophenyl)-NNO-azoxy]benzene¹: Synthesis of Cyclotrisazobenzene ( = (5E,6aZ,11E,12aZ,17E,18aZ)-5,6,11,12,17,18-Hexaazatribenzo[aei][1,3,5,7,9,11]cyclododeca-hexaene) Na₂S reduction of 1,2-bis[(Z)-(2-nitrophenyl)-NNO-azoxy]benzene ( 2 ) yielded 3 deoxygenated products: the (known) red 2,2′-((E,E)-1,2-phenylenbisazo)dianiline ( 3 , 23%), the orange 2-[2-((E)-2-aminophenylazo)phenyl]-2H-benzotriazol ( 4 , 55%) and the colorless 2,2′-(1,2-phenylene)di-2H-benzotriazol ( 5 , 13%). The constitutions of 3 – 5 and of 6 , the N-acetyl derivative of 4 , were deduced from their ¹H-NMR spectra (chemical shifts, couplings, and symmetry properties), and the configurations of 3 , 4 , and 6 at their N,N-double bonds are assumed to be the same as in 2 . Oxidation of 3 with 2 mol-equiv. of Pb(OAc)₄ afforded 5 (47%) and a novel, highly symmetrical macrocycle, called cyclotrisazobenzene ( 7 , 24%). The constitution of 7 as a tribenzo-hexaaza[12]annulene and its (E)-configuration at the N,N-bonds was confirmed by X-ray analysis. The molecular symmetry expressed by the ¹H-, ¹³C- and ¹⁵N-NMR spectra of 7 reveals a rapid torsional motion around the six N,C bonds. This implies that the N,N-double bonds in the cyclic 12π-electron system (or 24π-electron system if the benzene rings are included) of 7 are highly localized.     

Photochemical reactions, 136th communication. Photochemistry of conjugated epoxyenones and dienes of the ionone series: Influence of a neighboring spiro-oxirane function

The preparation and photolyses of the diepoxyenones (E)- 8 and (E)- 9 as well as the diepoxydiene (E)- 10 are described. On ¹π,π^*-excitation (λ = 254 nm), the diastereoisomeric diepoxyenones (E)- 8 and (E)- 9 undergo isomerization via the ylide intermediate f and the carbene intermediate g leading to the primary photoproducts 17A and 18–21 (Scheme 8). On ¹n, π^*-excitation (λ > 347 nm), (E)- 8 shows behaviour typical of epoxyenones undergoing C(γ), O-bond cleavage of the oxirane and isomerization to compounds 22 , (E/Z)- 23 and (E)- 24 (Scheme 10). On singlet excitation, the diepoxydiene (E)- 10 , is cleaved to the carbonyl ylide j and the carbenes 1 and m (Scheme 11). The carbonyl ylide j fragments via the dipolar intermediate k to the acetylenic dienone (E)- 31 . The carbene 1 , showing behaviour typical of vinyl carbenes, furnishes the cyclopropene 30 . The alternative carbene m , however, undergoes an insertion reaction into the neighboring oxirane C,C-bond leading to the proposed but not isolated oxetene 43 , which is further transformed to the products 33A _ B by an intramolecular cycloaddition.     

Synthesis and Structural Studies of 2-(hydroxyimino)-1-methylpropylideneamino-phenyliminobutan-2-one Oxime, Ligand and its Complexes with Cu(II) and Ni(II)

A new ligand incorporating a dioxime moiety, (2E,3E)-3-[(2-{[(1E,2E)-2-(hydroxyimino)-1-methylpropylidene]amino}phenyl)imino]butan-2-one oxime, (H₂Phmdo) (3) has been prepared by reacting 2,3-butanedionemono-{O-[4-(1-methyl-2-oxo-propylideneaminooxy)-2,3-bis-(1-methyl-2-oxo-propylideneaminooxy-methyl)-but-2-enyl]-oxime} (2) with 1,2-phenylenediamine. Mono-, di- and trinuclear copper(II) and/or nickel(II) complexes of H₂Phmdo were characterized by elemental analyses, magnetic moments, ¹H-n.m.r. and ¹³C-n.m.r., i.r. and mass spectral studies. The mononuclear copper(II) and nickel(II) complexes of H₂Phmdo were found to have a 1:1 metal:ligand ratio. Elemental analyses, stoichiometric and spectroscopic data of the metal complexes indicated that the metal ions are coordinated to the oxime and imine nitrogen atoms (C=N). In the dinuclear complexes, in which the first Cu^(II) or Ni^(II) ion was complexed with nitrogen atoms of the oxime and imine groups, the second Cu^(II) ion is ligated with dianionic oxygen atoms of the oxime groups and are linked to the 1,10-phenanthroline nitrogen atoms. The data support the proposed structure of H₂Phmdo and its complexes.     

