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47.1.1.1.5.2.2 Variation 2: With Phosphole-Derived Ylides

DOI: 10.1055/sos-SD-047-00004

Schobert, R.; Hölzel, C.; Barnickel, B.Science of Synthesis, (20104762.

The synthesis[‌371‌] of dibenzophosphole-based ylides such as 131 and their application in consistently E-selective alkenations of aldehydes[‌371‌] and ketones with sufficiently different branching at the α- and α-carbons has been reported.[‌470‌] Such ketones give rise to trisubstituted alkenes 132 with E/Z ratios of up to 98:2 (Scheme 45). Salt-free dibenzophosphole ethylide 131 can be generated in tetrahydrofuran by deprotonation of the phosphonium salt, which is available from commercial tetraphenylphosphonium bromide, with potassium hexamethyldisilazanide, as shown in Scheme 45.[‌371‌] The cooled ylide solution is then treated with the ketone, warmed to room temperature, and finally heated under nitrogen at 100°C for some time in order to cleave the oxaphosphetane intermediate and to induce alkene formation. A second drawback, aside from the necessity to heat the reaction mixture, is the fact that only one of the two identical P-alkyl substituents in the precursor phosphonium salt is utilized in the Wittig reaction. Bridged tetrahydrophosphole ylides such as 133 circumvent these problems.[‌372‌] They are available from a known bicyclic phosphine.[‌471‌]

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