Geometrical effects on the intramolecular quenching of pi,pi* aromatic ketones by phenols and indoles

Laser flash photolysis of a series of bichromophoric compounds 1-12 containing the 2-benzoylthiophene (BT) and phenol (PhOH) or indole (InH) moieties has been used to determine the possible geometrical effects in the intramolecular quenching of triplet excited ketones, resulting in formal hydrogen abstraction. The results are compared with those obtained in the intermolecular process. In both cases, substitution either at the thienyl or the phenyl moiety has a marked influence on the photoreactivity. Time-resolved experiments showed that the rate constants for bimolecular quenching by phenol and indole of 2-benzoylthiophene substituted at the thienyl 5-position were lower than those for BT substituted at the phenyl p-position, which agrees with the higher energy found for the excited triplet state of the latter compounds. However, the rate constant for hydrogen abstraction in the bichromophoric compounds by the pi,pi* triplet state of the derivatives with the spacer linked to the thienyl 5-position are higher than those of their regioisomers. These results indicate a possible geometry-dependence in the intramolecular quenching process. Theoretical DFT studies have been carried out in order to estimate the optimum conformation for hydrogen abstraction in two pairs of phenolic and indolic bichromophoric regioisomers. The energy profile for photoactivation/deactivation of the aromatic ketone and the structures of the triplet states and biradicals involved in the process have been determined. The observed regiodifferentiation in the experimental studies is consistent with a dependence of the rate constant on orbital overlap between the carbonyl oxygen and the X-H bonds.     

Diels-Alder reaction between indoles and cyclohexadienes photocatalyzed by pi,pi aromatic ketones

[structure: see text] Diels-Alder reactions between indoles and cyclohexadienes can be photocatalyzed by benzoylthiophenes. Both experimental and theoretical studies show that the reactions occur through ternary excited triplet state complexes (triplet triplexes).     

Stereodifferentiation in the formation and decay of the encounter complex in bimolecular electron transfer with photoactivated acceptors

Experimental evidence has been obtained for the involvement of encounter complexes between both enantiomers of a pi,pi* triplet excited ketone and a chiral phenol or indole. Determination of the pre-equilibrium constants (K(EC)) and the intrinsic decay rate constants (kd) indicates a significant stereodifferentiation in both steps of the quenching process.     

Palladium-catalyzed intramolecular hydroalkylation of alkenyl-beta-keto esters, alpha-aryl ketones, and alkyl ketones in the presence of Me3SiCl or HCl

Reaction of 3-butenyl beta-keto esters or 3-butenyl alpha-aryl ketones with a catalytic amount of [PdCl2(CH3CN)2] (2) and a stoichiometric amount of Me3SiCl or Me3SiCl/CuCl2 in dioxane at 25-70 degrees C formed 2-substituted cyclohexanones in good yield with high regioselectivity. This protocol tolerated a number of ester and aryl groups and tolerated substitution at the allylic, enolic, and cis and trans terminal olefinic positions. In situ NMR experiments indicated that the chlorosilane was not directly involved in palladium-catalyzed hydroalkylation, but rather served as a source of HCl, which presumably catalyzes enolization of the ketone. Identification of HCl as the active promoter of palladium-catalyzed hydroalkylation led to the development of an effective protocol for the hydroalkylation of alkyl 3-butenyl ketones that employed sub-stoichiometric amounts of 2, HCl, and CuCl2 in a sealed tube at 70 degrees C.     

The inter-and intramolecular addition of aromatic ketone triplets to quadricyclenes: Comparison with the corresponding reactivities of norbornadiene,methylenenorbornene, methylenenorbornane and norbornene

Irradiation of acetophenone or benzophenone in the presence of norbornadiene or quadricyclene gives 1: 1-adducts. These arise via reaction of the ketone triplet with the quadricyclene. Similar reactivity is exhibited in an intramolecular sense by 7-phenacylnorbornadiene and the corresponding quadricyclene. Addition of triplet benzophenone to 5-methylene norbornene is regiospecific in contrast to expectations based on a comparison of the reactivities of 2-methylenenorbornane and norbornene. The possibility is raised that, in contrast to the simple olefins and the quadricyclenes, exciplexes formed by reaction of the rigid homoconjugated dienes with the triplet ketone are endo orientated with participation of both double bounds.     

