Lewis Acid Catalysis of a Diels−Alder Reaction in Water

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.

Oligonucleotides containing 7-vinyl-7-deazaguanine as a facile strategy for expanding the functional diversity of DNA

A modified nucleobase 7-vinyl-7-deazaguanine ((V)G) produced adducts with maleimides through Diels-Alder cycloaddition under very mild conditions. By this method, post-synthetic modification to oligonucleotides with diverse functionality (carboxylic acid, pyrene, benzophenone, succinimidyl ester, nitroxide and biotin) was accomplished.

The origins of noncovalent catalysis of intermolecular Diels-Alder reactions by cyclodextrins, self-assembling capsules, antibodies, and RNAses

The catalysis of Diels-Alder reactions by noncovalent binding by synthetic, protein, and nucleic acid hosts has been surveyed and compared. These catalysts consist of binding cavities that form complexes containing both the diene and the dienophile; the cycloaddition reaction occurs in the cavity. The binding requires no formation of covalent bonds and is driven principally by the hydrophobic (or solvophobic) effect. A molecular mechanics and dynamics study of the cyclodextrin catalysis of a Diels-Alder reaction is used to exemplify and probe this form of catalysis. Detailed kinetic data is available for catalysis by antibodies, RNA, cyclodextrins, and Rebek's tennis ball capsules. Some of these catalysts stabilize the reactants more than the transition state and consequently will only have catalytic effect under conditions of low substrate-to-catalyst ratios. None of the hosts achieve significant specific binding of transition states that is the hallmark of enzyme catalysis.

Effects of host-guest recognition on kinetics of Diels-alder reaction of quinocrown ethers with cyclopentadiene

Diels-Alder reactions of 15-21-membered quinocrown ethers 1a-c and 18-membered quinobenzocrown ether 1d with cyclopentadiene were catalyzed by the addition of alkali, alkaline earth metal and ammonium perchlorates, and scandium trifluoromethane-sulfonate. The alkali metal and ammonium ions brought about a fairly selective rate-acceleration for each crown ether due to the size-fitted ion-in-the-hole complexation. However, such a hole-size-selectivity was not observed for the reactions catalyzed by divalent alkaline-earth (Mg2+ to Ba2+) and trivalent Sc3+ ions. The wrapping complexation played a significant role in rate-acceleration in such a way that the smallest Mg2+ caused 160 times rate-enhancement for the most flexible 1c and the Sc3+ performed maximal 3700 times rate-increment for the 18-membered quinobenzocrown 1d. These effects of cation recognition were rationalized by the reduction of LUMO energy that is favored by the orbital interaction with the HOMO of cyclopentadiene. The magnitude of rate-enhancement was discussed in terms of the cation binding affinity and coordination geometry of quinocrown ethers as well as the valence of cations.

Rhodium chemzymes: Michaelis-Menten kinetics in dirhodium(II) carboxylate-catalyzed carbenoid reactions

Rhodium carboxylate-mediated reactions of diazoketones involving cyclopropanation, C-H insertion, and aromatic C-C double bond addition/electrocyclic ring opening obey saturation (Michaelis-Menten) kinetics. Axial ligands for rhodium, including aromatic hydrocarbons and Lewis bases such as nitriles, ethers, and ketones, inhibit these reactions by a mixed kinetic inhibition mechanism, meaning that they can bind both to the free catalyst and to the catalyst-substrate complex. Substrate inhibition can also be exhibited by diazocompounds bearing these groupings in addition to the diazo group. The analysis of inhibition shows that the active catalyst uses only one of its two coordination sites at a time for catalysis. Some ketones exhibit the interesting property that they selectively bind to the catalyst-substrate complex. The similarity of the kinetic constants from different types of reactions with similar diazoketones, regardless of the linking unit or the environment of the reacting alkene, suggests that the rate-determining step is the generation of the rhodium carbenoid. A very useful rhodium carboxylate catalyst for asymmetric synthesis, Rh(2)(DOSP)(4), shows slightly slower kinetic parameters than the achiral catalysts, implying that enantioselectivity of this catalyst is based on slowing reactions from one of the enantiotopic faces of the reactant, rather than any type of ligand-accelerated catalysis. A series of rhodium catalysts derived from acids with pK(a)s spanning 4 orders of magnitude give very similar kinetic constants.

