Addition of Aromatic C−H Bonds to Terminal Metal−Carbonyl Groups. A New Method for Cyclocarbonylation of Aromatic Groups

Abstract

Substitutive carbonylation of the aryl C-H bond to form an aryl ketone group is a useful reaction in organic synthesis. 1 Such syntheses are generally carried out through treatment of aromatic compounds with carboxylic acid derivatives in the presence of acid catalysts (Friedel-Crafts acylation) as shown in Scheme 1 (eq 1). 1 Intramolecular Friedel-Crafts reaction is an important tool for the syntheses of natural pericyclic ketones. 2 Nucleophilic attack of a metal-coordinated carbonyl group is an important step in metal-mediated organic syntheses. 3,4 Although this synthetic method covers a wide range of nucleophiles, 3,4 to our best knowledge, there is no precedent for attack of a neutral aryl group at a metal-carbonyl ligand to effect a substitutive carbonylation (Scheme 1, eq 2). A cationic metalcarbonyl group may serve as a good candidate to realize this carbonylation reaction because its structure resembles the intermediate in Friedel-Crafts reaction. We report here a new cyclocarbonylation for propargyltungsten compounds having a tethered aryl group. These cyclizations show the feasibility for addition of aryl C-H bonds at coordinated carbonyl groups.As shown in Scheme 2, alkynyltungsten compound 2 was conveniently prepared from the corresponding propargyl tosylate 1 and CpW(CO) 3 Na in 80% yield. 5 Treatment of compound 2 with triflic acid (25 mol %) in cold CH 2 Cl 2 (-95°C) gave tungsten-π-allyl compound 3a and 3b in 40% and 32% yields, respectively, after separation from a silica column. The molecular structure 6 of 3b shows that the molecule has a cyclopentenone fragment fused with a phenyl ring, and the CpW(CO) 2 fragment is bound to the ketone ring in a metal-π-allyl fashion. This information indicates that the aryl C-H bond of compound 2 is cleaved and inserted by a CO group to effect a substitutive carbonylation. We also examined the same operation on alkynyltungsten complex 4 (triflic acid, 0.25 equiv, -95°C) to give two cyclohexenonyl derivatives, 5a and 5b, in 21% and 24% yields, respectively, after separation on a silica column. The molecular structures of 5a and 5b 7,8 are also elucidated by X-ray diffraction studies to confirm the formation of a tungsten-π-cyclohexenonyl complex.We also prepared various alkynyltungsten compounds 6-11 tethered with an aromatic group to generalize the cyclizations; the yields in Table 1 are estimated after separation on a silica column. Cyclocarbonylation of these alkynyl compounds 6-11 follows the same reaction sequence involving the use of triflic acid catalyst (0.25 equiv) in cold CH 2 Cl 2 (-95°C). Entry 1 shows an intramolecular carbonylation of a tethered 2,5-dimethoxyphenyl group to yield the tungsten-π-cyclopentenonyl complex 12 in 55% yield. The same reaction sequence on compound 7 gave a 44% yield of π-cyclohexenonyl complex 13, as shown in entry 2. Similarly, treatment of alkynyltungsten species 8 and 9 with triflic acid catalysts (0.25 equiv) gave the cyclocarbonylation products 14 and 15 in 52% and 45% yields, respectively. † National Tsing Hua Un...