The first Ru(II)-catalysed asymmetric hydrogen transfer reduction of aromatic ketones in aqueous media

The first water-soluble asymmetric hydrogen-transfer ruthenium(II) catalyst system consisting of [Ru(p-cymene)Cl2]2, (S)-proline amide, and sodium formate, which gives high conversion rates with high ee values up to 95.3% and is reusable, has been developed.     

Nonradiative inter- and intramolecular energy transfer from the aromatic donor anisole to a synthesized photoswitchable acceptor system

We report steady state and time resolved fluorescence measurements on acetonitrile (ACN) solutions of the model compounds, energy donor anisole (A) and a photoswitchable acceptor N,N′-1,2-phenylene di-p-tosylamide (B) and the multichromophore (M) where A and B are connected by a spacer containing both rigid triple (acetylenic) and flexible methylene bonds. Both steady state and time correlated single photon counting measurements demonstrate that though intermolecular energy transfer, of Forster type, between the donor and acceptor moieties occurs with rate 10⁸ s^?1 but when these two reacting components are linked by a spacer (multichromophore, M) the observed transfer rate (～10¹¹ s^?1) enhances. This seemingly indicates that the imposition of the spacer by inserting a triple bond may facilitate in the propagation of electronic excitation energy through bond. The time resolved fluorescence measurements along with the theoretical predictions using Configuration interaction singles (CIS) method by using 6-31G (d,p) basis set, implemented in the Gaussian package indicate the formations of the two excited conformers of B. The experimental findings made from the steady state and time resolved fluorescence measurements demonstrate that, though two different isomeric species of the acceptor B are formed in the excited singlet states, the prevailing singlet–singlet nonradiative energy transfer route was found from the donor A to the relatively longer-lived isomeric species of B.     

Trapping the intermediate involved in the intramolecular cyclisation of cyclopropyl ketones. A convenient preparation of open-chain γ-hydroxy ketones

The concerted mechanism for the stannic chloride catalysed cyclisation of aryl cyclopropyl ketones is disproved by trapping the intermediate. The reaction provides a facile route to γ-hydroxyketones.     

Asymmetric reduction of aromatic and hetero cyclic ketones by hydrosilylation and hydrogen transfer in the presence of optically active rhodium catalysts

A study has been made of enantioselective hydrosilylation and reduction, by hydrogen transfer, of prochiral alkyl phenyl ketones or alkyl hetaryl ketones over various optically active catalysts. A total of 14 aromatic and heterocyclic carbinols were synthesized with preparative yields of 54–100%. The most effective catalytic systems were found to be complexes of RhCl₃ and [Rh(cod)Cl]₂ with the known optical inductor (S,S)-i-Pr-Pybox, with which we have obtained for the first time a series of heterocyclic secondary alcohols with an enantioselectivity of 20–63%.Latvian Institute of Organic Synthesis, Riga, LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 342–358, March, 1996. Original article submitted December 11, 1995.     

Diironcarbonyl-coumarin complex: preparation, intramolecular electron transfer, and electro-generation of hydrogen

A new organometallic complex coupling photoactive coumarin to a diironhexacarbonyl unit has been successfully prepared and its composition and electronic structure confirmed by elemental and spectroscopic analyses. Emission spectral analysis of the complex reveals photoinduced intramolecular electron transfer from coumarin to the iron-carbonyl moiety. The compound is electrochemically reduced at −1.24 V vs. Fc/Fc⁺. This reduction is irreversible, attesting to the instability of the complex. Electrochemical evolution of hydrogen in the presence of the complex has been studied and results are discussed.      

Proton magnetic resonance spectra and the intramolecular hydrogen bonding of o-alkoxy- phenols and -benzenethiols

The PMR spectra of several methoxy substituted benzenethiols and related phenols have been examined in various solvents over a wide range of concentrations. The positions of the sulfhydryl proton resonance signals (δ_SH) of o-methoxybenzenethiol and 2,6-dimethoxybenzenethiol in inert solvents, and the δ_SH values of the thiols carrying o-OMe groups are less affected by the interaction with the solvents than those without o-OMe groups. The significant differences in the behaviour of chemical shifts of these compounds have been best interpreted by the intramolecular S---HO H-bonding. Additional evidence for the intramolecular H-bonding in o-OMe substituted benzenethiols have been obtained from the IR spectroscopic data.     

