The biaryl structural motif is a predominant feature in many pharmaceutically relevant and biologically active compounds. As a result, for over a century 1 organic chemists have sought to develop new and more efficient aryl -aryl bond-forming methods. Although there exist a variety of routes for the construction of aryl -aryl bonds, arguably the most common method is through the use of transition-metalmediated reactions. 2-4 While earlier reports focused on the use of stoichiometric quantities of a transition metal to carry out the desired transformation, modern methods of transitionmetal-catalyzed aryl -aryl coupling have focused on the development of high-yielding reactions achieved with excellent selectivity and high functional group tolerance under mild reaction conditions. Typically, these reactions involve either the coupling of an aryl halide or pseudohalide with an organometallic reagent (Scheme 1), or the homocoupling of two aryl halides or two organometallic reagents. Although a number of improvements have developed the former process into an industrially very useful and attractive method for the construction of aryl -aryl bonds, the need still exists for more efficient routes whereby the same outcome is accomplished, but with reduced waste and in fewer steps. In particular, the obligation to use coupling partners that are both activated is wasteful since it necessitates the installation and then subsequent disposal of stoichiometric activating agents. Furthermore, preparation of preactivated aryl substrates often requires several steps, which in itself can be a time-consuming and economically inefficient process.

http://aether.cmi.ua.ac.be/artikels/Artikels%20Gitte/Artikels/Reviews/Heck-Type-lautens.pdf

Aryl-aryl bond formation by transition-metal-catalyzed direct arylation.

The direct functionalization of C-H bonds in organic compounds has recently emerged as a powerful and ideal method for the formation of carbon-carbon and carbon-heteroatom bonds. This Review provides an overview of C-H bond functionalization strategies for the rapid synthesis of biologically active compounds such as natural products and pharmaceutical targets.

C-H bond functionalization: emerging synthetic tools for natural products and pharmaceuticals

Selected patented cross-coupling reaction technologies.

During the last two decades there has been considerable growth in the development of catalytic reactions capable of activating unreactive C–H bonds. These methods allow for the synthesis of complex molecules from easily available and cheaper precursors in a fewer number of steps. Naturally, the development of C–H activation methods for direct functionalization of heterocyclic molecules, invaluable building blocks for pharmaceutical and synthetic chemistry and material science, has received substantial attention as well. This critical review summarizes the progress made in this field until November 2006 (117 references).

Direct transition metal-catalyzed functionalization of heteroaromatic compounds

Large-scale applications of transition metal-catalyzed couplings for the synthesis of pharmaceuticals.

3-Pyridylboronic acid was prepared in high yield and bulk quantity from 3-bromopyridine via a protocol of lithium-halogen exchange and "in situ quench". This technique was further studied and evaluated on other aryl halides in the preparation of arylboronic acids.

An improved protocol for the preparation of 3-pyridyl- and some arylboronic acids.

Palladium-Catalyzed Regioselective Arylation of Imidazo[1,2-a]pyrimidine

Synthesis of an anti-methicillin-resistant Staphylococcus aureus (MRSA) carbapenem via stannatrane-mediated Stille coupling.

A practical preparation of an αvβ3 antagonist is reported. The antagonist consists of three key components, a tetrahydronaphthyridine moiety, a β-alanine moiety, and a central imidazolidone moiety. The tetrahydronaphthyridine component was prepared using two different methods, both of which relied on variations of the Friedlander reaction to establish the desired regiochemistry. The β-alanine component was prepared using Davies' asymmetric 1,4-addition methodology as the key stereo-defining step. The central imidazolidone portion was created from these two components using an effective three-step cyclization protocol. Thus, a highly convergent process for the drug candidate was defined.

An Efficient Synthesis of an αvβ3 Antagonist

A practical synthesis of 2-[3-(4-fluoro-3-pyridin-3-yl-phenyl)-imidazo[1,2-a]pyrimidin-7-yl]-propan-2-ol (1), an oral GABAA α2/3-selective agonist, is described. The five-step process, which afforded 1 in 40% overall yield, included imidazopyrimidine 2 and pyridine boronic acid 4 as key fragments. The synthesis is highlighted by consecutive Pd-catalyzed coupling steps to assemble the final free base 1 in high yield and regioselectivity. A novel method for Pd removal in the final step is also described.

Mark S. Jensen

Papers

An improved protocol for the preparation of 3-pyridyl- and some arylboronic acids.

Palladium-Catalyzed Regioselective Arylation of Imidazo[1,2-a]pyrimidine

Synthesis of an anti-methicillin-resistant Staphylococcus aureus (MRSA) carbapenem via stannatrane-mediated Stille coupling.

An Efficient Synthesis of an αvβ3 Antagonist

Efficient Synthesis of a GABAA α2,3-Selective Allosteric Modulator via a Sequential Pd-Catalyzed Cross-Coupling Approach