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Engineering Orthogonal Methyltransferases to Create Alternative Bioalkylation Pathways
S-adenosyl-l-methionine (SAM)-dependent methyltransferases (MTs) catalyse the methylation of a vast array of small metabolites and biomacromolecules. Recently, rare carboxymethylation pathways have been discovered, including carboxymethyltransferase enzymes that utilise a carboxy-SAM (cxSAM) cofactor generated from SAM by a cxSAM synthase (CmoA). We show how MT enzymes can utilise cxSAM to catalyse carboxymethylation of tetrahydroisoquinoline (THIQ) and catechol substrates. Site-directed mutagenesis was used to create orthogonal MTs possessing improved catalytic activity and selectivity for cxSAM, with subsequent coupling to CmoA resulting in more efficient and selective carboxymethylation. An enzymatic approach was also developed to generate a previously undescribed co-factor, carboxy-S-adenosyl-l-ethionine (cxSAE), thereby enabling the stereoselective transfer of a chiral 1-carboxyethyl group to the substrate.
Bennett, Matthew R.,Cronin, Victoria A.,Herbert, Abigail J.,Micklefield, Jason,Shepherd, Sarah A.,Sung, Rehana
supporting information
p. 14950 - 14956
(2020/07/04)

Temperature-dependent regioselectivity of nucleophilic aromatic photosubstitution. Evidence that activation energy controls reactivity
Irradiation (λ > 330 nm) of 2-chloro-4-nitroanisole (1) at 25 C in aqueous NaOH forms three substitution photoproducts: 2-methoxy-5-nitrophenol (2), 2-chloro-4-nitrophenol (3), and 3-chloro-4-methoxyphenol (4), in chemical yields of 69.2%, 14.3%, and 16.5%. The activation energies for the elementary steps from the triplet state at 25 °C were determined to be 1.8, 2.4, and 2.7 kcal/mol, respectively. The chemical yields of each of the three products were determined for exhaustive irradiations at 0, 35, and 70 °C. The variation with temperature of the experimental yields is reproduced almost exactly by the yields calculated with the Arrhenius equation. This indicates that activation energy is the fundamental property related to regioselectivity in nucleophilic aromatic photosubstitution of the SN2 Ar* type. The many methods proposed for predicting regioselectivity in reactions of this type have had limited success and have not been related to activation energy.
Wubbels, Gene G.,Tamura, Ryo,Gannon, Emmett J.
p. 4834 - 4839
(2013/07/11)

The element effect and nucleophilicity in nucleophilic aromatic photosubstitution (SNAR*). Local atom effects as mechanistic probes of very fast reactions
(Chemical Equation Presented) Photoreactions of 4-nitroanisole and the 2-halo-4-nitroanisoles (halogen = F, Cl, Br, and I) with the nucleophiles hydroxide ion and pyridine have been investigated quantitatively to extend the findings recently communicated for cyanide ion. The halonitroanisoles on excitation form triplet π,π* states, which undergo substitution of the halogen by nucleophiles. Chemical yields of photoproducts, Stern-Volmer kinetic plots, triplet lifetimes, and triplet yields are reported for the five compounds with the three nucleophiles. Following a standard kinetic treatment, 73 rate constants are determined for elementary reactions of the triplets including quenching and various nucleophilic addition processes. The photoadditions are roughly 14 orders of magnitude faster than thermal counterparts. Rate constants for attack at the fluorine-bearing carbon of triplet 2-fluoro-4-nitroanisole are 2.9 × 109, 1.3 × 109, and 6.3 × 108 M-1 s-1 for cyanide ion, hydroxide ion, and pyridine, respectively. The relative rates for attack at the halogen-bearing carbons for F/Cl/Br/I are 27:1.9:1.9:1 (cyanide ion), 29:2.6:2.4:1 (hydroxide ion), and 39:3.9: 3.5:1 (pyridine), respectively. The relative nucleophilicities vary somewhat with the attack site; they are about 5:2:1 for cyanide ion, hydroxide ion, and pyridine for attack at the halogen-bearing carbons. The trend of the element effect opposes that of aliphatic substitution and elimination but is similar in size and parallel to that of thermal nucleophilic aromatic substitution. Relative nucleophilicities in the photoreactions are also similar to those of comparable but vastly slower thermal reactions. The findings imply that the efficiency-determining step of the halogen photosubstitution is simple formation of a σ-complex through electron-paired bonding within the triplet manifold.
Wubbels, Gene G.,Brown, Toby R.,Babcock, Travis A.,Johnson, Kandra M.
p. 1925 - 1934
(2008/09/19)

