M.A. Weber, P.C. Ford / Journal of Molecular Catalysis A: Chemical 416 (2016) 81–87
83
and analyzed by GC TCD with quantities determined by peak
integration calibrated against authentic samples.
the unreacted diol. (SI Fig. S-2). When the analogous open system
reaction was carried out for 18 h in refluxing diglyme (∼162 ◦C), the
yield of the ␣-HK increased to ∼40%. No products were apparent in
the absence of catalyst.
In order to improve the GC analysis of less volatile products,
some liquid products were treated with tert-butyldimethylsilyl
chloride to convert alcohols (ROH) to more the volatile silyl ethers
tBuMe2SiOR for GC–MS analysis. Liquid product mixtures were dis-
solved in 30 mL of dichloromethane under argon and cooled to
0 ◦C, and to this solution, tert-butyldimethylsilyl chloride (0.397 g,
2.644 mmol, 1.3 equiv.) and imidazole (0.224 g, 3.30 mmol, 1.5
equiv.) were added. The mixture was warmed to rt and allowed to
stir overnight with monitoring by thin layer chromatography. The
reaction mixture was then either filtered through celite and con-
centrated, or washed with water (2×) then brine (1×), dried over
anhydrous Na2SO4, and concentrated. An aliquot of the mixture
was dissolved in DMSO-d6 and a proton NMR recorded to confirm
conversion of the hydroxide groups to silyl ethers. A second aliquot
was dissolved in MeOH and analyzed by GC–MS methods. After sily-
lation the diols typically gave two GC peaks, representing mono-
and di-silylated derivatives.
3.2. Reactions of vicinal diols with a hydrogen acceptor:
A solution of 1,2-butanediol (0.180 g, 2.0 mmol), the hydro-
gen acceptor cyclohexanone (0.396 g, 4.0 mmol) and 1 (1.3 mg,
2.3 mol) was added to a J. Young valve adapted NMR tube. This was
degassed by fpt cycles then sealed, after which the tube heated in an
oil bath at 150 ◦C for 30 min, after which the system was quenched
by cooling to ambient temperature. The 13C NMR spectrum of
the neat solution showed resonances at ı = 211.026 (C O), 67.598
(CH2OH), 31.105 (CH2), and 7.019 (CH2) ppm (SI Fig. S-3) consis-
d6 (ı= 5.03 (t, CH2OH), 4.05 (d, CH2OH), 2.41 (q, CH3CH2−) and
0.93 (t, CH3CH2−) ppm, SI Fig. S-4) confirmed formation of 1-
hydroxybutanone as the major dehydrogenation product according
to the SBDS database [15]. Integration of the 1H NMR spectrum,
using benzene as an internal standard, revealed 1.16 mmol of the
ketone formed, corresponding to 56% conversion and 510 cata-
lyst turnovers. Peaks at ı = 4.55 (d, OHCH−), 3.25 (m, OHCH−), 1.45
(m, C5H10) and 1.16 (m, C5H10) ppm in the 1H NMR spectrum
corresponded to formation of 1.41 mmol of cyclohexanol. Reso-
nances indicating the presence of unreacted 1,2-butanediol and
cyclohexanone were apparent, and the spectra of these last three
components were confirmed by comparison to the spectra of these
compounds in DMSO-d6 (SI Figs. S-4 & S-5).
3. Results and discussion
The following diols were studied: 1,2-butanediol, 1,2-
propanediol, 1-phenyl-1,2-ethanediol, 1,3-butanediol, and
1,3-propanediol. Dehydrogenations were carried out: (a) with
neat substrate in a closed system: (b) with a hydrogen acceptor to
increase catalysis turnover; (c) with deuterated substrates to probe
potential isomerization mechanisms (d) with the rhodium decar-
bonylation catalyst 2 to evaluate whether aldehyde intermediates
could be intercepted.
3.1. Acceptor-free dehydrogenation of 1,2-propanediol
A deaerated solution of 1 (6.9 mg, 12 mol) in neat 1,2-
propanediol (11.1 g, 0.146 mol) was stirred at 150 ◦C in
a
Fisher–Porter bottle for 18 h. The solution color was initially a
golden yellow but turned lighter and browner as the reaction pro-
gressed. GC-TCD analysis of the gas phase display a negative peak
eluting at 1.72 min (H2), integration of which demonstrated that
0.37 mmol of H2 was produced. This corresponded to 31 catalyst
turnovers, but only to 0.25% conversion of the diol. Distillation
of the reaction mixture yielded a clear liquid. The 1H NMR spec-
resonances at ı = 2.05 (s, CH3(CO)CH2OH), 4.04 (d, CH3(CO)CH2OH)
and 5.09 (t, CH3(CO)CH2OH) consistent with the ␣-HK product 1-
hydroxyacetone as confirmed by comparison with the spectrum
listed in the SDBS database [15]. There was no indication of the
␣-HA product 2-hydroxypropanal over the 9–11 ppm region char-
acteristic of an aldehyde proton.
Dehydrogenation was then attempted by heating in an open sys-
tem so that any H2 formed was released. In the first experiment,
a solution prepared from 1,2-propanediol (0.381 g, 5 mmol) and
1 (5.7 mg, 10 mol) in toluene (6.7 mL) was refluxed (115 ◦C) for
24 h during which time the solution remained bright yellow. After
removing the solvent by distillation, the 1H NMR spectrum of the
product showed only a small amount of the ␣-HK (1.3% yield based
on NMR integration, corresponding to ∼7 catalyst turnovers) plus
An identical sample was heated at 150 ◦C for 10 min intervals,
and monitored by recording the 13C NMR spectra. After the first
interval, it was determined that 64% conversion had occurred.
However, no additional conversion was observed as the heating
continued, so a steady state was apparently reached. From these
data it is apparent that the hydrogen transfer process indicated by
Eq. (3) occurs with much greater net catalytic activity than does the
acceptorless dehydrogenation.
The vicinal diol 1-phenyl-1,2-ethanediol was studied simi-
larly. This substrate (0.215 g, 1.56 mmol), cyclohexanone (0.602 g,
6.14 mmol) and 1 (0.9 mg, 1.5 mol) were heated together in
a J. Young valve adapted NMR tube at 150 ◦C for 30 min. The
13C NMR spectrum of the resulting solution (␦ = 197.927 (C O),
133.468 (C6H5), 132.934 (C6H5), 127.165 (C6H5), 126.232 (C6H5)
and 64.833 (CH2OH) ppm) as well as the 1H NMR spectrum in
DMSO-d6 (ı = 7.93 (d, C6H5), 7.64 (t, C6H5), 7.53 (t, C6H5), 4.53
(t, (CO)CH2OH), 3.42 (t, (CO)CH2OH)) (SI Figs. S-6 & S-7) indi-
cated the formation of 1-hydroxyacetophenone (Eq. (4)). The 1H
NMR spectrum of this product was confirmed by comparison to
the SDBS database.15 Integration of these proton resonances, refer-
enced to a mesitylene standard, indicated formation of 0.79 mmol
of this product (70% conversion and 562 catalyst turnovers). The
13C and 1H NMR spectra also indicated the presence of cyclohex-