RuH2(CO)(PPh3)3-catalyzed arylation of aromatic esters using arylboronates via C-H bond cleavages

The RuH₂(CO)(PPh₃)₃-catalyzed C-H functionalization of aromatic esters with 5,5-dimethyl-2-aryl-[1,3,2]dioxaborinanes (arylboronates) gave the ortho arylation products. This coupling reaction can be performed with various combinations of isopropyl benzoate derivatives and arylboronates. Introduction of CF₃ group in the aromatic ring increased the reactivity of the esters. Pinacolone effectively served as an acceptor of a hydride generated by C-H bond cleavage, and the amount of pinacolone used also affected the yield of the arylation product.     

Reactions of CF3-substituted boranes with α-diazocarbonyl compounds

Boranes substituted with a CF₃-group can be generated from methyl boronic esters RB(OMe)₂ and Me₃SiCF₃/KF followed by treatment with Me₃SiCl. These boranes are stable only in coordinating solvents, and due to the increased Lewis acidity of boron, react rapidly with α-diazocarbonyl compounds to give the products of transfer of the organic group from boron. Alkyl, aryl, vinyl, and alkynyl boronic esters can be used in this reaction.     

Substituent Effects on Oxidation‐Induced Formation of Quinone Methides from Arylboronic Ester Precursors

A series of arylboronic esters containing different aromatic substituents and various benzylic leaving groups (Br or N⁺Me₃Br^?) have been synthesized. The substituent effects on their reactivity with H₂O₂ and formation of quinone methide (QM) have been investigated. NMR spectroscopy and ethyl vinyl ether (EVE) trapping experiments were used to determine the reaction mechanism and QM formation, respectively. QMs were not generated during oxidative cleavage of the boronic esters but by subsequent transformation of the phenol products under physiological conditions. The oxidative deboronation is facilitated by electron‐withdrawing substituents, such as aromatic F, NO₂, or benzylic N⁺Me₃Br^?, whereas electron‐donating substituents or a better leaving group favor QM generation. Compounds containing an aromatic CH₃ or OMe group, or a good leaving group (Br), efficiently generate QMs under physiological conditions. Finally, a quantitative relationship between the structure and activity has been established for the arylboronic esters by using a Hammett plot. The reactivity of the arylboronic acids/esters and the inhibition or facilitation of QM formation can now be predictably adjusted. This adjustment is important as some applications may benefit and others may be limited by QM generation.     

Merging Electron Deficient Boronic Centers with Electron-Withdrawing Fluorine Substituents Results in Unique Properties of Fluorinated Phenylboronic Compounds

Fluorinated boron species are a very important group of organoboron compounds used first of all as receptors of important bioanalytes, as well as biologically active substances, including Tavaborole as an antifungal drug. The presence of substituents containing fluorine atoms increases the acidity of boronic compounds, which is crucial from the point of view of their interactions with analytes or certain pathogen’s enzymes. The review discusses the electron acceptor properties of fluorinated boronic species using both the acidity constant (pK_a) and acceptor number (AN) in connection with their structural parameters. The NMR spectroscopic data are also presented, with particular emphasis on ¹⁹F resonance due to the wide range of information that can be obtained from this technique. Equilibria in solutions, such as the dehydration of boronic acid to form boroxines and their esterification or cyclization with the formation of 3-hydroxyl benzoxaboroles, are discussed. The results of the latest research on the biological activity of boronic compounds by experimental in vitro methods and theoretical calculations using docking studies are also discussed.     

Tuning the Lewis Acidity of Boranes in Frustrated Lewis Pair Chemistry: Implications for the Hydrogenation of Electron‐Poor Alkenes

An analysis of the metal‐free reduction of electron deficient olefins by frustrated Lewis pairs indicates that the rate‐determining step might be either the heterolytic cleavage of H₂ to form an ‐onium borohydride salt, or the subsequent transfer of the hydride moiety to the substrate following a Michael‐type addition reaction. While the use of strong Lewis acids such as B(C₆F₅)₃ facilitates the first of these processes, hydride transfer to the olefin should be contrarily favoured by the use of weak Lewis acids which, for this very same reason, might be unable to promote the prior H₂ split. After systematic testing of several boranes of different Lewis acidity (assessed by using the Childs’ method) and steric demand, an optimal situation that employs tris(2,4,6‐trifluorophenyl)borane was reached. Mixtures of this borane with 1,4‐diazabicyclo[2.2.2]octane (DABCO) exhibited excellent catalytic activity for the hydrogenation of alkylidene malonates. In fact, this transformation could be achieved under milder conditions than those we reported previously. Moreover, the reaction scope could be expanded to other electron deficient olefins containing esters, sulfones or nitro functionalities as electron‐withdrawing substituents.     