Photochemical nucleophile mapping: Identification of Tyr311 within the catalytic domain of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase
Photochemical mapping of nucleophiles in close proximity to the active site Cys149 of rabbit glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was demonstrated based on the nucleophilic aromatic photosubstitution reaction using two regioisomers of alkoxy-fluoro-nitro-substituted benzenes. Two photophores were covalently attached to the active site SH group of GAPDH and the protein was subjected to photolysis then to the cyanogen bromide cleavage reaction. The advantage of this method is the capability to chase labeled products by monitoring absorption at 380 nm because of the chromogenic property of photophore. HPLC separation identified a large labeled peptide fragment that was further digested by V8 protease for Edman sequence analysis. From the recent X-ray crystallography of rabbit GAPDH, Tyr311, His176, Ser238 and Lys183 are closely located to catalytic Cys149. Among these nucleophiles, Tyr311 was preferentially labeled with 2-fluoro-4-nitrophenoxy photophore and no label was identified with the isomeric 4-fluoro-2-nitrophenoxy photophore. The result clearly reflects the distance between Cys149 and nucleophiles to distinguish the nearest Tyr311. As photophores show great reactivity even with water under neutral conditions, the distance between nucleophiles and photophores is important for photo-induced nucleophilic aromatic substitution. The method will provide a useful technique to survey nucleophiles within the catalytic domain.
Hatanaka, Yasumaru,Kaneda, Masaki,Tomohiro, Takenori
p. 213 - 217
(2008/02/01)

Rates of reductive elimination of substituted nitrophenols from the (indol-3-yl)methyl position of indolequinones
A series of indolequinones bearing substituted nitrophenols on the (indol-3-yl)methyl position was synthesised. The nitrophenol leaving groups were appropriately substituted to give a wide range (4 units) in phenolic pKa value. The rate of redu
Swann,Moody,Stratford,Patel,Naylor,Vojnovic,Wardman,Everett
p. 1340 - 1345
(2007/10/03)

Etherification of phenols catalysed by solid-liquid phase transfer catalyst PEG400 without solvent
Aromatic ethers were synthesised in excellent yields(above 86 %) and purity by the etherification of phenols with dimethyl sulfate or alkyl halides, catalysed by phase transfer catalyst PEG400 under solvent-free conditions, and the effects of some key reaction conditions were also studied.
Cao, Yu-Qing,Pei, Ben-Gao
p. 1759 - 1766
(2007/10/03)

Photosubstitution-photoreduction mechanistic duality in the SET photoreactions of nitrophenyl ethers with amines. The role of the steps that follow the ET
Nitrophenyl ethers are photoreduced by primary amines in water through a mechanism initiated by single electron transfer that is in direct competition with the single electron transfer photosubstitution mechanism (S(N)Ar*-SET). Our results indicate that the preferred pathway does not depend on the electron donor or proton donor ability of the amine. The key factor that determines the progress of the photoreaction is the structure of the carbon skeleton of the amine, particularly the type of hydrogens on the carbon α to the amino group. A mechanistic rationale that includes hydrogen atom transfer as a key step is discussed.
Mir, Miquel,Marquet, Jordi,Massot, Oriol
p. 12603 - 12614
(2007/10/03)

