Iminium Ion Formation Research Articles

On the Roles of Protic Solvents in Imidazolidinone‐Catalyzed Transformations

Step by step: The effect of protic solvents on the rate and stereochemical outcome of the imidazolidinone-catalyzed Diels–Alder cycloaddition was rationalized. The following picture emerges: the solvent accelerates iminium ion formation (step 1), the Diels–Alder cycloaddition is reversible (step 2), and the solvent intercepts the iminium ion adduct (step 3

ChemInform Abstract: Secondary and Primary Amine Catalysts for Iminium Catalysis

AbstractReview: 237 refs.

ChemInform Abstract: The Polonovski Reaction

In connection with their interest in the isolation and medicinal properties of the “gen alkaloids” (N-oxides), Max and Michel Polonovski reported in 1927 their discovery that the treatment of a tertiary amine N-oxide with acetic anhydride or acetyl chloride results in a rearrangement in which one of the alkyl groups attached to nitrogen is cleaved, and the N-acetyl derivative of the corresponding secondary amine and aldehyde are obtained. As the original work by the Polonovskis was mainly carried out on bicyclic tropane N-oxide derivatives, the products of the reaction were the demethylated amides and formaldehyde. The reaction was thus looked upon as a means of effecting N-demethylation of tertiary amines. As such it was, and still remains, a viable alternative to the use of cyanogen bromide, alkyl chloroformates, azocarboxylic esters, or nitrous acid for this purpose. These methods often require more drastic conditions and/or promote unwanted side reactions. The central feature of the Polonovski reaction is the transformation of an N-oxide to an iminium ion intermediate. Depending on the structure of the substrate and the acid anhydride or other activating reagent employed, iminium ion formation can occur through loss of an α hydrogen, or through fragmentation of a Cαcarbon bond. Again, depending on conditions, the reaction will either stop at this stage and iminium ions become the Polonovski products, or proceed to give enamines or tertiary amides and/or secondary amines and aldehydes. The often close relationship between structure and reaction conditions, which determine both product types and reaction regiochemistry, is discussed. In its initial stages the Polonovski reaction resembles other reactions in which a tertiary amine is oxidized through interaction of the pair of electrons on nitrogen with an agent X followed by elimination of the elements of HX. Reagents like lead tetraacetate, N-bromosuccinimide and in particular mercuric acetate have been employed for this purpose. However, the Polonovski reaction can offer certain advantages in selectivity and experimental ease. In principle, any reagent capable of activating the N-oxide oxygen could promote the Polonovski reaction. However, three major types of activating agents, acid anhydrides and chlorides (including chloroformate esters), iron salts and complexes, and sulfur dioxide, are usually employed. Perhaps the most important contribution to the use of the Polonovski reaction in modern organic synthesis was the discovery in the 1960s that on replacing acetic anhydride by trifluoroacetic anhydride the reaction could be stopped at the iminium ion stage. Considering the rich chemistry of iminium ions, many applications of this modified, or Polonovski–Potier, reaction have appeared in the literature, the most spectacular of which was the first successful approach to the synthesis of the indole antitumor agents of the vinblastine group. In this chapter the discussion is limited to examples of the Polonovski reaction in which the abovementioned activating reagents are employed. The coverage is restricted to tertiary amine oxides in which at least one of the substituents on nitrogen is an alkyl group. Both acyclic and cyclic amine oxides such as piperidine N-oxide fall into this category, whereas heteroaromatic N-oxides do not. With the exception of one special case, nitrones are also omitted from the discussion. The literature coverage includes articles appearing before the end of August 1988. Keywords: Polonovski reaction; scope; limitations; acylating agents; iron-based reagents; sulfur dioxide; carbon-hydrogen eliminations; synthetic applications; CC fragmentations; iron salts; silicon; selenium; experimental procedures; indole alkaloids

ChemInform Abstract: X=Y-ZH Systems as Potential 1,3-Dipoles. Part 40. Chiral Azomethine Ylides from Homochiral Cyclic α-Amino Esters. Unusual Regiospecific Deprotonation of Iminium Ions.

