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6.1.37.10 Method 10: Cyclopropane Ring Formation via Hydroboration of Allylic Halides

DOI: 10.1055/sos-SD-006-00890

Abu Ali, H.; Dembitsky, V. M.; Srebnik, M.Science of Synthesis, (200561093.

γ-Haloalkylboranes are the major intermediate products during the formation of cyclopropanes from the hydroboration of allylic halides. This cyclization of hydroborated allylic chlorides to form the corresponding cyclopropanes was discovered in 1958[‌11‌] and subsequently applied to the synthesis of various cyclopropanes.[‌37‌] If 9-borabicyclo[3.3.1]nonane is used as the hydroborating agent of haloalkenes or haloalkynes, the yield of cyclopropanes is increased.[‌29‌] γ-Haloalkylboranes such as 50 are intermediates (Scheme 20). Propargyl bromide has also been used[‌30‌,‌31‌] for dihydroboration with 9-borabicyclo[3.3.1]nonane, resulting in formation of 9-cyclopropyl-9-borabicyclo[3.3.1]nonane (51) via the open-chain γ-bromoalkylborane 50 (X=Br) intermediate (Scheme 20). Treatment of the intermediate adduct with aqueous sodium hydroxide gives the cyclopropyl(hydroxy)borate complexes, which undergo efficient palladium-catalyzed cross coupling to produce a variety of aryl- and vinylcyclopropanes in good to excellent yields.[‌32‌]

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