Azimine IV. Kinetik und Mechanismus der thermischen Stereoisomerisierung von 2,3-Diaryl-1-phthalimido-aziminen

Azimines IV. Kinetics and Mechanism of the Thermal Stereoisomerization of 2,3-Diaryl-1-phthalimido-azimines¹) Mixtures of (1E, 2Z)- and (1Z, 2E)-2-phenyl-1-phthalimido-3-p-tolyl-azimine ( 3a and 3b , resp.) and (1E, 2Z)- and (1Z, 2E)-3-phenyl-1-phthalimido-2-p-tolylazimine ( 4a and 4b , resp.) were obtained by the addition of oxidatively generated phthalimido-nitrene (6) to (E)- and (Z)-4-methyl-azobenzene ( 7a and 7b , resp.). Whereas complete separation of the 4 isomers 3a, 3b, 4a and 4b was not possible, partial separation by chromatography and crystallization led to 5 differently composed mixtures of azimine isomers. The spectroscopic properties of these mixtures (UV., ¹H-NMR.) were used to determine the ratios of isomers in the mixtures, and served as a tool for the assignment of constitution and configuration to those isomers which were dominant in each of these mixtures, respectively. Investigation of the isomerization of the azimines 3a, 3b, 4a and 4b within the 5 mixtures at various concentrations by ¹H-NMR.-spectroscopy at room temperature revealed that only stereoisomers are interconverted ( 3a ? 3b; 4a ? 4b) and that the (1E, 2Z) ? (1Z, 2E) stereoisomerization is a unimolecular reaction. These observations exclude an isomerization mechanism via an intermediate 1-phthalimido-triaziridine (2) or via dimerization of 1-phthalimido-azimines (1) , respectively. The 3-p-tolyl substituted stereoisomers 3a and 3b isomerized slightly slower than the 3-phenyl substituted ones 4a and 4b , an effect which is consistent with the assumption that the rate determining step of the interconversion of (1E, 2Z)- and (1Z, 2E)-1-phthalimido-azimines (1a ? 1b) is the stereoisomerization of the stereogenic center at N(2), N(3), either by inversion of N(3) or by rotation around the N(2), N(3) bond. The total isomerization process is assumed to occur via the thermodynamically less stable (1Z, 2Z)- and (1E, 2E)-isomers 1c and 1d , respectively, as intermediates in undetectably low concentrations which stay in rapidly established equilibria with the observed, thermodynamically more stable (1E, 2Z)- and (1Z, 2E)-isomers 1a and 1b , respectively. At higher temperatures, the azimines 3 and 4 are transformed into N-phenyl-N,N′-phthaloyl-N′-p-tolyl-hydrazine (8) with loss of nitrogen.     

Photochemische Reaktionen. 118. Mitteilung [1] Zur vinylogen β-Spaltung von Enonen. UV.-Bestrahlung von 4-(3',7',7'-Trimethyl-2'-oxabicyclo [3.2.0]hept-3'-en-1'-yl)but-3-en-2-on

