Synthesis of α-Aminophosphonic Acids Sy Pd(0)Alkylation of Diethyl Aminomethylphosphonate Schiff Bases

Abstract

Aminoalkyl phosphonic compounds have many interesting biological properties: they are used as herbicides, antibiotics and enzyme inhibitors. Several approaches to the preparation of α-aminophosphonic acids have been reported. One efficient method using the schiff base of diethyl amino methylphosphonate 1 has been developped, allowing alkylation at the α carbon after deprotonation using strong bases such as LDA. In our continuing program dealing with the synthesis of unusual α-aminoacids which relies on C-C bond formation by transition metal catalyzed reactions (l), we report here a method based on the palladium-promoted alkylation of diethyl aminophosphonate schiff bases 2 and 3 with n³-allyl species 5 generated in situ from the allylic derivative 4:     

带电组氨酸侧链与DNA碱基间非键作用强度的理论研究

采用MP2方法和6-31+G(d,p)基组优化得到了带有一个正电荷的组氨酸侧链与4个DNA碱基间形成的18个氢键复合物的气相稳定结构, 从文献中获取了组氨酸侧链与DNA碱基间形成的12个堆积和T型复合物的气相稳定结构, 使用包含基组重叠误差(BSSE)校正的MP2方法和aug-cc-pVTZ基组及密度泛函理论M06-2X-D3方法和aug-cc-pVDZ基组计算了这些复合物的结合能. 研究结果表明, 包含BSSE校正的M06-2X-D3方法和aug-cc-pVDZ基组能够给出较准确的结合能; 气相条件下, 组氨酸侧链与同种DNA碱基间的离子氢键作用明显强于堆积作用和T型作用, 组氨酸侧链最易通过离子氢键与胞嘧啶C和鸟嘌呤G作用形成氢键复合物, 组氨酸与胞嘧啶C和鸟嘌呤G间的T型作用强于与腺嘌呤A和胸腺嘧啶T间的离子氢键作用; 水相条件下, 组氨酸侧链与同种DNA碱基间的离子氢键作用仍明显强于堆积作用和T型作用, 组氨酸侧链更易与胞嘧啶C和鸟嘌呤G相互作用形成氢键复合物, 但是最强的组氨酸侧链与胞嘧啶C间的T型作用明显弱于与腺嘌呤A和胸腺嘧啶T间的离子氢键作用, 说明水相条件下组氨酸侧链与DNA碱基间主要通过离子氢键作用形成氢键复合物.     

Controlling the Coordination Sphere of Alkyllithiums Results in Selective Reactions with Allylic Amines

Described herein is a selective way to control the reaction of allylic amines with metalorganic bases depending on the amine handle as well as the metalorganic base is used. Depending on the number of coordinating groups within the amine handle either a selective carbometalation or deprotonation reaction can be performed. By changing the alkali metal within the base from lithium to either sodium or potassium, a change of chemoselectivity takes place and the reaction of piperidinoallylamine can be controlled.     

Effective synthesis of bicyclodienes via palladium-catalyzed asymmetric allylic alkylation and ruthenium-catalyzed cycloisomerization

[n.3.0]Bicycles (n = 3–6) can be synthesized using palladium-catalyzed asymmetric allylic alkylation followed by ruthenium-catalyzed cycloisomerization. New types of triarylphosphino-1,2-diaminooxazoline ligands show the same high levels of enantioselectivity observed with Trost ligand when employed in Pd-catalyzed allylic alkylation reactions. The enyne products of these allylic alkylation reactions were further elaborated using a Ru-catalyzed redox isomerization process, for which a mechanism is proposed.     

The rate ladder of proton-coupled tyrosine oxidation in water: a systematic dependence on hydrogen bonds and protonation state

Proton coupled electron transfer (PCET) from tyrosine covalently linked to Ru(bpy)32+ has been studied with laser flash-quench techniques. Two new complexes with internal hydrogen bonding bases to the phenolic proton have been synthesized. Depending on the hydrogen bonding and protonation situation the rate constant of PCET spanned over 5 orders of magnitude and revealed a systematic dependence on pH. This resulted in a previously predicted "rate ladder" scheme: (i) pH dependent concerted electron-proton transfer (CEP) with deprotonation to bulk water, giving low PCET rates, (ii) pH independent CEP with deprotonation to the internal base, giving intermediate PCET rates, and (iii) pure electron transfer from tyrosinate, giving high rates. This behavior is reminiscent of Yz oxidation in Mn-depleted and native photosystem II. The study also revealed important differences in rates between phenols with strong and weak hydrogen bonds, and for the latter a hydrogen bond-gated PCET was observed.     