Electronically Driven Regioselective Iridium-Catalyzed C−H Borylation of Donor-π-Acceptor Chromophores Containing Triarylboron Acceptors

We observed a surprisingly high electronically driven regioselectivity for the iridium-catalyzed C−H borylation of donor-π-acceptor (D -π-A) systems with diphenylamino ( 1 ) or carbazolyl ( 2 ) moieties as the donor, bis(2,6-bis(trifluoromethyl)phenyl)boryl ( B(^FXyl)₂ ) as the acceptor, and 1,4-phenylene as the π-bridge. Under our conditions, borylation was observed only at the sterically least encumbered para-positions of the acceptor group. As boronate esters are versatile building blocks for organic synthesis (C−C coupling, functional group transformations) the C−H borylation represents a simple potential method for post-functionalization by which electronic or other properties of D -π-A systems can be fine-tuned for specific applications. The photophysical and electrochemical properties of the borylated ( 1-(Bpin)₂ ) and unborylated ( 1 ) diphenylamino-substituted D -π-A systems were investigated. Interestingly, the borylated derivative exhibits coordination of THF to the boronate ester moieties, influencing the photophysical properties and exemplifying the non-innocence of boronate esters.     

Reversible Oxidative Addition at Carbon

The reactivity of N‐heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs) with arylboronate esters is reported. The reaction with NHCs leads to the reversible formation of thermally stable Lewis acid/base adducts Ar‐B(OR)₂⋅NHC ( Add1 – Add6 ). Addition of cAAC^Me to the catecholboronate esters 4‐R‐C₆H₄‐Bcat (R=Me, OMe) also afforded the adducts 4‐R‐C₆H₄Bcat⋅cAAC^Me ( Add7 , R=Me and Add8 , R=OMe), which react further at room temperature to give the cAAC^Me ring‐expanded products RER1 and RER2 . The boronate esters Ar‐B(OR)₂ of pinacol, neopentylglycol, and ethyleneglycol react with cAAC at RT via reversible B−C oxidative addition to the carbene carbon atom to afford cAAC^Me(B{OR}₂)(Ar) ( BCA1 – BCA6 ). NMR studies of cAAC^Me(Bneop)(4‐Me‐C₆H₄) ( BCA4 ) demonstrate the reversible nature of this oxidative addition process.     

Molecular complexes of non-chelating polydentate Lewis bases with group 13 Lewis acids: crystal structure and computed energy of stepwise donor–acceptor bond formation

The crystal structures of five donor–acceptor (DA) complexes of 1:1 and 2:1 composition between E(C₆F₅)₃ (E = B, Al, Ga and In) and pyrazine (pyz) as a non-chelating bidentate nitrogen-containing donor, as well as the GaI₃ pyz GaI₃ complex have been established for the first time. A joint analysis of the experimental structural data and the results of computations at the M06-2X/def2-TZVP level of theory reveals that with an increase in the number of acceptor molecules in the DA complex, the DA bond distances increase, while the DA bond energies and Wiberg bond indexes decrease, indicating a weaker bonding. The previously reported ‘inverse’ relationship between the Lewis acidity and the capacity of a polydentate donor to complex with multiple Lewis acids is not confirmed.     

Inorganic Esters of Ethylene Glycol as Macrocyclic Ligands

Six different inorganic esters of ethylene glycol: B(OR)₃, P(OR)₃, OS(OR)₂, OP(OR)₃, OPH(OR)₂ and As(OR)₃, where R = CH₂CH₂OCH₃ were obtained. Their structures were studied by multinuclear NMR. These compounds can complex metal cations and behave like macrocyclic ligands. The influence of metal cation complexation on spectra were investigated by ¹, ¹³C, ¹⁷O, ¹¹B, ⁷Li, ⁸⁷Rb and ³¹P NMR.     

