Efficient, practical synthesis of symmetrically α,α-disubstituted α-amino acids

Ni(II)-complex derived from glycine Schiff base with 2-[N-(α-picolyl)amino]benzophenone (PABP) was found to be an ideal equivalent of nucleophilic glycine in the reactions with various alkyl halides affording an efficient, generalized and practically useful method for preparing symmetrically α,α-disubstituted α-amino acids.     

Michael addition reactions between chiral Ni(II) complex of glycine and 3-(trans-enoyl)oxazolidin-2-ones. A case of electron donor-acceptor attractive interaction-controlled face diastereoselectivity

This study has demonstrated that the readily available and inexpensive 3-(trans-3'-alkyl/arylpropenoyl)oxazolidin-2-ones, featuring high electrophilicity and conformational homogeneity, are synthetically superior Michael acceptors over the conventionally used alkyl enoylates, allowing for a remarkable improvement in reactivity and, in most cases, diastereoselectivity of the addition reactions with a Ni(II) complex of the chiral Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone. Kinetically controlled diastereoselectivity in the corresponding Michael addition reactions between the Ni(II) complex of glycine and the oxazolidin-2-ones was systematically studied as a function of steric, electronic, and position effects of the substituents on the starting Michael acceptor. In both aliphatic and aromatic series the simple diastereoselectivity was found to be virtually complete, affording the products via the corresponding TSs with the approach geometry like. The face diastereoselectivity of the reactions between the Ni(II) complex of glycine and the 3-(trans-3'-alkylpropenoyl)oxazolidin-2-ones was found to depend exclusively on the steric bulk of the alkyl group on the starting Michael acceptor. In contrast, the face diastereoselectivity in the reactions of aromatic oxazolidin-2-ones with the Ni(II) complex of glycine was shown to be controlled predominantly by the electronic properties of the aryl ring. In particular, the additions of the Ni(II) complex of glycine with 3-(trans-3'-arylpropenoyl)oxazolidin-2-ones, bearing electron-withdrawing substituents on the phenyl ring, afforded the (2S,3R)-configured products with synthetically useful diastereoselectivity and in quantitative chemical yields, thus allowing for an efficient access to the sterically constrained beta-aryl-substituted pyroglutamic and glutamic acids.     

Michael addition reactions between chiral equivalents of a nucleophilic glycine and (S)- or (R)-3-[(E)-enoyl]-4-phenyl-1,3-oxazolidin-2-ones as a general method for efficient preparation of beta-substituted pyroglutamic acids. Case of topographically controlled stereoselectivity

This paper describes a systematic study of addition reactions between the chiral Ni(II) complex of the Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone and (S)- or (R)-3-[(E)-enoyl]-4-phenyl-1,3-oxazolidin-2-ones as a general and synthetically efficient approach to beta-substituted pyroglutamic acids and relevant compounds. These reactions were shown to occur at room temperature in the presence of nonchelating organic bases and, most notably, with very high (>98% diastereomeric excess (de)) stereoselectivity at both newly formed stereogenic centers. The stereochemical outcome of the reactions was found to be overwhelmingly controlled by the stereochemical preferences of the Michael acceptors, and the chirality of the glycine complex influenced only the reaction rate. Thus, in the reactions of both the (S)-configured Ni(II) complex and the Michael acceptors, the reaction rates were exceptionally high, allowing preparation of the corresponding products with virtually quantitative (>98%) chemical and stereochemical yields. In contrast, reactions of the (S)-configured Ni(II) complex and (R)-configured Michael acceptors proceeded at noticeably lower rates, but the addition products were obtained in high diastereo- and enantiomeric purity. To rationalize the remarkably high and robust stereoselectivity observed in these reactions, we consider an enzyme-substrate-like mode of interaction involving a topographical match or mismatch of two geometric figures. Excellent chemical and stereochemical yields, combined with the simplicity and operational convenience of the experimental procedures, render the present method of immediate use for preparing various beta-substituted pyroglutamic acids and related compounds.     

Heterocycles from substituted amides. VII Oxazoles from 2-isocyanoacetamides

Certain heretofore unknown tertiary α-isocyanacetanilides 1 are reported for the first time, prepared by methods necessarily different from those employed to prepare the previously cited aliphatic α-isocyano-acetamides 2 . Like 2, 1 has been shown to readily undergo dialkylation with alkyl halides. Moreover, 1 also gives rise to oxazole and oxazoline derivatives upon reaction with acyl halides and aldehydes, respectively, resembling the similarly activated α-isocyano esters and sulfones. Further, in a demonstrable ring-chain tautomeric equilibration, apparently restricted to tertiary amides, 1 readily cyclizes, providing the only known synthesis of 5-amino-2,4-unsubstituted oxazoles 3 . In other examples 4-chloro and 4-alkyl-5-amino-oxazoles are produced. The materials and reactions are characterized, and compared with previous related studies.     

