Micellar catalysis of Diels-Alder reactions: substrate positioning in the micelle

We have studied the kinetics of the Diels-Alder reactions of cyclopentadiene, sorbyl alcohol, and sorbyltrimethylammonium bromide with a series of N-substituted maleimides in micellar media. Micellar rate constants have been determined and were found to be 20-40 times lower than the respective aqueous rate constants. Nevertheless, it was found that upon addition of sodium dodecyl sulfate the observed rate constants could be enhanced up to a factor of about 4.5. The low micellar rate constants can be attributed to the relatively apolar (water-poor) region of the micelle, in which the reactions take place. NMR experiments indicate that the reactants usually reside near the alpha- or beta-CH2 groups of the surfactant molecules in the micelle. Comparison of the micellar rate constants with rate constants in water/1-propanol mixtures suggests a concentration of water of 10-15 M in the micellar region where the diene and dienophile react.     

Hyperconjugative effects in the stereoselective ring-opening reactions of oxetenoxides

[reaction: see text] Unexpectedly, the pattern of the stereoselectivity in the ring-opening reactions of lithium oxetenoxides is not consistent with the bulkiness of substituents, and both the bulkier tert-butyl and silyl substituents favor inward rotation. With the aid of B3LYP calculations, the hyperconjugative interaction between the breaking C(1)-O sigma and its anti-periplanar Z-Me (Z = Si or C) sigma orbital is found to be responsible among the secondary orbital interactions of the substituents and the oxetene moiety.     

Recent development of heterogeneous catalysis in ring-opening,biocatalysis, and selective partial oxidation reactions on metal oxides

In this review article, we analyze the state of the art and future developments in three important domains of heterogeneous catalysis, namely ring opening, biocatalysis, and partial oxidation on metal oxides. After recollecting the scientific bases of each domain, we consider several examples, some recent improvements/developments, and some prospective views.     

Divergent regioselectivity in the base-promoted reactions of cyclic eight-membered alpha-ketols with activated halides

[reaction: see text] Deprotonation of 2-hydroxycyclooctanone followed by exposure to an allylic or benzylic halide proceeds very selectively to give the product of C-alkylation. The effect of Delta(5,6)-unsaturation is to promote instead the formation of the O-alkylated derivative. This crossover in kinetic preference is attributed to an inability of the olefinic system to attain a conformation conducive to proton abstraction at C-2.     

Redox catalysis versus heterogeneous one-electron catalysis. Mixed reduction of arenes and alkyl halides at specific palladized electrodes

Reduction of aromatic compounds (A) is achieved in the presence of alkyl halides RX (X = I or Br) at Ag–Pd electrodes in organic solvents. Those electrodes allow the one-electron reduction of RX with the selective formation of free radicals R. This new process (heterogeneous one-electron catalysis, H1EC) was used to alkylate in situ arenes. This mode of alkylation leads to re-visit previous results concerning redox catalysis published by Henning Lund when more conventional electrodes (like glassy carbon or mercury) were used and afforded similar results within a totally different potential scale. These preliminary results underline the reactivity differences between the molecular electrode (A⁻) and the almost ideal catalysis process (facile and specific liberation of free alkyl radicals).     

Hydrophobic substituent effects on proline catalysis of aldol reactions in water

Derivatives of 4-hydroxyproline with a series of hydrophobic groups in well-defined orientations have been tested as catalysts for the aldol reactions. All of the modified proline catalysts carry out the intermolecular aldol reaction in water and provide high diastereoselectivity and enantioselectivity. Modified prolines with aromatic groups syn to the carboxylic acid are better catalysts than those with small hydrophobic groups (1a is 43.5 times faster than 1f). Quantum mechanical calculations provide transition structures, TS-1a(water) and TS-1f(water), that support the hypothesis that a stabilizing hydrophobic interaction occurs with 1a.     

On the importance of specific solvent effects in electron trransfer reactions

The intramolecular electron transfer reaction within the binuclear complex formed between tetraamminepyrazinecarboxylatocobalt(III) and aquopentacyanoferrate(II) ions is studied in various binary aqueous media at different temperatures. Results could not be explained on the basis of a primitive model. Experimental data were fitted by using a multiparameter regression which shows the important role played by specific interactions on reactivity. © 1993 John Wiley & Sons, Inc.     