Vinylogous β-Cleavage of Enones: UV.-irradiation of 4-(3′,7′,7′-trimethyl-2′-oxabicyclo[3.2.0]hept-3′-ene-1′-yl)but-3-ene-2-on On ¹π,π*-excitation (λ = 254 nm) in acetonitrile (E/Z)- 2 is converted into the isomers 4–9 and undergoes fragmentation yielding 10 ; in methanol (E/Z)- 2 gives 7–10 and is transformed into 11 by incorporation of the solvent. On ¹π,π*-excitation (λ λ?347 nm; benzene-d₆) (E)- 2 is isomerized into (Z)- 2 , which is converted into the isomers 3 and 4 by further irradiation. ¹π,π*-Excitation (λ = 254 nm; acetonitrile) of 4 gives 6 and (E)- 9 , whereas UV.-irradiation (λ = 254 nm; acetonitrile-d₃) of 5 yields (E)- 7 and 8 . On ¹π,π*-excitation (λ = 254 nm; acetonitrile) of (E/Z)- 12 the compounds (E)- 14 and (E)- 15 are obtained.     

氮苄叉基苯胺分子的平面扭曲驱动力的DFT研究

为了探索DFT方法中氮苄叉基苯胺分子的扭曲驱动力, 通过把非平面氮苄叉基苯胺(NBA)分子的DFT能量分成π和σ的方法, 分析了垂直离域能ΔE^V(θ)及σ-π轨道作用能ΔE^σπ(θ)的失稳定性, 并讨论了在扭曲过程中它们所起的作用. 在B3LYP/6-31G*, 6-311G*, 6-31G(2d), 6-311G(2d)水平下的计算结果显示: 与经典观点不同, π电子的离域是失稳定的, 且平面时失稳定性最强, 是分子扭曲的动力; 但σ-π轨道作用也是失稳定的, 随着扭角的增大其失稳定性增强, 是分子扭曲的阻力. NBA分子的大扭角构象, 是包含π-π, σ-π轨道作用在内的各种电子相互作用共同作用的结果.     

Volumetric properties of binary mixtures of alcoxyethanols with tert-butyl ethyl ether at various temperatures

Densities at 293.15, 298.15, 303.15, 308.15 and 313.15 K of the binary liquid mixtures made of tert-butyl ethyl ether with either 2-ethoxyethanol, or 2-(2-ethoxy)ethoxyethanol, or 2-[2-(2-ethoxy)ethoxy]ethoxyethanol have been measured over the whole mixture compositions. These data have been used to compute the excess molar volumes (V^E). The excess molar volumes always are negative over the entire range of composition for all the binary mixtures investigated. The changes of V^E with variations of the composition and the chain-length of the alkyl groups in the alkoxyethanol molecules are discussed in terms of possible intermolecular interactions.     

Organic Diodes Based on Redox-Polymer Bilayer-Film-Modified Electrodes

The synthesis of four electropolymerizable 2,2′-bipyridinium salts with tuned reduction potential (E₁^°) is described (N,N′-ethylene-4-methyl-4′-vinyl-2,2′-bipyridinium dibromide ( 4 ; E₁^° ?–0.48 V), 4-methyl-N, N′-(trimethylene)-4′-vinyl-2,2′-bipyridinium dibromide ( 5 ; E₁^°? ?0.66 V), N,N′-ethylene-4-methyl-4′-[2-(1H-pyrrol-1-yl)ethyl]-2, 2′-bipyridinium bis(hexafluorophosphate) ( 6b ; E₁^°? ?0.46 V), and 4-methyl-4′-[2-(1H-pyrrol-1-yl)ethyl]-N, N′-(trimethylene)-2,2′-bipyridinium bis(hexafluorophosphate) ( 7b ; E₁^°? ?0.66 V)). E₁^°-Tuning is based on the torsional angle C(3)–C(2)–C(2′)–C(3′), imposed by the N,N′-ethylene and N,N′-(trimethylene) bridge. The vinylic compounds 4 and 5 undergo cathodic, the pyrrole derivatives 6b and 7b anodic electropolymerization on glassy carbon electrodes from MeCN solutions, yielding thin, surface-confined films with surface concentrations of redox-active material in the range 5 · 10^?9 < Γ < 2.10^?8 mol/cm², depending on experimental conditions. The modified electrodes exhibit reversible ‘diquat’ electrochemistry in pure solvent/electrolyte. Copolymerization of 6b or 7b with pyrrole yields most stable electrodes. Bi ayer-film-modified electrodes were prepared by sequential electropolymerization of the monomers. The assembly electrode/poly- 6b /poly- 7b behaves as a switch, it transforms – as a Schmitt trigger – an analog input signal (the electrode potential) into a digital output signal (redox state of the outer polymer film). Forward-(electrode/poly- 7b /poly- 6b ) and reverse-biased assemblies (electrode/poly- 6b /poly- 7b ) were coupled to the electrochemical reduction of redox-active solution species, e.g. N- (cyanomethyl)-N′-methyl-4,4′-bipyridinium bis(hexafluorophosphate) ( 8 ). Zener-diode-like behavior was observed. Aspects of redox-polymer multilayer-film assemblies, sandwiched between two electronic conductors, are discussed in terms of molecular electronic devices.     

