An Alkyne Metathesis-Based Route toortho-Dehydrobenzannulenes

Abstract

An application of alkyne metathesis to 1,2-di(prop-1-ynyl)arenes, producing dehydrobenzannulenes, is described. An efficient method for selective Sonogashira couplings of bromoiodoarenes under conditions of microwave irradiation is also reported. Keywords:alkyne metathesis, 1,2-di(prop-1-ynyl)arenes, dehydrobenzannulenes, microwave irradiation, Sonogashira coupling Alkyne metathesis (Scheme 1) has not been used widely in organic syntheses until recently, even though the first reports of its homogeneous, catalytic version date back to as early as 1974 with results of Mortreux et al. 1 A resurgence of the field occurred in the 1990s, based on the work by the groups of Bunz 2 and Fürstner, 3 which includes examples of ring closures, one notably leading to a meta-dehydrobenzannulene system. 2e We report here the investigation of the potential of alkyne metathesis in the construction of dehydrobenzannulenes (Scheme 2). 4 This class of molecules is interesting in several respects -it provides attractive ligands to transition metal complexes, 5 models for subunits of graphyne, a novel allotrope of carbon, 6 precursors to ordered carbon nanostructures, 7 scaffolds for molecules on which to study supramolecular phenomena, 8 and materials with interesting photophysical properties. 9 Our need for an efficient synthetic entry into substituted derivatives stems from our quest for the first members of the circular phenylenes, such as antikekulene. 10 As representative targets, we directed our initial efforts to examples of tribenzocyclyne 1, as in Scheme 3, and subsequently the more challenging tetrabenzocyclyne 2, as well as the fused systems 3 and 4. The parent 1a and its derivatives have received considerable attention as synthetic targets, 4 whereas a substituted version of 2 8f and parent 3 6c have been prepared for the first time only recently, the latter two through relatively long sequences.

Controlled Microwave Heating in Modern Organic Synthesis

“…[85] Essentially the same experimental protocol was employed by Vollhardt and co-workers to synthesize o-dipropynylated arene 8, which served as the precursor to tribenzocyclyne 9 through an alkyne metathesis reaction (Scheme 8). [86] In this case the Sonogashira reaction was carried out in a prepressurized (ca. 2.5 atm of propyne) sealed microwave vessel.…”

Konjugierte Makrocyclen: Konzepte und Anwendungen

“…[55] Vollhardt und Mitarbeiter beschrieben die Synthese von 65 a mithilfe einer sechsfachen intermolekularen Methathesereaktion in nur drei Stufen in 6 % Ausbeute. [56] Aufgrund von Lçslichkeits-problemen konnten die Eigenschaften von 65 a allerdings nicht bestimmt werden. Iyoda und Mitarbeiter synthetisierten die octabutylsubstituierte Verbindung 65 b durch eine kupfervermittelte Kreuzkupplung in Gegenwart von PPh 3 (1 %).…”

Let the Best Ring Win: Selective Macrocycle Formation through Pd‐Catalyzed or Cu‐Mediated Alkyne Homocoupling

“…Precursor 6 was constructed by Sonogashira crosscoupling of diyne 7 a [5a, 14] to 1,2-dibromo-4,5-diiodobenzene, [15] thus providing an ortho substitution of the alkynes on the central ring. Protiodesilylation of the triisopropylsilyl groups by Bu 4 NF and MeOH followed by slow injection of this precursor into a pyridine solution of Cu(OAc) 2 furnished DBA 4 in a low yield (24 %) along with large amounts of oligomeric/polymeric by-products.…”