Hydrosilylation of aldehydes and ketones catalyzed by Nickel PCP-Pincer hydride complexes

Article abstract of DOI:10.1021/om800948f

Nickel PCP-pincer hydride complexes catalyze chemoselective hydrosilylation of C=O bonds of aldehydes and ketones in the presence of other functional groups. The mechanism involves C=O insertion into a nickel-hydrogen bond, followed by cleavage of the new

Full text of DOI:10.1021/om800948f

582
Organometallics 2009, 28, 582–586
Hydrosilylation of Aldehydes and Ketones Catalyzed by Nickel
PCP-Pincer Hydride Complexes
Sumit Chakraborty, Jeanette A. Krause, and Hairong Guan*
Department of Chemistry, UniVersity of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172
ReceiVed October 1, 2008
Nickel PCP-pincer hydride complexes catalyze chemoselective hydrosilylation of CdO bonds of
aldehydes and ketones in the presence of other functional groups. The mechanism involves CdO insertion
into a nickel-hydrogen bond, followed by cleavage of the newly formed Ni-O bond with a silane.
Nickel hydride complexes are of great importance in the
Details of their existence during catalytic processes largely rely
on computational studies.4f,g,5b In contrast, a number of discrete
and stable nickel hydride complexes have been prepared and
many stoichiometric transformations involving these hydrides
have been reported.4c,11-15 However, very few of these com-
plexes are catalytically competent; of the known well-defined
catalytic systems, nickel hydride complexes are solely used as
catalysts for olefin isomerization and oligomerization.15f,16 This
has prompted us to study new reactivity of nickel hydride
complexes and to explore their potential as relatively inexpensive
metal catalysts for various organic reactions.
research areas of homogeneous catalysis, coordination chemistry,
and enzymatic reaction mechanisms. They are often postulated
as key intermediates in a variety of nickel-catalyzed organic
transformations.1-10 These hypothesized nickel hydrides are
usually too reactive to allow direct observation of reaction
intermediates or thorough investigation of reaction mechanisms.
* To whom correspondence should be addressed. E-mail:
hairong.guan@uc.edu.
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We have focused our initial studies on nickel hydrides
supported by pincer ligands. Such a ligand set has great
flexibility in terms of steric and electronic modifications. In
addition, square-planar d8 metals with pincer ligands have shown
numerous applications in organic synthesis, polymerization, and
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10.1021/om800948f CCC: $40.75
2009 American Chemical Society
Publication on Web 12/31/2008