The Behavior of 3,3-Diphenylindan-1,2-dione Towards Phosphonium Ylides

Summary. The reaction of alkoxycarbonyl- and cyanomethylene(triphenyl)phosphoranes with 3,3-diphenylindan-1,2-dione in dry benzene at room temperature for about 5 h led to the formation of a mixture of (E)- and (Z)-diastereomers. On the other hand, treatment of the dione with acetylmethylene(triphenyl)phosphorane afforded a mixture of (E)-3,3-diphenyl-1-(2-oxo-2-methylethylidene)indan-2-one and unexpected product (E)-3-(3,3-diphenyl-2-oxoindan-1-ylidene)-4-(triphenyl-λ⁵-phosphanylidene)hexane-2,5-dione, whereas with benzoylmethylene(triphenyl)phosphorane gave a mixture of (E)-3,3-diphenyl-1-(2-oxo-2-phenylethylidene)indan-2-one, [(2R ^*,3S ^*)-3-benzoyl-8,8-diphenyl-3,8-dihydro-2H-indeno{2,1-b}furan-2-yl]phenylmethanone and 1,4-diphenyl-2-(3,3-diphenyl-2-hydroxy-3H-inden-1-yl)but-2-ene-1,4-dione. The reaction mechanisms are considered and structural assignments of the new compounds are based on spectroscopic evidence. The molecular structures of the two diastereomers and the unexpected product were elucidated by X-ray crystallography.     

Configurational assignments of oximes derived from 5-formyl and 5-acyl-1,2,4-triazines

The configuration of (3-substituted)-1,2,4-triazin-5-ylcarbaldoximes and (3-substituted)alkyl-1,2,4-triazin-5-ylketoximes was determined by means of ¹H-nmr, ¹³C-nmr, ¹⁵N-nmr and homonuclear NOE-difference spec-troscopy. Oximes resulting from reaction of 1,2,4-triazines with nitroalkanes were found to be either pure E-isomers or E/Z-mixtures with the amount of E-isomer greatly predominating. Detailed ¹³C-nmr data of the oximes investigated are presented.     

Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates

The irreversible-reduction potentials of 26 alkylcob(III)alamins (RCbl^III 1a – z ) and 26 alkylcob(III)yrinates (R‘Cby’^III; 2a – z ) (E_p 1a – z and E_p 2a – z , resp.) have been measured in situ by single-scan voltammetry of hydroxocob(III)alamin hydrochloride (vitamin B_12b- HCl; 1 ) or heptamethyl cob(II)yrinate perchlorate (ClO₄‘Cby’^II; 2 ) in presence of the corresponding alkyl halides (RX; 3a – z ) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half-life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E°(R? Co^III + e^??R? + Co^I). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCbl^III and, R‘Cby’^III shift E_p( 1a – z ) and E_p ( 2a – z ) towards positive potentials. Linear correlations have been found between E_p( 1a – z ) (E_p( 2a – z )) of RCbl^III (R‘Cby’^III) and the pK_a of RH (or the Taft σ*- or the Hammett σ-values of R) within each class of R, i. e. MeCbl^III (Me‘Cby’^III), primary RCbl^III (R‘Cby’^III) and secondary RCbl^III (R‘Cby’^III). The correlations allow to distinguish between electronic effects of the Co-bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light-induced formal insertion of an olefin into the Co, C-bond of an alkylcobalamin (Scheme 2, 1a → 1u ), a key step in the vitamin-B₁₂-catalized C, C-bond formation.