(N -Phosphinoamidinate)Iron pre-catalysts for the room temperature hydrosilylation of carbonyl compounds with broad substrate scope at low loadings

Article abstract of DOI:10.1021/om400883u

The synthesis and structural characterization of three-coordinate iron(II) and cobalt(II) complexes supported by new N-phosphinoamidinate ligands is reported, along with the successful application of these complexes as precatalysts for the challenging room-temperature hydrosilylation of carbonyl compounds to afford alcohols upon workup. Under the rigorous screening conditions employed (0.015 mol % Fe) for the reduction of acetophenone, the well-defined iron(II) amido precatalyst 2b, featuring bulky N-2,6- diisopropylphenyl and di-tert-butylphosphino moieties within the N-phosphinoamidinate ligand, exhibited exceptional catalytic performance. Further experimentation revealed that the yield achieved in the hydrosilylation of acetophenone employing 2b was unaltered when conducting reactions in the absence of light, in the presence of excess mercury, or under solvent-free conditions. Notably, precatalyst 2b was found to exhibit the broadest substrate scope reported to date for such room-temperature iron-catalyzed carbonyl hydrosilylations en route to alcohols, enabling the chemoselective reduction of structurally diverse aldehydes and ketones, as well as for the first time esters, at remarkably low loadings (0.01-1.0 mol % Fe) and using only 1 equiv of phenylsilane reductant.

Full text of DOI:10.1021/om400883u

Article
pubs.acs.org/Organometallics
(N‑Phosphinoamidinate)Iron Pre-Catalysts for the Room
Temperature Hydrosilylation of Carbonyl Compounds with Broad
Substrate Scope at Low Loadings
†
‡
†
†
†
,‡
Adam J. Ruddy, Colin M. Kelly, Sarah M. Crawford, Craig A. Wheaton, Orson L. Sydora,*
,†
,†
Brooke L. Small, Mark Stradiotto,* and Laura Turculet*
†
Department of Chemistry, Dalhousie University, 6274 Coburg Road, P.O. Box 15000, Halifax, Nova Scotia B3H 4R2, Canada
Research and Technology, Chevron Phillips Chemical Company, 1862 Kingwood Drive, Kingwood, Texas 77339, United States
‡
*
S Supporting Information
ABSTRACT: The synthesis and structural characterization of
three-coordinate iron(II) and cobalt(II) complexes supported
by new N-phosphinoamidinate ligands is reported, along with
the successful application of these complexes as precatalysts for
the challenging room-temperature hydrosilylation of carbonyl
compounds to afford alcohols upon workup. Under the
rigorous screening conditions employed (0.015 mol % Fe) for
the reduction of acetophenone, the well-defined iron(II)
amido precatalyst 2b, featuring bulky N-2,6-diisopropylphenyl
and di-tert-butylphosphino moieties within the N-phosphinoamidinate ligand, exhibited exceptional catalytic performance.
Further experimentation revealed that the yield achieved in the hydrosilylation of acetophenone employing 2b was unaltered
when conducting reactions in the absence of light, in the presence of excess mercury, or under solvent-free conditions. Notably,
precatalyst 2b was found to exhibit the broadest substrate scope reported to date for such room-temperature iron-catalyzed
carbonyl hydrosilylations en route to alcohols, enabling the chemoselective reduction of structurally diverse aldehydes and
ketones, as well as for the first time esters, at remarkably low loadings (0.01−1.0 mol % Fe) and using only 1 equiv of
phenylsilane reductant.
INTRODUCTION
emerged as a useful protocol for the synthesis of alcohols that is
complementary to reductions employing molecular hydrogen
or hydrogen-transfer reagents. Despite the diversity of recent
■
The reduction of carbonyl compounds is among the most
widely employed transformations in synthetic organic chem-
8
1
contributions made toward the development of the iron-
istry. Such protocols are employed broadly on both benchtop
9
catalyzed (asymmetric) hydrosilylation of ketones and
and industrial reaction scales and have been shown to
accommodate a diversity of substrate classes and functional
1
0−14
aldehydes to afford alcohols by the groups of Nishiyama,
15−17
18
19
20,21
2
Beller,
Gade, Nikonov, Adolfsson,
Darcel and
groups. Among the myriad reduction protocols that have been
22−25
26
27
28
29,30
31
Sortais,
and others,
temperature employing low loadings (≤1 mol % Fe) are
Glorius, Guan, Plietker, Royo,
Driess,
established, platinum-group-metal-catalyzed reductions have
proven to be particularly effective, providing high yields and
selectivity under mild conditions at low catalyst loading.
However, the high cost and toxicity of the platinum-group
metals has prompted the development of alternative classes of
carbonyl reduction catalysts involving first-row transition
5
,6
reports of such transformations at room
1
−3
32,33
limited to a total of three reports from the groups of Chirik
34
32
and Tilley. In 2008, Chirik and co-workers reported on the
use of well-defined (bis(imino)pyridine)iron complexes as
precatalysts for aldehyde (1 example) and ketone (12
examples) hydrosilylation. Efficient room-temperature carbonyl
reductions were observed employing 0.1−1.0 mol % of Fe and
using 2 equiv of either PhSiH or Ph SiH as the reducing
4
elements, most notably iron. Iron is a desirable substitute for
this application, as it is both abundant and inexpensive and is
significantly less toxic than the platinum-group metals in
pharmaceutical applications. The recent emergence of highly
effective iron-based catalysts for carbonyl reductions can be
attributed in part to the development and application of
suitable supporting multidentate ancillary ligands, including
those featuring combinations of phosphorus and nitrogen
3
2
2
agent, with varying levels of chemoselectivity in carbonyl
reductions involving enone substrates. Asymmetric variants of
such reactions employing a borane cocatalyst were reported
3
3
34
subsequently. In 2010, Yang and Tilley reported on the use
35
5
−7
of [Fe{N(SiMe ) } ] as a precatalyst for the room-temper-
donors.