Manganese compounds in the catalysis of organic reactions

The review summarizes published data on the use of manganese compounds as catalysts in oxidation of alkanes, alcohols, aldehydes, sulfides, and amines and carbometalation, cross coupling, telomerization, chlorination, hydrosilylation, hydrohydrazination, and other reactions.     

A simple theoretical model for predicting the effects of benzannelation and bismethylenation on the rates of four-membered ring-opening reactions

A model based on simple Hückel MO theory is applied to the ring-openings of several cyclobutene derivatives and their benzo- and bis-methylene-analogs. The calculated changes in activation enthalpy upon benzannelation and bismethylenation are found to be in qualitative accord with experimental results. Application of the model to Dewar benzene and bicyclo[4.2.0]octa-2,4,7-triene ring-opening is discussed.     

Micellar effects of cetyltrimethylammonium dibromobromate in phosphoryl group transfer reactions

The nucleophilic reactivity of cetyltrimethylammonium dibromobromate (CTABr₃) with respect to organophosphorus compounds-4-nitrophenyl esters of diethylphosphoric (NPDEP) and diethylphosphonic acids (NPDEPN)-has been studied. Solution of CTABr₃ in aqueous alkali solutions generated the hypobromite anion, which permitted the formation of a micellar system with reactive α-nucleophilic counterion. The kinetics of this system was described in terms of a simple pseudo-phase model, the half-life for the conversion of the organophosphorus substrates into reaction products was 95 s (NPDEP) and 12 s (NPDEPN). __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 4, pp. 226–230, July–August, 2007.     

Conrotatory ring-opening reactions of cyclopropyl anions in monocyclic and tricyclic systems

[reaction: see text] Only conrotatory transition structures were located by B3LYP6-311++G(3df, 2p) computations for the electrocyclic ring opening of cyclopropyl anions having different substituents (H, Me, CN). The transition structure for the similarly substituted cyclopropyl anion fused to a bicyclic system exhibits the same features, in apparent contradiction with the observed product. A reaction path where the direction of twist changes after the transition state provides an explanation alternative to those proposed in recent reports.     

Bidentates versus monodentates in asymmetric hydrogenation catalysis: synergic effects on rate and allosteric effects on enantioselectivity

C 1-Symmetric phosphino/phosphonite ligands are prepared by the reactions of Ph 2P(CH 2) 2P(NMe 2) 2 with ( S)-1,1'-bi-2-naphthol (to give L A ) or ( S)-10,10'-bi-9-phenanthrol (to give L B ). Racemic 10,10'-bi-9-phenanthrol is synthesized in three steps from phenanthrene in 44% overall yield. The complexes [PdCl 2( L A,B )] ( 1a, b), [PtCl 2( L A,B )] ( 2a, b), [Rh(cod)( L A,B )]BF 4 ( 3a, b) and [Rh( L A,B ) 2]BF 4 ( 4a, b) are reported and the crystal structure of 1a has been determined. A (31)P NMR study shows that M, a 1:1 mixture of the monodentates, PMePh 2 and methyl monophosphonite L 1a (based on ( S)-1,1 '-bi-2-naphthol), reacts with 1 equiv of [Rh(cod) 2]BF 4 to give the heteroligand complex [Rh(cod)(PMePh 2)( L 1a )]BF 4 ( 5) and homoligand complexes [Rh(cod)(PMePh 2) 2]BF 4 ( 6) and [Rh(cod)( L 1a ) 2]BF 4 ( 7) in the ratio 2:1:1. The same mixture of 5- 7 is obtained upon mixing the isolated homoligand complexes 6 and 7 although the equilibrium is only established rapidly in the presence of an excess of PMePh 2. The predominant species 5 is a monodentate ligand complex analogue of the chelate 3a. When the mixture of 5- 7 is exposed to 5 atm H 2 for 1 h (the conditions used for catalyst preactivation in the asymmetric hydrogenation studies), the products are identified as the solvento species [Rh(PMePh 2)( L 1a )(S) 2]BF 4 ( 5'), [Rh(S) 2(PMePh 2) 2]BF 4 ( 6') and [Rh(S) 2( L 1a ) 2]BF 4 ( 7') and are formed in the same 2:1:1 ratio. The reaction of M with 0.5 equiv of [Rh(cod) 2]BF 4 gives exclusively the heteroligand complex cis-[Rh(PMePh 2) 2( L 1a ) 2]BF 4 ( 8), an analogue of 4a. The asymmetric hydrogenation of dehydroamino acid derivatives catalyzed by 3a, b is reported, and the enantioselectivities are compared with those obtained with (a) chelate catalysts derived from analogous diphosphonite ligands L 2a and L 2b , (b) catalysts based on methyl monophosphonites L 1a and L 1b , and (c) catalysts derived from mixture M. For the cinnamate and acrylate substrates studied, the catalysts derived from the phosphino/phosphonite bidentates L A,B generally give superior enantioselectivities to the analogous diphosphonites L 2a and L 2b ; these results are rationalized in terms of delta/lambda-chelate conformations and allosteric effects of the substrates. The rate of hydrogenation of acrylate substrate A with heterochelate 3a is significantly faster than with the homochelate analogues [Rh( L 2a )(cod)]BF 4 and [Rh(dppe)(cod)]BF 4. A synergic effect on the rate is also observed with the monodentate analogues: the rate of hydrogenation with the mixture containing predominantly heteroligand complex 5 is faster than with the monophosphine complex 6 or monophosphonite complex 7. Thus the hydrogenation catalysis carried out with M and [Rh(cod) 2]BF 4 is controlled by the dominant and most efficient heteroligand complex 5. In this study, the heterodiphos chelate 3a is shown to be more efficient and gives the opposite sense of optical induction to the heteromonophos analogue 5.     

