Visible‐Light Photoredox‐Catalyzed C−H Difluoroalkylation of Hydrazones through an Aminyl Radical/Polar Mechanism

An unprecedented visible‐light‐induced direct C?H bond difluoroalkylation of aldehyde‐derived hydrazones was developed. This reaction represents a new way to synthesize substituted hydrazones. The salient features of this reaction include difluorinated hydrazone synthesis rather than classical amine synthesis, extremely mild reaction conditions, high efficiency, wide substrate scope, ease in further transformations of the products, and one‐pot syntheses. Mechanistic analyses and theoretical calculations indicate that this reaction is enabled by a novel aminyl radical/polar crossover mechanism, with the aminyl radical being oxidized into the corresponding aminyl cation through a single electron transfer (SET) process.     

Photoinduced Electron Transfer Reactions of 3,3‐Dialkylated 4,5‐Diphenyl‐3H‐Pyrazoles: A New Route to the Formation of the Solvent Adducts

3,3‐Dialkyl‐4,5‐diphenyl‐3H‐pyrazoles undergo readily photoinduced electron transfer (PET) reaction with 2,4,6‐triphenylpyrylium tetrafluoroborate (TPP⁺) in acetonitrile to produce cyclopropenes and 2H‐pyrroles. During prolonged irradiation, the new ring‐closure products derived from 2H‐pyrroles as the secondary photoproducts are also produced. However, the corresponding ester analog exhibits different behavior to obtain the cyclopropene as the primary photoproduct and a [2+2] dimer of the cyclopropene as the secondary photoproduct. A rationale for the different behavior is offered.     

Cyclopropyl alkynes as mechanistic probes to distinguish between vinyl radical and ionic intermediates

[reaction: see text] The reactions of (trans-2-phenylcyclopropyl)ethyne, 1a, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, 1b, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1c, with either aqueous sulfuric acid or tris(trimethylsilyl)silane (or tributyltin hydride) and AIBN have been investigated. Protonation and addition of the silyl (or stannyl) radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively. Under both reaction conditions, 1a yielded products derived from ring opening toward the phenyl substituent. Alkynes 1b and 1c, however, gave different products depending on whether radical or cationic conditions were used. When radical conditions were employed, products derived from regioselective ring opening toward the phenyl substituent were obtained. In contrast, when cationic conditions were employed, products derived from selective ring opening toward the methoxy substituent were isolated. The corresponding alpha-cyclopropyl-substituted vinyllithium derivatives were also synthesized and were found to be stable toward rearrangement. An estimate of the rate constants for ring opening of the alpha-cyclopropylvinyl cations was also made: values of 10(10)-10(12) s(-1) were found for the vinyl cations derived from protonation of the terminal carbon of alkynes 1a-c. Based on these results, cyclopropyl alkynes 1a-c can be classified as hypersensitive mechanistic probes for the detection of vinyl radical or cationic intermediates generated adjacent to the cyclopropyl ring and, in the case of 1b and 1c, the distinction between a radical or cationic intermediate is possible.     

Quinazolines and 1,4-benzodiazepines. LXXIII. The ring expansion of 2-chloromethylquinazoline 3-oxides with nitromethane

The reaction of 2-chloromethylquinazoline 3-oxides (3) with the anion of nitromethane gave the 2-nitromethylene benzodiazepine oxides (4) as a result of ring expansion. Other nucleophiles led only to products derived from substitution of the halogen. The ring expansion of 2,2-dichloro-1,2-dihydroquinazolines with nitromethane anion is also described.     

The Formation of Phosphacycloheptatrienes in Ring Enlargement Reaction

Abstract

The forrmation of substituted phosphacycloheptatrienes in ring expansion reaction(s) is describzd. From thc: reac-tion of 3,4-dimethyl-l-phenyl-3-phospholene-l-oxide(1,R¹ = c₆H₅, R₂=R₃ = CH₃) with dichlorocarbene under liquid-liquid phase transfer circumstances not the expected adduct but the appropriate phoshacycloheptatrisne (4, R_l, R₂,R₃ as above) was prepared. The formation of this product can be explained assuming two ring expansions effected by two series of dichlorocarbene addition and cyclopropane ring opening. In the similar reaction of the methoxy-phospholenc derivative (1, R¹=CH₃O, R² = R³ = CH3 four other products are also formed beside the phosphacyclohcptatriene. Again phosphacycloheptatrienes (4) are formed as the result of dichlorocarbene addition to the regioisoners of dihidrophosphorins (2) obtained from the phospholene-dichlorocarbene adducts by thermolysis. The same product can be derived from each regioisomeric pair.     