3 2 2
Among the various classes of transition-metal-catalyzed
carbonyl reductions that have been developed, hydrosilylation
followed by Si−O bond cleavage upon aqueous workup has
Received: September 3, 2013
Published: September 26, 2013
©
2013 American Chemical Society
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Organometallics
Article
ature hydrosilylation of aldehydes (2 examples) and ketones (8
examples) at 0.03−2.7 mol % Fe loading and using 1.6 equiv of
PhSiH , Ph SiH , or PhMeSiH as the reducing agent.
Scheme 1. Synthesis of N-Phosphinoamidines and N-
Phosphinoamidinate Complexes
3
2
2
2
Importantly, control experiments ruled out the presence of
trace platinum-group-metal impurities as contributing signifi-
cantly to the observed catalytic activity. Notwithstanding these
breakthroughs from the groups of Chirik and Tilley, the scope
of reactivity featured in these reports is limited; notably absent
are examples of synthetically relevant substrates such as
benzophenones, sterically demanding acyclic dialkyl ketones,
and heteroaryl acetophenones. Furthermore, while the first
reports of the iron-catalyzed reduction of esters to alcohols
3
6,37
appeared only very recently,
(
relatively high catalyst loading
Information). While these phenomena are presently not well
understood, it is evident that a single isomeric form of 1a,b is
preferred due to the steric and/or electronic demands of the di-
tert-butylphosphino moiety. The apparent phosphorus coupling
5 mol % Fe) and temperature (100 °C) were required in these
38
systems in order to achieve efficient catalysis. In this context,
the identification of new iron-based catalysts for the hydro-
silylation of carbonyl compounds to alcohols that operate under
mild conditions (room temperature; ≤1 mol % of Fe; ≤1 equiv
of silane) and with expanded substrate scope represents an
important challenge in current catalysis research.
1
observed for the N−H resonances in the H NMR spectra of
1a,b supports the connectivity proposed in Scheme 1; this was
further substantiated via the crystallographic characterization of
b (Figure 1; Table 1), whereby the (dipp)N−C distance
1.2807(19) Å) was found to be significantly shorter than the
P(tBu) }N−C distance (1.3679(19) Å).
1
(
In seeking to address some of the aforementioned challenges
in iron-catalyzed carbonyl reductions, we turned our attention
to the examination of well-defined iron complexes derived from
N-phosphinoamidinesa new (pre)ligand class that was
recently introduced in the development of highly active
chromium-based ethylene tri-/tetramerization catalysts. In
particular, we envisioned that three-coordinate (N-
phosphinoamidinate)iron(amido) species might exhibit some
{
2
39
34
of the desirable reactivity properties of [Fe{N(SiMe ) } ],
3
2 2
while at the same time providing a means of enhancing catalyst
40
stability and performance via ancillary ligand modification.
Herein we report on the successful application of a newly
developed three-coordinate (N-phosphinoamidinate)iron-
(
amido) precatalyst in the room-temperature hydrosilylation
of aldehydes, ketones, and esters to alcohols, at remarkably low
loadings (0.01−1.0 mol % Fe), employing only 1 equiv of silane
relative to the carbonyl compound, and with the broadest
substrate scope reported to date for such iron-catalyzed
transformations.
RESULTS AND DISCUSSION
■
Amidines can be prepared conveniently from nitriles and
4
1
anilines, and we have subsequently shown that these can be
transformed into the corresponding N-phosphinoamidines
upon treatment with n-BuLi, followed by quenching with a
39
chlorophosphine. In an effort to access ligand variants
featuring significant steric demand at both the nitrogen and
phosphorus termini to support low-coordinate iron complexes,
we targeted the preparation of new N-phosphinoamidines
pairing bulky N-aryl (aryl = 2,6-dimethylphenyl (dmp), 2,6-
Figure 1. Crystallographically determined structures of 1b and 2a−c
employing 50% ellipsoids and with hydrogen atoms omitted for
clarity).
(
diisopropylphenyl (dipp)) and P(tBu) functionalities (Scheme
2
1). Such reactions proceeded smoothly, affording 1a,b in 70
Subsequent treatment of 1a,b with [Fe{N(SiMe ) } ]
3 2 2
and 86% isolated yields, respectively. The existence of
tautomers has been observed previously in solution for N-
phosphinoamidines featuring alternative substitution patterns;
for example, the previously reported diphenylphosphino
analogue of 1a is observed as a 65:35 tautomeric mixture,
whereby in the major species the proton resides on the nitrogen
resulted in the apparent loss of HN(SiMe ) and concomitant
3 2
formation of isolable, crystalline green-yellow solids that were
identified as being the respective (N-phosphinoamidinate)iron-
(N(SiMe ) ) complexes 2a (60%) and 2b (62%). As a point of
3
2
comparison in catalysis (vide infra) we sought to prepare an
analogous Co(II) variant of 2b; we were pleased to find that
39
35
that is connected to phosphorus, N(1). However, only a
under analogous conditions employing [Co{N(SiMe ) } ]
3
2 2
single major isomeric form for both 1a and 1b was observed at
and 1b, the (N-phosphinoamidinate)cobalt(N(SiMe ) ) com-
3 2
1
13
31
300 K by use of H, C, and P NMR spectroscopy, with any
plex 2c was obtained as a red crystalline solid in 68% isolated
yield. Elemental analysis data confirmed the absence of
additional coligands in these paramagnetic complexes, and
alternative minor isomeric structures contributing much less
significantly to the equilibrium mixture (see the Supporting
5
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Table 1. Selected Interatomic Distances (Å) and Angles (deg) for 1b and 2a−c
a
a
a
a
1
b
2
2b
2c
M−P
2.4313(6)
1.9978(18)
1.9111(19)
1.6862(19)
1.309(3)
1.363(3)
132.86(6)
81.10(6)
144.15(9)
358.1
2.4434(6)
2.0013(15)
1.9106(16)
1.6804(17)
1.316(2)
1.353(2)
132.59(5)
81.12(5)
145.84(7)
360.0
2.3873(7)
1.9762(18)
1.8939(19)
1.674(2)
1.316(3)
1.351(3)
132.04(6)
81.56(6)
146.39(8)
360.0
M−N(2)
M−N(3)
P−N(1)
N(1)−C(9)
1.7174(14)
1.3679(19)
1.2807(19)
(aryl)N(2)−C(9)
P−M−N(3)
P−M−N(2)
N(2)−M−N(3)
∑
angles at M
a
Representative data for only one of the two crystallographically independent molecules.