Nickel PCP-Pincer Hydride Complexes
Organometallics, Vol. 28, No. 2, 2009 583
Figure 1. Structure of [2,6-(tBu2PO)2C6H3]NiH (3b) (50% probability level). Selected bond lengths (Å) and angles (deg): Ni-H ) 1.37(3),
Ni-C1 ) 1.892(3), Ni-P1 or Ni-P1A ) 2.1160(5); P1-Ni-P1A ) 166.26(3), C1-Ni-H ) 180.000(9), P1-Ni-C1 or P1A-Ni-C1 )
83.132(17).
molecular sensing.17 The first synthesis of a nickel pincer
hydride complex, specifically [2,6-(tBu2PCH2)2C6H3]NiH, was
reported by Shaw in 1976, although it was not fully character-
ized.18 Recently, the Ozerov group15i and the Liang group15k,m
have independently isolated nickel hydride complexes with
amido diphosphine (PNP-pincer) ligands. Liang and co-workers
have also demonstrated that this class of compounds is capable
of activating arene C-H bonds intermolecularly15k and under-
goes CdC bond insertion.15m In any case, we are not aware of
any reactions catalyzed by nickel pincer hydride complexes.
Herein we describe our preliminary results on the reactivity of
new nickel pincer hydrides that led to the discovery of efficient
nickel catalysts for hydrosilylation of aldehydes and ketones.
To the best of our knowledge, this catalytic system represents
one of the very few examples of nickel-catalyzed hydrosilylation
of carbonyl compounds.19,20
Results and Discussion
Encouraged by the excellent reactivity seen in Pd-catalyzed
cross-coupling21 and Ir-catalyzed alkane dehydrogenation reac-
tions,22 we chose diphosphinites 1a-c as the pincer ligands for
the synthesis of nickel hydrides. First, nickel chlorides 2a,c were
prepared, as reported in the literature,23 through cyclometalation
of the diphosphinites with NiCl2. Nickel chloride 2b was
synthesized similarly and characterized by 1H NMR, 31P NMR,
elemental analysis, and X-ray crystallography (see the Support-
ing Information). The desired nickel hydrides 3a,b were isolated
as yellow-orange solids in good isolated yields from treatment
1
of nickel chlorides with LiAlH4 (Scheme 1). The H NMR
spectra of 3a,b in C6D6 revealed characteristic hydride reso-
nances as triplets at δ -7.89 (JHP ) 55.2 Hz) and δ -7.96 (JHP
) 53.2 Hz), respectively. The hydride ligand in 3b was also
located by X-ray diffraction of its single crystal (Figure 1).24
(15) Other well-defined nickel hydride systems: (a) Srivastava, S. C.;
Bigorgne, M. J. Organomet. Chem. 1969, 18, P30–P32. (b) Green, M. L. H.;
Saito, T. Chem. Commun. 1969, 208. (c) Schunn, R. A. Inorg. Chem. 1970,
9, 394–395. (d) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 4217–4222.
(e) Nesmeyanov, A. N.; Isaeva, L. S.; Lorens, L. N. J. Organomet. Chem.
1977, 129, 421–427. (f) Rigo, P.; Bressan, M.; Basato, M. Inorg. Chem.
1979, 18, 860–863. (g) Darensbourg, D. J.; Darensbourg, M. Y.; Goh, L. Y.;
Ludvig, M.; Wiegreffe, P. J. Am. Chem. Soc. 1987, 109, 7539–7540. (h)
Chen, W.; Shimada, S.; Tanaka, M.; Kobayashi, Y.; Saigo, K. J. Am. Chem.
Soc. 2004, 126, 8072–8073. (i) Ozerov, O. V.; Guo, C.; Fan, L.; Foxman,
B. M. Organometallics 2004, 23, 5573–5580. (j) Clement, N. D.; Cavell,
K. J.; Jones, C.; Elsevier, C. J. Angew. Chem., Int. Ed. 2004, 43, 1277–
1279. (k) Liang, L.-C.; Chien, P.-S.; Huang, Y.-L. J. Am. Chem. Soc. 2006,
128, 15562–15563. (l) She, L.; Li, X.; Sun, H.; Ding, J.; Frey, M.; Klein,
H.-F. Organometallics 2007, 26, 566–570. (m) Liang, L.-C.; Chien, P.-S.;
Lee, P.-Y. Organometallics 2008, 27, 3082–3093.
(19) A comprehensive review covering transition metal-catalyzed hy-
drosilylation of carbonyl compounds and imines: D´ıez-Gonza´lez, S.; Nolan,
S. P. Org. Prep. Proced. Int. 2007, 39, 523–559.
(20) NiCl2/additive-catalyzed hydrosilylation of carbonyls accompanied
by dehydrogenative hydrosilylation and other side reactions: (a) Frainnet,
E.; Martel-Siegfried, V.; Brousse, E.; Dedier, J. J. Organomet. Chem. 1975,
85, 297–310. (b) Frainnet, E.; Bourhis, R.; Simonin, F.; Moulines, F. J.
Organomet. Chem. 1976, 105, 17–31. Hydrosilylation of both carbonyls
and alkenes (thus non-selective hydrosilylation) catalyzed by activated nickel
metal: (c) Lee, S. J.; Kim, T. Y.; Park, M. K.; Han, B. H. Bull. Korean
Chem. Soc. 1996, 17, 1082–1085. A brief description of ketone hydrosi-
lylation catalyzed by (indenyl)Ni(PPh3)+: (d) Fontaine, F.-G.; Nguyen, R.-
V.; Zargarian, D. Can. J. Chem. 2003, 81, 1299–1306. Hydrosilylation of
mixtures of ketones catalyzed by Ni(COD)2/DIOP: (e) Irrgang, T.; Schareina,
T.; Kempe, R. J. Mol. Catal. A: Chem. 2006, 257, 48–52. Hydrosilylation
of carbonyls with 1,1′-bis(dimethylsilyl)ferrocene catalyzed by Ni(PEt3)4:
(f) Kong, Y. K.; Kim, J.; Choi, S.; Choi, S.-B. Tetrahedron Lett. 2007, 48,
2033–2036.
(16) (a) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 6777–6784. (b)
Green, M. L. H.; Munakata, H. J. Chem. Soc., Dalton Trans. 1974, 269–
272.
(17) (a) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. Acc. Chem.
Res. 1998, 31, 423–431. (b) Albrecht, M.; van Koten, G. Angew. Chem.,
Int. Ed. 2001, 40, 3750–3781. (c) Singleton, J. T. Tetrahedron 2003, 59,
1837–1857. (d) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103,
1759–1792. (e) Liang, L.-C. Coord. Chem. ReV. 2006, 250, 1152–1177. (f)
Nishiyama, H. Chem. Soc. ReV. 2007, 36, 1133–1141.
(21) (a) Bedford, R. B.; Draper, S. M.; Scully, P. N.; Welch, S. L. New
J. Chem. 2000, 24, 745–747. (b) Morales-Morales, D.; Grause, C.; Kasaoka,
K.; Redo´n, R.; Cramer, R. E.; Jensen, C. M. Inorg. Chim. Acta 2000, 300-
302, 958–963.
(18) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020–1024.