Effects of superoxide anion generated from aromatic radical anions produced in nucleophilic aromatic photosubstitution reactions
Superoxide anion is generated from aromatic radical anions produced in nucleophilic aromatic photosubstitutions when the reactions are carried out in non-deoxygenated solutions of polar aprotic solvents. Superoxide anion thus generated displaces cyanide anion from acetonitrile and benzyl cyanide, ethoxide anion from ethyl acetate, and methanesulfenate anion from dimethyl sulfoxide. Hence, non-deoxygenated polar aprotic solvents should be avoided in nucleophilic aromatic photosubstitution reactions.
Cervera, Maria,Marquet, Jordi
p. 966 - 969
(2007/10/03)

Phosphomonoesters and phosphodiesters derived from the photohydrolysis of 2-methoxy-5-nitrophenyl substituted phosphotriesters
Phosphotriesters composed of one or two 2-methoxy-5-nitrophenyl group(s) can be quantatively photohydrolyzed in aqueous acetonitrile to yield the desired phosphodiester or phosphomonoester, respectively. Photohydrolysis occurs by attack of hydroxide at both the phosphoryl phosphorus and at the ipso-carbon in the triplet exited state of the 2-methoxy-5-nitrophenyl substituted phosphoesters. The photophysical studies described within imply that this type of reaction may be synthetically useful.
Graciani, Nilsa R.,Swanson, Daniel S.,Kelly, Jeffery W.
p. 1077 - 1086
(2007/10/02)

Topologically Controlled Coulombic Interactions, a New Tool in the Developing of Novel Reactivity. Photochemical and Electrochemical Cleavage of Phenyl Alkyl Ethers
The hypothesis that a specific placement of a positive charge would dramatically alter the behavior of a charged intermediate has been tested.Phenyl ethers substituted by electron-attracting groups do not undergo reductive fragmentation.However, related α-piperidino-ω-(4-substituted-phenoxy)alkanes give alkyl ether photocleavage when the linker between the redox centers is short, or the usual substitution-reduction photochemistry when it is long.Mechanistic experiments suggest that the photofragmentation process operates through space intramolecular electron transfer to the triplet aromatic chromophore and that a coplanar relative orientation of the alkyl ether bond and the phenyl ring is compulsory for the photofragmentation to be observed.Configuration interaction AM1 calculations justify the described facts, indicating that the fragmentation process is only operative when a Coulombic stabilization of a ?* intramolecular electron transfer excited state is produced.Electrochemical studies carried out with the corresponding quaternary salts (intermolecular generation of the phenyl ether radical anion) confirm the conclusions derived from the photochemical experiments.
Marquet, Jorge,Cayon, Eduard,Martin, Xavier,Casado, Francisco,Gallardo, Iluminada,et al.
p. 3814 - 3825
(2007/10/02)

Regioselective cleavage reaction of the aromatic methylenedioxy ring. VI. Synthesis of phenothiazine analogues by using the cleavage reaction with sodium methoxide-thiols in dimethyl sulfoxide and evaluation of their biological activities
The reactions of aromatic methylenedioxy compounds containing electron- withdrawing groups with sodium methoxide-thiols in dimethyl sulfoxide gave 3- and 4-hydroxybenzene derivatives in good yield by regioselective attack of the thiolate ions on the methylenedioxy ring. The formation mechanism and the reactivity of thiolate ions in the cleavage reaction of the methylenedioxy ring are discussed. Various biologically active compounds, 32a, 32d, 36b, 38h, 41b and 44-47, were prepared from the 4-hydroxybenzene derivatives and their Ca2+ antagonistic activities were evaluated. Among these compounds, 2-(2-bromophenylthiomethoxy)-10-(2-diethylaminoacetyl)-3-methoxyphenothiazi ne (46) showed the most potent Ca2+ antagonistic activity. Biological activity could be conveniently evaluated by measurement of the peak height of the vanadyl ion (+4 oxidation ion) signal produced by redox reaction between the phenothiazine derivatives and vanadate ion +5 oxidation ion) with ESR spectroscopy.
Imakura,Konishi,Uchida,Sakurai,Kobayashi,Haruno,Tajima,Yamashita
p. 500 - 511
(2007/10/02)

