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Contents 1. Introduction 49 2. Three-Membered Ring Formation 50 2.1. [2 + 1] Cycloadditions Using Stoichiometric Amounts of Zinc Carbenoids (Simmons−Smith Type) 50 2.2. [2 + 1] Cycloadditions with Stoichiometric Amounts of Samarium Carbenoids 52 2.3. [2 + 1] Cycloadditions with Transition Metal-Stabilized Alkyl Carbenes of Cu, Pd, Ni, Co, and Rh 52 2.4. Cyclopropanations with Metal Carbenes Derived from R-Diazocarbonyl Compounds 54 2.5. [2 + 1] Cycloadditions with Vinylcarbenes 54 2.6. [2 + 1] Cycloadditions with Palladium Complexes of Oxatrimethylenemethane 56 2.7. Transition Metal-Catalyzed Aziridinations 57 3. Four-Membered Ring Formation 58 3.1. [2 2π + 2 2π ] Cycloadditions of Strained Alkenes with Electron-Deficient Olefins 58 3.2. [2 2π + 2 2π ] Cycloadditions of Strained Alkenes with Acetylenes 59 3.3. Intramolecular [2 2π + 2 2π ] Cycloadditions 60 4. Five-Membered Ring Formation 60 4.1. [3 2σ + 2 2π ] Cycloadditions with Metallacyclobutanes 60 4.2. [3 2σ + 2 2π ] Cycloadditions with Pd−Trimethylenemethane Complexes 61 4.3. Intramolecular [3 2σ + 2 2π ] Cycloadditions with Pd−TMM Complexes 62 4.4. [3 2σ + 2 2π ] Cycloadditions with Methylenecyclopropane 64 4.5. Intramolecular [3 2σ + 2 2π ] Cycloadditions with Methylenecyclopropanes 66 4.6. [3 4π + 2 2π ] Cycloadditions with Metal Carbenoids 67 4.7. [4 4π + 1 2π ] Cycloadditions 69 5. Six-Membered Ring Formation 69 5.1. [2 2π + 2 2π + 2 2π ] Cycloadditions 69 5.2. Acetylene [2 2π + 2 2π + 2 2π ] Cyclotrimerization Reactions 70 5.3. Acetylene−Olefin [2 2π + 2 2π + 2 2π ] Cyclotrimerizations 71 5.4. Hetero [2 2π + 2 2π + 2 2π ] Cyclotrimerizations 72 5.5. Homo-Diels−Alder [2 2π + 2 2π + 2 2π ] Cycloadditions 72 5.6. Bis-Homo-Diels−Alder [2 2π + 2 2π + 2 2π ] Cycloadditions 74 5.7. [3 2π + 3 2σ ] Cycloadditions 74 5.8. [4 4π + 2 2π ] Cycloadditions 75 6. Seven-Membered Ring Formation 78 6.1. [4 4π + 3 2σ ] Cycloadditions 78 6.2. [5 2π+2σ + 2 2π ] Cycloadditions 79 7. Eight-Membered Ring Formation 79 7.1. [2 2π + 2 2π + 2 2π + 2 2π ] Cycloadditions 80 7.2. [4 4π + 2 2π + 2 2π ] Cycloadditions 80 7.3. [4 4π + 4 4π ] Cycloadditions 81 7.4. [6 6π + 2 2π ] Cycloadditions 83 8. Ten-Membered Ring Formation 85 9. Conclusion and Remarks 87 49

Among the most common ligands found on transition metal catalysts are halide ions. Of the commercially available catalysts or pre‐catalysts, most are halo–metal complexes. In recent years, manipulation of this metal‐halide functionality has revealed that this can be used as a highly valuable method of tuning the reactivity of the complex. Variation of the halide ligand will usually not alter the nature of the system to the extent that it becomes unreactive but will impart sufficiently large changes that differences in reactivity or selectivity occur. These differences are a product of the steric and electronic properties of the halide ligand which has the ability to donate electron density to the metal occurs in a predictable manner. Despite the common perception in asymmetric catalysis that halide ligands are of secondary importance compared to chiral ligands, halide ligands have been found to exert dramatic effects on the enantioselectivity of asymmetric transformations. While the mechanism of action is known for relatively few of the cases, many intriguing and potentially synthetically useful trends are apparent. This review discusses the physical properties of the halides and their effects on stoichiometric and catalytic transition metal processes. The metal‐halide moiety thus emerges as a tunable functionality on the transition metal catalyst that can be used in the development of new catalytic systems.

Enormous effort has gone into the development of metal-catalyzed cross-coupling reactions with alkyl halides as electrophilic coupling partners. Whereas a wide array of primary alkyl halides can now be used effectively in cross-coupling reactions, the synthetic potential of secondary alkyl halides is just beginning to be revealed. This Minireview summarizes selected examples of the use of secondary alkyl halides as electrophiles in cross-coupling reactions. Emphasis is placed on the transition metals employed, the mechanistic pathways involved, and implications in terms of the stereochemical outcome of reactions.

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