Trimethylsilylcyanid als Umpolungsreagens, XXI. Regioselektivität der Reaktionen von substituierten α‐Siloxy‐α‐cyanallyl‐Anionen mit α,β‐ungesättigten Ketonen

Abstract

Trimethylsilyl Cyanide — A Reagent for Umpolung, XXI. — Regioselectivity of the Reaction between Substituted α‐Siloxy‐α‐cyanoallylic Anions and α,β‐Unsaturated KetonesAmbident allylic anions A and ambident α,β‐unsaturated ketones B combine to the four isomeric products D — G (Scheme 1). Systematic variation of substituents in A, obtained by deprotonation of 1a — c with LDA, and in B (→ 2a — d) as well as variation of the reaction medium and temperature reveals the following results: In diethyl ether (DE) at –78°C the kinetic products 3 (1,2/α) prevail, except with 2d. On warming rearrangement occurs to the thermodynamic products 4 (1,4/α) and/or 6 (1,4/γ) depending strongly on the substituents in both 1 and 2 (Tables 1 – 3). Adducts 6 (1,4/γ) are supposed to be formed by anionic oxy‐Cope rearrangement, since the isolated products 4 rearrange thermally to 6 in rates which reflect the yields of 6 isolated (Table 7). Substitution of DE by THF (and addition of HMPT) strongly favours adducts 4 and 6 (Table 4). With 1a + 2c a series of Lewis acids was also tested (Table 5). With MgBr2 the so far missing 1,2/γ product 5ac becomes the major one. In contrast, ClTi(NEt2)3 preserves the kinetic 1,2/α product 3ac totally even at 0°C. This behaviour is found also with other combinations of 1 + 2 (Table 6). By proper choice of the conditions the reaction between a given pair of 1 and 2 can be steered to produce one of the four possible adducts predominantly.