the measured room-temperature magnetic moments indicated
collected in Table 2. Whereas the dmp-functionalized iron
complex 2a performed rather poorly under these test
that the iron complexes 2a,b feature S = 2 spin centers, while
3
the cobalt complex 2c features an S = / spin center. The
2
crystallographic characterization of 2a−c (Figure 1 and Table
Table 2. Preliminary Catalytic Screening Results
1
) confirms the three-coordinate, trigonal-planar nature of
these species. Significant differences in the interatomic
distances are observed between the neutral (protonated)
proligand 1b and the resultant anionic P,N ligands featured
in 2a−c. Notably, the P−N(1) and N(1)−C(9) distances are
shorter in the complexes relative to 1b, while conversely the
N(2)−C(9) distances in 2a−c are significantly longer than that
found in 1b. These structural data suggest that the complexes
are best viewed as featuring phosphine-amido ligands, with the
formal negative ligand charge residing on N(2), as represented
in Scheme 1. The sum of the bond angles at the metal
confirmed a trigonal-planar geometry in each complex (2a,
a
entry
Fe precatalyst (0.015 mol % M)
n
3a (%)
1
2
3
4
5
6
7
8
2a
1.6
1.6
1.6
1.6
1.6
0.6
0.4
1.0
25
>99
<5
2b
2c
[Fe{N(SiMe ) } ]
19
3
2 2
[Co{N(SiMe ) } ]
<5
3
2 2
2b
2b
2b
>99
27
358.1°; 2b, 360.0°; 2c, 360.0°), and no significant variation in
b
>99
the P−Fe−N(2) bond angle (i.e., chelating ligand bite angle)
was observed between the structurally related complexes 2a,b,
which feature dmp and dipp substituents at nitrogen,
respectively. Within 2a−c, the ligands are not symmetrically
distributed within the trigonal plane, displaying the following
progression: P−M−N(2) (ca. 81°) < P−M−N(3) (ca. 132°) <
N(2)−M−N(3) (ca. 145°). The overall metrical parameters in
a
b
GC conversion. No change in conversion to 3a was observed in the
case of reactions conducted in the absence of light, in the presence of
00 equiv of Hg, or under solvent-free conditions. See the Supporting
1
Information for additional details.
conditions (entry 1), we were pleased to observe quantitative
conversion to the desired 1-phenylethanol (3a) when using the
more sterically hindered dipp-functionalized precatalyst 2b
2
a,b mirror those found in related three-coordinate (N∼N)FeX
42
complexes supported by bis(phosphinimino)methanide or β-
4
0,43,44
diketiminate ligands,
ligand bite angle (112° and 95°,
a,b). While we were unable to identify three-coordinate
with the exception of the chelating
(
entry 2). Interestingly, the use of 2cthe Co(II) structural
42
40,43,44
respectively; cf. 81° in
analogue of 2bunder analogous conditions afforded no
2
detectable conversion of the acetophenone starting material
iron(II) complexes featuring chelating phosphine-amido
ligation in the chemical literature, the Fe−P and Fe−N
distances in recently reported four-coordinate complexes of this
type are comparable to those involving the chelating ligands
(
entry 3), thereby supporting the key role of iron in catalyzing
the carbonyl reduction chemistry featured herein. Under the
challenging test conditions employed, only low conversion was
achieved when using [Fe{N(SiMe ) } ] (entry 4), and
negligible conversion was observed when using [Co{N-
(SiMe ) } ] (entry 5). While the use of alternative silanes
provided some conversion to 3a at higher 2b loadings, under
the conditions featured in entry 2, the use of Ph SiH ,
3
2 2
4
5
within 2a,b.
We then turned our attention to screening the room-
temperature catalytic performance of 2a−c relative to the
diamido complexes [M{N(SiMe ) } ] (M = Fe, Co) in
3
2 2
3
2
2
2
2
hydrosilylation chemistry. We selected acetophenone as a test
substrate in combination with 1.6 equiv of phenylsilane, in
keeping with the conditions reported by Yang and Tilley.
PhMeSiH , Et SiH , or polymethylhydrosiloxane in place of
2 2 2
PhSiH afforded negligible conversion of the acetophenone
3
34
starting material. In evaluating the influence of silane
stoichiometry on the observed catalysis when using 2b, we
found that the use of 0.6 equiv of phenylsilane was well
tolerated (entry 6), while a significant drop in conversion was
observed when using 0.4 equiv of phenylsilane (entry 7). On
this basis, and in an effort to circumvent the need for further
optimization with respect to the amount of phenylsilane used
when exploring other carbonyl substrates, we opted to use 1
equiv of phenylsilane as the reductant in all subsequent
However, in an effort to employ particularly stringent test
conditions, we opted to conduct our screening using only 0.015
mol % of Fe, measuring the progress of the reaction after 4 h.
To place these conditions in context, the lowest catalyst loading
reported to date for the iron-catalyzed room-temperature
hydrosilylation of acetophenone involved the use of 0.03 mol %
34
[
Fe{N(SiMe ) } ], whereby 98% conversion was achieved
3 2 2
after 18 h. The results of our preliminary catalyst screen are
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Organometallics
Article
experimentation. Notably, the ability to use only 1 equiv of
phenylsilane at low catalyst loadings in successful reductions
involving the precatalyst 2b represents a practical improvement
relative to previously reported iron-based catalyst systems for
the room-temperature hydrosilylation of ketones and aldehydes
to alcohols, where larger quantities (1.6−2.0 equiv) of silane are
3
2−34
employed.