Diels--Alder bioconjugation of diene-modified oligonucleotides

In an effort to offer complementary technology for covalent biomolecule modification (bioconjugation), we have developed a method that exploits the aqueous acceleration of Diels--Alder reactions for this purpose. Three different diene phosphoramidite reagents have been synthesized that enable diene modification of synthetic oligonucleotides prepared by the phosphoramidite method. Clean and efficient Diels--Alder cycloaddition of these diene oligonucleotides with maleimide dieneophiles was carried out, and the labeled oligonucleotide bioconjugates were characterized by HPLC and electrospray mass spectrometry. Dieneophile stoichiometry, temperature, and pH are all parameters that were shown to influence the efficiency of the process.

Lewis-acid catalyzed organic reactions in water. The case of AlCl(3), TiCl(4), and SnCl(4) believed to be unusable in aqueous medium

Classical Lewis acids such as AlCl(3), TiCl(4), and SnCl(4), believed to be unusable as catalysts in aqueous medium, efficiently catalyzed regio- and stereoselective azidolysis and iodolysis of alpha,beta-epoxycarboxylic acids in water at pH 4.0 and 1.5, respectively. The concept of water-tolerant metal-salt is reexamined in direct relationship to the aqua ion hydrolysis constant.

Azidolysis of α,β-Epoxycarboxylic Acids. A Water-Promoted Process Efficiently Catalyzed by Indium Trichloride at pH 4.0

The catalytic efficiency of InCl(3), Yb(OTf)(3), and Sc(OTf)(3) in the azidolysis of alpha,beta-epoxycarboxylic acids has been studied in water and in organic solvents, for comparison using NaN(3) and Me(3)SiN(3) as the source of the azido group. In water, the catalytic effectiveness of these metal salts strongly depends on the pH of the aqueous medium and on the type of Lewis acid catalyst. In water their catalytic activity is mostly due to the corresponding aqua ion species, the concentration of which becomes significant when the pH of the aqueous medium is below the corresponding pK(1,1) hydrolysis constant. The process is more efficient in water than in organic solvents. At pH 4.0, InCl(3) is a far better catalyst than Yb(OTf)(3) or Sc(OTf)(3) and allows the highly regio- and diasteroselective preparation of beta-azido-alpha-hydroxycarboxylic acids, which can be isolated in pure form in very high yields.

Efficient catalysis of a Diels-Alder reaction by metallo-vesicles in aqueous solution

Vesicles have been prepared from a cyclic phosphate ester (5,5-di-n-dodecyl-2-hydroxy-1,3,2-dioxaphosphorinan-2-one) with copper(II) counterions (Cu(dDP)(2)). They form a highly efficient aqueous Lewis acid catalyst system. The reaction of two azachalcon derivatives (1a, 1b) with cyclopentadiene (2) was studied to elucidate the catalytic potential of this system.

Chiral bis(oxazoline) copper(II) complexes: versatile catalysts for enantioselective cycloaddition, Aldol, Michael, and carbonyl ene reactions

A bis(oxazoline) (box) copper(II) complex and its hydrated counterpart (1 and 2) function as enantioselective Lewis acid catalysts for carbocyclic and hetero Diels-Alder, aldol, Michael, ene, and amination reactions with substrates capable of chelation through six- and five-membered rings. X-ray crystallography of the chiral complexes reveals a propensity for the formation of distorted square planar or square pyramidal geometries. The sense of asymmetric induction is identical for all the processes catalyzed by [Cu((S,S)-t-Bu-box)](X)(2) complexes 1 and 2 (X = OTf and SbF) resulting from the intervention of a distorted square planar catalyst-substrate binary complex. These catalyzed processes exhibit excellent temperature-selectivity profiles. Reactions catalyzed by [Cu(S,S-Ph-pybox)](SbF(6))(2) and their derived chelation complexes are also discussed.

Retro-Diels-Alder Reaction in Aqueous Solution: Toward a Better Understanding of Organic Reactivity in Water

The retro-Diels-Alder (RDA) reaction of anthracenedione 1a proceeds considerably faster in aqueous solutions than in organic solvents. Addition of organic solvents to water retards the reaction, whereas glucose induces a modest acceleration. SDS micelles induce a considerable retardation, but even at high concentrations of surfactant (complete micelle-substrate binding), the cycloreversion is not fully inhibited. Correlation with data for solvatochromic indicators strongly suggest that the origin of the water-induced acceleration involves primarily enhanced hydrogen bonding of water to the activated complex for the RDA reaction of 1a. Activation parameters support this view. A comparison of the present results with previous kinetic data for bimolecular and intramolecular Diels-Alder reactions provides insights into the contributions of hydrogen-bond and hydrophobic interactions to the aqueous accelerations of the latter two types of reactions.