Crossover point between dialkoxy disulfides (ROSSOR) and thionosulfites ((RO)2S=S): prediction, synthesis, and structure

Isomeric preference between cyclic dialkoxy disulfides and thionosulfites is governed by the ring size of the heterocycle. Rings smaller than seven atoms prefer the thionosulfite connectivity, whereas larger rings or acyclic analogues favor the unbranched dialkoxy disulfide structure. Density functional calculations were employed to predict the crossover point at which both constitutional isomers are of comparable stability. Follow-up synthesis provides the previously unknown eight-membered ring dialkoxy disulfide 14 and seven-membered ring thionosulfite 15 from the same reaction. X-ray crystallography for all but one of the reaction products and complementary NMR analysis furnishes insights into both solid-state and solution conformations. A long-standing issue regarding the concerted vs catalyzed isomerization pathway between XSSX and X(2)S=S has been addressed for X = RO and shown to be acid dependent.     

The micro-optical ring electrode. 3: Transient photocurrent studies of photophysical-electrochemical and photophysical-chemical-electrochemical systems

The micro-optical ring electrode (MORE) is a photoelectrochemical device based on a ring microelectrode that uses the insulating material interior to the ring electrode as a light guide. In this paper, we describe the preparation and characterization of very thin ring MOREs with (ring inner radius)/(ring outer radius) > 0.99. Theoretically, we derive asymptotic analytical expressions for the time dependence of the diffusion-limited transient light-on photocurrent generated by two general types of photoelectrochemical systems: (a) the PE (photophysical-electrochemical) system, wherein the photoexcited species itself is directly detected on the ring; (b) the PCE (photophysical-chemical-electrochemical) system, wherein the photoexcited species undergoes a homogeneous electron transfer reaction prior to electrochemical detection. Experimentally, we establish that it is possible to use such MOREs to study the wavelength dependence of photocurrents derived from photoelectrochemically active systems, such as the Ru(bipy)3 2+/Fe3+ PCE system, demonstrating the potential utility of the MORE as a selective electroanalytical probe. We also use our expressions for the time dependence of photocurrents at the MORE to derive values for the photoelectrochemical kinetic parameters of this system, including the rate coefficient for the back reaction of photogenerated Ru(bipy)3 3+ (0.115 s(-1)) and the quantum efficiency for the primary redox products, Ru(bipy)3 3+ and Fe2+, escaping cage recombination, phi(CE) = 0.099.     

Oxidative 5-endo cyclization of enamides mediated by ceric ammonium nitrate

[reaction: see text] Ceric ammonium nitrate mediates the oxidative 5-endo radical-polar crossover reactions of beta-enamide esters to give 5,5-C,O-disubstituted-gamma-lactams. Trapping of the intermediate cations leads to 5-hydroxy- or 5-alkoxy-gamma-lactams depending upon the reaction conditions. The methodology was used to synthesize the basic heterocyclic ring fragments of the natural products L-755,807, Quinolacticin C, and PI-091.     

