Transition-metal-catalyzed C−H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C−H bond activation, the functionalization of proximal C−H bonds has gained utmost popularity. Unlike the activation of proximal C−H bonds, the distal C−H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C−H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C−H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Transition metal–catalyzed aryl C−H activation is a powerful synthetic tool as it offers step and atom-economical routes to site-selective functionalization. Compared with proximal ortho-C−H activation, distal (meta- and/or para-) C−H activation remains more challenging due to the inaccessibility of these sites in the formation of energetically favorable organometallic pretransition states. Directing the catalyst toward the distal C−H bonds requires judicious template engineering and catalyst design, as well as prudent choice of ligands. This review aims to summarize the recent elegant discoveries exploiting directing group assistance, transient mediators or traceless directors, noncovalent interactions, and catalyst and/or ligand selection to control distal C−H activation.

Direct catalytic transformation of C–H bonds to new functionalities has provided a powerful strategy to synthesize complex molecular scaffolds in a straightforward way. Unstinting efforts of the synthetic community have helped to overcome the long-standing major challenge of regioselectivity by introducing the directing group concept. However, the full potential of the strategy cannot be realized unless the activated C–H bonds are stereochemically controlled. The enantioselective C–H bond functionalization could provide an imperative tool for a sustainable way of synthesizing chiral complex molecular scaffolds. Despite the intrinsic challenges in achieving stereocontrol, the synthetic community has developed different tools in order to achieve stereoselective C–H bond functionalization. In this review, we discuss the remarkable recent advances in the emerging area of enantioselective C­(sp2)–H bond functionalization to highlight the challenges and opportunities, emphasizing the different techniques developed so far.

Theoretical and experimental investigations have been carried out to examine the in uence of laser power and on time on the weld thermal cycle and weld metal microstructure and hardness during laser spot welding of low alloy steel. A transient heat transfer model that takes into account the temperature dependence of material properties and latent heat of phase transformation is employed to simulate thermal cycles and cooling rates experienced by the material under various combinations of power and on times. Two models for predicting the microstructure and hardness of the weld pool metal from the cooling rates are used to evaluate the results.

The present work reports on a two-dimensional axisymmetric finite element analysis of heat flow during laser spot welding, taking into account the temperature dependence of the physical properties and latent heat of transformations. An analysis based on conduction heat transfer alone, but using the ‘double ellipsoidal’ representation of the laser beam, seems to be sufficient to estimate the transition to keyhole formation during laser spot welding, although the ‘double ellipsoidal’ representation requires an a priori knowledge of the expected weld pool dimensions. Transient temperature isotherms and the weld pool dimensions are predicted using the model; the latter are found to compare well with measurements of weld bead dimensions. The results show that the keyhole mode is stimulated using either a high laser power and low on-time or a low laser power and high on-time. The outcomes are found to be sensitive to the assumed absorptivity and the assumed weld pool depth used to define the ‘double ellipsoidal’ heat source.