Alkyl halides (also referred to as haloalkanes) are compounds in which one or more hydrogen atoms have been replaced by a halogen atom (fluorine, chlorine, bromine or iodine) in an alkane. Depending on how the halogen atoms are positioned on the carbon chain, alkyl halides are divided into different classes and there are some chemical differences between the various types.
Applications
Cross-coupling reaction: A new era of cross-coupling methodology dawned when the work of Suzuki, Knochel and their coworkers demonstrated the general feasibility of metal-catalyzed cross-coupling reaction with the unactivated alkyl halide. Suzuki and his coworkers reported the first palladium-catalyzed alkyl-alkyl cross-coupling of alkyl iodides with alkylboranes. Various functional groups are tolerable, and β-hydride elimination is greatly suppressed. Another important general method, which is based on the Suzuki reaction, for alkyl-alkyl cross-coupling reactions is developed by Fu and coworkers. In the past few years, the palladium-catalyzed cross-coupling reactions of alkylboranes with primary alkyl bromides, chlorides and tosylates in the presence of a large number of catalysts with a large number of electron-rich phospholene ligands have confirmed the broad scope of the reactions.
Dehydrohalogenation reaction: Dehydrohalogenation is a chemical reaction that involves the elimination of hydrogen halide from the substrate. The reaction is usually related to the synthesis of olefins, but it has broader applications. Traditionally, alkyl halides are the dehydrohalogenated substrates. Alkyl halides must be capable of forming alkenes, so methyl halides and benzyl halides are not suitable substrates. When treated with a strong base, many alkyl chlorides are converted to the corresponding alkenes. Ethyl chloride reacts with potassium hydroxide dissolved in ethanol to produce ethylene. Similarly, 1-chloropropane and 2-chloropropane give propylene as product.
Polymerization reaction: Alkyl halides can act as initiators to initiate cationic ring-opening polymerization of cyclic carbonates, with no decarboxylation taking place during this period. During the polymerization, decarboxylation occurs to yield the corresponding polycarbonate with ether units in the main chain. A decrease of molecular weight of the polymer and an increase in the ether unit ratio are observed, which strongly depends on the kind of initiators. So, alkyl halides have proven to be effective initiators for the polymerization of cyclic carbonates, which would control the decarboxylation observed usually in the cationic ring-opening polymerization of cyclic carbonate with catalysts.
Fig.1. Optimized palladium-catalyzed Suzuki coupling with alkyl bromides
Fig.2. Dehalogenation to give an alkene
References
- Frisch A C, Beller M. Catalysts for Cross‐Coupling Reactions with Non-activated Alkyl Halides[J]. Angewandte Chemie International Edition, 2005, 44(5): 674-688.
- Ariga T, Takata T, Endo T. Cationic ring-opening polymerization of cyclic carbonates with alkyl halides to yield polycarbonate without the ether unit by suppression of elimination of carbon dioxide[J]. Macromolecules, 1997, 30(4): 737-744.