Design of new organic superacids with fused and isolated pyrrole and cyclopentadiene rings and assessment of effect of –BX2 (X = H,F, Cl,CN) substituents on the acidity enhancement: A DFT analysis

New classes of organic Brønsted acids were designed with pyrrole and cyclopentadiene scaffolds, and their acidity was assessed theoretically by the B3LYP/6-311++G(d,p) method. The hydrogen atom of NH group in pyrrole was substituted by an –BX₂ (X = H, F, Cl, CN, CF₃). The boron atom stabilizes the conjugated bases by interaction with the center of negative charge after deprotonation. The acidity of the compounds was promoted by substitution of the hydrogen atoms of the rings with CN moiety as a strong electron withdrawing group. Also, after deprotonation, delocalization of the negative charge in both pyrrole and cyclopentadiene rings causes stability of the conjugated bases and consequently enhances the acidity. The charge delocalization in the neutral acids and their conjugated bases was compared using nucleus-independent chemical shift index. Enthalpies and Gibbs free energies of deprotonation in gas phase, ∆H_acid and ∆G_acid, were used as a measure of acidity. Both compounds with isolated and fused pyrrole and cyclopentadiene rings were investigated and it was found that the formers are more acidic. Using these strategies, several acids and superacids with wide range of acidity with ∆G_acid values of 244 to 328 kcal mol⁻¹ were obtained.     

Observation of Main‐Group Tricarbonyls [B(CO)3] and [C(CO)3]+ Featuring a Tilted One‐Electron Donor Carbonyl Ligand

A combined experimental and theoretical study on the main‐group tricarbonyls [B(CO)₃] in solid noble‐gas matrices and [C(CO)₃]⁺ in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η¹(μ₁‐CO)‐bonded carbonyl ligand, which serves as an unprecedented one‐electron donor ligand. Thus, the central atoms in both complexes still retain an 8‐electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two‐electron donors that display classical σ‐donation and π‐back‐donation following the Dewar–Chatt–Duncanson model. The tilted CO ligand is a formal one‐electron donor that is bonded by σ‐donation and π‐back‐donation that involves the singly occupied orbital of the radical fragments [B(CO)₂] and [C(CO)₂]⁺.     

Reductions of Carboxylic Acids and Esters with NaBH4 in Diglyme at 162°C

Abstract

Aromatic esters, including the extremely sterically hindered ester: t-amyl 2-chlorobenzoate, are readily reduced to the corresponding benzyl alcohols in high yield with NaBH₄ in refluxing diglyme (162°C). In sharp contrast, aliphatic esters usually gave only low yields of alcohols. Instead, diglyme fragmentation products are formed which undergo transesterification reactions, producing complex product mixtures including products such as RCOOCH₂CH₂OCH₃. The mechanism of this process involves sodium borohydride-induced S_N2 cleavage of diglyme (hydride attack) at high temperatures. However, when the extremely electron rich, 3,4,5-trimethoxybenzoic acid is treated with NaBH₄/diglyme at 162°C (with or without an equivalent of LiCl), no 3,4,5-trimethyoxybenzyl alcohol is formed. The electron rich and hindered ester, t-amyl-3,4,5-trimethoxybenzoate, also does not reduce under these conditions (with or without LiCl). However, both methyl and isopropyl 3,4,5-trimethoxybenzoate esters were converted into 3,4,5-trimethyoxybenzyl alcohol in good yields in NaBH₄/diglyme/LiCl at 162°C. These reductions did not occur unless LiCl was present, illustrating the electron releasing effect of the three methoxy functions which reduce the carbonyl group's reactivity.     

1H‐NMR Analysis of Intra‐ and Intermolecular H‐Bonds of Alcohols in DMSO: Chemical Shift of Hydroxy Groups and Aspects of Conformational Analysis of Selected Monosaccharides,Inositols, and Ginkgolides

