Complexes & Reagents

Reagents and metal complexes are the enabling tools of synthetic medicinal chemistry, determining whether a desired bond can be formed efficiently, selectively, and at a scale compatible with drug discovery programmes. The most widely used reactions in small-molecule drug synthesis, including amide coupling, palladium-catalysed cross-coupling, reductive amination, asymmetric reduction, and olefin metathesis, each depend on a distinct set of reagents and catalyst complexes that have been refined over decades to maximise chemoselectivity, functional group tolerance, and operational simplicity. The Suzuki-Miyaura and Buchwald-Hartwig methodologies are the two most common coupling reactions in medicinal chemistry, valued for their versatility in generating carbon-carbon and carbon-heteroatom bonds across diverse substrates, with the Suzuki-Miyaura coupling also used in the large-scale synthesis of approved drugs including losartan and abemaciclib. Amide bond formation, present in the majority of top-selling small-molecule drugs, relies on uronium and phosphonium-based coupling reagents, most notably HATU, HBTU, PyBOP, and EDC, used together with HOBt or HOAt as racemisation-suppressing additives and a tertiary amine base. Asymmetric synthesis of chiral drug candidates is enabled by the Noyori BINAP-Ru complexes and related chiral bisphosphine-metal systems that deliver ketone hydrogenation with high enantioselectivity, while Grubbs and Hoveyda-Grubbs ruthenium carbene catalysts provide access to macrocyclic and ring-closing metathesis products in complex natural product-derived drug scaffolds. The emergence of photoredox and electrochemical synthesis has introduced an entirely new class of reagents and catalytic systems, including iridium polypyridyl complexes, ruthenium bipyridyl complexes, metal-free organic dyes, and electrode-based oxidants, which access radical intermediates and redox-activated bond formations inaccessible by thermal methods. Together, these reagents and complexes define the synthetic toolbox available to the medicinal chemist and set the boundaries of what chemical space can be rapidly and reliably explored in a drug discovery campaign.

Reagents and complexes in common organic chemistry reactions
The most frequently employed reagents in medicinal chemistry synthesis fall into several well-defined categories corresponding to the reactions they enable. For amide bond formation, the single most common transformation in drug synthesis, the key reagents are carbodiimide activators (DCC, DIC, EDC) and the more reactive uronium and phosphonium coupling salts. The most common syntheses of amides use stoichiometric quantities of activating agents such as DCC, DIC, and EDC, and coupling via guanidinium and uronium salts such as HATU, HBTU, TBTU, CDI, and T3P. HATU provides faster and more efficient couplings with less epimerisation and under milder reaction conditions than its structural analogue HBTU, attributed to the formation of a stabilising hydrogen bond in the reaction intermediate, making it the reagent of choice for difficult or sterically hindered couplings. For palladium-catalysed cross-coupling, the choice of palladium precatalyst and phosphine or N-heterocyclic carbene (NHC) ligand is critical. The most widely used palladium sources are Pd(OAc)₂ and Pd₂(dba)₃, paired with biarylphosphine ligands including SPhos, RuPhos, and XPhos developed by the Buchwald group, or with NHC-ligated PEPPSI-type complexes. Whether one uses Pd₂(dba)₃ or Pd(OAc)₂ as a precatalyst can have a profound effect on the success of any given transformation, with dba playing an active role as a ligand in controlling the rates of oxidative addition as well as the concentration of catalytically active species. NHC ligands, particularly IPr and IMes, offer strong σ-donation and steric shielding that stabilise coordinatively unsaturated palladium species and enable coupling of challenging electrophiles including aryl chlorides and sterically hindered substrates. NHC complexes are easily obtained by deprotonating imidazolium or benzimidazolium salts and are relatively stable to air and moisture; they are weak π-acceptors and strong σ-donors that form strong M-C bonds with transition metal ions compared to trivalent phosphine ligands. For reductive amination, the most common hydride reductants are sodium cyanoborohydride (NaBH₃CN) and sodium triacetoxyborohydride (NaBH(OAc)₃), both of which selectively reduce iminium ions in the presence of the unreacted aldehyde and other functional groups. The Dess-Martin periodinane (DMP) and Swern conditions (oxalyl chloride/DMSO/Et₃N) provide mild, selective alcohol oxidation to aldehydes and ketones, while the Grubbs second-generation catalyst, a ruthenium benzylidene complex bearing an NHC ligand and a tricyclohexylphosphine, performs ring-closing metathesis with excellent functional group tolerance across a wide range of drug-like substrates.

Reagents and complexes in emerging photochemistry and electrochemistry
The development of photoredox catalysis and electrochemical synthesis has introduced a new generation of reagents and catalytic complexes that operate through single-electron transfer (SET) rather than the two-electron polar mechanisms of classical organic chemistry. The triplet state of fac-Ir(ppy)₃ lies 56 kcal mol⁻¹ above the ground state, giving this iridium(III) tris(phenylpyridine) complex a capacity to act simultaneously as both an oxidant and a reductant in its excited state; this unique property enables a wide array of novel synthetic transformations and provides access to previously elusive mechanistic pathways. The ruthenium complex [Ru(bpy)₃]²⁺ was among the earliest photoredox catalysts applied to preparative organic synthesis, enabling enantioselective α-alkylation of aldehydes in combination with chiral imidazolidinone organocatalysts, establishing the principle of dual photoredox/organocatalysis. Metal-free organic dyes, including eosin Y, rose bengal, acridinium salts, and the carbazole-based 4CzIPN, have emerged as cost-effective and structurally tunable alternatives to iridium and ruthenium complexes, with their ground and excited-state redox potentials readily modulated by substitution of the core chromophore. Nickel/photoredox dual catalysis is efficacious in activating inert bonds and creating reaction pathways not achievable under single-catalyst conditions; the single-electron transfer processes and versatile oxidation states of nickel, coupled with organic and metal-based photocatalysts, underpin dual catalytic cycles that enable enantioselective C-N and C-O coupling reactions and C(sp²)-C(sp³) bond construction. In practice, Ni(cod)₂ or NiCl₂(dme) paired with dtbbpy or bipyridyl ligands constitutes the nickel component of these dual systems, with the photocatalyst typically used at sub-stoichiometric loading of 1-5 mol%. Electrochemical synthesis replaces stoichiometric chemical oxidants and reductants with controlled electrode potential, eliminating reagent waste and enabling reactions at a continuously tuneable potential. Electrosynthetic methods, which obviate the need for hazardous chemical oxidants or reductants, offer unprecedented control of reactions through the continuous variation of applied potential; this capability is particularly advantageous for late-stage functionalisation of drug candidates, where chemoselectivity over multiple sensitive functional groups is essential. The key electrochemical reagents include conducting electrolytes, most commonly tetrabutylammonium tetrafluoroborate (Bu₄NBF₄) or hexafluorophosphate salts dissolved in DMF, MeCN, or MeOH, which carry current through the cell without participating in the substrate reaction. Selectfluor serves a dual role in electrochemical C-H fluorination, functioning both as the electrophilic fluorine source and as a redox mediator through its reducible N-F bond, enabling selective fluorination of unactivated C(sp³)-H bonds that cannot be achieved by classical electrophilic methods. Hypervalent iodine reagents, including PhI(OAc)₂ and its cyclic variants such as the Koser reagent and Togni reagents, can be generated electrochemically in situ from aryl iodides, providing an oxidant-free approach to α-oxygenation, trifluoromethylation, and C-H acetamidation, transformations that install or modify functional groups present across a wide range of drug scaffolds.