José M. Lassaletta

Functional group directed C-H borylation.

The direct borylation of hydrocarbons via C-H activation has reached an impressive level of sophistication and efficiency, emerging as a fundamental tool in synthesis because of the versatility offered by organoboron compounds. As a remarkable particularity, the catalytic systems originally developed for these reactions are relatively insensitive to directing effects, and the regioselectivity of the borylations is typically governed by steric factors. Likely stimulated by the great synthetic potential of the expected functionalised organoboranes, however, many groups have recently focused on the development of complementary strategies for directed, site-selective borylation reactions where a directing group controls the course of the reaction. In this tutorial review, the different strategies and findings related to the development of these directed borylation reactions via C(sp(2))-H or C(sp(3))-H activation will be summarized and discussed.

C2-Symmetric Bis-Hydrazones as Ligands in the Asymmetric Suzuki-Miyaura Cross-Coupling

Glyoxal bis-hydrazone derived from (S,S)-1-amino-2,5-diphenylpyrrolidine behaves as an excellent ligand for phosphine-free, asymmetric Suzuki-Miyaura cross couplings, thereby affording a variety of enantiomerically enriched biaryls with different substitution patterns. The high catalytic activity of the [PdCl2(bis-hydrazone)] complex allows reactions to be performed at room temperature, affording products with excellent enantioselectivities in all cases.

Axially chiral triazoloisoquinolin-3-ylidene ligands in gold(I)-catalyzed asymmetric intermolecular (4 + 2) cycloadditions of allenamides and dienes.

The first highly enantioselective intermolecular (4 + 2) cycloaddition between allenes and dienes is reported. The reaction provides good yields of optically active cyclohexenes featuring diverse substitution patterns and up to three stereocenters. Key to the success of the process is the use of newly designed axially chiral N-heterocyclic carbene-gold catalysts.

Use of Hemilabile N,N Ligands in Nitrogen‐Directed Iridium‐Catalyzed Borylations of Arenes

We thank the Spanish “Ministerio de Ciencia e Innovacion” (grants CTQ2010-15297 and CTQ2010-14974, and fellowship to B.E.), the European FEDER funds and the Junta de Andalucia (grants 2008/FQM-3833 and 2009/FQM-4537) for financial support. A.R. thanks the EU for a Marie Curie Reintegration Grant (FP7-PEOPLE-2009-RG-256461) and R.L.-R. thanks CSIC for a JAE predoctoral fellowship.

Phosphino hydrazones as suitable ligands in the asymmetric Suzuki-Miyaura cross-coupling

Phosphino hydrazones derived from C(2)-symmetric hydrazines exhibit excellent catalytic activity and provide good enantioselectivities in the asymmetric Suzuki-Miyaura cross-coupling to axially chiral biaryls, in particular for the most challenging reactions of monocyclic, functionalized aryl bromides and triflates. X-ray analysis of preformed [Pd(P/N)Cl(2)] precatalysts [(P/N) = phosphino hydrazone] revealed a strong n-π conjugation in the hydrazone moiety, identified by a high planarity degree at the pyrrolidine N(sp(3)) atom, that makes rotations around N-N bonds inconsequential. The complexes are also characterized by an envelope-like conformation with the Pd atom placed at the opposite side to the 2-phenyl group on the nearest stereogenic center of the pyrrolidine group. The isolation and structural analysis of oxidative addition intermediates indicate that the configurational stability of Pd-C(Ar) bonds is dependent on the substitution pattern in the aryl bromide.

Asymmetric formal carbonyl-ene reactions of formaldehyde tert -butyl hydrazone with α-keto esters: Dual activation by bis-urea catalysts

The dual activation of α-keto esters and formaldehyde tert-butyl hydrazone by BINAM-derived bis-ureas is the key to achieve high reactivity and excellent enantioselectivities in nucleophilic addition (formal carbonyl-ene reaction) to functionalized tertiary carbinols. Ensuing high-yielding diazene-to-aldehyde tranformations and subsequent derivatizations provides a direct entry to a variety of densely functionalized products.

Isoquinolin-1-ylidenes as electronically tuneable ligands

The novel isoquinolin-1-ylidene ligands, introduced into Rh(I) complexes by exploiting the carbene-like reactivity of adducts , exhibit ligand properties similar to those of classic NHCs, and their electronic properties can be tuned by the introduction of electron-withdrawing or donating groups in the benzene ring.

Synthesis of Enantiopureα-Alkoxy-α-Trifluoromethyl Aldehydes and Carboxylic Acids from Trifluoromethyl Ketones

Formaldehyde dialkylhydrazones behave as neutral d1 synthons in their uncatalyzed reaction with trifluoromethyl ketones (see reaction). Both racemic and optically pure 1,2-adducts were obtained in good yields. Efficient deprotection of the hydrazone moiety afforded interesting fluorinated quaternary compounds such as 1 and 2.

Pyridine–Hydrazones as N,N′-Ligands in Asymmetric Catalysis: Pd(II)-Catalyzed Addition of Boronic Acids to Cyclic Sulfonylketimines

The design, synthesis, and coordination features of a novel class of chiral pyridine-hydrazone ligands are described. As a first application, L/Pd(TFA)2 complexes served as catalysts in the 1,2-addition of arylboronic acids to saccharin-derived cyclic ketimines, affording products in high yields and enantioselectivities. The method was also applied to more challenging 3,4-disubstituted 1,2,5-thiadiazole 1,1-dioxides, affording again high yields and enantioselectivities along with high regioselectivities for unsymmetrically substituted derivatives.

Strongly Emissive and Photostable Four‐Coordinate Organoboron N,C Chelates and Their Use in Fluorescence Microscopy

Six strongly fluorescent four-coordinate organoboron N,C chelates containing an aryl isoquinoline skeleton were prepared. Remarkably, the fluorescence quantum yields reach values of up to 0.74 in oxygen-free toluene. The strong B-N interaction was corroborated by the single-crystal X-ray analysis of two dyes. The intramolecular charge-transfer character of the fluorophores was evidenced by solvatochromism studies and time-dependent DFT calculations at the PCM(toluene)/CAM-B3LYP/6-311++G(2d,p)//PCM(toluene)/B3LYP/6-311G(2d,p) level of theory. The compounds combine high chemical stability with high photostability, especially when equipped with electron-donating substituents. The strong fluorescence and the large Stokes shifts predestine these compounds for use in confocal fluorescence microscopy. This was demonstrated for the imaging of the N13 mouse microglial cell line. Moreover, significant two-photon absorption cross sections (up to 61 GM) allow the use of excitation wavelengths in the near-infrared region (>800 nm).