The search for biochemical photoprobes. III. The photoreactions of 4-nitroveratrole and 2-fluoro-4-nitroanisole with bovine pancreatic ribonuclease A and with model nucleophiles
The photoreactions of 4-nitroveratrole (NVT) and 2-fluoro-4-nitroanisole (FNA) with bovine pancreatic ribonuclease A (RNAase A) and with model nucleophiles are described. From them it is concluded that 2-fluoro-4-nitrophenyl ethers constitute improved alternative biochemical photoprobes, specially for proteins where a number of good nucleophiles are present.
Marquet,Rafecas,Cantos,Moreno-Manas,Cervera,Casado,Nogues,Cuchillo
p. 1297 - 1306
(2007/10/02)

Regioselective cleavage reaction of the aromatic methylenedioxy ring. V. Cleavage with sodium alkoxides-alcohols, potassium tert-butoxide-alcohols, dimsyl anion-methyl alcohol, metallic sodium-alcohols, and sodium cyanide in dipolra aprotic solvents
The reaction of aromatic methylenedioxy compounds containing electron-withdrawing groups with dimsyl anion-methyl alcohol, potassium tert-butoxide-alcohols, and metallic sodium-alcohols in dimethyl sulfoxide (DMSO), and with sodium alkoxides-alcohols in hexamethylphosphoramide, gave 3- and 4-hydroxybenzene derivatives in good yield by regioselective attack of the alkoxide ions on the methylenedioxy ring. The formation mechanism of alkoxide ions and the effect of DMSO in the cleavage reaction of the methylenedioxy ring are discussed on the basis of proton nuclear magnetic resonance (1H-NMR) spectra. The reactions of aromatic methylenedioxy compounds (3 and 22) with sodium cyanide in dipolar aprotic solvents gave 4-cyano-3-hydroxybenzene derivatives (23 and 24) by regioselective attack of the cyanide ion on the methylenedioxy ring. The reactions of aromatic methylenedioxy compounds (28-30) containing no electron-withdrawing group with MeONa-MeOH in dipolar aprotic solvents gave non-regioselective cleavage products (31 and 34).
Imakura,Okimoto,Konishi,Hisazumi,Yamazaki,Kobayashi,Yamashita
p. 1691 - 1696
(2007/10/02)

Solid state photochemistry of ternary cyclodextrin complexes: Total selectivity in the photoreduction of nitrophenyl ethers by 1-phenylethylamine
The irradiation of the ternary complexes of β-cyclodextrin with several nitrophenyl ethers, and 1-phenylethylamine in the solid state, produces selective photoreduction to the corresponding nitroso derivatives.
Mir, Miquel,Marquet, Jorge,Cayon, Eduard
p. 7053 - 7056
(2007/10/02)

REGIOSELECTIVE CLEAVAGE REACTION OF THE METHYLENEDIOXY RING IN AROMATIC COMPOUNDS CONTAINING ELECTRONWITHDRAWING GROUPS WITH SODIUM ALKOXIDES-ALCOHOLS IN DIMETHYL SULFOXIDE
The reaction of 6-bromopiperonal (1) with sodium alkoxides (MeONa or PhCH2ONa)-alcohols (MeOH or PhCH2OH), and sodium alkoxides (MeONa, PhCH2ONa or PhONa)-phenol (PhOH) in dimethyl sulfoxide gave 3-hydroxybenzene derivatives (3, 5 and 6) and 4-hydroxybenzene derivative (4), respectively.The reactivity and formational mechanism of various nucleophilic reagents (alkoxide anions) formed from the alcohols and phenol by sodium alkoxides in the regioselective cleavage reactions are discussed.
Imakura, Yasuhiro,Okimoto, Kazuto,Gorohata, Chizuru,Kobayashi, Shigeru,Kihara, Masaru,Yamashita, Shinsuke
p. 1067 - 1076
(2007/10/02)

