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25.8.1.1.16.2 Variation 2: Cross-Coupling Reactions

DOI: 10.1055/sos-SD-025-00531

Escher, I.; Glorius, F.Science of Synthesis, (200725758.

3-Halopropenals, which are readily available by the VilsmeierHaackArnold reaction (see Section 25.8.2.1.4.1), are valuable building blocks in transition-metal-catalyzed cross-coupling reactions (Scheme 33). For example, palladium-catalyzed cross-coupling of a variety of arylboronic acids with 3-chloropropenals can be carried out in aqueous media, to give 2-enals such as 1-(2-methoxyphenyl)-3,4-dihydronaphthalene-2-carbaldehyde (67),[‌177‌] in yields that are normally good to excellent. Trialkylated alkynylstannanes react by palladium-catalyzed cross-coupling reactions with 3-iodopropenals to give alkynylated enals 68, as do the corresponding alkynylsilanes. Purification of the products is more difficult in those reactions that use trialkylated alkynylstannanes, while the alkynylsilanes are less-reactive, and need to be activated with tetrabutylammonium fluoride prior to their use in the coupling step.[‌178‌,‌179‌] Simple terminal alkynes also undergo palladium-catalyzed cross-coupling with 3-bromopropenals under Sonogashira reaction conditions to afford alkynylated enals 69.[‌180‌] 1-Halo-2-nitroarenes may also be successfully cross-coupled with α-halogenated enals in a palladium-catalyzed Ullmann cross-coupling reaction. Thus, in a specific example of this type, the stilbene 70 is available from 1-bromo-2-nitrobenzene and α-iodocinnamaldehyde (Scheme 33).[‌181‌]

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