Toward Efficient Palladium‐Catalyzed Allylic C?H Alkylation

Recent breakthroughs have proved that direct palladium(II)‐catalyzed allylic C? H alkylation can be achieved. This new procedure shows that the inherent requirement for a leaving group in the Tsuji–Trost palladium‐catalyzed allylic alkylation can be lifted. These initial reports hold great promise for the development of allylic C? H alkylation into a widely applicable methodology, thus providing a means to enhance synthetic efficiency in these reactions.     

Enamines: efficient nucleophiles for the palladium-catalyzed asymmetric allylic alkylation

Enamines were tested to be efficient nucleophiles for palladium-catalyzed asymmetric allylic alkylation, avoiding the use of unstablilized ketone enolates formed by strong bases. The influence of the chiral metallocene-based ligands upon this reaction was studied in detail. It was shown that planar chirality played an important role in enantioselectivities. Meanwhile, different kinds of enamines and allylic acetate to the reactions were also investigated. High catalytic activity and excellent enantioselectivity (up to 99% ee) were obtained with pyrrolidine enamines of both aliphatic and aromatic ketone.     

Intra/intermolecular direct allylic alkylation via Pd(II)-catalyzed allylic C-H activation

The first catalytic direct alkylation of allylic C-H bonds via Pd(II)-catalysis is described in the absence of base. Polysubstituted cyclic compounds can also be constructed by the intramolecular direct allylic alkylation.     

Knoevenagel Adducts as Trimethylenemethane Dipole Surrogates

Knoevenagel adducts derived from readily available acetoxyacetone and malonic acid derivatives served as trimethylenemethane surrogates for formal 1,3‐difunctionalization through a sequence of selective γ‐deprotonation/α‐alkylation and palladium(0)‐catalyzed allylic alkylation. Herein, we report the discovery and development of a three‐component 1,3‐difunctionalization of Knoevenagel adducts as well as a unique palladium(0)‐catalyzed branch‐selective allylic alkylation.     

Acidity constants from vertical energy gaps: density functional theory based molecular dynamics implementation

The question of how to compute acidity constants (pK(a)) treating solvent and solute at the same level of theory remains of some interest, for example in the case of high or low pH conditions. We have developed a density functional theory based molecular dynamics implementation of such a method. The method is based on a half reaction scheme computing free energies of dissociation from the vertical energy gaps for insertion or removal of protons. Finite system size effects are important, but largely cancel when half reactions are combined to full reactions. We verified the method by investigating a series of organic and inorganic acids and bases spanning a wide range of pK(a) values (20 units). We find that the response of the aqueous solvent to vertical protonation/deprotonation is almost always asymmetric and correlated with the strength of the hydrogen bonding of the deprotonated base. We interpret these observations in analogy with the picture of solvent response to electronic ionization.     

Metal Amides as the Simplest Acid/Base Catalysts for Stereoselective Carbon–Carbon Bond‐Forming Reactions

In this paper, new possibilities for metal amides are described. Although typical metal amides are recognized as strong stoichiometric bases for deprotonation of inert or less acidic hydrogen atoms, transition‐metal amides, namely silver and copper amides, show interesting abilities as one of the simplest acid/base catalysts in stereoselective carbon–carbon bond‐forming reactions.     

The deprotonation of internally hydrogen-bonded acids. Mechanistic conclusions from the influence of general bases

The reactions between alizarin yellow G and six different bases B (including OH^?) and between tropaeolin 0 and eight different bases have been investigated at 25°C and an ionic strength of 0.5M, using the temperature-jump method. From the form of the log k_B versus ΔpK curves it is concluded that for alizarin yellow G the observed relaxation time is due chiefly to a diffusion-controlled reaction between the base and that fraction which is present in the “open” non-hydrogen-bonded form, whereas for tropaeolin 0 the base attacks the hydrogen bridge. The dissociation constants for the internally bound hydrogen have been measured under the same conditions of temperature and ionic strength, using a spectrophotometric method.     