Abstract A1,3-Strain and charge accelerated 1,5-H shifts are important factors in the stereospecific formation of iminium ions from cyclic secondary α-amino esters and bifunctional carbonyl compounds and their regiospecific or regioselective conversion to homochiral azomethine ylides. The endo-exo selectivity in the cycloadditions of these azomethine ylides is sensitive to the bifunctional carbonyl compound employed.

Stereochemical Models for Discussing Additions to α,β‐Unsaturated Aldehydes Organocatalyzed by Diarylprolinol or Imidazolidinone Derivatives – Is There an ‘(E)/(Z)‐Dilemma’?

AbstractThe structures of iminium salts formed from diarylprolinol or imidazolidinone derivatives and α,β‐unsaturated aldehydes have been studied by X‐ray powder diffraction (Fig. 1), single‐crystal X‐ray analyses (Table 1), NMR spectroscopy (Tables 2 and 3, Figs. 2–7), and DFT calculations (Helv. Chim. Acta 2009, 92, 1, 1225, 2010, 93, 1; Angew. Chem., Int. Ed. 2009, 48, 3065). Almost all iminium salts of this type exist in solution as diastereoisomeric mixtures with (E)‐ and (Z)‐configured +NC bond geometries. In this study, (E)/(Z) ratios ranging from 88 : 12 up to 98 : 2 (Tables 2 and 3) and (E)/(Z) interconversions (Figs. 2–7) were observed. Furthermore, the relative rates, at which the (E)‐ and (Z)‐isomers are formed from ammonium salts and α,β‐unsaturated aldehydes, were found to differ from the (E)/(Z) equilibrium ratio in at least two cases (Figs. 4 and 5, a, and Fig. 6, a); more (Z)‐isomer is formed kinetically than corresponding to its equilibrium fraction. Given that the enantiomeric product ratios observed in reactions mediated by organocatalysts of this type are often ≥99 : 1, the (E)‐iminium‐ion intermediates are proposed to react with nucleophiles faster than the (Z)‐isomers (Scheme 5 and Fig. 8). Possible reasons for the higher reactivity of (E)‐iminium ions (Figs. 8 and 9) and for the kinetic preference of (Z)‐iminium‐ion formation are discussed (Scheme 4). The results of related density functional theory (DFT) calculations are also reported (Figs. 10–13 and Table 4).

Formation and Structure of an (Iminium ion)ruthenium Complex and Reaction of the Iminium Ion Moiety with Alcohols

The (iminium ion)ruthenium(II) complex mer(Cl,Cl,Cl)-[RuCl3{η2-NCHCH2py(C2H4py)C2H5}] was synthesized via a C−H activation of the bridging ethyl group of an ethylbis(2-pyridylethyl)amine ligand and characterized by X-ray structural analysis. Oxidations of alcohols by the (iminium ion)ruthenium(II) complex occurred with generation of mer-[RuIIICl3(ebpea)].

α-Aminoester-Derived Imidazoles by 1,5-Electrocyclization of Azavinyl Azomethine Ylides

An efficient and practical method for the preparation of alpha-imidazol-1-yl esters from 1,2-diaza-1,3-dienes (DDs), alpha-amino esters, and aldehydes is described. The overall sequence features a Michael-type conjugate addition between the alpha-amino ester and the DD, followed by iminium ion formation via condensation with the aldehyde and 1,5-electrocyclization of the resulting thermally generated azavinyl azomethine ylide to afford eventually alpha-imidazol-1-yl esters. Such a protocol allows access to enantiomerically pure imidazoles from optically pure alpha-amino esters.

Facile Formation of Cyclic Aminals through a Brønsted Acid-Promoted Redox Process

Cyclic aminals were prepared through a Brønsted acid-promoted reaction. This redox neutral process involves iminium ion formation, 1,5 H-transfer, followed by ring closure.

Broad Scope Organocatalytic Aza-Michael Addition

A procedure for asymmetric cinchona alkaloid (1, 2) catalyzed aza-Michael addition of enones 3 and alkoxyamines 4 is reported. The scope of the reaction was demonstrated in several examples with different substituted substrates, affording products in moderate to good yields and good to excellent enantioselectivities. Moreover, the authors provide two possible activation modes. Both involve hydrogen-bonding interactions of the tertiary amine with alkoxyamines 4 and iminium-ion formation between the primary amine and the enones 3.

