Perspectives of introducing aryl substituents into 1,3-functionalized cyclobutane ring by C - C cross-coupling reactions of trifluoroborates

Hryshchuk, OV
Tymtsunik, AV
1Moskvina, VS
Grygorenko, OO
1Taras Shevchenko National University of Kyiv
Dopov. Nac. akad. nauk Ukr. 2020, 5:61-69
Section: Chemistry
Language: Ukrainian

Preparative procedures for the synthesis of 1,3-functionalized cyclobutane-containing trifluoroborates bearing a protected amino- or carboxylic group are developed. The method included the reduction of the corresponding 3-functionalized cyclobutanones (i.e. tert-butyl (3-oxocyclobutyl)carbamate and methyl 3-oxocyclo butanecarboxylate) with sodium borohydride in methanol, giving the corresponding secondary alcohols. Their further Appel reaction with tetrabromomethane and triphenylphosphine provides 1,3-functionalized cyclobutane- derived bromides (57 and 43 % for two steps, respectively). The reaction of these bromides with bis(pinacolato) diboron in the presence of copper (I) bromide — triphenylphosphine complex and lithium tert-butylate, followed by treatment with potassium hydrofluoride gives the target trifluoroborates (63 and 47 % for two steps, respectively). These products are obtained with moderate diastereoselectivity (dr = 2 : 1 to 3 : 1). For the case of coupling with bromobenzene, it is shown that the obtained derivatives do not undergo the Suzuki—Miyaura reaction neither with classical tetrakis(triphenylphosphino)palladium (no reaction occurs) nor even upon the application of highly active palladium catalysts based on di(1-adamantyl)(n-butyl)phosphine (CataXCium® A) (a complex mixture of products is formed, presumably due to the β-elimination in intermediate palladium complexes). Nevertheless, the photoredox coupling is possible in the presence of dual nickel-iridium catalyst (namely, iridium complex with the 3,5-difluoro-2-[5-(trifluoromethyl)-2-pyridinyl]phenyl (dF(CF3)ppy) and 4,4′-ditert- butyl-2,2′-dipyridyl (dtbpy) ligands, [Ir{dF(CF3)ppy}2(dtbpy)]PF6, as well as nickel complex, Ni(1,2-dimethoxyethane) Cl2 — dtbpy) in the presence of cesium carbonate upon irradiation with a fluorescent lamp, which gives the target products after the removal of the protective groups in 32—43 % yield (per two steps). It is shown that, unlike for 1,2-difunctionalized cyclobutane derivatives, the C—C coupling in the case of 1,3-isomers occurs without any diastereoselectivity (dr = 1 : 1).

Keywords: cyclobutane, organoboron compounds, photoredox coupling, small carbocycles, Suzuki—Miyaura reaction, trifluoroborates

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