Phospha-Michael Additions to Activated Internal Alkenes: Steric and Electronic Effects

Abstract

The addition of P(O)-H bonds to internal alkenes has been accomplished under solvent-free conditions without the addition of a catalyst or radical initiator. Using a prototypical secondary phosphine oxide, a range of substrates including cinnamates, crotonates, coumarins, sulfones, and chalcones were successfully functionalized. Highly activated acceptors such as isopropylidenemalononitrile and ethyl 2-cyano-3-methyl-2-butenoate underwent the phospha-Michael reaction upon simple trituration of the reagents at room temperature, whereas less activated substrates such as ethyl cinnamate and methyl crotonate required heating (>150 °C) in a microwave reactor to achieve significant consumption of the starting alkenes. For the latter alkenes, a competing reaction involving disproportionation of the ditolylphosphine oxide into ditolylphosphinic acid and ditolylphosphine was observed at the high temperatures needed to promote the addition reaction.

Crystal Structure of Bis(2,4,6-trimethylphenyl)-phosphine Oxide

“…These compounds are precursors to phosphinoyl radicals; they are effective reagents in phosphorylation reactions [10][11][12] due to the accessible P-H bond dissociation energies (ca. 80 kcal mol −1 ) [13], and also participate in olefin addition reactions [14,15]. Given the utility of these compounds, it seemed pertinent to report the single crystal structure of the title compound bis(2,4,6-trimethylphenyl)phosphine oxide, 1.…”

Aerobic addition of secondary phosphine oxides to vinyl sulfides: a shortcut to 1-hydroxy-2-(organosulfanyl)ethyl(diorganyl)phosphine oxides

“…Conventionally, the activation of the P–H bonds in this reaction is achieved by using radical initiators [13–15], Brønsted/Lewis acids [16–17] and bases [18–20] as well as transition metal catalysts [21–23]. Also, examples of the microwave-assisted [24–25] and photoinduced [26] addition are described.…”

Catalyst‐ and Additive‐Free Decarboxylative C‐4 Phosphorylation of Coumarin‐3‐Carboxylic Acids at Ambient Conditions

“…An extensive literature survey revealed just two earlier reports on the direct C‐4 phosphorylation of coumarins to access C‐4 phosphorylated chroman‐2‐ones [12] . In 2012, Lenker et al [12a] . for the first‐time reported the microwave‐assisted synthesis of only two such derivatives.…”