Interestingly, the quantitative conversion
achieved in the absence of added solvent (entry 8) suggests
that this catalyst system could also be optimized for use under
more environmentally benign, solvent-free reaction conditions.
In monitoring the progress of acetophenone hydrosilylation
employing phenylsilane (1.0 equiv) in combination with
precatalyst 2b (entry 8), we observed 59% conversion to 3a
over the course of only 10 min, which equates to a noteworthy
−1
turnover frequency of approximately 23600 h . Preliminary
control experiments in which analogous reactions were
conducted with the rigorous exclusion of light or in the
presence of added mercury resulted in no loss of catalytic
performance, thereby suggesting that such transformations are
neither photochemically promoted nor heterogeneous in
nature.
Having identified 2b as being a superior catalyst for the
hydrosilylation of acetophenone at room temperature, we
surveyed the reaction scope with other ketones and aldehydes
(
Figure 2). We were pleased to find that a broad range of
substrates was successfully accommodated, employing only 1
equiv of phenylsilane and ≤1 mol % of 2b. In building on the
successful reduction of acetophenone to afford 3a, alternative
para-substituted acetophenones featuring electron-withdrawing
or electron-donating substituents were successfully accommo-
dated, as were pyridyl and thiophenyl variants, affording very
high conversion to the corresponding secondary alcohols (3b−
f). Sterically hindered ortho-substituted acetophenones, α-
tetralone, as well as branched/benzo-fused variants were also
reduced efficiently (3g−l). Structurally diverse dialkyl ketones
were suitable substrates when using 2b as a precatalyst,
resulting in high conversion to both cyclic (3m−o,q) and
acyclic (3p,r−t) secondary alcohols. Benzophenones were also
efficiently reduced when using 2b (3u−w), as were
benzaldehydes featuring either ortho substitution or para
substitution involving an electron-donating or electron-with-
drawing substituent (3x−z). While for some substrates the
catalytic abilities of [Fe{N(SiMe ) } ] were found to be
3
2 2
comparable to those of 2b under analogous reaction conditions
Figure 2. Hydrosilylation of ketones and aldehydes. Reaction
conditions: carbonyl substrate (0.4 or 1.0 mmol), phenylsilane (0.4
or 1.0 mmol), toluene (250 or 625 μL), 4 h (unoptimized). Percent
GC conversions are given under the compounds in Roman type,
amounts of 2b in mole percent are given in parentheses, and isolated
yields are given in italics. Calibrated GC data confirmed that
differences between GC yield and isolated yield can be attributed
entirely to losses upon isolation, rather than due to byproduct
formation. See the Supporting Information for additional details.
Legend: (a) GC conversion (mol % of [Fe{N(SiMe ) } ]).
(
(
3j,m,x), significantly inferior performance of [Fe{N-
SiMe ) } ] was noted in the case of apparently more
3
2 2
challenging substrates including 4-chloroacetophenone, α-
tetralone, and pivalone (3b,i,p). Further comparisons to the
32
published work of Chirik and co-workers regarding the use of
bis(imino)pyridine)iron complexes for the room-temperature
(
hydrosilylation of ketones revealed a similar trend. For example,
whereas 82% conversion of α-tetralone over the course of 3 h at
3
2 2
0
.1 mol % Fe was described in a report by Chirik and co-
32
workers, near-quantitative conversion at 0.05 mol % Fe was
achieved herein over the course of 4 h using 2b (3i; Figure 2).
A brief survey regarding chemoselective hydrosilylations
confirmed the propensity of 2b in promoting the selective
reduction of ketones to alcohols in the presence of alkene
functionalities (Scheme 2). In this regard, both 2-cyclo-
hexenone and 5-hexen-2-one were cleanly transformed into
the corresponding secondary alcohols (3aa,ab) over the course
of 4 h at room temperature. Notably, the performance of 2b in
the reduction of the latter substrate leading to 3ab is more
efficient than analogous reactions employing either [Fe{N-
34
32
(SiMe ) } ] or (bis(imino)pyridine)iron catalyst com-
3
2 2
plexes.
Encouraged by the remarkably broad scope at low catalyst
loadings exhibited by 2b in the iron-catalyzed, room-temper-
ature hydrosilylation of ketones and aldehydes, we turned our
attention to developing the first examples of analogous
reductions of esters to alcohols. The iron-catalyzed reduction
36
of esters to alcohols was disclosed by the groups of Darcel
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Article
a
Scheme 2. Chemoselective Reduction of Enones
corresponding benzyl alcohol derivatives (4b−e). Further
functional group tolerance was demonstrated in the reduction
of a pyridine-containing ester, affording 4f. Finally, the ability to
promote the reduction of esters featuring only aliphatic
substitution was achieved in the reduction of ethyl octanoate,
which afforded n-octanol (4g) in high isolated yield. The
application of [Fe{N(SiMe ) } ] as a precatalyst for the
3
2 2
reduction of esters has not been reported in the literature
previously. However, we demonstrate herein that while the
conversion achieved by use of [Fe{N(SiMe ) } ] can in some
a
1
Values given under 3aa,ab are percent H NMR conversion, mole
3
2 2
percent of 2b (in parentheses), and isolated yield (in italics).
Calibrated GC data confirmed that differences between GC yield
and isolated yield can be attributed entirely to losses upon isolation,
rather than to byproduct formation.
cases parallel that obtained when using 2b under similar
conditions (e.g., 4a from methyl phenylacetate; Figure 3), this
simple amide precatalyst proved inferior to 2b with alternative
substrates, as in the formation of 4g, thereby confirming the key
role of the N-phosphinoamidine ligand in enhancing catalytic
performance. Furthermore, where direct substrate comparisons
can be made with the literature, the room-temperature catalytic
performance of 2b (≤1 mol % Fe) using 1 equiv of
phenylsilane as the reductant was found to be competitive
3
7
and Beller during the course of our studies presented herein;
however, both high catalyst loading (5 mol % Fe) and
temperature (100 °C) were employed in these published
catalyst systems. Our successful application of 2b (≤1 mol % of
Fe) in the first iron-catalyzed room-temperature reduction of
esters to alcohols is summarized in Figure 3. Methyl, ethyl, and
3
6
with or superior to that of both [CpFe(PCy )(CO) ]BF and
3
2
4
37
Fe(stearate) /NH CH CH NH , which operate under much
2
2
2
2
2
more forcing conditions (5 mol % of Fe, 100 °C, 2−5 equiv of
silane). Mechanistic studies directed toward understanding the
selectivity for alcohol formation, rather than ether formation, in
this chemistry are ongoing.
