Aryl Group Transfer Research Articles

Catalytic Enantioselective Aryl Transfer to Aldehydes Using Chiral 2,2’-Bispyrrolidine-Based Salan Ligands

Chiral C2-symmetric diamines have emerged as versatile auxiliaries or ligands in numerous asymmetric transformations. Chiral 2,2’-bispyrrolidine-based salan ligands were prepared and applied to the asymmetric aryl transfer to aldehydes with arylboronic acids as the source of transferable aryl groups. The corresponding diarylmethanols were obtained in high yields with moderate to good enantioselectivitives of up to 83% ee.

Mononuclear Bis(imino)arylcopper(I) N‐Heterocyclic Carbene Complex: Synthesis, Structure, and Reaction with Organic Azide

AbstractMononuclear bis(imino)arylcopper(I) N‐heterocyclic carbene compound [(IPM)CuL] {1, L = 2,6‐(RN=CH)2‐4‐tBuC6H2, R = 2,6‐iPr2C6H3, IPM = C[N(iPr)CMe]2} has been synthesized, structurally characterized, and its reaction with 1‐adamantyl azide investigated. The reaction of a less bulky arylcopper(I) compound [(IPM)CuPh] with the azide was also studied. The formation of the corresponding products [(IPM)CuN(1‐ad)NN(L)] (2) and [(IPM)CuN(Ph)NN(1‐ad)] (3) (1‐ad = 1‐adamantyl) reveals an aryl group transfer (L in 2 and Ph in 3) reactivity pattern mediated at CuI with the support of the N‐heterocyclic carbene. However, a Cu–N1‐ad σ‐bond and a Cu···NL weak bond were observed in the structure of 2, significantly different to the bonding in 3 in which a Cu–NPh σ‐bond and a Cu···N1‐ad weak bond were indicated. This may suggest an initial NT (NT, terminal N atom of the azide) coordination of the azide at the Cu center followed by aryl group transfer to NT. Complex 2 may undergo further bond rearrangement of the Cu···N bonds within a pseudo‐CuN3 four‐membered ring as a result of the large bulk of the ligand L.

New types of asymmetrical bromonium salts [RF(RF′)Br]Y where RF and/or RF′ represent perfluorinated aryl, alkenyl, and alkynyl groups

A series of previously unknown asymmetrical fluorinated bis(aryl)bromonium, alkenyl(aryl)bromonium, and alkynyl(aryl)bromonium salts was prepared by reactions of C6F5BrF2 or 4-CF3C6H4BrF2 with aryl group transfer reagents Ar′SiF3 (Ar′=C6F5, 4-FC6H4, C6H5) or perfluoroorganyl group transfer reagents RF′BF2 (RF=C6F5, trans-CF3CFCF, C3F7C≡C) preferentially in weakly coordinating solvents (CCl3F, CCl2FCClF2, CH2Cl2, CF3CH2CHF2 (PFP), CF3CH2CF2CH3 (PFB)). The presence of the base MeCN and the influence of the adducts RF′BF2·NCMe (RF=C6F5, CF3C≡C) on reactions aside to bromonium salt formation are discussed. Reactions of C6F5BrF2 with AlkF′BF2 in PFP gave mainly C6F5Br and AlkF′F (AlkF′=C6F13, C6F13CH2CH2), presumably, deriving from the unstable salts [C6F5(AlkF′)Br]Y (Y=[AlkF′BF3]−). Prototypical reactivities of selected bromonium salts were investigated with the nucleophile I–and the electrophile H+. [4-CF3C6H4(C6F5)Br][BF4] showed the conversion into 4-CF3C6H4Br and C6F5I when reacted with [Bu4N]I in MeCN. Perfluoroalkynylbromonium salts [CnF2n+1C≡C(RF)Br][BF4] slowly added HF when dissolved in aHF and formed [Z–CnF2n+1CFCH(RF)Br][BF4].

Enantioselective Arylations Catalyzed by Carbohydrate‐Based Chiral Amino Alcohols

AbstractThe application of carbohydrate‐derived amino alcohols in the asymmetric arylation of aldehydes by using arylboronic acids as the source of transferable aryl groups is described. The best ligand is derived from the readily available sugar D‐xylose and it mediates the addition of a range of arylboronic acids to various aromatic aldehydes in excellent yields and high enantiomeric excesses.