Geminal dicarboxylates as carbonyl surrogates for asymmetric synthesis. Part II. Scope and applications.

An enantioselective synthesis of allylic esters has been achieved by a novel asymmetric alkylation of allylic gem-dicarboxylates. The catalyst derived from palladium(0) and R,R-1,2-di(2'-diphenylphosphinobenzamido)cyclohexene efficiently induced the alkylation process with a variety of nucleophiles to provide allylic esters as products in good yield. High regio- and enantioselectivities were observed in the alkylation with most nucleophiles derived from malonate, whereas a modest level of ee's was obtained in the reactions with less reactive nucleophiles such as bis(phenylsulfonyl)ethane. In the latter case, a slow addition procedure proved effective, leading to significantly improved ee's. The utility of the alkylation products was demonstrated by several synthetically useful transformations including allylic isomerizations, allylic alkylations, and Claisen rearrangements. Using these reactions, the chirality of the initial allylic carbon-oxygen bond could be transferred to new carbon-oxygen, carbon-carbon, or carbon-nitrogen bonds in a predictable fashion with high stereochemical fidelity. The conversion of gem-diesters to chiral esters by the substitution reaction is the equivalent of an asymmetric carbonyl addition by stabilized nucleophiles. In conjunction with the subsequent reactions that occur with high stereospecificity, allylic gem-dicarboxylates serve as synthons for a double allylic transformation.     

Annulative Allylic Alkylation Reactions between Dual Electrophiles and Dual Nucleophiles: Applications in Complex Molecule Synthesis

Metal-catalyzed allylic alkylation reactions between dual nucleophiles and dual electrophiles represent a powerful set of methods for the synthesis of small-, medium-, and even large-sized rings. Using this strategy, a handful of simple allylic diol derivatives can be transformed into a broad array of complex carbo- and heterocycles of varying ring sizes in just a single step. Because of their ability to rapidly generate complexity, annulative allylic alkylation reactions between dual nucleophiles and dual electrophiles have been extensively employed in the total synthesis of both natural products and pharmaceutical compounds.     

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C−N Bond Forming Reactions of Amines with CO2

Organocatalysts promote a range of C?N bond forming reactions of amines with CO₂. Herein, we review these reactions and attempt to identify the unifying features of the catalysts that allows them to promote a multitude of seemingly unrelated reactions. Analysis of the literature shows that these reactions predominantly proceed by carbamate salt formation in the form [BaseH][RR′NCOO]. The anion of the carbamate salt acts as a nucleophile in hydrosilane reductions of CO₂, internal cyclization reactions or after dehydration as an electrophile in the synthesis of urea derivatives. The reactions are enhanced by polar aprotic solvents and can be either promoted or hindered by H‐bonding interactions. The predominant role of all types of organic and salt catalysts (including ionic liquids, ILs) is the stabilization of the carbamate salt, mostly by acting as a base. Catalytic enhancement depends on the combination of the amine, the base strength, the solvent, steric factors, ion pairing and H‐bonding. A linear relationship between the base strength and the reaction yield has been demonstrated with IL catalysts in the synthesis of formamides and quinazoline‐2,4‐diones. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyze the reactions. However, a physical limit to the extent of a purely base catalyzed reaction mechanism should exist, which needs to be identified, understood and overcome by synergistic or alternative methods.     

Organocatalytic asymmetric transformations of modified Morita-Baylis-Hillman adducts

Chiral Lewis basic tertiary amines or phosphines can enable properly modified Morita-Baylis-Hillman (MBH) adducts to undergo asymmetric allylic substitutions with a wide range of nucleophiles. In addition, assisted by a Br?nsted base, chiral Lewis bases can also catalytically convert modified MBH adducts into allylic ylides, which can be engaged in a variety of asymmetric annulation reactions. This tutorial review will focus on such chiral Lewis base-catalysed asymmetric transformations of MBH adducts, especially those developed over the past five years, allowing for the rapid construction of densely functionalised chiral molecules with high levels of regio- and stereoselectivities.     