A quantum mechanical study of the reactivity of (SiO)2-defective silica surfaces

The reactivity of the strained (SiO)(2)-four atom ring defect at the silica surfaces has been studied in a cluster approach adopting the ONIOM2[B3LYP6-31+G(d,p):MNDO] method to compute the ring opening reaction by interaction with H(2)O and NH(3). The vibrational "fingerprints" of the isolated defect are computed at 921, 930, and 934 cm(-1) in reasonable agreement with experimental evidence on amorphous silica outgassed at T>900 K. The opening of the (SiO)(2)-four-member ring by the considered molecules is exergonic and the actual value depends on the possible constraints enforced on the reaction products by the silica surrounding. The free kinetic energy barriers result from the interplay between the nucleophilic/electrophilic character of the adsorbed molecule and are 22 and 25 kcal mol(-1) for NH(3) and H(2)O, respectively. All free energy profiles envisage an activated complex in which the nucleophilic part of the molecule interacts on the coordinatively strained silicon atom of the (SiO)(2) defect followed by the proton transfer from the coordinated molecule towards the oxygen of the defective ring. Calculations show that this step can be speed up by the presence of more than one adsorbed molecule or even more (about seven orders of magnitude), by the copresence of water molecules acting as "proton transfer helpers." In these cases, the free energy barriers decrease to approximately 13 and 15 kcal mol(-1) for NH(3) and H(2)O, respectively. For the case of H(2)O adsorption, benchmark test calculations reveal that MP2, BLYP, and B3LYP energy profiles are in very good agreement with each other, whereas for PBE, both the reaction energy and the activation barrier are underestimated. Present data also show that the molecular model mimicking the (SiO)(2) defect is far less reactive than what appears to occur on the real defect at the surface of amorphous silica. So, only a combination of some further geometrical strains imparted by the solid on the (SiO)(2) defect, not accounted for by the cluster models, and higher adsorbate loadings are needed to reharmonize experiment and simulation. Notwithstanding, the vibrational features of the reaction products have been characterized and support the available experimental measurements.     

Addition Mechanisms of Phenol toward Formaldehyde under Acidic Condition： a Theoretical Investigation

The reaction mechanisms of phenol with formaldehyde in the first and second addition at the ortho- and para-position in acid solution were theoretically investigated at the PW91/DNP level with solvent effects included. The reaction of phenol with protonated methanediol firstly forms an adduct intermediate, via a SN2 mechanism with a water molecule as the leaving group. From the adduct intermediate, there are two reaction channels involving a proton transfer to form the addition products. One is that a proton directly transfers via a four-membered ring transition state with a notable energy barrier (Four-member mechanism). Another mechanism involving a water molecule as catalyst to mediate the proton transfer (WCP mechanism), is a barrierless process, indicating that the formation of the adduct intermediate, the first reaction step, is rate-limiting. The reaction products are free hydroxymethyl phenols and/or hydroxybenzy carbocation (HOC6H4CH2+) which plays an important role in the following formation of methylene and methylene ether linkages. The second addition reactions between formaldehyde and hydroxymethyl phenol at all possible reaction sites of the phenol ring in acid solution were also investigated and discussed.     

Reactions of azirines with sulfur nucleophiles. 4. Treatment of 2H-azirine with mercaptosubstituted acids. reactions of aziridinyl alkyl sulfides with carboxylic acids and acyl chloride derivatives

Treatment of 2H-azirines with mercaptosubstituted acids and their derivatives leads to -ketoamides and 2-aziridinyl alkyl sulfides, respectively. 2-Aziridinyl alkyl sulfides, in turn, react with carboxylic acids to give -ketoamides and substituted ethanethiol derivatives. Acylation of 2-aziridinyl alkyl sulfides with acyl halides generates a variety of products, depending on the reaction conditions; either products derived from cleavage and isomerization of the aziridinyl ring or (1-acylaziridinyl-2) alkyl sulfides are obtained.For communication 3, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 206–211, February, 1986.     

Quantum Chemical Approach for Determining Degradation Pathways of Phenol by Electrical Discharge Plasmas

This study uses density functional theory (DFT) simulations to predict the main pathways by which hydroxyl (OH) radicals oxidize phenol into monohydroxylated products during an electrical discharge directly in or contacting water. The calculated activation energies and reaction rate constants indicate that phenol ring H abstraction is less likely to occur than OH addition, which will be the fastest in the ortho and para positions. The chain propagation with molecular oxygen of such formed ortho and para radicals will result in the production of hydroquinone and catechol, which are, concurrently, the most likely products of phenol degradation by OH radicals. Electron transfer reactions between dihydroxycyclohexadienyl radicals and plasma oxidative species are another important reaction mechanism which may be contributing significantly to the formation of products. Good agreement between computed kinetic and experimental data demonstrates the feasibility of applying DFT to investigate chemical reaction mechanisms.     