Catalytic enantioselective arylation of aryl aldehydes by chiral aminophenol ligands

The catalytic enantioselective arylation of aryl aldehydes using boronic acids as the source of transferable aryl groups is described; the reaction is found to proceed in good yields and in good to high enantioselectivities (up to 99% ee) in the presence of a chiral aminophenol ligand.

Probing the Steric Limits of Carbon−Gold Bond Formation: (Dialkylbiarylphosphine)gold(I) Aryls

Aryl-group transfer from arylboronic acids to gold has emerged as a functionally tolerant alternative to classical lithiation- and magnesiation-based synthesis of arylgold(I) species. Here, the scope of the reaction is explored with attention to the sterics of the boronic acid starting material and the supporting ligand on gold. Dicyclohexylbiaryl phosphines are selected as supporting ligands on gold(I) because of their substantial bulk. Aryl-group transfer is compatible with steric buildup on either reaction partner. The structural preference of gold(I) for linear, two-coordinate geometries circumvents potential steric clashes. The new organometallics likely gain added stability through dative interactions with the flanking phosphine biaryl arm and, in three cases, through π-interactions with the aryl ligand σ-bonded to gold. The new compounds are characterized by multinuclear NMR and optical spectroscopy, X-ray diffraction crystallography, and combustion analysis. All compounds absorb ultraviolet light at wavelengths λ < 325 nm. Time-dependent density-functional theory calculations find that multiple singlet−singlet transitions account for the absorption profile, which has both intraligand and (ligand−metal)-to-ligand charge-transfer character.

ChemInform Abstract: Catalytic Enantioselective Arylations. Boron to Zinc Exchange as a Powerful Tool for the Generation of Transferable Aryl Groups

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Catalytic enantioselective arylations: boron to zinc exchange as a powerful tool for the generation of transferable aryl groups

The transmetalation between boron and zinc is of great importance for application in organic synthesis, since it allows the formation of new carbon-carbon bonds between organometallic units and electrophiles. The direct arylation of aldehydes or more scarcely ketones, in a catalytic, enantioselective manner using chiral catalysts has been described recently. The enantiomerically enriched diarylmethanols obtained in these reactions are valuable precursors for important bioactive molecules. This review provides a synopsis of this ever-growing field and highlights some of the challenges that still remain.

A New Methodology for Synthesis of Aryl Bismuth Compounds: Arylation of Bismuth(III) Carboxylates by Sodium Tetraarylborate Salts

Sodium tetraarylborate salts Na[BAr4] (Ar = C6H5 (Ph), C6H4-Me-4 (tolyl), C6H4-F-4) are found to be efficient arylating species for a range of bismuth(III) aromatic and aliphatic carboxylates including Bi(Hsal*)3 (Hsal* = 2-HO-C6H4CO2− (Hsal); 4-Me-2-HO-C6H3CO2− (Hsal4Me); or 3-MeO-2-HO-C6H3CO2− (Hsal3OMe)) and Bi(O2CR)3 (R = Me, CMe3, and CF3) to produce triaryl bismuth compounds. The reactions may be carried out in ethanol, tetrahydrofuran, or acetone. The arylbismuth bis(salicylates) BiPh(Hsal)2 and Bi(tolyl)(Hsal)2 exhibit similar reactivity in refluxing THF and may be used to produce mixed arylbismuthines BiPhx(tolyl)3−x. Formation of the known arylbismuth compounds was confirmed by IR spectroscopy, X-ray crystallography, NMR spectroscopy (1H, 11B, 13C, and 19F), and mass spectrometry. This is a facile synthesis of both symmetrical and unsymmetrical triarylbismuthines involving the aryl group transfer from the tetraarylborate ions to bismuth(III) under mild conditions.

Modular chiral thiazolidine catalysts in asymmetric aryl transfer reactions

Modular chiral thiazolidine derivatives were synthesized in a single step from inexpensive and commercially available starting materials. These ligands catalyzed enantioselective arylation of different aldehydes using aryl boronic acids as a source of transferable aryl groups. The products were obtained in excellent yields and good enantioselectivities.

