A Domino Approach to Dibenzopentafulvalenes by Quadruple Carbopalladation

Abstract

Owing to their special electronic properties cross-conjugated systems have experienced a renaissance in recent years, [1] and also pentafulvalene and its substituted analogues have come back into focus. [2,3] Originally regarded as laboratory curiosities displaying nonplanar structures and unusual bond lengths and angles, [4] they have attracted attention as ligands [5] for transition metals and as two-electron acceptors affording aromatic dianions. [6] Whereas the parent fulvalene is unstable at temperatures above À78 8C, [7] the perchloro derivative shows completely reversible redox behavior. [6b] Further investigations with fulvalene vinylogues [6b] indicate properties conducive to tuning the performance of organic-based batteries on a molecular level.The preparation of symmetrical pentafulvalenes has been realized for the most part by the coupling of two cyclopentadienyl rings, involving strong lithium bases and often molecular oxygen as the oxidant or CuCl 2 as the coupling promoter. [7,8] Other couplings have been performed by the Grignard addition to ketones [9] and homo couplings employing halogenated species and transition metals. [10] Also diazo compounds, which readily release nitrogen, have served as starting materials. [11] An alternative synthetic pathway investigated in the last decade accesses dibenzopentafulvalenes 1 by reductive Bergman-type cyclizations of cross-conjugated enediynes 2 (Scheme 1, top). [12] Highly substituted unsaturated oligocyclic systems can also be prepared by carbopalladation reactions. In the last two decades a number of domino carbopalladation sequences have been developed, yielding homo-and heterocyclic systems such as lactones, chromanes, molecular switches, and further complex systems in one single step from relatively simple starting materials. [13][14][15] Herein we disclose a novel dibenzopentafulvalene synthesis which affords the pentafulvalene framework in only one step through a quadruple carbopalladation process. Our retrosynthetic strategy relied on a readily available biphenylacetylene and two tethered alkyne moieties 4 (Scheme 1, bottom). We envisioned that a sequence starting with the oxidative addition of Pd into the CÀBr bond followed by three carbopalladation steps to the CÀC triple bonds and finishing with a final carbopalladation to the newly formed alkene would provide the dibenzopentafulvalene skeleton 3.To test our approach, we chose the biphenyl derivative 7 which was coupled with various diynes (Scheme 2). Biphenyl 7 was synthesized in two steps by the Suzuki-Miyaura coupling of the trisubstituted arene 5 and boronic acid 6 followed by removal of the silyl protecting groups in 75 % overall yield. Various diynes were obtained by Sonogashira cross-coupling of terminal diyne 8; the respective alcohols were transformed by Appel reaction into propargylic iodides 9. Final nucleophilic substitution of 9 by the phenolic hydroxy group in 7 afforded the domino substrates 4 in good to excellent yields.We initially chose substrate 4 a to optimize the rea...

Intramolecular trans‐Dicarbofunctionalization of Alkynes by a Formal anti‐Carbopalladation/Stille Cascade

“…Since the attack of the organopalladium species [Pd]‐R commonly takes place in a syn ‐fashion, Pd and R are located on the same side of the emerging double bond . Embedded in domino reactions, syn ‐carbopalladations of alkyne units have paved the way to a variety of (oligo)cyclic compounds such as steroid‐like scaffolds, chromanes, fenestranes, pentafulvalenes, molecular switches, truncated helicenes, persubstituted cyclooctatetraenes, hetero‐ and spirocycles, and several natural products…”

Access to Indene Derivatives by a Sequence of Intermolecular anti‐Carbopalladation, Heck Reaction, and Electrophilic Attack

“…Using water as additive or using other bases instead of triethylamine also resulted in lower yields ( Table 1, entries 8-10). The use of other solvents such as DMSO, DME, or toluene was also unsuccessful, so DMF was confirmed as the solvent of choice (Table 1, entries [11][12][13][14]. Furthermore, when bromobenzene was used instead of the corresponding iodobenzene, the domino product was only obtained in trace amounts, as detected by GC.…”

“…Such a species commonly undergoes further transformation, e.g., a second carbopalladation or a terminating process such as a cross-coupling reaction. Complex scaffolds such as molecular switches, [7] fenestranes, [8] chromans, [9] biphenyls, [10] pentafulvalenes, [11] and oligoene-based π-helicenes, [12] and also other heterocyclic systems [13] and natural products [14] have been accessed by such an approach.…”

Intramolecular Pd‐Catalyzed Formal anti‐Carboalkoxylation of Alkynes: Access to Tetrasubstituted Enol Ethers

“…Initially, carbopalladation reactions of alkynes were integrated in cascades (domino processes) leading to steroid derivatives and highly substituted arenes . Recently, novel domino processes with one or several carbopalladations as key reactions have been designed affording highly complex natural products or natural product analogs, and also other more specific targets such as spirocycles, (oligo)cyclic compounds, dibenzopentafulvalenes, helical oligoenes, fenestranes, persubstituted cyclooctatetraenes, and molecular switches . Because the emerging organopalladium species, which is formed at the beginning of such a cascade, commonly attacks the carbon–carbon triple bond in a syn ‐fashion, both residues are located at the same side of the newly generated double bond…”