SN2 reactions in the gas phase. Alkyl group structural effects

The gas phase heats and entropies of S_N2-like association of bromide ions with a series of alkyl bromides are reported. Comparison of this data with published data for the same reactions in solution suggests that the alkyl group structural effects on S_N2 reactivity in solution are controlled entirely by the solvent.     

Photochemical ring-opening and intramolecular hydrogen shift reactions in sulfur analogues of alpha-pyrone

A combined matrix isolation FTIR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the photochemistry of sulfur analogues of alpha-pyrone [2H-thiopyran-2-one (TP) and 2H-pyran-2-thione (PT)] was carried out. The vibrational spectra of monomers of the compounds isolated in low-temperature argon matrixes were studied experimentally and assigned completely on the basis of theoretical calculations. UV irradiation (lambda > 337 nm) of the studied compounds isolated in low-temperature matrixes results mainly in the ring-opening reaction by means of the cleavage of the alpha-bond. Other photoprocesses, not involving the alpha-bond-cleavage step (such as generation of Dewar valence isomer), correspond to the minor reaction channels in both studied compounds. The ring-opening photoreaction in PT represents the first reported case of an alpha-bond cleavage in a compound with a C=S group attached to a six-membered ring, in which the internal strain practically does not exist, whereas the corresponding reaction in TP (a cleavage of a C-S bond in the alpha position with respect to a carbonyl group) is now reported for the first time. Following the ring-opening reactions, isomerization processes and intramolecular hydrogen shift reactions were observed, enabling production of TP from PT and vice versa. A detailed study of such processes was undertaken, and kinetical and mechanistical data are presented and discussed.     

Synthesis of functionalized 2,3-dihydroisoxazoles by domino reactions in water and unexpected ring-opening reactions of 2,3-dihydroisoxazoles

α,α-Dicyanoolefins react with hydroxylamine to afford 2,3-dihydroisoxazoles (2,3-dihydroisoxazoles can be easily isolated by filtration) in excellent yields under mild and environmentally benign conditions. A one-pot reaction in tandem with an unexpected ring-opening of 2,3-dihydroisoxazoles has been developed as well.     

Superacid catalyzed ring-opening reactions involving 2-oxazolines and the role of superelectrophilic intermediates

A variety of 2-oxazolines are found to react with arenes in superacidic triflic acid, CF₃SO₃H. It is proposed that the 2-oxazolines are protonated twice in triflic acid and the resulting intermediates undergo ring-opening reactions to produce reactive, dicationic species. These superelectrophiles are capable of reacting with benzene and o-dichlorobenzene in high yields by Friedel-Crafts type reactions.