The application of chiral, non-racemic N-alkylephedrine and N, N-dialkylnorephedrine as ligands for the enantioselective aryl transfer reaction to aldehydes

The catalytic enantioselective arylation of several aldehydes using arylboronic acids as the source of transferable aryl groups is described; the reaction is found to proceed in excellent yields and high enantioselectivities (up to 96% ee) in the presence of a chiral amino alcohol derived from ephedrines and congeners.

Transmetalation of arylpalladium and platinum complexes. Mechanism and factors to control the reaction

This article reviews recent studies on intra- and intermolecular transfer of the aryl ligand bonded to Pd(II) and Pt(II). Cationic arylpalladium complexes with bpy and THF ligands undergo intermolecular aryl group transfer to produce biaryl via a diarylpalladium intermediate. This reaction is applied to cyclization of cationic dinuclear arylpalladium complexes, affording the crown ether derivative with biphenylene units. Analogous arylplatinum complexes do not form diaryl complexes via transmetalation, while they react with CO and phenylallene to cause replacement of the coordinated solvent and insertion of the small molecules into the Pt–C bond, respectively. Conproportionation of PtCl 2(cod) and PtPh 2(cod) produces PtCl(Ph)(cod), which is induced by dissociation of a Cl ligand from the former complex. PtCl 2(cod) reacts also with diarylplatinum complexes with bpy and dppe. Disproportionation of PtPh(CH 2COMe)(cod) and conproportionation of PtPh 2(cod) and Pt(CH 2COMe) 2(cod) take place at 50 °C, but the rates of apparently reversible reactions differ from each other. Addition of OH − to a solution of PtI(Ph)(cod) causes intermolecular phenyl ligand transfer to produce PtPh 2(cod). The dinuclear intermediate complex with bridging OH ligand is prepared from an independent route and fully characterized. The complex causes transmetalation of aryl group of aryl boronic acid.

Chemical Properties of Mononuclear and Dinuclear Phenylplatinum(II) Hydroxo Complexes with Cod Ligands. Transmetalation of Arylboronic Acids, Coupling of the Phenyl Ligands, and Carbonylation

The reaction of bpy with [{Pt(Ph)(cod)}2(μ-OH)](BF4) (1-BF4; cod = 1,5-cyclooctadiene) in acetone splits a Pt−O bond to yield a mixture of Pt(CH2COMe)(Ph)(cod) (2) and [Pt(Ph)(bpy)(cod)](BF4) (3-BF4), whereas a similar reaction in toluene produces 3-BF4 and Pt(OH)(Ph)(cod) (4). The complex 4 was obtained in solution and was not isolated as analytically pure crystals because of its gradual disproportionation, giving PtPh2(cod) (5). The dinuclear complex 1-BF4 reacts with ArB(OH)2 (Ar = Ph, C6H2F3-2,4,6) to form 5 and Pt(C6H2F3-2,4,6)(Ph)(cod) (6), respectively, via aryl group transfer from boron to platinum. The accompanying formation of B(OH)3 has been confirmed by 11B{1H} NMR spectroscopy. The reaction of BF3·Et2O with 1-BF4 followed by the addition of NH4Cl(aq) produces biphenyl and PtCl2(cod), which takes place possibly via a dinuclear intermediate. The cationic dinuclear complex 1-BF4 reacts with CO (1 atm) to form benzophenone. Since the reactions of AgBF4 with Pt(COPh)(I)(cod) (7) and CO with [Pt(Ph)(THF)(cod)](BF4) also yield benzophenone, the above carbonylation of 1-BF4 is considered to involve mononuclear intermediates.

Synthesis of Functionalized Benzylsilanes from Arylzinc Compounds and (Iodomethyl)trimethylsilane by Means of a Novel Rh Catalysis

[reaction: see text] The catalytic activity of a Rh complex in cross-coupling between ArZnI and TMSCH2I was examined in which the Rh complex, generated in situ from [RhCl(1,5-cyclooctadiene)]2 and 1,1'-bis(diphenylphosphino)ferrocene, exhibited excellent catalytic activity for the production of various functionalized benzylsilanes in good yields. From 31P NMR studies of various solutions containing several of the reaction components, confirmation of the rapid and quantitative transfer of aryl groups from ArZnI to the Rh complex to form arylrhodium species was ascertained. A catalytic cycle, commencing with the transmetalation, was thus proposed for the reaction.