CONCLUSION
■
An investigation examining the catalytic utility of new three-
coordinate iron(II) and cobalt(II) N-phosphinoamidinate
complexes in the challenging room-temperature hydrosilylation
of carbonyl compounds to alcohols established the superior
performance of the well-defined iron(II) amido precatalyst
(
2b), featuring sterically demanding N-2,6-diisopropylphenyl
and di-tert-butylphosphino moieties within the N-phosphinoa-
midinate ligand, in such applications. Notably, precatalyst 2b
operates at very low catalyst loadings (0.01−1.0 mol % Fe),
requires the use of only 1 equiv of phenylsilane reductant, and
exhibits the broadest scope (37 examples total) of any reported
iron precatalyst for carbonyl hydrosilylation at room temper-
ature. Included for the first time in the room-temperature
catalytic survey reported herein are examples of iron-catalyzed
hydrosilylations of benzophenones, sterically demanding acyclic
dialkyl ketones, heteroaryl acetophenones, and esters en route
to alcohols. While we are still working toward developing a
better understanding of the factors that contribute to the
desirable catalytic profile exhibited by 2b, it is feasible that
redox noninnocence involving the N-phosphinoamidinate
ligand may figure prominently in this regard, as has been
implicated in the case of alternative classes of carbonyl
Figure 3. Hydrosilylation of esters to alcohols. Reaction conditions:
ester (0.4 mmol), phenylsilane (0.4 mmol), toluene (250 μL), 4 h
(
unoptimized). Percent GC conversions are given under the
compounds in Roman type, amounts of 2b in mole percent are
given in parentheses, and isolated yields are given in italics. Calibrated
GC data confirmed that differences between GC yield and isolated
yield can be attributed entirely to losses upon isolation, rather than
due to byproduct formation. See the Supporting Information for
additional details. Legend: (a) GC conversion (mol % of [Fe{N-
reduction catalysts featuring diiminodiphosphine and bis-
5,6,9,32
(
imino)pyridine coligands.
Our efforts to evaluate these
and other mechanistic possibilities will be the subject of future
reports.
(
SiMe ) } ]).
3 2 2
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all experiments
were conducted under nitrogen in a glovebox apparatus or using
standard Schlenk techniques. Dry, deoxygenated solvents were used
unless otherwise indicated. Toluene, pentane, and benzene were
deoxygenated and dried by sparging with nitrogen and subsequent
passage through a double-column solvent purification system with one
column packed with activated alumina and one column packed with
activated Q5. Tetrahydrofuran and diethyl ether were dried over Na/
phenethyl phenylacetates were efficiently reduced to 2-
phenylethanol (4a) in the presence of catalytic amounts of
2
accommodated without over-reduction to toluene, as were
para-substituted variants featuring chloro, methoxy, and
trifluoromethyl groups, enabling high conversions to the
b (0.25−1.0 mol %). The parent methyl benzoate was well
5
585
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benzophenone and distilled under nitrogen. Benzene-d (Cambridge
reflux condenser attached and placed under vacuum. After 20 min, the
white solid was dissolved in ca. 150 mL of THF and cooled to −78 °C.
6
Isotopes) was degassed via three repeated freeze−pump−thaw cycles.
n
CDCl (Cambridge Isotopes) was used as received. Acetophenone and
BuLi (2.85 mL, 2.5 M in hexanes, 7.13 mmol) was added to the
3
α-tetralone were distilled and then degassed via three repeated freeze−
pump−thaw cycles. All other liquid ketones, aldehydes, and esters
were degassed via three repeated freeze−pump−thaw cycles. All
solvents, liquid ketones, aldehydes, and esters were stored over
activated 4 Å molecular sieves for a minimum of 12 h prior to use.
Silanes (Gelest) were stored as received over 4 Å molecular sieves. All
solid ketones, aldehydes, and esters were degassed under vacuum for a
minimum of 1 h and stored under nitrogen. Flash column
chromatography was performed on silica gel (SiliaFlash P60, Silicycle).
cooled solution dropwise over 10 min. Upon complete addition of
n
BuLi, the white suspension was warmed to room temperature over 2
t
h. After 2 h, ClP( Bu)2 (1.36 mL, 7.13 mL) was added to the
suspension dropwise at room temperature over 10 min. The reaction
mixture was then stirred at reflux temperature for 18 h. The resulting
clear yellow reaction mixture was cooled to room temperature, and the
volatile components were removed in vacuo. The residue was
extracted with 20 mL of benzene and filtered through Celite, and
the filtrate was concentrated in vacuo. The sample was then triturated
with pentane (3 × 2 mL) and dried in vacuo to yield 1a as an off-white
1
13
31
H, C, and P NMR characterization data for compounds 1a,b were
collected at 300 K on a spectrometer operating at 500.1, 125.8, and
02.5 MHz (respectively), with chemical shifts reported in parts per
1
amorphous solid (1.85 g, 70%). Mp: 96−98 °C. H NMR (500 MHz,
2
benzene-d ): δ 7.92 (d, 2H, J = 8 Hz, H ), 7.24 (apparent t, 2H, J =
6
arom
1
13
million downfield of SiMe (for H, C) and 85% H PO in D O (for
4
3
4
2
8 Hz, Harom), 7.18 (apparent d, 1H, J = 8 Hz, Harom), 7.11 (apparent d,
3
1
1
13
P). H and C NMR chemical shift assignments are based on data
2
H, J = 8 Hz, Harom), 6.95 (apparent t, 1H, J = 8 Hz, Harom), 5.15 (br d,
obtained from C-DEPTQ, 1H− H COSY, H− C HSQC, and
13
1
1
13
3
1H, NH), 2.36 (s, 6H, ArMe ), 0.81 (d, 18H, J = 12 Hz, PCMe3).