Combined Organo- and Transition-Metal Catalysis Cascade to Cyclopentenes

The authors report an efficient synthesis of various cyclopentenones from α,β-unsaturated ketones 1 and alkyne-tethered malonate pro-nucleophiles 2. The reaction presumably occurs via a complex mechanism, combining pyrrolidine and a copper(II) species as catalysts. Iminium-ion formation from α,β-unsaturated ketones 1 and pyrrolidine is followed by Michael addition of malonate nucleophiles 2. Subsequently the resulting enamine attacks the adjacent alkyne functionality, which is activated by the copper(II) catalyst. Carbon-carbon bond formation, followed by hydrolysis and protonolysis provides products 3 and recycles the catalysts.

The Strecker reaction — an examination in terms of no-barrier theory

For those examples of the Strecker reaction where information about both rate and equilibrium is available, we have been able to calculate rate constants for the addition of cyanide ion to the iminium ion by the no-barrier theory (NBT) approach. Both experimental and calculated values are for reaction in aqueous solution. Only for the reactions of benzaldehyde with benzyl or allyl amines and HCN are the equilibrium constants and rate constants for the final, rate-determining, step directly available from the literature. For the reactions of acetone with ammonia, methylamine, or dimethylamine and HCN rate constants for the retro-Strecker cleavage and the equilibrium constants for the overall Strecker reaction have been reported. These equilibrium constants, combined with equilibrium constants for iminium ion formation, which can be extracted from information in the literature, allow calculation of the equilibrium constants for the final step of these Strecker reactions. No-barrier theory has already been applied to carbonyl additions, including cyanohydrin formation; this report provides further evidence for the generality of this approach for calculating rate constants without using any kinetic information.Key words: Strecker reaction, rate constant, equilibrium constant, no-barrier theory, computation.

Glycine Enolates: The Effect of Formation of Iminium Ions to Simple Ketones on α-Amino Carbon Acidity and a Comparison with Pyridoxal Iminium Ions

Equilibrium constants in D2O were determined by 1H NMR analyses for formation of imines/iminium ions from addition of glycine methyl ester to acetone and from addition of glycine to phenylglyoxylate. First-order rate constants, also determined by 1H NMR, are reported for deuterium exchange between solvent D2O and the alpha-amino carbon of glycine methyl ester and glycine in the presence of increasing concentrations of ketone and Brønsted bases. These rate and equilibrium data were used to calculate second-order rate constants for deprotonation by DO- and by Brønsted bases of the alpha-imino carbon of the ketone adducts. Formation of the iminium ion between acetone and glycine methyl ester and between phenylglyoxylate and glycine is estimated to cause 7 unit and 15 unit decreases, respectively, in the pKa's of 21 and 29 for deprotonation of the parent carbon acids. The effect of formation of iminium ions to phenylglyoxylate and to 5'-deoxypyridoxal (DPL) [Toth, K.; Richard, J. P. J. Am. Chem. Soc. 2007, 129, 3013-3021] on the carbon acidity of glycine is similar. However, DPL is a much better catalyst than phenylglyoxylate of deprotonation of glycine, because of the exceptionally large thermodynamic driving force for conversion of the amino acid and DPL to the reactive iminium ion.

Modular Asymmetric Synthesis of Functionalized Azaspirocycles Based on the Sulfoximine Auxiliary

A modular asymmetric synthesis of the functionalized azaspirocycles 6 (m = 2, n = 1), 7 (m = 1, n = 2), 8 (m = n = 2), 12, 20, and 24 from the cyclic allylic sulfoximines 1 is described. The synthetic strategy is based on the stereoselective construction of the carbocycle 4 containing the amino-substituted tertiary C atom from 1 followed by the generation of the azaspirocycle. Three different routes have been followed for the synthesis of the heterocyclic ring: N,C-dianion cycloalkylation, ring-closing metathesis, and N-acyl iminium ion formation.