Catalytic Enantioselective Arylation of Aldehydes: Boronic Acids as a Suitable Source of Transferable Aryl Groups.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic enantioselective arylation of aldehydes: boronic acids as a suitable source of transferable aryl groups

The catalytic enantioselective arylation of several aldehydes using boronic acids as the source of transferable aryl groups is described; the reaction is found to proceed in excellent yields and high enantioselectivities (up to 97% ee) in the presence of a chiral amino alcohol.

C6F5-Group Transfer from [MeB(C6F5)3]- to the Metal Center of L2MMe+ (M = Ti, Zr) as a Deactivation Pathway in Olefin Polymerization Catalysis: A Combined Density Functional Theory and Molecular Mechanics Investigation

We have carried out a combined density functional theory and molecular mechanics study of aryl group transfer reactions from [MeB(C6F5)3]- to the metal center of L2MMe+ (M = Ti, Zr). This reaction,...

A comparative study of base-free arylcopper reagents for the transfer of aryl groups to boron halides

A comparative study on the reactivity and selectivity of arylcopper species in reactions with boron halides was performed. Mesitylcopper reacts with BX 3 (X=Cl, Br) in toluene at low temperature under highly selective formation of the monosubstituted boranes MesBX 2. The dimesitylboranes Mes 2BX are gradually formed with a twofold excess of the organocopper reagent at elevated temperature. In contrast, pentafluorophenylcopper shows a tendency for formation of B(C 6F 5) 3 in reactions with BX 3 irrespective of the stoichiometry used, suggesting a strong impact of electronic factors on the selectivity of the aryl transfer reaction. New procedures for the synthesis of the pentafluorophenylboron halides C 6F 5BX 2 (X=Cl: 57%; X=Br: 62%) and of tris(pentafluorophenyl)borane (80%) and related mixed-substituted triarylboranes from the base-free isolable pentafluorophenylcopper precursor have been developed.

Ortho substituent effects on transfer of aryl groups from oxygen to carbon in adducts from diaryloxycarbenes and DMAD

Diaryloxycarbenes, generated by thermolysis of 2,2-diaryloxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines in benzene, reacted with dimethyl acetylenedicarboxylate (DMAD) to afford triesters. The triesters arose by aryl-group transfer from oxygen to carbon (ipso substitution). A cyano group in the para position had been shown earlier to favour the ipso substitution whereas a para-methoxy group disfavoured it. Surprisingly, the ortho-methoxy substituent also favoured ipso substitution. As a tentative rationale, we had proposed that a vinylogous diaryloxycarbene intermediate, with dipolar properties, is formed when a diaryloxycarbene attacks at the sp-carbon of DMAD and that an ortho substituent, with one or more unshared electron pairs, can stabilize the intermediate by donating electron density to the positive site. In the present work, support for that interpretation and for the importance of a steric effect, was provided by examining the competition between (i) p-tolyl and phenyl; (ii) o-tolyl and phenyl; (iii) o-iodophenyl and phenyl; (iv) o-iodophenyl and p-chlorophenyl; and (v) o-bromophenyl and p-chlorophenyl as potential migrating groups.Key words: aromatic substitution, carbene, ipso, diaryloxycarbene, oxadiazoline.

Mechanism of ipso aromatic substitution by reaction of aryloxy(methoxy)carbenes and diaryloxycarbenes with DMAD

Some aryloxy(methoxy)carbenes and diaryloxycarbenes attack dimethyl acetylenedicarboxylate (DMAD) with aryl group transfer to an alkyne carbon of DMAD. In this study diaryloxycarbenes with different aryl groups that could be transferred competitively, were generated in the presence of DMAD to probe for the mechanism of that ipso aromatic substitution. It was found that a para electron-withdrawing substituent, relative to an electron-donating substituent, facilitated migration of an aryl group. Mechanisms in accord with these findings involve initial nucleophilic attack by the carbene at an alkyne carbon of DMAD. That step is followed by either nucleophilic, ipso attack on the aromatic ring or by electron transfer, from the side chain of the aromatic ring into the ring itself.Key words: aromatic substitution, diaryloxycarbene, DMAD, ipso, nucleophilic.