2 PH
1
13
1
13
13
1
H− C HMBC NMR experiments. H and C NMR characterization
C{ H} NMR (300 K, 125.8 MHz, benzene-d ): δ 157.9 (d, C
J =
arom,
6
2
data for compounds 2a−c and alcohol products 3a−z,aa,ab and 4a−g
were collected at 300 K on a spectrometer operating at 300.1 and 75.5
MHz (respectively) with chemical shifts reported in parts per million
15 Hz), 147.4 (Carom) 137.7 (sp Camidine), 130.7 (CHarom), 129.9
CHarom), 129.4 (CHarom), 129.2 (Carom), 128.2 (overlapped with
C D , CHarom) 123.9 (CHarom), 34.7 (d, J = 24 Hz, PCMe ), 28.4 (d, J
(
6
6
3
35
39
3
1
1
downfield of SiMe . [M{N(SiMe ) } ] (M = Fe, Co) and amidines
4
3
2
2
= 15 Hz, PCMe ), 19.0 (ArMe ). P{ H} NMR (300 K, 202.5 MHz,
3 2
−
1
were prepared according to literature procedures. Infrared spectra
benzene-d ): δ 61.9. IR (thin film, cm ): 3315 (m, N−H), 1623 (s,
6
+
were recorded as thin films between KBr plates using an FT-IR
NC). HRMS (ESI): [M + H] calcd for C H N P 369.2454,
23
34
2
−1
spectrometer at a resolution of 4 cm . X-ray data collection was
carried out by Dr. Robert MacDonald and Dr. Michael J. Ferguson at
the University of Alberta X-ray Crystallography Laboratory,
Edmonton, Alberta, Canada. Magnetic moments (Evans method)
found 369.2465.
1
Synthesis of 1b. N -(2,6-Diisopropylphenyl)benzamidine (6.20 g,
2
2.1 mmol) was placed in an oven-dried round-bottom flask with a
reflux condenser attached and placed under vacuum. After 20 min, the
white solid was dissolved in ca. 250 mL of THF and cooled to −78 °C.
46
were determined according to literature procedures. GC data were
obtained on an instrument equipped with an Astec CHIRALDEX B-
PH 30 m, 0.25 mm i.d. column. The following method was used: 90
n
BuLi (8.84 mL, 2.5 M in hexanes, 22.1 mmol) was added to the
cooled solution dropwise over 10 min. Upon complete addition of
°
C, 5 min; 10 °C/min to 180 °C; 180 °C, 10 min.
n
BuLi, the white suspension was warmed to room temperature over 2
General Procedure for Determining Conversions of Carbon-
t
h. After 2 h, ClP Bu (3.99 g, 4.20 mL) was added to the suspension
2
yl Substrates (GP1). In an inert-atmosphere glovebox, the carbonyl
substrate (0.4 mmol), phenylsilane (49 μL, 0.4 mmol), and a stirbar
were placed in an oven-dried screw-capped vial. 2b (0.01−1 mol %)
was then added as a stock solution (0.16−16 mM) in toluene (250
μL), the vial was sealed with a cap containing a PTFE septum, and the
contents were stirred in the glovebox for 4 h. After 4 h, the vial was
removed from the glovebox and the contents were hydrolyzed with
dropwise at room temperature over 5 min. The reaction mixture was
then stirred at reflux temperature for 18 h. The resulting clear yellow
reaction mixture was cooled to room temperature, and the volatile
components were removed in vacuo. The residue was extracted with
2
0 mL of benzene and filtered through Celite, and the filtrate was
concentrated in vacuo. The sample was then washed with pentane (3
2 mL) and dried in vacuo to yield 1b as an off-white amorphous
×
10% NaOH (1 mL) and stirred for 1 h. The organic layer was
1
solid (8.10 g, 86%). Mp: 97−99 °C. H NMR (500 MHz, benzene-
extracted with Et O (3 × 2 mL), dried over MgSO , and concentrated
2
4
d ): δ 7.92 (d, 2H, J = 8 Hz, H ), 7.21−7.26 (m, overlapping
6
arom
under reduced pressure. The crude residue was then analyzed by GC
1
resonances, 4H, Harom), 7.12−7.15 (m, 2H, Harom), 5.15 (d, 1H, NH, J
or H NMR to determine the conversion of the substrate.
3
=
9 Hz), 3.34 (m, 2H, CH
), 1.38 (d, 6H, J = 7 Hz, CH3isopropyl),
isopropyl
General Procedure for Isolation of Carbonyl Substates (0.4
mmol scale) (GP2). In an inert-atmosphere glovebox, the carbonyl
substrate (0.4 mmol), phenylsilane (49 μL, 0.4 mmol), and a stirbar
were placed in an oven-dried screw-capped vial. 2b (0.01−1 mol %)
was then added as a stock solution (0.16−16 mM) in toluene (250
μL), the vial was sealed with a cap containing a PTFE septum, and the
contents were stirred in the glovebox for 4 h. After 4 h, the vial was
removed from the glovebox and the contents were hydrolyzed with
3
3
1
.32 (d, 6H, J = 7 Hz, CH
), 0.84 (d, 18H, J = 12 Hz,
3isopropyl
PH
13
1
PCMe3). C{ H} NMR (300 K, 125.8 MHz, benzene-d ): δ 157.2 (d,
Carom, J = 16 Hz), 144.8 (Carom), 139.2 (Carom), 137.6 (sp Camidine),
6
2
1
30.4 (CHarom), 129.6 (CHarom), 128.3 (overlapped with C D ,
6 6
CHarom), 124.4 (CHarom), 123.9 (CHarom), 34.5 (d, J = 23 Hz,
PCMe ), 29.4 (CH
), 28.3 (d, J = 15 Hz, PCMe ), 24.8
3
isopropyl
3
31
1
(
CH3isopropyl), 22.3 (CH3isopropyl). P{ H} NMR (300 K, 202.5 MHz,
−1
benzene-d ): δ 61.4. IR (thin film, cm ): 3315 (m, N-H), 1625 (s,
1
0% NaOH (1 mL) and stirred for 1 h. The organic layer was
6
+
NC). HRMS (ESI): [M + H] calcd for C H N P 425.3100,
extracted with Et O (3 × 2 mL), dried over MgSO , and concentrated
27 42
2
2
4
found 425.3080. A single crystal suitable for X-ray diffraction analysis
was grown from a concentrated pentane solution at −35 °C.