Synthesis of 14C‐labeled piperidines and application to synthesis of [14C]SCH 351125, a CCR5 receptor antagonist

Abstract1‐Benzyl‐4‐hydroxy[2‐14C]piperidine, a useful intermediate in labeled compound synthesis, was prepared from [14C]formaldehyde in high yield. The distribution pattern of 14C in the product is consistent with a mechanism involving reversible iminium ion formation and rapid equilibration of the iminium ion through a cationic aza‐Cope rearrangement. These steps precede the rate‐determining intramolecular cyclization step. SCH 351125 is a potent, selective CCR5 receptor antagonist with potential as a treatment for HIV infection. [14C]SCH 351125, required for metabolism studies, was prepared from 1‐benzyl‐4‐hydroxy[2‐14C]piperidine in six steps. [14C]SCH 351125 is a mixture of four atropisomers. Preparation of [14C]SCH 351125 besylate salt of the desired atropisomer pair is also described. Copyright © 2007 John Wiley & Sons, Ltd.

A New Imidazole‐Containing Imidazolidinone Catalyst for Organocatalyzed Asymmetric Conjugate Addition of Nitroalkanes to Aldehydes

AbstractHerein we report a new organocatalyst for the asymmetric Michael addition of nitroalkanes to α,β‐unsaturated aldehydes. This catalyst incorporates a basic imidazole group in addition to the secondary amine responsible for activation of the α,β‐unsaturated carbonyl compounds via iminium ion formation. The new organocatalyst is capable of catalyzing the enantioselective carbon‐carbon bond formation with a high degree of enantiocontrol providing products in enantiomeric excesses of up to 92 % and yields of up to to 91 %. These results constitute the best results so far reported for organocatalyzed Michael additions of nitroalkanes to α,β‐unsaturated aldehydes, and provide proof of principle that organocatalysts incorporating two internal basic moieties may find broad application in organocatalyzed Michael additions.

Inhibitory Effects of Neurotransmitters and Steroids on Human CYP2A6

Human CYP2A6 catalyzes the metabolism of nicotine, cotinine, and coumarin as well as some pharmaceutical drugs. CYP2A6 is highly expressed in liver and, also, in brain and steroid-related tissues. In this study, we investigated the inhibitory effects of neurotransmitters and steroid hormones on CYP2A6 activity. We found that coumarin 7-hydroxylation and cotinine 3'-hydroxylation by recombinant CYP2A6 expressed in baculovirus-infected insect cells were competitively inhibited by tryptamine (both K(i) = 0.2 microM), serotonin (K(i) = 252 microM and 167 microM), dopamine (K(i) = 49 microM and 22 microM), and histamine (K(i) = 428 microM and 359 microM). Cotinine formation from nicotine was inhibited by tryptamine (K(i) = 0.7 microM, competitive), serotonin (K(i) = 272 microM, noncompetitive), dopamine, noradrenaline, and adrenaline (K(i) = 11 microM, 54 microM, and 81 microM, uncompetitive). Estrogens (K(i) = 0.6-3.8 microM), androgens (K(i) = 60-149 microM), and corticosterone (K(i) = 36 microM) also inhibited cotinine formation, but coumarin 7-hydroxylation and cotinine 3'-hydroxylation did not. Nicotine-Delta(5'(1'))-iminium ion formation from nicotine was not affected by these steroid hormones, indicating that the inhibition of cotinine formation was due to the inhibitory effects on aldehyde oxidase. The nicotine-Delta(5'(1'))-iminium ion formation was competitively inhibited by tryptamine (K(i) = 0.3 microM), serotonin (K(i) = 316 microM), dopamine (K(i) = 66 microM), and histamine (K(i) = 209 microM). Thus, we found that some neurotransmitters inhibit CYP2A6 activity, being related with inter- and intraindividual differences in CYP2A6-dependent metabolism. The inhibitory effects of steroid hormones on aldehyde oxidase may also contribute to interindividual differences in nicotine metabolism.

Organocatalytic synthesis of chiral benzopyrans

Benzopyrans, or chromenes, are widespread in nature and are considered to be a privileged scaffold in medicinal chemistry. Herein, we report the first organocatalyzed asymmetric synthesis of chiral benzopyrans. The benzopyran unit is constructed through a domino reaction involving an oxa-Michael attack of salicylic aldehyde derivatives onto α,β-unsaturated aldehydes, activated through iminium-ion formation with the organocatalyst, followed by an intramolecular aldol reaction and subsequent elimination of water. This overall reaction sequence provides benzopyrans with aromatic C-2 substituents in up to 60% yield and 60% enantioselectivity, while C-2 aliphatic analogues can be obtained in 90% enantiomeric excess, but with only 20% yield. The role of additives, as well as the possible racemization of the benzopyran, was also investigated.