Ruthenium-catalyzed transfer hydrogenation of aromatic ketones with aminophosphine or bis(phosphino)amine ligands derived from isopropyl substituted anilines

A series of new highly active Ru(II) complexes with two new (N-diphenylphosphino)isopropylanilines, having an isopropyl substituent at carbon 2- (1) or 2,6- (2) and two new bis(diphenylphosphino)isopropylanilines, having an isopropyl substituent at carbon atom 2- (3) or 4- (4), were prepared starting from the dimeric complex [Ru(η⁶-p-cymene)(μ-Cl)Cl]₂. All the compounds have been fully characterized by microanalysis, IR, ³¹P{1H} NMR, ¹H NMR and ¹³C NMR spectroscopies. Following activation by NaOH, complexes 5–8 were tested in the transfer hydrogenation of acetophenone derivatives with iso-PrOH as the hydrogen source. Catalytic studies showed that the complexes are excellent catalytic precursors for the transfer hydrogenation of acetophenone derivatives.     

Co(III)/Zn(II)-catalyzed dearomatization of indoles and coupling with carbenes from ene-yne ketones via intramolecular cyclopropanation

A straightforward and efficient protocol for dearomatizing indoles is described. The reaction, catalyzed by an inexpensive Co(III)/Zn(II) catalyst, starts from easily accessible N-pyrimidinyl indoles and ene-yne ketones. Mild reaction conditions, high diastereoselectivity, a broad substrate scope, effective functional group tolerance, and reasonable to remarkable yields were observed.     

Contribution of the intramolecular hydrogen bond to the shift of the pKa value and the oxidation potential of phenols and phenolate anions

Intramolecularly OHO[double bond, length as m-dash]C hydrogen bonded phenols, 2-HO-C6H2-3,5-(t-Bu)2-CONH-t-Bu (1-OH), 2-HO-C6H2-5-t-Bu-1,3-(CONH-t-Bu)2 (2-OH) and 2-HO-C6H2-3,5-(t-Bu)2-NHCO-t-Bu (4-OH), were synthesized and their phenolate anions were prepared as tetraethylammonium salts (-1O-(NEt4+), 2-O-(NEt4+) and 4-O-(NEt4+)) with intramolecular NHO(oxyanion) hydrogen bonds. 4-HO-C(6)H(2)-3,5-t-Bu(2)-CONH-t-Bu (3-OH) and its phenolate anion, 3-O-(NEt4+), were synthesized as non-hydrogen bonded references. The presence of intramolecular hydrogen bonds was established through the crystallographic analysis and/or (1)H NMR spectroscopic results. Intramolecular NHO(phenol) hydrogen bonds shift the pK(a) of the phenol to a more acidic value. The results of cyclic voltammetry show that the intramolecular OH...O=C hydrogen bond negatively shifts the oxidation potential of the phenol. In contrast, the intramolecular NHO(oxyanion) hydrogen bond positively shifts the oxidation potential of the phenolate anion, preventing oxidation. These contributions of the hydrogen bond to the pKa value and the oxidation potentials probably play an important role in the formation of a tyrosyl radical in photosystem II.     

Evidence supporting a single electron transfer pathway in the reduction of aromatic ketones by metal alkoxides. Lithium isopropoxide,an excellent reducing agent for aromatic ketones.

Reactions of various metal alkoxides with aromatic ketones have been shown to produce radical intermediate. Lithium isopropoxide has been found to be an excellent reducing agent for aromatic ketones and reduces benzophenone at a faster rate than does aluminum isopropoxide.     

Selective hydrogen transfer reactions from alcohols to ketones promoted by rhodium complexes with 1,10-phenanthroline and related ligands

Substituted cyclohexanones are effectively reduced to the corresponding alcohols using propan-2-ol as hydrogen source and Rh(I) complexes with 1,10-phenanthroline (phen) and substituted derivatives as catalyst precursors.Rh(I) derivatives with phen, 4,7-Ph₂phen and 4,7-Mc₂phen (chel/Rh ⩾ 2) promote the preferential formation of the axial alcohol (substrate = 4-t-butylcyclohexanone), while the 4,7-(MeO)₂phen derivative gives in higher yield the equatorial isomer. Hindered substrates such as 2-methylcyclohexanone and 3,3,5-Me₃cyclohexanone are reduced to the corresponding axial alcohol with a selectivity ⩾ 98% (chel = 4,7-Me₂phen).