Virtually complete control of simple and face diastereoselectivity in the Michael addition reactions between achiral equivalents of a nucleophilic glycine and (S)- or (R)-3-(E-enoyl)-4-phenyl-1,3-oxazolidin-2-ones: practical method for preparation of beta-substituted pyroglutamic acids and prolines

This study demonstrates a new strategy for controlling the stereochemical outcome of the Michael addition reactions between nucleophilic glycine equivalents and alpha,beta-unsaturated carboxylic acid derivatives: The addition reactions between achiral Ni(II)-complex of the Schiff base of glycine with o-[N-alpha-pycolylamino]acetophenone and (S)- or (R)-3-(E-enoyl)-4-phenyl-1,3-oxazolidin-2-ones were shown to occur at room temperature in the presence of nonchelating organic bases and, most notably, with very high stereoselectivity at both newly formed stereogenic centers. Thus, the chiral 4-phenyl-1,3-oxazolidin-2-one moiety was found to control efficiently both face diastereoselectivities of the glycine derived enolate and the C,C double bond of the Michael acceptor. The new strategy developed in this work is methodologically superior to previous methods, most notably in terms of generality and synthetic efficiency. Excellent chemical yields and diastereoselectivities, combined with the simplicity of the experimental procedures, render the present method of immediate use for preparing various 3-substituted pyroglutamic acids and related amino acids (glutamic acids, glutamines, prolines, etc.) available via conventional transformations of the former.     

Cobalt-mediated cross-coupling reactions of primary and secondary alkyl halides with 1-(trimethylsilyl)ethenyl- and 2-trimethylsilylethynylmagnesium reagents

[reaction: see text] This paper describes cobalt-mediated cross-coupling reactions of alkyl halides with 1-(trimethylsilyl)ethenylmagnesium bromide and 2-(trimethylsilyl)ethynylmagnesium bromide, respectively. The cobalt system allows for employing secondary as well as primary alkyl halides as the substrates. The reactions offer facile formations of alkyl-alkenyl and alkyl-alkynyl bonds. The reaction mechanism would include single-electron transfer from a cobalt complex to alkyl halide to generate the corresponding alkyl radical. The cobalt system thus enables sequential radical cyclization/alkenylation and cyclization/alkynylation reactions of 6-halo-1-hexene derivatives.     

Asymmetric Michael addition reactions of chiral Ni(II)-complex of glycine with (N-trans-enoyl)oxazolidines: improved reactivity and stereochemical outcome

Application of the (N-trans-enoyl)oxazolidines as Michael acceptors in the kinetically controlled additions with a Ni(II)-complex of the chiral Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone 1 was shown to be synthetically advantageous over the alkyl enoylates, allowing for remarkable improvement in reactivity and, in most cases, diastereoselectivity of the reactions. While the stereochemical outcome of the Michael additions of the aliphatic (N-trans-enoyl)oxazolidines with complex 1 depended on the steric bulk of the alkyl group on the starting oxazolidines, the diastereoselectivity of the aromatic (N-trans-enoyl)oxazolidines reactions was found to be controlled by the electronic properties of the aryl ring. In particular, the additions of complex 1 with (N-cinnamoyl)oxazolidines, bearing electron-withdrawing substituents on the phenyl ring, afforded the (2S,3R)-configured products with synthetically useful selectivity and in quantitative chemical yield, thus allowing an efficient access to sterically constrained β-substituted pyroglutamic acids and related compounds.     

Asymmetric Synthesis of Chiral Trifluoromethyl Containing Heterocyclic Amino Acids

An operationally convenient, asymmetric synthesis of chiral trifluoromethyl containing heterocyclic amino acids has been developed via Michael addition reaction of chiral equivalent of Ni(II)‐complex of glycine and β ‐trifluoromethylated‐α ,β ‐unsaturated ketones. The simplicity of the experimental procedures and high stereochemical outcome of the presented method render these heterocyclic amino acids readily available for systematic medicinal chemistry studies and de novo peptide design.     

Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted radical alkenylation of alkyl halides: a complement to the Heck reaction

A cobalt complex, [CoCl2(dpph)] (DPPH = [1,6-bis(diphenylphosphino)hexane]), catalyzes an intermolecular styrylation reaction of alkyl halides in the presence of Me3SiCH2MgCl in ether to yield beta-alkylstyrenes. A variety of alkyl halides including alkyl chlorides can participate in the styrylation. A radical mechanism is strongly suggested for the styrylation reaction. The sequential isomerization/styrylation reactions of cyclopropylmethyl bromide and 6-bromo-1-hexene provide evidence of the radical mechanism. Crystallographic and spectroscopic investigations on cobalt complexes reveal that the reaction would begin with single electron transfer from an electron-rich (diphosphine)bis(trimethylsilylmethyl)cobalt(II) complex followed by reductive elimination to yield 1,2-bis(trimethylsilyl)ethane and a (diphosphine)cobalt(I) complex. The combination of [CoCl2(dppb)] (DPPB = [1,4-bis(diphenylphosphino)butane]) catalyst and Me3SiCH2MgCl induces intramolecular Heck-type cyclization reactions of 6-halo-1-hexenes via a radical process. On the other hand, the intramolecular cyclization of the prenyl ether of 2-iodophenol would proceed in a fashion similar to the conventional palladium-catalyzed transformation. The nonradical oxidative addition of carbon(sp2)-halogen bonds to cobalt is separately verified by a cobalt-catalyzed cross-coupling reaction of alkenyl halides with Me3SiCH2MgCl with retention of configuration of the starting vinyl halides. The cobalt-catalyzed intermolecular radical styrylation reaction of alkyl halides is applied to stereoselective variants. Styrylations of 1-alkoxy-2-bromocyclopentane derivatives provide trans-1-alkoxy-2-styrylcyclopentane skeletons, one of which is optically pure.     

ESR studies on reactions of dicyclopentadienyl dicarbonyl titanium with organic halides

ESR method was used to elucidate the mechanism of the reactions of alkyl, allyl or benzyl halides with dicyclopentadienyldicarbonyl titanium. The paramagnetic [intermediates of the reactions were identified during the course of the reactions. The reaction mechanism based on ESR findings and the products analyses is postulated to operate on radical pathways. When alkyl halides were used to react with the organometallic compound 1, the intermediate found was [Cp₂Ti(CO)X] (C), and the main product was identified to be dicyclopentadienyl-acyl-halo titanium (3), an insertion of TiCO into R-X, i.e. [Cp₂Ti-C(0)R] X. When allyl or benzyl halides were used, the intermediate found was [Cp₂TiX] (B), and the main products were identified to be the dicyclopentadienyl titanium dihalides and the coupling products of allyl or benzyl groups.     

Use of hexamethylphosphoramide (HMPA) in the alkylation of aromatic amines.

The convenience of using HMPA as solvent in the mono and dialkylation of anilines was demonstrated through the study of the reaction of p-toluidine with alkyl halides, tosylates and epoxides.     

Intramolecular rearrangement of 1,3-bistriflate ester of thiacalix[4]arene to 1,2-counterpart: an efficient di-O-protection method for the stereoselective synthesis of anti-1,2-diethers

A net anti-selective dialkylation of the proximal hydroxy groups of thiacalix[4]arene 2 is achieved for the first time via the initial protection of the two proximal hydroxy groups of compound 2 with Tf moieties by intramolecular rearrangement of easily preparable 1,3-bistriflate ester 3 to 1,2-counterpart 4, followed by anti-selective dialkylation of the remaining hydroxy groups with alkyl halides or under the Mitsunobu conditions and subsequent removal of the Tf moieties.     

Rational design of highly diastereoselective, organic base-catalyzed, room-temperature Michael addition reactions

Via the rational design of a single-preferred transition state, stabilized by electron donor-acceptor-type attractive interactions, structural and geometric requirements for the corresponding starting compounds have been determined. The Ni(II) complex of the Schiff base of glycine with o-[N-alpha-picolylamino]acetophenone, as a nucleophilic glycine equivalent, and N-(trans-enoyl)oxazolidin-2-ones, as derivatives of an alpha,beta-unsaturated carboxylic acid, were found to be the substrates of choice featuring geometric/conformational homogeneity and high reactivity. The corresponding Michael addition reactions were found to proceed at room temperature in the presence of catalytic amounts of DBU to afford quantitatively the addition products with virtually complete diastereoselectivity. Acidic decomposition of the products followed by treatment of the reaction mixture with NH4OH gave rise to the diastereomerically pure 3-substituted pyroglutamic acids.     

