CL-140885
Received: September 22, 2014 | Accepted: October 16, 2014 | Web Released: January 5, 2015
Reactions of a Stable Phosphinyl Radical with Stable Aminoxyl Radicals
Shintaro Ishida,* Fumiya Hirakawa, and Takeaki Iwamoto*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578
(E-mail: iwamoto@m.tohoku.ac.jp)
Reaction of stable phosphinyl radical 1a with AZADO gave
indicated formation of a sole diamagnetic product.7,8 The NMR
data confirmed that the observed species at ¹40 °C was
aminoxyphosphine 3; the observed NMR spectra are consistent
with the existence of two kinds of trimethylsilyl groups,
phosphacyclopentane ring, and 2-azaadamantyl moiety. More-
over, the singlet 31P signal observed at 186.1 ppm was well
reproduced by the calculated 31P chemical shift (213.4 ppm) of
the optimized structure of 3 (3opt) by DFT calculations. With
increasing temperature of the resulting solution, compound 3
decomposed to cyclic phosphorane 4, silyl phosphinate 5, and
aminophosphine 6 in 82%, 7%, and 5% yields, respectively,
with trace amounts of unidentified products. Interestingly, when
a similar reaction was conducted at room temperature, the yields
of the products were considerably changed (4, 28%; 5, 27%; 6,
35%). The reaction of 1a with two equivalents of AZADO gave
compound 5 in 61% yield. Products 4-6 did not undergo further
reactions with AZADO at room temperature.
aminoxyphosphine 3 as the primary product by selective radical
coupling at ¹40 °C. Compound 3 decomposed to phosphorane
4, silyl phosphinate 5, and aminophosphine 6 at room temper-
ature. The molecular structures of 4-6 were determined by X-ray
structural analysis. The homolytic N-O bond cleavage of 3 and
the subsequent silyl migration of the resulting phosphinoyl
radical 7 would be key steps in the reaction.
Phosphorus-centered dicoordinate radicals (phosphinyl
radicals, R2P•) have a bifunctionality owing to the lone pair
and unpaired electron on the same phosphorus atom, whose
reactivity deserves much attention.1-3 Although reactions of
phosphinyl radicals with alkenes, C-X bonds (X = halogen,
hydrogen), tin hydride, oxygen, disulfides, and diselenide have
been reported,3 selective radical coupling between phosphinyl
radicals and other radicals to provide functionalized phosphines
has not been investigated. We successfully synthesized stable
dialkylphosphinyl radical RH2P• (1a) [RH2 = 1,1,4,4-tetrakis(tri-
methylsilyl)butane-1,4-diyl] that exists as a monomer both in
solution and in the solid state (Chart 1).4 Recently, we have
reported the synthesis of persistent heavier pnictogen-centered
radicals RH2Pn• [Pn = Sb (1b) and Bi (1c)] in solution by
utilizing facile dissociation of the corresponding dimer and their
reactions with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)
to afford 2b and 2c as selective radical coupling products.5 Thus,
we anticipated selective radical coupling of 1a with other stable
radicals. We wish to report herein reactions of stable phosphinyl
radical 1a with stable aminoxyl radicals AZADO (2-azaada-
mantane-N-oxyl)6 and TEMPO, which give unexpected oxida-
tion products, cyclic phosphorane 4, silyl phosphinate 5, and
aminophosphine 6. Product analysis and theoretical studies
indicate that oxygen transfer from the aminoxyl radicals to 1a
and subsequent silyl migration are key steps.
Me3Si
SiMe3
AZADO
P
O
N
1a
SiMe3
Me3Si
3
SiMe3
OSiMe3
SiMe3
OSiMe3
Me3Si
Me3Si
SiMe3
P
+
P
+
P
N
N
O
SiMe3
SiMe3
Me3Si
Me3Si
SiMe3
4
5
6
Scheme 1. Reaction of 1a with AZADO.
Compounds 4-6 were characterized by NMR, MS, elemen-
tal analysis, and X-ray diffraction study; their molecular
structures in the solid state are shown in Figures 1, 2, and S17
(in Supporting Information).9 Compound 4 has a siloxy(ami-
no)phosphorane skeleton. The lengths of P1-O1 [1.6149(12) ¡]
and P1-N1 [1.6612(14) ¡] bonds are within the standard range
of P-O and P-N single bond lengths.10 The P1-C4 bond
[1.6764(17) ¡] is considerably shorter than the P1-C7 bond
[1.8404(17) ¡] and is similar to the reported ylide P=C bond
of phosphorane H2C=P(NMe2)3 [1.655(6) ¡].11 The C4 atom
adopts planar trigonal geometry; the sum of the bond angles
around C4 is 360.0(1)°. In the 13C{1H} NMR spectrum of 4,
two quaternary carbon nuclei resonate at 20.3 and 29.0 ppm as
Me3Si
Me3Si
Me3Si
SiMe3
SiMe3
Pn
SiMe3
Pn
O
N
N
O
N
O
SiMe3
Me3Si
2b (Pn = Sb)
2c
1a
1b
(Pn = P)
(Pn = Sb)
TEMPO
AZADO
(Pn = Bi)
1c (Pn = Bi)
Chart 1. Stable dialkylphosphinyl radical 1a and related
compounds.
1
a doublet due to the 31P nuclei, with JPC values of 146 and
75.4 Hz, respectively. Based on the large 1JPC value, the signal at
20.3 ppm is assignable to the ylide carbon (C4).
Phosphinyl radical 1a was treated with one equivalent of
AZADO in a sealed NMR tube with toluene-d8 at ¹40 °C, and
the reaction was monitored by NMR spectroscopy (Scheme 1).
The 31P{1H} NMR spectrum of the resulting solution (Figure S5
in Supporting Information) showed one singlet signal, which
The P1-O1 and P1-O2 bond lengths of 5 being 1.4751(14)
and 1.5889(14) ¡ indicate silyl phosphinate functionality. The
C1-C2 bond length [1.340(3) ¡] and planar geometry around C1
(the sum of the bond angles around C1 of 360.0(1)°) indicate
that a double bond exists between the C1 and C2 atoms.
© 2015 The Chemical Society of Japan