Synthesis of 2a. A solution of 1a (0.150 g, 0.42 mmol) in ca. 2 mL
under reduced pressure. The crude residue was then purified via flash
column chromatography.
General Procedure for Isolation of Carbonyl Substrates (1
mmol scale) (GP3). In an inert-atmosphere glovebox, the carbonyl
substrate (1 mmol), phenylsilane (123 μL, 1 mmol), and a stirbar were
placed in an oven-dried screw-capped vial. 2b (0.01−1 mol %) was
then added as a stock solution (0.16−16 mM) in toluene (625 μL),
and the vial was sealed with a cap containing a PTFE septum and
stirred in the glovebox for 4 h. After 4 h, the vial was removed from the
glovebox and the contents were hydrolyzed with 10% NaOH (1 mL)
of pentane was added via pipet to a solution of [Fe{N(SiMe ) } ]
3 2 2
(0.158 g, 0.42 mmol) in ca. 1 mL of pentane. The reaction mixture was
allowed to sit at room temperature, and a color change from pale green
to amber was observed over 1 h. The reaction mixture was then
filtered through Celite to remove a small amount of a fine white
precipitate and concentrated to ca. 0.5 mL in vacuo. The concentrated
solution was placed in the freezer at −35 °C and was allowed to sit for
1 h. After 1 h the brown supernatant was decanted and the green-
yellow solid was washed with cold (−35 °C) pentane (2 × 0.5 mL).
The volatile components were removed in vacuo to yield 2a as a
and stirred for 1 h. The organic layer was extracted with Et O (3 × 3
2
mL), dried over MgSO , and concentrated under reduced pressure.
4
The crude residue was purified via flash column chromatography.
1
Synthesis of 1a. N -(2,6-Dimethylphenyl)benzamidine (2.00 g,
.13 mmol) was placed in an oven-dried round-bottom flask with a
crystalline (parallelipiped) green-yellow solid (0.146 g, 60%). Mp:
1
7
107−109 °C. H NMR (300.1 MHz, benzene-d ): δ 98.7 (1H), 46.1
6
5
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Article
4
7
(
2H), 31.6 (2H), 28.8 (18H), 7.5 (2H) −14.2 (6H), −44.0 (18H),
Hg Test for Homogeneity.
In an inert-atmosphere glovebox,
−
65.9 (1H). μ (benzene-d ): 5.04 μ , S = 2. Anal. Calcd for
acetophenone (47 μL, 0.4 mmol), phenylsilane (49 μL, 0.4 mmol), Hg
(8.0 g, 40 mmol), and a stirbar were placed in an oven-dried screw-
capped vial. Precatalyst 2b (0.015 mol %) was then added as a stock
solution (0.24 mM) in toluene (250 μL), the vial was sealed with a cap
containing a PTFE septum, and the contents were stirred in the
glovebox for 4 h. After 4 h, the vial was removed from the glovebox
and the contents were hydrolyzed with 10% NaOH (1 mL) and stirred
for 1 h. The organic layer was extracted with Et O (3 × 2 mL), dried
over MgSO , and concentrated under reduced pressure. The crude
residue was then analyzed by GC to determine the conversion of the
substrate.
Solvent-Free Reaction. In an inert-atmosphere glovebox, 2b
eff
6
B
C H FeN PSi : C, 59.67; H, 8.63; N, 7.20. Found: C, 58.61; H, 8.60;
29
50
3
2
N, 7.02. A single crystal suitable for X-ray diffraction analysis was
grown from a concentrated pentane solution at −35 °C.
Synthesis of 2b. A solution of 1b (0.150 g, 0.35 mmol) in ca. 1
mL of pentane was added via pipet to a solution of [Fe{N(SiMe ) } ]
3
2 2
(
0.133 g, 0.35 mmol) in ca. 1 mL of pentane. The reaction mixture was
allowed to sit at room temperature, and a color change from pale green
to amber was observed over 1 h. The reaction mixture was then
filtered through Celite to remove a small amount of a fine white
precipitate and concentrated to ca. 0.5 mL in vacuo. The concentrated
solution was placed in the freezer at −35 °C and was allowed to sit for
2
4
(
0.015 mol %) was placed in an oven-dried screw-capped vial as a
1
h. After 1 h the brown supernatant was decanted and the green-
stock solution (0.24 mM) in toluene (250 μL) and the solvent was
removed in vacuo. Acetophenone (47 μL, 0.4 mmol), phenylsilane (79
μL, 0.4 mmol), and a stirbar were added to the vial, the vial was sealed
with a cap containing a PTFE septum, and the contents were stirred in
the glovebox for 4 h. After 4 h, the vial was removed from the glovebox
and the contents were hydrolyzed with 10% NaOH (1 mL) and stirred
yellow solid was washed with cold (−35 °C) pentane (2 × 0.5 mL).
The volatile components were removed in vacuo to yield 2b as a
crystalline (paralelipiped) green-yellow solid (0.140 g, 62%). Mp:
1
1
(
−
(
43−145 °C. H NMR (300.1 MHz, benzene-d ): δ 98.8 (1H), 45.9
6
2H), 31.7 (2H), 24.1−30.5 (overlapping resonances, 26H), 8.8 (2H),
43.2 (18H), −55.5 to −57.6 (overlapping respnances, 7H). μ
eff
for 1 h. The organic layer was extracted with Et O (3 × 2 mL), dried
2
benzene-d ): 4.95 μ , S = 2. Anal. Calcd for C H FeN PSi : C,
6 B 33 58 3 2
over MgSO , and concentrated under reduced pressure. The crude
4
61.95; H, 9.14; N, 6.57. Found: C, 61.29; H, 9.02; N, 6.45. A single
residue was then analyzed by GC to determine the conversion of the
substrate.
crystal suitable for X-ray diffraction analysis was grown from a
concentrated pentane solution at −35 °C.
