1,2,3-Triazole-Containing Azamacrocycles from Chiral Triazolopeptoids: Synthesis and Solid-State Studies

Two new chiral 1,2,3-triazole-containing macrocyclic oligoamides (i. e.: triazolopeptoid 4 and 5) were obtained through solid-phase synthesis of linear precursors followed by high dilution macrocyclization reaction. Theoretical (DFT) and spectroscopic (NMR) studies revealed the intricate interplay between the Nα-chiral side chains and their conformational attitudes. BH3-mediated reduction of the tertiary amide groups of known 1-3 and newly synthesized 4 gave novel azamacrocycles 6-9. Detection of borane complexes of azamacrocycles 6 and 9 (i. e.: 10 and 11), corroborated by X-ray diffraction studies, demonstrated the peculiar properties of 1,2,3-triazole-containing macrorings.

Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Synthesis and reactions of N-heterocyclic carbene boranes

Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently. NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes. They do not exhibit "borane chemistry", but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts. They have significant potential for use in organic synthesis and in polymer chemistry. They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry. Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions. This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes.

Substitution reactions at tetracoordinate boron: synthesis of N-heterocyclic carbene boranes with boron-heteroatom bonds

Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (dipp-Imd-BH(2)X, X = halide or sulfonate) have been prepared from the parent borane dipp-Imd-BH(3) by (1) substitution reactions with R-X (X = halide or sulfonate), (2) reactions with electrophiles (like I(2) or NIS), or (3) acid/base reactions with HX (provided that HX has a pK(a) of about 2 or less). Dipp-Imd-BH(2)I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH(2)OTf is best prepared by reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and many have been characterized by X-ray crystallography. Several are members of very rare classes of functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.).

CAAC Boranes. Synthesis and characterization of cyclic (alkyl) (amino) carbene borane complexes from BF₃ and BH₃

In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF₃•Et₂O provided the first two examples of CAAC–BF₃ complexes: 1-(2,6-diisopropylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF₃ complexes are robust compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spectroscopy and X-ray crystallography. In contrast, a CAAC complex with borane (BH₃) was readily formed in situ according to ¹H and ¹¹B NMR analysis, but did not survive the workup conditions. These results set the stage for further studies of the chemistry of CAAC boranes.

Competition between hydrogen and dihydrogen bonding: interaction of B2H6 with CH3OH and CH(n)X(3-n)OH derivatives

Ab initio calculations were used to analyze the interactions between a molecule of B(2)H(6) with CH(3)OH and CH(n)X(3-n)OH (X = F, Cl and n = 0,1,2) derivatives at the MP2/6-311++G(d,p) computational level. Interaction of B(2)H(6) with CH(3)OH occurs through its bridged protons to form a hydrogen bond cluster. On the other hand, CH(n)X(3-n)OH molecules interact with B(2)H(6) by a H(t)...H dihydrogen bond along with a weak H(b)...X interaction. The structures obtained have been analyzed with the atoms in molecules (AIMs) methodology. AIM calculations indicate van der Waal's interactions of X with H(b) of B(2)H(6). The stability of the clusters depends on the type and number of X derivatives.

Kinetic acidity of supramolecular imidazolium salts-effects of substituent, preorientation, and counterions on H/D exchange rates

The deprotonation of imidazolium salts to N-heterocyclic carbenes is often a decisive step in modern catalytic reactions. Therefore, we studied the H/D exchange of the C2 H of 15 imidazolium-substituted calix[4]arenes and 11 nonmacrocyclic model compounds in methanol/water (97:3). The influence of the counterion, substitution directly on the imidazolium unit or on the preorientating calixarene backbone could be studied. The observed exchange rates might give a rational for the suitability of the imidazolium salts as precursors in the Suzuki coupling.