Electrophilic activation of propargyl silanes is known to induce the 1,2-silyl shift,1 liberating stabilized allylic cations as intermediates. In previous studies these cationic intermediates have been subjected to 1) deprotonation for the synthesis of silyl dienes2 (occurs in the absence of strong nucleophiles (R=alkyl, aryl)) and 2) reaction with external nucleophiles, resulting in 1,3-difunctionalization.3 In this work we demonstrate the use internal nucleophiles (O-, S-, N-) to trap the in situ generated intermediate allylic cations, leading to the formation of 5- and 6-membered heterocycles with a highly stereodefined olefin side chain (scheme 1). Various electrophiles, such as Brønsted acids (HOTf, HNTf2), electrophilic halogen sources (TsNBr2, NBS, NIS) and selenyl chloride (PhSeCl), were used to induce this transformation, providing diverse functionalization for the resulting olefin side chain. The synthetic utility of the obtained products was demonstrated by double-bond geometry-preserving electrophilic substitution (C=C-Si → C=C-Hal) and cross-coupling reactions (C=C-Hal → C=C-Aryl), giving access to styrene derivatives.4 REFERENCES (1) Beļaunieks, R.; Puriņš, M.; Turks, M. Manifestation of the β-Silicon Effect in the Reactions of Unsaturated Systems Involving a 1,2-Silyl Shift. Synthesis (Stuttg). 2020, 52 (15), 2147–2161. (2) Puriņš, M.; Mishnev, A.; Turks, M. Brønsted Acid Catalyzed 1,2-Silyl Shift in Propargyl Silanes: Synthesis of Silyl Dienes and Silyl Indenes. J. Org. Chem. 2019, 84 (6), 3595–3611. (3) Beļaunieks, R.; Puriņš, M.; Līpiņa, R. A.; Mishnev, A.; Turks, M. 1,3-Difunctionalization of Propargyl Silanes with Concomitant 1,2-Silyl Shift: Synthesis of Allyl Functionalized Vinyl Silanes. Org. Lett. 2023. (4) Kroņkalne, R.; Beļaunieks, R.; Ubaidullajevs, A.; Mishnev, A.; Turks, M. 1,2-Silyl Shift-Induced Heterocyclization of Propargyl Silanes: Synthesis of Five-Membered Heterocycles Containing a Functionalized Olefin Side Chain. J. Org. Chem. 2023, 88 (19), 13857–13870.