Communications
Table 3. Borinic acid catalyzed reduction of secondary phosphine
oxides.[a]
9
Ar
10
Conv.[b] [%]
9l
4-F3CC6H4
Ph
4-tBuC6H4
4-MeOC6H4
10l
81
53
65
50
9m
9n
9o
10m
10n
10o
[a] Reaction conditions: phosphine, 6 (2.5 mol%), PhSiH3 (2.4 equiv.), dry
toluene, under an argon atmosphere, 808C, 18 h. [b] Conversions were
determined by 31P NMR spectroscopy.
Owing to their useful reactivity, amines N-oxides and sulfox-
ides are frequently encountered in synthetic organic chemis-
try,[18] and their use is usually related to efficient deoxygenation
methods to obtain amines and sulfides.
Scheme 3. Borinic acid catalyzed reduction of N-oxides.
The reduction of sulfoxides and amine N-oxides is well
known if methods involving the use of a stoichiometric
amount of the reducing species are considered.[19] However,
very few methods rely on the use of a catalyst to promote the
mild deoxygenation of sulfoxides and amine N-oxides, as illus-
trated by the scarcity of metal-based procedures.[20] Moreover,
only a few methods involving hydrosilanes have been reported
to date.[21] Accordingly, a mild and catalytic method involving
hydrosilanes was envisaged to be of high utility.
First, the potential involvement of the Brønsted acidity of 6
was ruled out by a control experiment involving the use of
2,6-di-tert-butylpyridine (5 mol%) as a proton scavenger, which
afforded 97% conversion after 96 h.
As a consequence, the Lewis acid–base interactions between
borinic acid 6 and various phosphine oxides were investigated:
equimolar quantities of 6 and Ph3PO or Cy3PO (Cy=cyclohexyl)
were mixed in deuterated toluene and heated at 808C for sev-
eral hours, but careful analysis by 31P NMR spectroscopy did
not reveal any new signals, whereas analysis by 11B NMR spec-
troscopy revealed minor protodeborylation leading to boronic
acid 7.[22]
Upon adding phenylsilane at room temperature, significant
evolution of hydrogen was observed over 10 min. Whereas
Ph3PO gave puzzling NMR spectroscopy data, Cy3PO delivered
a clearer picture: a new 31P singlet at d=69.8 ppm (in
[D8]toluene) and complete conversion of the starting oxide
(d=45 ppm) were observed.[23] Additionally, the 11B NMR spec-
trum of the mixture displayed complete conversion of borinic
The scope of the reduction of sulfoxides was explored with
the previously developed reaction conditions after a control
experiment, which showed no conversion in the absence of
borinic acid 6. Rapid reduction at 808C was observed, which
gave sulfides 11 a and 11 b in yields of 92 and 98%, respective-
ly (Scheme 2). In contrast, methyl(phenyl)sulfide (11 c) was ob-
tained in 93% yield after a longer reaction time of 18 h
(Scheme 2).
Our reactions conditions were easily extended to the reduc-
tion of N-oxides. Then, the N-oxide derived from a tertiary al-
kylamine was markedly easier to reduce than pyridinium N-
oxides. After controlling the absence of reactivity without bor-
inic acid 6, corresponding N-methylmorpholine (13a) was ob-
tained in 82% yield at room temperature, whereas 2-methyl-
pyridine (13b) and 4-(dimethylamino)pyridine (DMAP, 13c)
were obtained in yields of 91 and 63%, respectively, at 808C
(Scheme 3).
acid
6 associated with a new characteristic triplet at
dÀ16.4 ppm (1J=85.0 Hz), which matched previously charac-
terized borohydride 16.[15c] These observations likely suggest
an ionic mechanism that involves fast dehydrogenative cou-
pling between 6 and PhSiH3 with the assistance of Cy3PO to
yield elusive boronate 14, which very rapidly abstracts an addi-
tional hydride to observed borohydride 16 (Scheme 4). The
nature of the phosphonium countercation was indicated by a
significant upfield shift in the 31P signal matching the structur-
ally similar phosphonium ion and the existence of an indicative
2JP,Si coupling constant of 29 Hz.[22]
Upon aging the mixture at room temperature for 48 h, a
new 31P NMR signal characteristic of phosphine–borane com-
plex 17[23] [d(31P)=15.4 ppm, d(11B)=À28.8 ppm, 1JP,B =65 Hz,
2JH,P =18 Hz] appeared very slowly.[24] After warming the mix-
ture at 808C for 2.5 h, the conversion smoothly increased to
50%. In the 11B NMR spectrum, a second minor signal ap-
peared {d(11B)=À42.9 ppm, HSQC [1H,11B] correlation} and was
tentatively attributed to intermediate borohydride 18.[21] Final-
Scheme 2. Borinic acid catalyzed reduction of sulfoxides.
ChemCatChem 2017, 9, 4460 –4464
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