Palladium-catalyzed borylation of ortho-substituted phenyl halides and application to the one-pot synthesis of 2,2'-disubstituted biphenyls

Borylation of unactivated aryl chlorides under mild conditions by using diisopropylaminoborane as a borylating reagent

The synthesis of arylboronic ester derivatives from aryl chlorides by using aryl(amino)boranes is described. Palladium-catalyzed coupling between aryl chlorides and diisopropylaminoborane leads to the formation of a CB bond under mild conditions. A wide range of functional groups are tolerated, making this method particularly useful for the borylation of functionalized aromatics.

Continuous coflow synthesis of hybrid palladium nanocrystals as catalysts for borylation reaction

The combination of highly active Pd nanocrystal (NC) types with tailored surface properties (various ligands) - e.g. organic-inorganic hybrid NCs - as catalysts opens avenues towards new synthetic pathways, implying a faster practical alternative for adjusting and screening the reaction conditions. Pd@dppf and Pd@PCy3 NCs have been successfully prepared via a continuous supercritical fluid assisted coflow route with promising results as catalysts in borylation reaction. It has been found that the ligands not only influence the catalytic properties of the systems, but also contribute to Pd metal core characteristics (size, shape and time stability).

Stereospecific approach to the synthesis of ring-A oxygenated sarpagine indole alkaloids. Total synthesis of the dimeric indole alkaloid P-(+)-dispegatrine and six other monomeric indole alkaloids

The first regio- and stereocontrolled total synthesis of the bisphenolic, bisquaternary alkaloid (+)-dispegatrine (1) has been accomplished in an overall yield of 8.3% (12 reaction vessels) from 5-methoxy-d-tryptophan ethyl ester (17). A crucial late-stage thallium(III) mediated intermolecular oxidative dehydrodimerization was employed in the formation of the C9-C9' biaryl axis in 1. The complete stereocontrol observed in this key biaryl coupling step is due to the asymmetric induction by the natural sarpagine configuration of the monomer lochnerine (6) and was confirmed by both the Suzuki and the oxidative dehydrodimerization model studies on the tetrahydro β-carboline (35). The axial chirality of the lochnerine dimer (40) and in turn dispegatrine (1) was established by X-ray crystallography and was determined to be P(S). Additionally, the first total synthesis of the monomeric indole alkaloids (+)-spegatrine (2), (+)-10-methoxyvellosimine (5), (+)-lochnerine (6), lochvinerine (7), (+)-sarpagine (8), and (+)-lochneram (11) were also achieved via the common pentacyclic intermediate 16.

An improved synthesis of [11C]MENET via Suzuki coupling with [11C]methyl iodide

[(11) C]MENET, a promising norepinephrine transporter imaging agent, was prepared by Suzuki cross coupling of 1 mg N-t-Boc pinacolborate precursor with [(11) C]CH3 I in DMF using palladium complex generated in situ from Pd2 (dba)3 and (o-CH3 C6 H4 )3 P together with K2 CO3 as the co-catalyst, followed by deprotection with trifluoroacetic acid. This improved radiolabeling method provided [(11) C]MENET in high radiochemical yield at end of synthesis (EOS, 51 ± 3%, decay-corrected from end of (11) CH3 I synthesis, n = 6), moderate specific activity (1.5-1.9 Ci/µmol at EOS), and high radiochemical (>98%) and chemical purity (>98%) in a synthesis time of 60 ± 5 min from the end of bombardment.

Synthesis of pinacol arylboronates from aromatic amines: a metal-free transformation

A metal-free borylation process based on Sandmeyer-type transformation using arylamines derivatives as the substrates has been developed. Through optimization of the reaction conditions, this novel conversion can be successfully applied to a wide range of aromatic amines, affording borylation products in moderate to good yields. Various functionalized arylboronates, which are difficult to access by other methods, can be easily obtained with this metal-free transformation. Moreover, this transformation can be followed by Suzuki-Miyaura cross-coupling without purification of the borylation products, which enhances the practical usefulness of this method. A possible reaction mechanism involving radical species has been proposed.

Scope of the two-step, one-pot palladium-catalyzed borylation/Suzuki cross-coupling reaction utilizing bis-boronic acid

The use of bis-boronic acid for the direct synthesis of boronic acids has greatly facilitated the two-step, one-pot borylation/Suzuki cross-coupling reaction between aryl and heteroaryl halides. With use of Buchwald's second-generation XPhos preformed catalyst, high yields of cross-coupled products were obtained for most substrates. The method also allows an efficient two-step, one-pot synthesis, providing access to three distinct cross-coupled products after column chromatography. The method also provides a rapid and convenient route to teraryl compounds.

o-(Trialkylstannyl)anilines and Their Utility in Stille Cross-Coupling: Direct Introduction of the 2-Aminophenyl Substituent

We have developed shelf- and air-stable ortho-stannylated aniline reagents that can directly be coupled with alkenyl and aryl halides via Stille cross-coupling. We report i) the efficient preparation of o-(tributylstannyl)aniline (2a) and o-(trimethylstannyl)aniline (2b), ii) the comparison of the reactivities of 2a and 2b with those of related organostannanes in cross-coupling reaction with an alkenyl halide, and iii) the cross-coupling of 2a and 2b with a series of arylhalides and triflate.

Methyl 2-(5,5-dimethyl-1,3,2-dioxa-borinan-2-yl)-4-nitro-benzoate

The six-membered boronate ester ring of the title compound, C₁₃H₁₆BNO₆, adopts an envelope conformation with the C atom bearing the dimethyl substituents at the flap. The O—B—C—C torsion angles between the boronate group and the benzene ring are 72.5 (2) and 81.0 (2)°. The 4-nitro­benzoate unit adopts a slightly twisted conformation, with dihedral angles between the benzene ring and the nitrate and methyl ester groups of 17.5 (2) and 14.4 (3)°, respectively. In the crystal, inversion-related pairs of mol­ecules show weak π–π stacking inter­actions [centroid–centroid distance = 4.0585 (9) Å and inter­planar spacing = 3.6254 (7) Å].