Second Comes First: Switching Elementary Steps in Palladium-Catalyzed Cross-Coupling Reactions

The electron-poor palladium(0) complex L₃Pd (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) reacts with Grignard reagents RMgX and organolithium compounds RLi via transmetalation to furnish the anionic organopalladates [L₂PdR]⁻, as shown by negative-ion mode electrospray-ionization mass spectrometry. These palladates undergo oxidative additions of organyl halides R′X (or related S_N2-type reactions) followed by further transmetalation. Gas-phase fragmentation of the resulting heteroleptic palladate(II) complexes results in the reductive elimination of the cross-coupling products RR′. This reaction sequence corresponds to a catalytic cycle, in which the order of the elementary steps of transmetalation and oxidative addition is switched relative to that of palladium-catalyzed cross-coupling reactions proceeding via neutral intermediates. An attractive feature of the palladate-based catalytic system is its ability to mediate challenging alkyl–alkyl coupling reactions. However, the poor stability of the phosphine ligand L against decomposition reactions has so far prevented its successful use in practical applications.     

Copper-catalyzed alkylation of benzoxazoles with secondary alkyl halides

Copper-catalyzed direct alkylation of benzoxazoles using nonactivated secondary alkyl halides has been developed. The best catalyst is a new copper(I) complex (1), and the reactions are promoted by bis[2-(N,N-dimethylamino)ethyl] ether.     

The interconversion of primary alkyl halides

Using tetra-alkylammonium halides dissolved in alkyl halides as reagents a variety of halogen exchange reactions can be carried out conveniently in high yield. In particular, it becomes practicable to use -CH₂Cl as a protected equivalent of -CH₂Br.     

Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization: theoretical and experimental study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(?-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(?-), which is shown here to be nonexistent.     

Pd-NHC catalyzed conjugate addition versus the Mizoroki-Heck reaction

Ace of base: A catalytic system is presented that, solely by choice of the base, selectively switches between conjugate addition and the Mizoroki-Heck reaction of aryl halides with Michael acceptors (see scheme; R, R' = alkyl, aryl). For conjugate addition reactions, this avoids the preparation and use of organometallics.     

A General Synthesis of 2-Phenylselenenylenones. The Reaction of Unsaturated Ketones with a Phenylselenenyl Chloride/Pyridine Complex

The generation of specific enolates via Michael addition of nucleophiles to unsaturated ketones has proven to be an extremely useful process in organic synthesis. With regard to this we felt that the incorporation of a phenylselenenyl group in the 2-position of an enone would (a) enhance the ability of the enone to undergo Michael addition, (b) provide, after Michael addition, a stabilized enolate for the subsequent introduction of a substituent in the 2-position and (c) allow for the eventual introduction of a new double bond via the well-known selenoxide β-elimination reaction.¹ While a few 2-phenylselenenylenones have been previously reported in the literature,² no general method currently exists for their preparation.³ We now wish to report a general method for the synthesis of these systems in good yield under extremely mild reaction conditions. In future communications we will demonstrate the versatility of this class of compounds in a variety of enone mono- and dialkylation reactions.     

A structure-activity study of Ni-catalyzed alkyl-alkyl Kumada coupling. Improved catalysts for coupling of secondary alkyl halides

A structure-activity study was carried out for Ni catalyzed alkyl-alkyl Kumada-type cross coupling reactions. A series of new nickel(II) complexes including those with tridentate pincer bis(amino)amide ligands ((R)N(2)N) and those with bidentate mixed amino-amide ligands ((R)NN) were synthesized and structurally characterized. The coordination geometries of these complexes range from square planar, tetrahedral, to square pyramidal. The complexes had been examined as precatalysts for cross coupling of nonactivated alkyl halides, particularly secondary alkyl iodides, with alkyl Grignard reagents. Comparison was made to the results obtained with the previously reported Ni pincer complex [((Me)N(2)N)NiCl]. A transmetalation site in the precatalysts is necessary for the catalysis. The coordination geometries and spin-states of the precatalysts have a small or no influence. The work led to the discovery of several well-defined Ni catalysts that are significantly more active and efficient than the pincer complex [((Me)N(2)N)NiCl] for the coupling of secondary alkyl halides. The best two catalysts are [((H)NN)Ni(PPh(3))Cl] and [((H)NN)Ni(2,4-lutidine)Cl]. The improved activity and efficiency was attributed to the fact that phosphine and lutidine ligands in these complexes can dissociate from the Ni center during catalysis. The activation of alkyl halides was shown to proceed via a radical mechanism.