Mechanism of Inactivation of Human Cytochrome P450 2B6 by Phencyclidine

The mechanism behind the observed inactivation of human P450 2B6 by phencyclidine (PCP) has been evaluated over the past 2 decades. The scope of the current investigation was to contribute to the fundamental knowledge of PCP oxidation and perhaps the mechanism behind P450 inactivation. To study the chemistry of PCP oxidation, we subjected PCP to the Fenton reagent. Under Fenton chemistry conditions, oxidation on all three PCP rings was observed by liquid chromatography/tandem mass spectrometry (LC-MS/MS). When PCP was incubated with the Fenton system in the presence of glutathione (GSH), three GSH-PCP conjugates were identified. Subsequent LC-MS/MS analysis of these conjugates revealed two species that had GSH attached to the cyclohexane ring of PCP and a third conjugate in which GSH was adducted to the piperidine ring. When PCP was incubated across a panel of P450 enzymes, several enzymes, including P450s 2D6 and 3A4, were able to catalyze the formation of the PCP iminium ion, whereas P450s 2B6 and 2C19 were exclusively able to hydroxylate secondary carbons on the cyclohexane ring of PCP. Subsequent mechanistic experiments revealed that only P450s 2B6 and 2C19 demonstrated loss of catalytic activity after preincubation with 10 microM PCP. Finally, investigation of P450 2B6 inactivation using structural analogs of PCP revealed that blocking the para-carbon atom on the cyclohexane ring of PCP from oxidation protected the P450 2B6 from inactivation, which suggests that a reactive intermediate generated during the hydroxylation of the cyclohexane ring may be linked to the mechanism of inactivation of P450 2B6 by PCP.

Computational study of iminium ion formation: effects of amine structure

Density functional calculations are used to explore the formation of iminium ions from secondary amines and acrolein and the subsequent reactivity of the resulting iminium ions. After establishing a feasible profile for this reaction in simulated experimental conditions, we focus on the effect of variation in amine structure on calculated barriers. This analysis shows that incorporation of a heteroatom (N or O) in the alpha-position to the reactive amine results in significantly reduced energy barriers, as does an electron-withdrawing group (carbonyl or thiocarbonyl) in the beta-position. Electron density analysis is used to monitor reactions at a detailed level, and to identify important intermolecular interactions at both minima and transition states. Barriers to reaction are linked to calculated proton affinities of secondary amines, suggesting that the relative ease of protonation-deprotonation of the amine is a key property of effective catalysts. Moreover, barriers for subsequent Diels-Alder reaction of iminium ions with cyclopentadiene are lower than for their formation, suggesting that formation may be the rate determining step in the catalytic cycle.

Cyanide Trapping of Iminium Ion Reactive Intermediates Followed by Detection and Structure Identification Using Liquid Chromatography−Tandem Mass Spectrometry (LC-MS/MS)

Secondary and tertiary alicyclic amines are widely found in pharmaceuticals and environmental compounds. The formation of iminium ions as reactive intermediates in the metabolic activation of alicyclic amines has previously been investigated in radiometric assays where radiolabeled cyanide is typically employed. In this paper, we report a relatively high throughput LC-MS/MS method for the detection of the nonradiolabeled cyanide adduct formed in rat or human liver microsomal incubations via constant neutral loss scan followed by structural characterization using product ion scan on a triple quadrupole mass spectrometer. A total of 14 alicyclic amine compounds were investigated with the cyanide trapping LC-MS/MS screen and also with the glutathione (GSH) trapping screen, a well-established and commonly employed technique for reactive metabolite screening. Our results are found to be in general agreement with the previous metabolism reports for these compounds, demonstrating the effectiveness, speed, and simplicity of the cyanide trapping LC-MS/MS method to study the iminium ion intermediates from alicyclic amines and its complementarities to GSH trapping method for reactive metabolite screenings.