Gold(I)‐Catalyzed Synthesis of Functionalized Cyclopentadienes

Abstract

mechanistic pathways: a concerted rearrangement (path a) involving direct addition of the olefin on the epoxide or a stepwise mechanism (path b) through a Nazarov cyclization of an oxypentadienyl cation (2, X=O ). The regioselectivity of the cyclization is dictated by donation of the oxyanion into the resulting cation leading to the formation of a ketone. Recently, stabilization of developing positive charge through back-bonding from phosphinegold(I) complexes has been implicated in a number of rearrangement reactions. Therefore, we hypothesized that coordination of cationic phosphinegold(I) complexes to a vinyl allene might mimic these reaction pathways through similar back-bonding, leading to metal–carbenoid intermediate 3 (X=R3PAu ). These intermediates would further rearrange into substituted cyclopentadienes, important building blocks in organic and organometallic chemistry. 7] In light of our recent success in using [Ph3PAuCl] with AgSbF6 in dichloromethane for carbon–carbon bond-forming reactions, we chose this system for preliminary studies of the proposed cycloisomerization (Table 1). Treatment of vinyl allene 4 with 2 mol% cationic triphenylphosphinegold(I) afforded the desired cyclopentadiene 5 as a single regioisomer in 97% yield after 1 min at 0 8C (Table 1, entry 1). Similar results were obtained when a lower temperature or lower catalyst loading were used (Table 1, entries 2 and 3). Control experiments employing either 5 mol% [Ph3PAuCl] or 5 mol% AgSbF6 as the sole catalyst did not lead to any conversion of 4 into 5 (Table 1, entries 4 and 5). Other transition-metal complexes showed no catalytic activity; however, gold(III) chloride rapidly consumed 4 to afford a small amount of 5 (Table 1, entry 6). With optimal conditions in hand, the scope of the gold(I)catalyzed cycloisomerization of vinyl allenes was examined. We were pleased to find that the reaction allowed for the regiospecific synthesis of functionalized cyclopentadienes in high yields with a variety of substitution patterns (Table 2). Substitution at the allene terminus was well tolerated, encompassing linear alkyl (Table 2, entries 8 and 9), oxygenated (entries 3–7), secondary benzyl (entry 1), and phenyl substituents (entry 2). Notably, the gold(I)-catalyzed reaction can be easily carried out on a gram scale albeit with a slightly diminished yield (Table 2, entry 1). Furthermore, the stability of acid-labile protecting groups, such as tetrahydropyranyl (Table 2, entry 9) and silyl ethers (entries 3, 4, 6, and 7), isopropylidene acetal (entry 5), and an N-Boc amine (entry 6), is a testament to the mildness of the reaction conditions. Bicyclic cyclopentadienes are readily produced from the cycloisomerization of vinyl allenes containing cyclic alkenes (Table 2, entries 1–6). Additionally, the gold(I)catalyzed reaction can be employed for the synthesis of cyclopentadienes with a quaternary carbon center (Table 2, entries 2 and 3). The use of a more electron-rich gold(I) complex, [tBu3PAuCl], as a catalyst gave improved yields for some vinyl allenes (Table 2, entries 5, 6, 8, and 9). For example, switching the gold catalyst from [Ph3PAuCl] to [tBu3PAuCl] resulted in an improved yield for the formation of cyclopentadiene 21 (Table 2, entry 8). Table 1: Catalyst optimization.