Palladium-Catalyzed Reactions of Hypophosphorous Compounds
catalyzed cross-coupling of an alkyl phosphinate with an allylic
halide (eq 1).7 Recently, Zhao reported a single example of a
Cu-cross-coupling between iodobenzene and ammonium hy-
pophosphite.8
alkylation with allylic halides,9 subsequent studies led us to
discover more efficient routes to prepare these compounds from
readily available starting materials and, ultimately, allylic alcohol
feedstocks (eq 2).10 Herein, we not only unveil new reactivity
patterns of hypophosphorous compounds with allylic electro-
philes and unsaturated substrates, but we also report on useful
Pd-catalytic methods to prepare allylic-H-phosphinic acids and
H-phosphinate esters, such as hydrophosphinylation, allylation,
rearrangement, and tandem allylation-esterification. The syn-
thetic flexibility of H-phosphinates is also illustrated with the
synthesis of P-heterocycles.
Although the allylic-H-phosphinates accessible through this
route could be more conveniently prepared by the base-promoted
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Results and Discussion
Reactivity of Allenes and Dienes in the Pd-Catalyzed
Hydrophosphinylation Reaction. Our work began with an
investigation of the reactivity of hypophosphorous acid (H3-
PO2-1a) and its anilinium salt (PhNH3OP(O)H2-1b) in the
Pd-catalyzed addition to allenes and dienes (Tables 1 and 2).
Pd2dba3/xantphos (1 mol %) was selected as the catalytic system
based on our previous work with alkenes and alkynes.5a The
reaction solvent proved also important: acetonitrile and DMF
gave the best results in reactions performed with H3PO2, while
DMF was required with amine salts of hypophosphorous acid.
Isolation of the products consisted of a simple acidic extractive
workup. Allenes were synthesized in moderate yields by
homologation of acetylenes11 or by Pd-catalyzed hydrogenolysis
of alk-2-ynyl carbonates with ammonium formate12 and then
reacted in the Pd-catalyzed hydrophosphinylation reaction. As
indicated in Table 1, a 3,3-disubstituted allene (entry 1) and
the aromatic monosubstituted phenylallene (entry 2) reacted
regioselectively and with high E-selectivity (>98:2 E/Z) to give
allylic-H-phosphinic acids in good yields. On the other hand,
the monosubstituted aliphatic cyclohexylallene reacted with low
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