Syntheses of novel diaryl[d,f ][1,3]diazepines via one-pot Suzuki coupling followed by direct ring closure with carboxylic acids

A series of novel and diverse diaryl[d,f][1,3]diazepines were designed and synthesized to expand the pharmaceutical utility of the [6,7]bicyclic molecular skeletons. The facile synthesis involved two key steps: a one-pot Suzuki coupling to construct the bi-aryl intermediates from corresponding halides, and a ring closure by direct condensation with carboxylic acids.

C-H activation: a complementary tool in the total synthesis of complex natural products

The recent advent of transition-metal mediated C-H activation is revolutionizing the synthetic field and gradually infusing a "C-H activation mind-set" in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of C-H activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of C-H activation as a strategic manoeuver are compared with their "non-C-H activation" counterparts, illuminating a new paradigm in strategic synthetic design.

Carbon-boron bond cross-coupling reaction catalyzed by -PPh(2) containing palladium-indolylphosphine complexes

This study describes the application of indolylphosphine ligands with a diphenylphosphino moiety to the palladium-catalyzed borylation of aryl chlorides. The combination of palladium metal precursor with PPh(2)-Andole-phos, which comprises an inexpensive -PPh(2) group, provides highly effective catalysts for the borylation of aryl chlorides. A range of functional groups such as -CN, -NO(2), -CHO, -COMe, -COOMe, and -CF(3) was compatible, and the catalyst loading down to 0.025 mol % of Pd can be achieved. The Pd/PPh(2)-Andole-phos system is able to catalyze both borylation reaction and Suzuki-Miyaura coupling reaction in a one-pot sequential manner for the direct synthesis of biaryl compounds in excellent yields.

Highly efficient one-pot access to functionalized arylboronic acids via noncryogenic bromine/magnesium exchanges

A general and convenient protocol for the electrophilic borylation of aryl Grignard reagents prepared from arylbromides by direct insertion of magnesium in the presence of LiCl or by Mg/Br exchange with (i)PrMgCl·LiCl has been developed. Various aryl boronic acids were synthesized in a straightforward manner in excellent yields at 0 °C.

Synthetic mimics of antimicrobial peptides from triaryl scaffolds

In this report, we describe the synthesis of a new series of small amphiphilic aromatic compounds that mimic the essential properties of cationic antimicrobial peptides using Suzuki-Miyaura coupling. The new design allowed the easy tuning of the conformational restriction, controlled by introduction of intramolecular hydrogen bonds, and the overall hydrophobicity by modifications to the central ring and the side chains. This approach allowed us to better understand the influence of these features on the antimicrobial activity and selectivity. We found that the overall hydrophobicity had a more significant impact on antimicrobial and hemolytic activity than the conformational stiffness. A novel compound was discovered which has MICs of 0.78 μg/mL against S. Aureus and 6.25 μg/mL against E. Coli, similar to the well-known antimicrobial peptide, MSI-78.

Intramolecular Suzuki-Miyaura reaction for the total synthesis of signal peptidase inhibitors, arylomycins A(2) and B(2)

Development of the total syntheses of arylomycins A(1) and B(2) is detailed. Key features of our approach include 1) formation of 14-membered meta,meta-cyclophane by an intramolecular Suzuki-Miyaura reaction; 2) incorporation of N-Me-4-hydroxyphenylglycine into the cyclization precursor, which avoids the late-stage low-yielding N-methylation step; 3) segment coupling of a fully elaborated peptide side chain to the macrocycle, which makes the synthesis highly convergent. Overall, arylomycin A(2) was obtained in 13 steps from L-Tyr for the longest linear sequence, in 13 % overall yield. Arylomycin B(2) was synthesized in 10 steps from L-3-nitro-Tyr, in 10 % overall yield.

Neopentylglycolborylation of ortho-substituted aryl halides catalyzed by NiCl(2)-based mixed-ligand systems

NiCl(2)-based mixed-ligand systems were shown to be very effective catalysts for the neopentylglycolborylation of aryl iodides, bromides, and chlorides bearing electron-rich and electron-deficient ortho-substituents. Although NiCl(2)-based single-ligand catalytic systems were able to mediate neopentylglycolborylation of selected substrates, they were not as effective for all substrates, highlighting the value of the mixed-ligand concept. Optimization of the Ni(II)-catalyzed neopentylglycolborylation of 2-iodoanisole and methyl 2-iodobenzoate demonstrated that, while the role of ligand and coligand in the conversion of Ni(II) precatalyst to Ni(0) active catalyst cannot be ignored, a mixed-ligand complex is likely present throughout the catalytic cycle. In addition, protodeborylation and hydrodehalogenation were demonstrated to be the predominant side reactions of Ni(II)-catalyzed borylation of ortho-substituted aryl halides containing the electron-deficient carboxylate substituents. Ni(II) complexes in the presence of H(2)O and Ni(0) are responsible for the catalysis of these side reactions.

PXPd-catalyzed borylation of aniline derivatives

Addition of pinacolborane (HBO2C2Me4) to 2-iodoaniline can be catalyzed using a number of palladium complexes, including [(t-Bu)2PCl]2PdCl2 (PXPd), to give the corresponding boronate ester 2-H2NC6H4(BO2C2Me4) in excellent yields. The PXPd system could also be used in the catalyzed borylation of substituted anilines to give the corresponding aminoboronate esters in moderate to high yields.