The interpretation of ¹H‐NMR chemical shifts, coupling constants, and coefficients of temperature dependence (δ(OH), J(H,OH), and Δδ(OH)/ΔT values) evidences that, in (D₆)DMSO solution, the signal of an OH group involved as donor in an intramolecular H‐bond to a hydroxy or alkoxy group is shifted upfield, whereas the signal of an OH group acting as acceptor of an intramolecular H‐bond and as donor in an intermolecular H‐bond to (D₆)DMSO is shifted downfield. The relative strength of the intramolecular H‐bond depends on co‐operativity and on the acidity of OH groups. The acidity of OH groups is enhanced when they are in an antiparallel orientation to a C−O bond. A comparison of the ¹H‐NMR spectra of alcohols in CDCl₃ and (D₆)DMSO allows discrimination between weak and strong intramolecular H‐bonds. Consideration of IR spectra (CHCl₃ or CH₂Cl₂) shows that the rule according to which the downfield shift of δ(OH) for H‐bonded alcohols in CDCl₃ parallels the strength of the H‐bond is valid only for alcohols forming strong intramolecular H‐bonds. The combined analysis of J(H,OH) and δ(OH) values is illustrated by the interpretation of the spectra of the epoxyalcohols 14 and 15 (Fig. 3). H‐Bonding of hexopyranoses, hexulopyranoses, alkyl hexopyranosides, alkyl 4,6‐O‐benzylidenehexopyranosides, levoglucosans, and inositols in (D₆)DMSO was investigated. Fully solvated non‐anomeric equatorial OH groups lacking a vicinal axial OR group (R=H or alkyl, or (alkoxy)alkyl) show characteristic J(H,OH) values of 4.5 – 5.5 Hz and fully solvated non‐anomeric axial OH groups lacking an axial OR group in β‐position are characterized by J(H,OH) values of 4.2 – 4.4 Hz (Figs. 4 – 6). Non‐anomeric equatorial OH groups vicinal to an axial OR group are involved in a partial intramolecular H‐bond (J(H,OH)=5.4 – 7.4 Hz), whereas non‐anomeric equatorial OH groups vicinal to two axial OR form partial bifurcated H‐bonds (J(H,OH)=5.8 – 9.5 Hz). Non‐anomeric axial OH groups form partial intramolecular H‐bonds to a cis‐1.3‐diaxial alkoxy group (as in 29 and 41 : J(H,OH)=4.8 – 5.0 Hz). The persistence of such a H‐bond is enhanced when there is an additional H‐bond acceptor, such as the ring O‐atom ( 43 – 47 : J(H,OH)=5.6 – 7.6 Hz; 32 and 33 : 10.5 – 11.3 Hz). The (partial) intramolecular H‐bonds lead to an upfield shift (relative to the signal of a fully solvated OH in a similar surrounding) for the signal of the H‐donor. The shift may also be related to the signal of the fully solvated, equatorial HO−C(2), HO−C(3), and HO−C(4) of β‐D ‐glucopyranose ( 16 : 4.81 ppm) by using the following increments: −0.3 ppm for an axial OH group, 0.2 – 0.25 ppm for replacing a vicinal OH by an OR group, ca. 0.1 ppm for replacing another OH by an OR group, 0.2 ppm for an antiperiplanar C−O bond, −0.3 ppm if a vicinal OH group is (partially) H‐bonded to another OR group, and −0.4 to −0.6 for both OH groups of a vicinal diol moiety involved in (partial) divergent H‐bonds. Flip‐flop H‐bonds are observed between the diaxial HO−C(2) and HO−C(4) of the inositol 40 (J(H,OH)=6.4 Hz, δ(OH)=5.45 ppm) and levoglucosan ( 42 ; J(H,OH)=6.7 – 7.1 Hz, δ(OH)=4.76 – 4.83 ppm; bifurcated H‐bond); the former is completely persistent and the latter to ca. 40%. A persistent, unidirectional H‐bond C(1)−OH⋅⋅⋅O−C(10) is present in ginkgolide B and C, as evidenced by strongly different δ(OH) and Δδ(OH)/ΔT values for HO−C(1) and HO−C(10) (Fig. 9). In the absence of this H‐bond, HO−C(1) of 52 resonates 1.1 – 1.2 ppm downfield, while HO−C(10) of ginkgolide A and of 48 – 50 resonates 0.5 – 0.9 ppm upfield.     

Dehydrogenative Carbon–Carbon Bond Formation Using Alkynyloxy Moieties as Hydrogen‐Accepting Directing Groups

In the presence of a catalyst system consisting of Pd(OAc)₂, PCy₃, and Zn(OAc)₂, the reaction of alkynyl aryl ethers with bicycloalkenes, α,ß‐unsaturated esters, or heteroarenes results in the site‐selective cleavage of two C? H bonds followed by the formation of C? C bonds. In all cases, the alkynyloxy group acts as a directing group for the activation of an ortho C? H bond and as a hydrogen acceptor, thus rendering the use of additives such as an oxidant or base unnecessary.     