THE SEARCH FOR NEW BIOCHEMICAL PHOTOPROBES. II. THE NUCLEOPHILIC PHOTOSUBSTITUTION OF 2-FLUORO-4-NITROANISOLE.
The photosubstitutions of 2-fluoro-4-nitroanisole with several amines are studied.The preparative results, the thermal stability of the photochemical substrate, and the limiting quantum yield values obtained from photoreactions with several nucleophiles suggest the possible usefulness of 2-fluoro-4-nitrophenyl ethers as biochemical photoprobes.
Pleixats, R.,Figueredo, M.,Marquet, J.,Moreno-Manas, M.,Cantos, A.
p. 7817 - 7826
(2007/10/02)

THE PHOTOREACTIONS OF 2-FLUORO-4-NITROANISOLE WITH AMINES. THE SEARCH FOR NEW BIOCHEMICAL PHOTOPROBES
The photosubstitutions of 2-fluoro-4-nitroanisole with several amines are studied from the preparative and the preliminary mechanistic points of view.The possible usefulness of this structure as biochemical photoprobe is discussed.
Figueredo, M.,Marquet, J.,Moreno-Manas, M.,Pleixats, R.
p. 2427 - 2428
(2007/10/02)

Dynamic Behavior of Photoexcited Solutions of 4-Nitroveratrole Containing OH(1-) or Amines
Kinetics and mechanism of the photoinduced reactions of 4-nitroveratrole (NV) with OH(1-) and a number of aliphatic amines have been studied by laser kinetic spectroscopy and by measurement of transient changes in electrical conductivity.In the case of OH(1-) only reactions with NV in its lowest triplet state (T0) are observed and these lead to substitution of the OCH3 group at atom C(2) of the aromatic molecule and to formation of a number of transient ?-complexes, which return to NV.In the case of the amines both excited singlet state and triplet state reactionsare observed.The singlet state reactions with methylamine and with n-hexylamine lead to substitution of the OCH3 group at atom C(2).Although piperidine quenches the excited singlet states involved, this does not lead to substitution at C(2).The triplet state reactions with the amines start with a discrete electron-transfer reaction in which the radical anion (E) of NV is formed.Subsequent reaction of E with the radical cation of the amine leads to substitution at C(1) and to a number of ?-complexes, which return to NV.
Eijk, Alexander M. J. van,Huizer, A. Herbert,Varma, Cyril A. G. O.,Marquet, Jorge
p. 88 - 95
(2007/10/02)

ON THE REGIOSELECTIVITY OF THE NUCLEOPHILIC AROMATIC PHOTOSUBSTITUTIONS OF 4-NITROVERATROLE. A THREEFOLD MECHANISTIC PATHWAY.
4-Nitroveratrole (NVT) is photosubstituted with primary amines, piperidine and hydroxide ion.Primary amines and hydroxide ion cause replacement of the methoxy group in meta position with respect to the nitro group whereas piperidine photosubstitutes the methoxy group in para to the nitro group.Photoreactions with piperidine and hydroxide ion involve attack of the amine upon a triplet excited state of NVT.Mechanistic evidences indicate that the reaction with hydroxide ion is probably a SN23Ar* process whereas the reaction with piperidine involves a radical ion pair formed via electron transfer from the amine to a triplet excited state.Photoreactions with primary amines involve attack of the amine upon a singlet excited state of NVT (SN21 Ar* process).These results are discussed and justified in the context of nucleophile ionization potential considerations and ground state donor-acceptor complex formation abilities.
Cantos, Albert,Marquet, Jorge,Moreno-Manas, Marcial,Castello, Assumpta
p. 2607 - 2618
(2007/10/02)

