Low‐Temperature Olefin Syntheses in View of Parent Fulvenes and Fulvalenes

Abstract

AbstractParent fulvenes and fulvalenes are thermally unstable cross‐conjugated olefins for which low‐temperature syntheses are indispensable. In this review 5 syntheses (in the temperature range between −100 and −10°) are discussed: 1. Reaction of sodium cyclopentadienide with 1‐acetoxy‐1‐chloroalkanes or 1‐acetoxy‐1‐bromoalkanes (26) gives acetoxy‐alkyl‐cyclopentadienes (27) which are easily converted to pentafulvenes (2) by low‐temperature HOAc‐elimination with NEt3. This synthesis has been applied to parent pentafulvene (2a), heptafulvene (3a), nonafulvene (4a) and sesquifulvalene (19a) (Schemes 8–11). 2. Based on a nearly quantitative oxidative coupling of cyclononatetraenide (8) to give dihydrononafulvalene (38) (Scheme 10), a general synthetic plan for fulvalenes has been outlined (Scheme 11) and applied to the synthesis of pentafulvalene (12), nonapentafulvalene (16) and nonafulvalene (14). Several applications of oxidative couplings of Hückel anions are discussed (Schemes 20 and 21). 3. Trifunctional cyclopropanes 67 (in most cases 1,1‐dibromo‐2‐X‐cyclopropanes) are attractive precursors of parent triafulvene (1a) and calicene (17) (Scheme 18). Contrary to classical procedures they are transformed into nucleophiles (67→68) by halogen‐lithium exchange, methylation (68→69) and HBr‐elimination to give 1‐methylidene‐2‐X‐cyclopropanes of type 71. By subsequent HX‐elimination triafulvene (1a) has been synthesized and trapped as a [4+2]‐cycloadduct 73 (Scheme 20). Furthermore, calicene precursors 77 are available by using cyclopentenone as an electrophilic cyclopentadiene equivalent. 4. Similarly, 1‐lithio‐1‐bromo‐2‐X‐cyclopropanes 68 are directly transformed into triafulvalene precursors 81 (Scheme 26) by a novel CuCl2‐catalyzed oxidative coupling. 5. In view of the synthesis of parent triafulvene (1a), triafulvalene (11) and calicene (17), retro‐DielsAlder reactions of stable precursors – prepared by low‐temperature reactions (described in chapters 3 and 4) – have been explored.