Dye-sensitized solar cells based on bisindolylmaleimide derivatives

Three organic dyes based on bisindolylmaleimide derivatives (I1, I2 and I3) were synthesized and investigated as sensitizers for the application in nanocrystalline TiO₂ solar cells. The indole group, maleimide group and carboxylic group functioned as electron donor, acceptor and anchoring group, respectively. Solar-to-electrical energy conversion efficiencies under simulated amplitude-modulated 1.5 irradiation (100 mW·cm⁻²) of 2.07% were obtained for solar cells based on I2 and of 1.87% and 1.50% for I3 and I1, respectively. The open circuit voltage V _oc was demonstrated to be enhanced by the introduction of dodecyl or benzyl moieties on the indole groups. The nonplanar structure of bisindolylmaleimide was proven to be effective in aggregation resistance. This work suggests that organic sensitizers with maleimide as electron acceptor are promising candidates as organic sensitizers in dye-sensitized solar cells.     

Phthalimide and Naphthalimide end‐Capped Diketopyrrolopyrrole for Organic Photovoltaic Applications

Two small molecules named PI‐DPP and NI‐DPP with a DPP core as the central strong acceptor unit and phthalimide/naphthalimide as the terminal weak acceptor were designed and synthesized. The effects of terminal phthalimide/naphthalimide units on the thermal behavior, optical and electrochemical properties, as well as the photovoltaic performance of these two materials were systematically studied. Cyclic voltammetry revealed that the lowest unoccupied molecular orbitals (LUMO) (~ ‐3.6 eV) of both molecules were intermediate to common electron donor (P3HT) and acceptor (PCBM). This indicated that PI‐DPP and NI‐DPP may uniquely serve as electron donor when blended with PCBM, and as electron acceptor when blended with P3HT, where sufficient driving forces between DPPs and PCBM, as well as between P3HT and DPPs should be created for exciton dissociation. Using as electron donor materials, PI‐DPP and NI‐DPP devices exhibited low power conversion efficiencies (PCEs) of 0.90% and 0.76% by blending with PCBM, respectively. And a preliminary evaluation of the potential of the NI‐DPP as electron acceptor material was carried out using P3HT as a donor material, and P3HT: NI‐DPP device showed a PCE of 0.6%, with an open circuit voltage (V _OC) of 0.7 V, a short circuit current density (J _SC) of 1.91 mA•cm^‐2, and a fill factor (FF) of 45%.     

Darstellung von fluorarsenigsäureestern

The preparation of arsinic acid esters F₂AsOR and FAs(OR)₂ by the alcoholysis of AsF₃ and F₂AsN(CH₃)₂ and the reaction of AsF₃ and BF₃ with As(OR)₃ are described.The reactions of AsF₃ with As(OR)₃ lead to the formation of fluoroesters containing various R groups. The ratio of the difluoro- and monofluoro-arsinic acid esters present in the system AsF₃/F₂AsOR/FAs(OR)₂/As(OR)₃ has been determined. ¹⁹F and ¹H NMR spectral data are presented.     