PHOTOREACTION OF 3,4-DIMETHOXY-1-NITROBENZENE WITH BUTYLAMINE. A pH DEPENDENCE OF REGIOSELECTIVITY
Irradiation of 3,4-dimethoxy-1-nitrobenzene in the presence of butylamine leads to the formation of both possible photosubstitution products, i.e., 2-methoxy-4-nitro-N-butylaniline and 2-methoxy-5-nitro-N-butylaniline with the predominance of the latter.Regioselectivity of the reaction as measured by molar ratio of the two isomeric products varies with pH of the solution, ranging from 3:1 at pH 10 to 12:1 at pH 12.The results are discussed in view of possible use of 3,4-dimethoxy-1-nitrobenzene moiety as a lysine-directed photoaffinity probe.
Kuzmic, Petr,Pavlickova, Libuse,Velek, Jiri,Soucek, Milan
p. 1665 - 1670
(2007/10/02)

PHOTOCHEMICAL HYDROLYSIS OF SOME NITROPHENYL ACETATES
Nitro substituent exhibits a meta-activating effect on the course of photochemical hydrolysis of phenyl acetates since UV photolysis of isomeric 4- and 3-nitrophenyl acetates in neutral aqueous solution leads to the formation of the corresponding phenols with quantum yields 0.002 and 0.006, respectively; 2-methoxy-4-nitro- and 2-methoxy-5-nitrophenyl acetates showed still greater difference in their photochemical reactivity ( Φr 0.002 and 0.129, respectively).Quenching of the photohydrolysis of the latter compound with 2,4-hexadienoic acid indicates the participation of a triplet state with the effective lifetime of 0.15 μs.The photoreaction is accelerated in acidic media which means that one of the early photochemical steps is the protonation of the excited state.No incorporation of 18O into the product molecule was observed after the photolysis of 2-methoxy-5-nitrophenyl acetate in H218O, which is an unambiguous evidence that the photoreaction proceeds as a light-induced hydrolysis of the ester bond.
Kuzmic, Petr,Pavlickova, Libuse,Soucek, Milan
p. 1293 - 1300
(2007/10/02)

THE NUCLEOPHILIC AROMATIC PHOTOSUBSTITUTION IN PHOTOAFFINITY LABELLING. A MODEL STUDY OF A CYCLOHEXIMIDE DERIVATIVE.
The photoreactions of several 4-nitrocatechol ethers with the relatively hard nucleophile methylamine occurs at the meta position respect to the nitro group.A derivative of the antibiotic cycloheximide carrying a 4-nitrocatechol ether moiety reacts with methylamine affording an in vitro model for photoaffinity labelling.
Castello, A.,Marquet, J.,Moreno-Manas, M.,Sirera, X.
p. 2489 - 2492
(2007/10/02)

APPLICATION OF LINEAR FREE ENERGY RELATIONSHIPS (LFER) IN THE AROMATIC HYDROXYLATION OF ANISOPLES BY CHROMIC ACID IN ACETIC ACID - WATER MIXTURES
Kinetics and mechanism of aromatic hydroxylation of anisoles by chromic acid has been investigated in 50percent aqueous acetic acid.The reaction followed secod order kinetics, first order each in and at constant and ionic strength.The effect of meta and para substituents on the reaction rates has been studied.The application of LFER to the rates reveals that the site of attack is at the ortho carbon atom but not at the oxygen atom.
Reddy, P. Musala,Jagannadham, V.,Sethuram, B.,Rao, T. Navaneeth
p. 115 - 118
(2007/10/02)

Synthesis of Nitrohydroxyxanthones and Oxidative Coupling of Phenols Using Cerium(IV) Ammonium Nitrate
Nitration of 1-hydroxyxanthone and oxidative coupling of phenol and o-methoxyphenol by cerium(IV)ammonium nitrate (CAN) have been reported for the first time.The reaction in each case leads to the formation of nitro-bis-phenols along with nitrophenols.
Chaudhuri, Kakoli (nee-Chakrabarty),Chawla, H. Mohindra
p. 1277 - 1279
(2007/10/02)

NITROPHENYL ETHERS AS POSSIBLE PHOTOAFFINITY LABELS. THE NUCLEOPHILIC AROMATIC PHOTOSUBSTITUTION REVISITED.
The photoreactions of nitrophenyl ethers with simple primary and secondary amines show a remarkable nucleophile influence on the reaction orientation.Calculations indicate a change from charge controlled to frontier orbital controlled reactions moving from smaller to larger amines.
Cervello, J.,Figueredo, M.,Marquet, J.,Moreno-Manas, M.,Bertran, J.,Lluch, J. M.
p. 4147 - 4150
(2007/10/02)

