Co-Catalyzed Tandem Pauson-Khand Type Reaction/Dimerization of Terminal Diynes in the Presence of Cyclopentadiene under CO Pressure

Abstract

Transition metal-catalyzed tandem reactions have attracted much attention due to their operational and economical efficiency in the synthesis of complex organic molecules. 1,2 In particular, if an intermediate is highly reactive, its further transformation in a single operation will broaden the utility of the highly reactive intermediate molecule in organic synthesis. Synthesis of novel cyclic compounds through cobalt carbonyl-catalyzed tandem cyclization including the Pauson-Khand and the related reaction has been one of our current research themes.3 Recently, we reported the utilization of the highly reactive cyclopentadienone, 4 which was produced by a Pauson-Khand reaction, for the synthesis of polycyclic compounds in a tandem fashion. Figure 1. Whereas internal diynes, produced only the [4+2] cycloaddition product in the Co-catalyzed tandem cycloaddition reaction with conjugated dienes, 3b it was revealed in this study that terminal diynes, such as 1,6-heptadiene, resulted in a [2+2+2] and [4+2] cycloaddition product mixture when the reaction occurred in the presence of cyclopentadiene. Higher reactivity of terminal alkynes compared to internal alkynes may be attributed to the competition of diyne with cyclopentadiene in the quenching step of the tandem catalysis. With this in mind, we carefully examined the tandem reaction for α,ω-diynes and mono-substituted diynes and found that the steric and electronic properties of the diyne could control the reaction pathway between the [4+2] and [2+2+2] cycloadditions in the second step of the reaction scheme (Scheme 1).Adding diyne 1 (0.8 mmol) and cyclopentadiene (5 equiv.) to dichloromethane (30 mL) containing Co 2 (CO) 8 The ratio of the products in the mixture varied according to the diyne used (Table 1).Competition between the diyne and cyclopentadiene at the second cycloaddition step after the production of a reactive cyclopentadienone intermediate by the carbonylative cycloaddition reaction in the presence of tandem catalysis was greatly influenced by the type of tether group in the terminal 1,6-diynes. In the reaction of the diynes containing a Ctether at the 4-position, carbonylative dimerization of the diyne occurred preferentially over carbonylative [4+2] cyclization of the diyne and cyclopentadiene (entries 1 and 2). Interestingly, cyclopentadiene did not take part as a diene in the cyclopentadienone quenching step in reaction of 1b at all owing to the higher reactivity of dimerization of the diyne compared to cyclization with cyclopentadiene. In the reaction of the diyne (1c) containing a N-tether at the 4-position, the chemoselectivity of the reaction was reversed to give the carbonylative [4+2] cycloadduct (3c) as a major product (entry 3). In order to investigate the chemoselectivity of the Communications to the Editor catalysis, mono-phenyl substituted diynes at one alkyne terminus were also tested as a substrate. As expected, the carbonylative [4+2] cycloadduct (3d) was the sole product in the reaction of mono-phenyl substituted diyne havin...