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Palladium-catalyzed bromoalkynylation of C-C double bonds: ring-structure-dependent synthesis of 7-alkynyl norbornanes and cyclobutenyl halides.

The development of efficient and sustainable procedures for the synthesis of complex molecules is an important task in modern organic chemistry. The direct cleavage of an alkynyl– halogen bond followed by the reconnection of both the alkynyl and halogen ions with the two carbon atoms of an unsaturated carbon–carbon bond provides ready access to highly functionalized products from simple alkynes with excellent atom economy. We previously achieved highly regioand stereoselective bromoalkynylation of internal alkynes for the synthesis of conjugated cis-bromoalkenynes. Subsequent research on this subject revealed a further use of bromoalkynes in complex molecule synthesis. Norbornene derivatives are an appealing group of organic molecules that are convenient starting materials for the synthesis of polymers, solar-energy-storage materials, and bioactive products. In addition, their strained structure and high potential to coordinate to transition metals, as well as their possible industrial applications have attracted considerable research interest. Thus, we decided to react phenylethynyl bromide (2a) with norbornene (1a), expecting to obtain 2-bromo-3-(2-phenylethynyl)bicyclo[2.2.1]heptane (3aa ; Scheme 1). However, when using the previously optimized conditions, we did not detect the formation of 3aa, instead we obtained 2-bromo-7-(2-phenylethynyl)bicyclo[2.2.1]heptane (3a) in excellent yield, as confirmed by H NMR spectroscopy. This unexpected result attracted our interest, since, to the best of our knowledge, no example of a direct 7-alkynyl bromonorbornane formation has been reported. From previous reports, we realized that after the formation of the nonclassical “norbornonium” cation, the C-7 functionalization can be achieved through a nucleophile rearrangement (Scheme 2). Our success in synthesizing C-7-