Intramolecular 1,8-hydrogen atom transfer reactions in disaccharide systems containing furanose units

A previously developed 1,8-hydrogen atom transfer (HAT) reaction promoted by 6-O-yl alkoxyl radicals between the two pyranose units in Hexp-(1→4)-Hexp disaccharides has been extended to other systems containing at least a furanose ring in their structures. In Penf-(1→3)-Penf (A) and Hexp-(1→3)-Penf (B) disaccharides, the 1,8-HAT reaction and concomitant cyclization to a 1,3,5-trioxocane ring are in competition with radical β-scission of the C4-C5 bond and formation of dehomologated products. The influence of the stereoelectronic β-oxygen effect on the β-scission and consequently on the 1,8-HAT reaction has been studied using the four possible isomeric d-furanoses. d-xylo- and d-lyxo-derivatives afforded preferentially 1,8-HAT products, whereas d-arabino- and d-ribo-derivatives gave exclusively direct β-scission of the alkoxyl radical. When the 6-O-yl radical is on a pyranose ring, as occurs in Penf-(1→4)-Hexp (C), it has been shown to provide the cyclized products exclusively.     

Mechanism of noncatalytic synthesis of tris(hydroxymethyl)phosphine. Nonempirical quantum-chemical approach

The gas-phase reaction of the synthesis of tris(hydroxymethyl)phosphine from phosphine and formaldehyde was studied using a calculation scheme based on the density functional theory with hybrid exchange-correlation functional B3LYP in the 6–311++G** basis. The reaction was shown to proceed with the participation of unstable intermediates containing three-membered ring. The transformation into final products includes opening of the three-membered ring and intramolecular proton transfer. The results can be useful at selecting catalysts and for explaining the mechanism of the catalytic reaction of tris(hydroxymethyl)phosphine synthesis.     

全氢菲在分子筛催化剂上环烷环开环反应的研究

在小型固定流化床(FFB)装置上考察了Y与ZSM-5分子筛催化剂以及Y分子筛催化剂上温度、剂油比对全氢菲裂化环烷环开环反应的影响。结果表明,全氢菲在分子筛催化剂上通过环烷环开环反应生成环己烷、十氢萘等单环或双环环烷烃;单环或双环环烷烃进一步侧链断裂生成2-甲基戊烷、甲基己烷等异构烷烃等,异构化生成二甲基环戊烷、甲乙基环戊烷等烷基环戊烷,氢转移生成苯、甲苯、二甲苯等烷基苯,进行深度氢转移反应生成萘、烷基萘等双环芳烃;另外,全氢菲也会通过脱氢缩合生成菲、芘等三环以上芳烃甚至焦炭等。由于扩散和吸附性能的影响,其裂化开环反应的选择性在Y分子筛催化剂上比在ZSM-5分子筛催化剂上高。因此,全氢菲环烷环开环与脱氢缩合反应的相对比例(s(NRO)/s(DHC))在Y分子筛催化剂上较高;在Y分子筛催化剂上,温度为475~550 ℃、剂油比为3.0~9.0,反应温度升高或者剂油比增加,双分子氢转移以及脱氢缩合反应增强,导致环烷环开环反应产物选择性降低。     

Density functional study on enantioselective reduction of keto oxime ether with borane catalyzed by oxazaborolidine

Chiral amino alcohols have interesting biological activities and are used widely as chiral ligands in metal-mediated organic reactions[1―3]. Although many amino alcohols can be derived from the available amino acids, the asymmetric synthesis is an important method to get novel amino alcohols. Tillyer et al.[4] reported a new, highly stereoselective synthesis of cyclic (1S,2R)-cis amino alcohols A from keto oxime ethers B, via the enantioselective reduction catalyzed by oxazaborolidine C in …     

Sydnones as Masked Hydrazines for Heterocycle Formation: Reactions of 3-(2-substituted phenyl)sydnones with HCl

In general, reaction of 3-(2-substituted phenyl)sydnones with HCl gives products derived from cleavage of the sydnone ring to the corresponding hydrazine and subsequent cyclization to the side chain. In one case, 3-(2-aminophenyl)sydnone ( 43 ), the product obtained, l-amino-lH-benzimidazole ( 47 ), apparently results from nucleophilic interception by the side chain prior to complete cleavage of the sydnone ring.     

