Potassium but-3-enyltrifluoroborate is an organotrifluoroborate salt, characterized by a potassium cation paired with a but-3-enyltrifluoroborate anion. This compound is a stable, crystalline solid that serves as a versatile and air-stable source of the but-3-enyl nucleophile, offering significant advantages over more pyrophoric traditional organometallic reagents like Grignard or organolithium compounds. Its primary application is in organic synthesis, particularly in transition-metal-catalyzed cross-coupling reactions, such as the Suzuki-Miyaura coupling. The terminal alkene group in its structure provides a valuable synthetic handle for further functionalization through reactions like hydroboration, epoxidation, or metathesis, enabling the rapid construction of complex molecular architectures. This makes it a valuable building block for medicinal chemistry and materials science research. As a bench-stable reagent, it facilitates safer and more convenient handling in laboratory settings. It is commonly used to introduce the but-3-enyl moiety into various aromatic and heteroaromatic systems, making it a fundamental tool for synthetic chemists exploring carbon-carbon bond formation strategies.