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

The systematic synthesis and photophysical, electrochemical and computational studies on an extended series of triphenylamine‐[C?C‐1,4‐C₆H₂(OR)₂]_n‐C?C‐diphenyl‐1,3,4‐oxadiazole dyad molecules (the OR groups are at 2,5‐positions of the para‐phenylene ring and R=C₆H₁₃; n=0–5, compounds 1 , 2 , 3 , 4 and 5 , respectively) are reported. Related molecules with identical end groups, triphenylamine‐C?C‐1,4‐C₆H₂(OR)₂‐C?C‐triphenylamine (R=C₆H₁₃; 6 ) and diphenyl‐1,3,4‐oxadiazole‐[C?C‐C₆H₂(OR)₂]₂‐C?C‐diphenyl‐1,3,4‐oxadiazole (R=C₆H₁₃; 7 ) were also studied. These D–B–A 1 – 5 , D–B–D 6 and A–B–A 7 (D=electron donor, B=bridge, A=electron acceptor) systems were synthesized using palladium‐catalysed cross‐coupling reactions of new p‐phenyleneethynylene building blocks. Steady‐state emission studies on the dyads 1 – 5 reveal a complicated behavior of the emission that is strongly medium dependent. In low polarity solvents the emission is characterized by a sharp high‐energy peak attributed to fluorescence from a locally excited (LE) state. In more polar environments the LE state is effectively quenched by transfer into an intramolecular charge‐transfer (ICT) state. The medium dependence is also observed in the quantum yields (QYs) which are high in cyclohexane and low in acetonitrile, thus also indicating charge‐transfer character. Low‐temperature emission spectra for 2 – 5 in dichloromethane and diethyl ether also reveal two distinct excited states, namely the LE state and the conventional ICT state, depending on solvent and temperature. Hybrid DFT calculations for 1 – 7 establish that the OPE bridge is involved in both frontier orbitals where the bridge character increases as the bridge length increases. Computed TD‐DFT data on 1 – 5 assign the emission maxima in cyclohexane as LE transitions. Each time‐resolved emission measurement on 2 – 7 in cyclohexane and diethyl ether reveals a wavelength dependent bi‐exponential decay of the emission with a fast component in the 5–61 ps range on blue detection and a slower approximately 1 ns phase, independent of detection wavelength. The fast component is attributed to LE fluorescence and this emission component is rate limited and quenched by transfer into an ICT state. The fast LE fluorescence component varies systematically with conjugation length for the series of D–B–A dyads 2 – 5 . An attenuation factor β of 0.15 Å^?1 was determined in accordance with an ICT superexchange mechanism.     

Computational Study on the Unconventional Hydrogen‐Bonded F–H···C Systems

The F–H···YZ₂ (Y = C, Si, BH, A1H;Z = H, PH₃) systems were examined using density functional theory calculations. The main focus of this work is to demonstrate that the chemistry of Y(PH₃)₂ exhibits a novel feature which is a central Y atom with unexpected high basicity. Further, the hydrogen bond strength can be adjusted by the substitution of H atoms of YH₂ by PH₃ groups. The FH···C(PH₃)₂ system has the strongest hydrogen bond interaction, which is larger than a conventional hydrogen bond. In addition to electrostatic interaction, donor‐acceptor interaction also plays an important role in determining the hydrogen bond strength. Therefore, a carbon atom can not only be the hydrogen bond acceptor but also can create an unusual stabilized hydrogen bond complex. Also, X₃B–YZ₂ (X = H, F; Y = C, Si, BH, A1H;Z = PH₃, NH₃) systems were examined, and it was found that the bond strength is controlled predominately by the HOMO‐LUMO gap (ΔIP). The smaller the ΔIP, the larger the bond dissociation energy of the B–Y bond. In addition, NH₃ is a better electron‐donating group than PH₃, and thus forms the strongest donor‐acceptor interaction between X₃B and Y(NH₃)₂.     

Glycosylidene Carbenes. Part 22. α-D-Selectivity in the Glycosidation by Carbenes Derived from 2-Acetamido-hexoses

Glycosylidene carbenes derived from the GlcNAc and AllNAc diazirines 1 and 3 were generated by the thermolysis or photolysis of the diazirines. The reaction of 1 with i-PrOH gave exclusively the isopropyl α-D -glycoside of 5 besides some dihydrooxazole 9 (Scheme 2). A similar reaction with (CF₃)₂CHOH yielded predominantly the α-D -anomer of 6 , while glycosidation of 4-nitrophenol (→ 7 ) proceeded with markedly lower diastereoselectivity. Similarly, the Allo-diazirine 3 gave the corresponding glycosides 12–14 , but with a lower preference for the α-D -anomers (Scheme 3). The reactions of the carbene derived from 1 with Ph₃COH (→ 8 ) and diisopropylideneglucose 10 (→ 11 ) gave selectively the α-D -anomers (Scheme 2). The αD -selectivity increases with increasing basicity (decreasing acidity) of the alcohols. It is rationalized by an intermolecular H-bond between the acetamido group and the glycosyl acceptor. This H-bond increases the probability for the formation of a 1,2-cis-glycosidic C–O bond. The gluco-intermediates are more prone to forming a N–H…?(H)OR bond than the allo-isomers, since the acetamido group in the N-acetylallosamine derivatives forms an intramolecular H-bond to the cis-oriented benzyloxy group at C(3), as evidenced by δ/T and δ/c experiments.