Cleavage of the Methylenedioxy Ring. III. Cleavage with Sodium Benzyloxide in Dimethyl Sulfoxide
Cleavage of the methylenedioxy ring in aromatic formyl (1-3), nitro (4 and 5), and acetyl (30) compounds with N-sodium benzyloxide-benzyl alcohol in dimethyl sulfoxide gave 3-hydroxybenzene derivatives (19, 22-24, 26, 27, and 33).In the case of the acetyl compound 30, the 4-hydroxybenzene derivative (34) was also obtained as a minor product.Regioselective cleavage of the ring in aromatic compounds having electronwithdrawing groups with nucleophilic oxide anions is discussed.Cleavage of the ring in 1-5 and 30 with 2 N sodium methoxide in dimethyl sulfoxide-dimethylformamide was found to be useful for the practical preparation of 3-hydroxybenzene derivatives (6-10 and 31).Keywords - Cleavage of methylenedioxy ring; regioselectivity; piperonals; 3,4-methylenedioxy-nitrobenzene; 3,4-methylenedioxy-acetophenone; sodium methoxide; sodium phenoxide; sodium benzyloxide; dimethyl sulfoxide; dimethylformamide
Kobayashi, Shigeru,Okimoto, Kazuto,Imakura, Yasuhiro
p. 1567 - 1573
(2007/10/02)

Photochemical Methoxide Exchange in Some Nitromethoxybenzenes. The Role of the Nitro Group in SN2Ar* Reactions
Photoinduced methoxide exchange has been measured for o-, m-, and p-nitromethoxybenzene as well as for the methoxy-labeled 4-nitroveratroles.In these reactions (of the SN23Ar* type), where the substituting and leaving groups are the same, the origin of the meta activation found cannot be otherwise than in the steps leading to the symmetrical ? complex.Next to considerations on the basis of charge distributions of the excited aromatic compounds and of electron densities in the HOMO and LUMO, a rationalization for the regioselectivity and activation is offered on the basis of the energy gap between the ground-state and the excited-state surfaces.Since the level of the ground-state ? complex corresponding to meta substitution as compared to that for ortho/para substitution is highest in energy and since the trajectory from the (triplet) excited state leading to the ? complex for meta substitution is lowest in energy, the energy gap between the excited-state and ground-state hypersurface is considerable smaller for the geometry that leads to the meta ? complex and thus to meta substitution.Meta direction and activation of photosubstitutions will therefore be outspoken in the case of a substituent that gives difficult or no substitution at the meta position in the ground state and brings with it low triplet energy.The NO2 group conforms nicely to these criteria.
Riel, H. C. H. A. van,Lodder, G.,Havinga, E.
p. 7257 - 7262
(2007/10/02)

Cleavage of Methylenedioxy Ring. II. Cleavage with Sodium Phenoxide and Methoxide in Dimethyl Sulfoxide
Regioselective cleavage of the methylenedioxy ring in piperonals (1, 6, and 7) and 3,4-methylenedioxynitrobenzenes (8 and 9) by oxide ions in dimethyl sulfoxide was achieved: the 4-hydroxybenzene derivatives (2, 10-13, and 22) were obtained with the phenoxide ion, while the 3-hydroxybenzene derivatives (4, 18-21, 23, 26, and 29) were obtained with the methoxide or benzyloxide ion.Keywords - cleavage of methylenedioxy ring; regioselectivity; piperonals; 3,4-methylenedioxynitrobenzenes; sodium phenoxide; sodium methoxide; dimethyl sulfoxide; 3-hydroxybenzene derivatives; 4-hydroxybenzene derivatives; NMR spectra
Kobayashi, Shigeru,Imakura, Yasuhiro,Horikawa, Ritsuko
p. 1287 - 1293
(2007/10/02)