Hydrocarbonylation reactions are pivotal in organic chemistry, with wide-ranging implications in fields like agriculture, polymer chemistry, petrochemistry, biochemistry, and medicinal chemistry. Particularly in drug discovery, alkylation reactions, especially involving methyl groups, play a key role in enhancing potency, modulating half-life, improving solubility, and altering binding affinity. However, conventional alkylating and alkenylating agents often pose substantial health and exposure risks due to their toxicity, mutagenicity, carcinogenicity, flammability, and explosiveness. Quaternary ammonium salts (QAS) present a promising alternative for these transformations while posing significantly lower hazards. QAS used for hydrocarbonylation reactions have short organic residues and inorganic anions, making them crystalline solids, easy-to-handle, air and moisture-stable, with reduced inhalation risk. They offer lower health risks, being non-cancerogenic, non-mutagenic, non-flammable, and non-corrosive. Despite their potential, their use in direct organic transformations remains underexplored.This thesis explores several strategies for harnessing QAS as alternative alkylating and allylating reagents. The development of these methodologies not only expands the toolbox of synthetic chemists but also contributes to safer and more sustainable practices in chemical synthesis.Phenyl trimethylammonium iodide was established as methylating agent for the mono-selective methylation at the α-carbon of aryl ketones, offering a safer alternative to toxic reagents like methyl iodide or dimethylsulfate. This methodology has demonstrated broad applicability across various substrates with different functional groups, giving the α-methylated products in up to 85 % yield. Moreover, the protocol has been extended to facilitate α-C-ethylation and -benzylation using phenyl triethylammonium iodide or benzyl trimethylammonium chloride, respectively.We further extended this protocol to facilitate the N-methylation and N-ethylation of primary amides. In general, nitrogen-containing motifs are prone to over-alkylation due to the increasing nucleophilicity of the nitrogen with higher degrees of substitution. To control the degree of substitution in primary amines, amides, and sulfonamides, several strategies have been developed and are comprehensively presented in the review on “Mono-Selective N-Methylation, N-Ethylation, and N-n-Propylation of Primary Amines, Amides, and Sulfonamides and Their Applicability in Late-Stage Modification”.Within the scope of this pre-doctoral study, we established a strategy for mono-selective alkylation of primary amides using phenyl trialkylammonium iodides. Presumably the steric bulk of this alternative alkylating agent hampers a second alkylation event, offering superior control over the degree of substitution compared to conventionally used alkylating agents like methyl iodide. This novel protocol could be further expanded to include indoles and related structures and proved suitable for late-stage methylation of bioactive compounds. In our most recent development, we leveraged the leaving group ability of quaternary ammonium moieties in allylic position for the allylation of O-, N-, and C-nucleophiles in a palladium-catalyzed Tsuji-Trost reaction. The reactions were conducted mechanochemically in a mixer mill, eliminating the use of solvent and significantly reducing the environmental impact. This approach offers short reaction times, low catalyst and ligand loading, mild conditions, high selectivity, and excellent functional group tolerance. Several structurally complex molecules, including bioactive compounds, could be allylated in excellent yields and high purity. Furthermore, the feasibility of an enantioselective reaction using chiral ligands has been demonstrated, setting a starting point for further research in this area.