Comparative Catalytic Experiments Employing [Fe{N-
Synthesis of 2c. A solution of 1b (0.200 g, 0.47 mmol) in ca. 1 mL
of pentane was added via pipet to a solution of [Co{N(SiMe ) } ]
(
SiMe ) } ]. In an inert-atmosphere glovebox, the carbonyl substrate
3 2 2
3
2 2
(
0.4 mmol), phenylsilane (49 μL, 0.4 mmol), and a stirbar were added
(
0.180 g, 0.47 mmol) in ca. 1 mL of pentane. A color change from
to an oven-dried screw-capped vial. [Fe{N(SiMe ) } ] (0.01−1 mol
3
2 2
deep green to red was observed immediately upon addition. The
reaction mixture was stirred for 1 h and was then filtered through
Celite and concentrated to ca. 0.5 mL in vacuo. The concentrated
solution was placed in the freezer at −35 °C and was allowed to sit for
%
) was then added as a stock solution (0.16−16 mM) in toluene (250
μL), and the vial was sealed with a cap containing a PTFE septum and
stirred in the glovebox for 4 h. After 4 h, the vial was removed from the
glovebox and the contents were hydrolyzed with 10% NaOH (1 mL)
1
h. After 1 h the deep red supernatant was decanted and the red solid
and stirred for 1 h. The organic layer was extracted with Et O (3 × 2
2
was washed with cold (−35 °C) pentane (2 × 0.5 mL). The volatile
mL), dried over MgSO , and concentrated under reduced pressure.
4
components were removed in vacuo to yield 2c as a crystalline
The crude residue was then analyzed by GC to determine the
conversion of the substrate.
1
(
(
(
paralelipiped) red solid (0.205 g, 68%). Mp: 144−146 °C. H NMR
300.1 MHz, benzene-d ): δ 108.6 (2H), 46.4 (2H), 35.0 (1H), 16.1
6
2H), 13.2 (overlapping resonances, 7H), 7.2−7.6 (overlapping
ASSOCIATED CONTENT
resonances, 19H), −44.4 (18H), −54.9 (1H), −55.7 (6H). μ
■
eff
3
(
benzene-d ): 4.08 μ , S = / . Anal. Calcd for C H CoN PSi : C,
6 B 2 33 58 3 2
S
*
Supporting Information
61.65; H, 9.09; N, 6.54. Found: C, 61.01; H, 9.01; N, 6.31. A single
Text, figures, tables, and CIF files giving detailed experimental
procedures, characterization data, NMR spectra, and crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
crystal suitable for X-ray diffraction analysis was grown from a
concentrated pentane solution at −35 °C.
TOF Measurements. In an inert-atmosphere glovebox, acetophe-
none (47 μL, 0.4 mmol), phenylsilane (49 μL, 0.4 mmol), and a stirbar
were added to each of six oven-dried, screw-capped glass vials.
Precatalyst 2b (0.015 mol %) was then added as a stock solution (0.24
mM) in toluene (250 μL) to each vial, the vials were sealed with caps
containing PTFE septa, and magnetic stirring was initiated. After 5
min, one vial was removed from the glovebox and the contents were
hydrolyzed with 10% NaOH (1 mL) and stirred for 1 h. This process
was repeated every 5 min until all six reaction mixtures had been
AUTHOR INFORMATION
■
Corresponding Authors
*
*
*
E-mail for O.L.S.: sydorol@cpchem.com
E-mail for M.S.: mark.stradiotto@dal.ca
E-mail for L.T.: laura.turculet@dal.ca
hydrolyzed. The organic layers were extracted with Et O (3 × 2 mL),
2
Notes
dried over MgSO , and concentrated under reduced pressure. The
4
crude residues were then analyzed by GC to determine the conversion
of the substrate at different intervals. A 59% conversion of
acetophenone was observed after 10 min, resulting in a calculated
The authors declare no competing financial interest.
−1
ACKNOWLEDGMENTS
TOF of 23600 h .
■
Reaction in the Absence of Light. In an inert-atmosphere
glovebox, acetophenone (47 μL, 0.4 mmol), phenylsilane (49 μL, 0.4
mmol), and a stirbar were placed in an oven-dried screw-capped vial
and wrapped in aluminum foil. 2b (0.015 mol %) was then added as a
stock solution (0.24 mM) in toluene (250 μL), the vial was sealed with
a cap containing a PTFE septum, and the contents were stirred in the
glovebox for 4 h. After 4 h, the vial was removed from the glovebox
and the contents were hydrolyzed with 10% NaOH (1 mL) and stirred
We thank Chevron Phillips Chemical Co. for supporting this
research and permission to publish. M.S. and L.T. are also
grateful to the NSERC of Canada and Dalhousie University for
their support of this work. Dr. Michael Lumsden (NMR-3,
Dalhousie) is thanked for ongoing technical assistance in the
acquisition of NMR data, and Mr. Xiao Feng (Maritime Mass
Spectrometry Laboratories, Dalhousie) is thanked for technical
assistance in the acquisition of mass spectrometric data.
Additionally, we thank Dr. Kevin Johnson and Prof. Paul
Hayes (University of Lethbridge) for elemental analysis
services.
for 1 h. The organic layer was extracted with Et O (3 × 2 mL), dried
2
over MgSO , and concentrated under reduced pressure. The crude
4
residue was then analyzed by GC to determine the conversion of the
substrate.
5
587
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Article
(
38) The iron-catalyzed reduction of esters to aldehydes under mild
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dx.doi.org/10.1021/om400883u | Organometallics 2013, 32, 5581−5588