Single operation palladium catalysed C(sp3)-H functionalisation of tertiary aldehydes: investigations into transient imine directing groups

Role of hexafluoroisopropanol in C–H activation

HFIP has developed immense importance in the C–H functionalization methodology. Both the reactivity and selectivity have been vastly improved using HFIP whose H-bonding to the substrate facilitates and accelerates C–H activation. This review summarizes the chronological development of the evolution of HFIP in C–H activation along with important mechanistic details.

Intramolecular palladium(ii)/(iv) catalysed C(sp3)-H arylation of tertiary aldehydes using a transient imine directing group

Palladium catalysed β-C(sp3)-H activation of tertiary aldehydes using a transient imine directing group enables intramolecular arylation to form substituted indane-aldehydes. A simple amine bearing a methyl ether (2-methoxyethan-1-amine) is the optimal TDG to promote C-H activation and reaction with an unactivated proximal C-Br bond. Substituent effects are studied in the preparation of various derivatives. Preliminary mechanistic studies identify a reversible C-H activation, product inhibition and suggest that oxidative addition is the turnover limiting step.

Ir(iii)-catalyzed ortho C-H alkylations of (hetero)aromatic aldehydes using alkyl boron reagents

Transition-metal-catalyzed C-H alkylation reactions directed by aldehydes or ketones have been largely restricted to electronically activated alkenes. Herein, we report a general protocol for the Ir(iii)-catalyzed ortho C-H alkylations of (hetero)aromatic aldehydes using alkyl boron reagents as the coupling partner. Featuring aniline as an inexpensive catalytic ligand, the method was compatible with a wide variety of benzaldehydes, heterocyclic aldehydes, potassium alkyltrifluoroborates as well as a few α,β-unsaturated aldehydes. An X-ray crystal structure of a benzaldehyde ortho C-H iridation intermediate was also successfully obtained.

Palladium-Catalyzed C(sp3 )-H Arylation of Primary Amines Using a Catalytic Alkyl Acetal to Form a Transient Directing Group

C-H Functionalization of amines is a prominent challenge due to the strong complexation of amines to transition metal catalysts, and therefore typically requires derivatization at nitrogen with a directing group. Transient directing groups (TDGs) permit C-H functionalization in a single operation, without needing these additional steps for directing group installation and removal. Here we report a palladium catalyzed γ-C-H arylation of amines using catalytic amounts of alkyl acetals as transient activators (e.g. commercially available (2,2-dimethoxyethoxy)benzene). This simple additive enables arylation of amines with a wide range of aryl iodides. Key structural features of the novel TDG are examined, demonstrating an important role for the masked carbonyl and ether functionalities. Detailed kinetic (RPKA) and mechanistic investigations determine the order in all reagents, and identify cyclopalladation as the turnover limiting step. Finally, the discovery of an unprecedented off-cycle free-amine directed ϵ-cyclopalladation of the arylation product is reported.

Ligand-Enabled γ-C(sp3)-H Activation of Ketones

We report the first example of Pd(II)-catalyzed γ-C(sp3)-H activation of ketones directed by a practical 2,2-dimethyl aminooxyacetic acid auxiliary. 2-Pyridone ligands are identified to enable C(sp3)-H activation for the first time. A rare six-membered palladacycle intermediate is isolated and characterized to elucidate the reaction mechanism. Both (hetero)arylation and vinylation of γ-C(sp3)-H bonds are demonstrated. Sequential β- and γ-C(sp3)-H (hetero)arylation of muscone showcases the utility of this method for late-stage diversification. A convenient Mn(II)-catalyzed auxiliary removal is also developed to further underscore the practicality of this transformation.

Pd-Catalyzed, ortho C-H Methylation and Fluorination of Benzaldehydes Using Orthanilic Acids as Transient Directing Groups

The direct, Pd-catalyzed ortho C-H methylation and fluorination of benzaldehydes have been accomplished using commercially available orthanilic acids as transient directing groups. In these reactions, the 1-fluoro-2,4,6-trimethylpyridinium salts can be either a bystanding F⁺ oxidant or an electrophilic fluorinating reagent. An X-ray crystal structure of a benzaldehyde ortho C-H palladation intermediate was obtained using triphenylphosphine as the stabilizing ligand.

Palladium-catalyzed site-selective arylation of aliphatic ketones enabled by a transient ligand

A direct arylation of C–H bonds of ketones enabled by a cheap and commercially available transient ligand with high site-selectivity and functional group compatibility is reported.

Palladium-catalyzed enantioselective C(sp2)–H arylation of ferrocenyl ketones enabled by a chiral transient directing group

A carboxylate-assisted palladium-catalyzed enantioselective C(sp²)–H arylation of ferrocenyl ketones is achieved through utilizing catalyticl-tert-leucine as a chiral transient directing group.

Combining transition metals and transient directing groups for C–H functionalizations

In the domain of synthetic chemistry, C–H bond activation has always remained in the spotlight for researchers over the last few decades. Although different strategies have been employed to chemically trigger unactivated C–H bonds, transition metal catalyzed directing group (DG) aided C–H bond activation is the most explored pathway of all because of its ability to perform diverse site selective functional metamorphosis. Despite its popularity, tedious synthetic methodology requiring additional steps for the installation and removal of DGs from the target substrate diminishes its efficacy. However, replacement of directing groups by transient directing groups (tDGs) reduces the hurdle to a greater extent without compromising the product yield and selectivity. In this report we have depicted the intense journey of transient directing groups with three (Rh, Ru, and Pd) prevalent second row transition metals.

This review is about the momentous evolution of transient directing group assisted C–H functionalizations promoted by three major second row transition metals.

Transient imines as ‘next generation’ directing groups for the catalytic functionalisation of C–H bonds in a single operation

This review describes recent developments in the use of catalytic transient directing groups, through imine linkages, which in combination with transition metal catalysts provide streamlined C–H functionalisation processes.

Pd-Catalyzed Ortho C-H Hydroxylation of Benzaldehydes Using a Transient Directing Group

The direct Pd-catalyzed ortho C-H hydroxylation of benzaldehydes was achieved using 4-chloroanthranilic acid as the transient directing group, 1-fluoro-2,4,6-trimethylpyridnium triflate as the bystanding oxidant, and p-toluenesulfonic acid as the putative oxygen nucleophile. The unusual C-H chlorination and polyfluoroalkoxylation reactions signaled the importance of external nucleophiles to the outcome of Pd(IV) reductive eliminations.

Methylene C(sp3)-H Arylation of Aliphatic Ketones Using a Transient Directing Group

Palladium-catalyzed methylene β-C(sp3)-H arylation of aliphatic ketones using a transient directing group is developed. The use of α-benzyl β-alanine directing group that forms a six-membered chelation with palladium is crucial for promoting the methylene C(sp3)-H bond activation.

Sulfinyl isobutyramide as an auxiliary for palladium(ii)-catalyzed C–H arylation and iodination of benzylamine derivatives

An original readily available and versatile bidentate 2-methylsulfinyl isobutyramide directing group has been developed for benzylamine derivatives.