Photoaddition of water and alcohols to the anthracene moiety of 9-(2'-hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer

The title compound undergoes efficient photoaddition of a molecule of a hydroxylic solvent (H(2)O, MeOH, (Me)(2)CHOH) across the 9- and 10-positions of the anthracene moiety to give isolable triphenylmethanol or triphenylmethyl ether type products. The reaction is believed to proceed via a mechanism involving water-mediated formal excited state intramolecular proton transfer (ESIPT) from the phenolic OH to the 10-position of the anthracene ring, generating an o-quinone methide intermediate that is observable by nanosecond laser flash photolysis, and is trappable with nucleophiles. A "water-relay" mechanism for proton transfer seems plausible but cannot be proven directly with the data available. Irradiation in deuterated solvents led to incorporation of one deuterium atom at the methylene position in the photoaddition product, and partial deuterium exchange of the 10-position of recovered starting material, consistent with the proposed formal excited state proton transfer mechanism. The deuterium exchange and photoaddition reach maximum quantum efficiency at approximately 5 M water (in CH(3)CN or CH(3)OH), with no reaction observed in the absence of a hydroxylic solvent, demonstrating the sensitivity of this type of ESIPT to solvent composition.     

Multistate reactivity in styrene epoxidation by compound I of cytochrome p450: mechanisms of products and side products formation

Density functional theoretical calculations are used to elucidate the epoxidation mechanism of styrene with a cytochrome P450 model Compound I, and the formation of side products. The reaction features multistate reactivity (MSR) with different spin states (doublet and quartet) and different electromeric situations having carbon radicals and cations, as well as iron(III) and iron(IV) oxidation states. The mechanisms involve state-specific product formation, as follows: a) The low-spin pathways lead to epoxide formation in effectively concerted mechanisms. b) The high-spin pathways have finite barriers for ring-closure and may have a sufficiently long lifetime to undergo rearrangement and lead to side products. c) The high-spin radical intermediate, (4)2(rad)-IV, has a ring closure barrier as small as the C--C rotation barrier. This intermediate will therefore lose stereochemistry and lead to a mixture of cis and trans epoxides. The barriers for the production of aldehyde and suicidal complexes are too high for this intermediate. d) The high-spin radical intermediate, (4)2(rad)-III, has a substantial ring closure barrier and may survive long enough time to lead to suicidal, phenacetaldehyde and 2-hydroxostyrene side products. e) The phenacetaldehyde and 2-hydroxostyrene products both originate from crossover from the (4)2(rad)-III radical intermediate to the cationic state, (4)2(cat,z(2) ). The process involves an N-protonated porphyrin intermediate that re-shuttles the proton back to the substrate to form either phenacetaldehyde or 2-hydroxostyrene products. This resembles the internally mediated NIH-shift observed during benzene hydroxylation.     

非对称氮杂环丙烷的亲核开环反应及其区域选择性

本文系统地总结了各类亲核试剂对非对称氮杂环丙烷(吖丙啶)的亲核开环反应及开环的区域选择性.氮杂环丙烷亲核开环的区域选择性是一种空间效应和电子效应平衡的结果,非芳基和非烯基取代的氮杂环丙烷的亲核开环通常发生在氮杂环丙烷取代少的碳原子上,空间效应起主导作用;而芳基和烯基取代的氮杂环丙烷的亲核开环通常发生在氮杂环丙烷芳甲位和烯丙位的碳原子上,电子效应起主导作用,烯基取代的氮杂环丙烷的亲核开环还可以发生在烯基的β-碳原子上;分子内的亲核开环反应主要受成环时环大小的控制,成环时的倾向是五元环>六元环>七元环.对于亲核试剂,一般的亲核试剂也同时受电子效应和空间效应的影响; 而亲核性强的亲核试剂通常只受空间效应的影响.容易生成稳定自由基的亲核试剂容易发生单电子转移机理的开环反应,生成相当于亲核试剂进攻氮杂环丙烷中取代多的碳原子得到的开环产物.