Catalytic Cyclization of o-Alkynylbenzaldehyde Acetals and Thioacetals. Unprecedented Activation of the Platinum Catalyst by Olefins. Scope and Mechanism of the Reaction

Abstract

A general protocol for the synthesis of functionalized indenes from o-alkynylbenzaldehyde acetals and thioacetals has been elaborated. Acetals uniformly give cyclization products having the alkyl group from the starting acetylene migrated to the alpha-position, whereas the cyclization of the corresponding thioacetals proceeds without alkyl migration. Optimization of the catalytic system for the cyclization of o-alkynylbenzaldehyde acetals revealed an unknown activation effect: PtCl(2) was found to be a better catalyst for the cyclization of acetals in the presence of olefins than without. A similar catalytic system (PtCl(2)/benzoquinone) has been found to be appropriate for the cyclization of cyclic acetals, whereas the optimal catalyst for the reaction of thioacetals is PdI(2). NMR monitoring of two reactions, acetal 3a + Pd(CH(3)CN)Cl(2) in CD(3)CN and thioacetal 5j + PdI(2) in CD(2)Cl(2), revealed that in both reactions similar cationic species are formed at the early stage of the transformation. Computational data (B3LYP/SDD level of theory) suggest that the difference in the reaction pathways for acetals and thioacetals can be rationalized by taking into account the relative stabilities of the corresponding vinylpalladium intermediates (22 vs 20 and 19 vs 21), which suggests a reversible thermodynamically controlled alkyl migration in the intermediate vinylcationic species.

Katalytische carbophile Aktivierung: Platin‐ und Gold‐π‐Säuren als Katalysatoren

“…Nur elektronenziehende und kinetisch labile Einheiten wie COD, [161] b-Pinen [164] und besonders CO, [151,166] haben zu signifikanten Verbesserungen geführt. Obgleich chirale kationische Platinkomplexe in der Literatur gut bekannt sind, [101] finden sich auffällig selten erfolgreiche Anwendungen auf die Aktivierung von Alkinen.…”

“…Anschließend wird einer der Substituenten vom Heteroatom auf die metallierte Position verschoben, was zum experimentell beobachteten Heteroatom!Kohlenstoff-Transfer führt. [160] Auch wenn der [151,166] oder Olefine (COD, b-Pinen, Benzochinon) [164] können sich dabei positiv auf die Effizienz auswirken. Es wurde vorgeschlagen, dass durch Verwendung von COD die polymere Struktur des eingesetzten PtCl 2 teilweise aufgebrochen wird, was die Zahl katalytisch aktiver Zentren in Lösung erhöhen sollte; [165] diese Interpretation steht aber unter Vorbehalt, da sich monomeres [PtCl 2 (cod)] als katalytisch inaktiv erwies.…”

Research Progress in Metal-Catalyzed Addition of Carbon-Hetero Bonds to Alkynes

“…Yamamoto 小组 [52] 图式 25 金催化 C-S 键对炔键的分子内加成 Scheme 25 Au-catalyzed intramolecular addition of C-S bond to alkynes 2012 年, Willis 小组 [57] 实现了铑催化的苯基甲基硫 醚 150 中的 C-S 键对端炔 151 的分子间加成(Eq. 2), 该 反应具有专一的区域选择性, 硫原子只加成到非末端炔 碳上, 该反应还具有专一的立体化学选择性, 只得到顺 式加成产物 152.…”

π-Conjugated carbocycles and heterocycles via annulation through C-H and X-Y activation across CC triple bonds

“…82 A similar catalytic system, PtCl 2 -benzoquinone, was also active for the cyclization of related acetals, whereas the optimal catalyst for the reaction of thioacetals was PdI 2 ; ortho-alkynylaryl thioacetals, which did not undergo a 1,2-alkyl shift, gave 1,3-di(alkylsulfonyl)indenes. 86 Contrary to the neutral palladium(II), ortho-alkynylaryl acetals 16 was converted to 1,1-dialkoxyindenes 18 with cationic palladium(II) catalyst system such as Pd(CH 3 CN) 4 (BF 4 ) 2 -PPh 3 (Scheme 5). 87 Mechanistic study using DFT calculations suggested that -coordination of cationic Pd + to the benzene ring would be a key step; a stepwise delivery of two methoxy groups would take place via the palladium cationic center coordinated to the aromatic ring.…”