THEORY OF GAS-PHASE ACIDITY(Ⅰ)CORRELATION OF DEPROTONATION ENERGY WITH HYDROGEN CHARGE

The charge distributions of various acids and their conjugate bases are calculated by using CNDO/2 and MNDO with full geometry optimization.The deprotonation energy is found to be better correlated with the charge on the acidic hydrogen than with the charge on the atom linking to the hydrogen and with the charge on the nucleophilic atom of the conjugate base.This shows that the charge on the acidic hydrogen has an important effect on acidity,whereas the effect of the charge on the linking atom and on the nucleophilic atom of the conjugate base is not important.It can thus be deduced that the view of stabilization of the anio'n due to dispersion of the negative charge is not a significant explanation of acidity.     

Cu(I)-catalyzed, α-selective, allylic alkylation reactions between phosphorothioate esters and organomagnesium reagents

Regiocontrol of allylic alkylation reactions involving hard nucleophiles remains a significant challenge and continues to be an active area of research. The lack of general methods in which α-alkylation is favored underscores the need for the development of new processes for achieving this type of selectivity. We report that Cu(I) catalyzes the allylic substitution of phosphorothioate esters with excellent α-regioselectivity, regardless of the nature of the Grignard reagent that is used. To the best of our knowledge, the Cu-catalyzed allylic alkylation of phosphorothioate esters has never been described. We have also developed a simple protocol for inducing high α selectivity starting from secondary allylic halides. This is accomplished by using sodium phosphorothioates as an additive.     

Detailing hydrogen bonding and deprotonation equilibria between anions and urea/thiourea derivatives

Spectrophotometric and 1H NMR titrations of N-methoxyethyl-N'-(4-nitrophenyl)thiourea (3) by Bu(4-)NOAc show that in DMSO deprotonation of the receptor and formation of a hydrogen-bonded complex with anion proceed simultaneously but in MeCN deprotonation requires the participation of the second acetate anion. The formation constants of hydrogen-bonded complexes were determined from titrations in the presence of added acetic acid, which suppressed deprotonation. These constants together with independently measured stability constants of (AcO)(2)H(-) complexes were employed for a rigorous numerical analysis of titration results in the absence of added acid, which allowed us to determine the equilibrium deprotonation constants as well as pKa values for 3 in both solvents. Although 3 appeared to be a weaker acid than AcOH in both solvents, it can be deprotonated by acetate in dilute solutions when the concentration of liberated acetic acid is low enough. With disubstituted N,N-bis(methoxyethyl)-N'-(4-nitrophenyl)thiourea 4 only deprotonation equilibrium is observed. In contrast, both parent urea derivatives 1 and 2 cannot be deprotonated by acetate anions. Independent of the real type of equilibrium, whether it is a deprotonation or a hydrogen bonding, titration plots always can be satisfactorily fitted to a formal 1:1 binding isotherm. A relationship between apparent "binding constants" and real equilibrium constants of hydrogen bonding association and deprotonation processes is discussed.     

Chromium-Catalyzed Alkylation of Amines by Alcohols

The alkylation of amines by alcohols is a broadly applicable, sustainable, and selective method for the synthesis of alkyl amines, which are important bulk and fine chemicals, pharmaceuticals, and agrochemicals. We show that Cr complexes can catalyze this C−N bond formation reaction. We synthesized and isolated 35 examples of alkylated amines, including 13 previously undisclosed products, and the use of amino alcohols as alkylating agents was demonstrated. The catalyst tolerates numerous functional groups, including hydrogenation-sensitive examples. Compared to many other alcohol-based amine alkylation methods, where a stoichiometric amount of base is required, our Cr-based catalyst system gives yields higher than 90 % for various alkyl amines with a catalytic amount of base. Our study indicates that Cr complexes can catalyze borrowing hydrogen or hydrogen autotransfer reactions and could thus be an alternative to Fe, Co, and Mn, or noble metals in (de)hydrogenation catalysis.