584 Organometallics, Vol. 28, No. 2, 2009
Chakraborty et al.
Scheme 1
Scheme 3
Scheme 4
Scheme 2
likely to undergo ꢀ-hydride elimination than primary alkoxides.
When a solution of hydride 3a in C6D6 was treated with an
equimolar amount of PhCOCH3 at room temperature for 24 h,
the insertion product consisted of only 17 mol % of all nickel
species. Reaction with PhCOPh under similar conditions did
not yield any alkoxide species. To complete a catalytic cycle,
complex 4a was mixed with various silanes to re-form hydride
3a. Both PhSiH3 and Ph2SiH2 were identified as excellent silyl
reagents for the nickel benzyloxide; complete regeneration of
3a and release of the silyl ether product were seen within a few
minutes (Scheme 2). Other silanes such as (EtO)3SiH and
poly(methylhydrosiloxane) (PMHS) regenerated hydride 3a after
longer reaction times (>1 h), while Et3SiH showed no reaction
with complex 4a.
Having established the protocols of aldehyde insertion and
hydride regeneration, we set out to investigate the catalytic
activity of hydrides 3a,b for the hydrosilylation of benzaldehyde
(Scheme 3). With PhSiH3 or Ph2SiH2 as the silyl reagents,
hydride 3a catalyzed the hydrosilylation reaction efficiently; the
reaction was complete within 2 h at room temperature with
catalyst loading as low as 0.2 mol %. Consistent with the
stoichiometric experiments, catalytic reactions with (EtO)3SiH,
PMHS, and Et3SiH were much slower than those with PhSiH3
and Ph2SiH2.28 A control experiment in the absence of the nickel
hydride 3a showed no significant hydrosilylation, even at 60
°C for 5 days. Hydride 3b also catalyzed the hydrosilylation
reaction, albeit with a slower rate.29
The scope of this catalytic system was studied using 0.2 mol
% of 3a in the presence of a slight excess of PhSiH3, and the
reduction products were isolated as alcohols following basic
hydrolysis of the silyl ethers (Scheme 4). As shown in Table 1,
the hydrosilylation reaction was tolerant of many functional
groups, including OMe (entry 2), NMe2 (entry 3), Cl (entry 4),
NO2 (entry 5), and CN (entry 10). It would be difficult to
rationalize the electronic effect of substituents on the relative
hydrosilylation rates, since both the aldehyde with an electron-
donating group (entry 2) and the one with an electron-
withdrawing group (entry 4) are less reactive than the unsub-
stituted benzaldehyde (entry 1). An explanation of these results
will require more detailed kinetic studies on individual steps of
the potential catalytic cycle for each substrate. These studies
will be reported in due course. For R,ꢀ-unsaturated aldehydes
(entries 9 and 12), only the 1,2-addition products were obtained.
These results are in contrast to other Ni-catalyzed30 or Cu-
catalyzed31 hydrosilylations of R,ꢀ-unsaturated carbonyl com-
The attempted synthesis of 3c (R ) Ph) from the reduction of
2c with LiAlH4, NaBH4, or LiEt3BH gave intractable products.
Nickel hydride 3a appeared inert toward CdC and Ct C bond
insertion; no appreciable insertion product was detected when
a solution of 3a in toluene-d8 was treated with 1-hexene, trans-
3-hexene, styrene, 3-hexyne, 2,3-dimethyl-1,3-butadiene, or
methyl methacrylate at room temperature. On the other hand,
fast CdO insertion of PhCHO into the same nickel hydride was
observed under similar conditions (Scheme 2). 1H NMR
spectroscopy showed the formation of nickel benzyloxide 4a;
the intensity of a new resonance at δ 4.88 (singlet) increased
as the intensities of both hydride 3a (δ -7.90) and PhCHO
(δ 9.62) decreased. The insertion reaction was complete within
30 min and provided 4a in >95% NMR yield. Complex 4a
was also independently generated from the metathesis between
2a and PhCH2ONa, although the isolation of compound 4a in
an analytically pure form was unsuccessful. Nevertheless, the
reaction shown in Scheme 2 is the first directly observed
insertion of an organic carbonyl group into a nickel-hydrogen
bond.25 The reverse step of aldehyde insertion, ꢀ-hydride
elimination of a metal alkoxide, is more commonly observed
for late transition metals.26 It is possible that the insertion of
PhCHO into 3a is reversible, with an equilibrium favoring the
nickel benzyloxide 4a.27 As expected, ketones were less reactive
than PhCHO, as the resulting secondary alkoxides are more
(22) Go¨ttker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem.
Soc. 2004, 126, 1804–1811.
(23) Synthesis of nickel chloride 2a: (a) Pandarus, V.; Zargarian, D.
Organometallics 2007, 26, 4321-4334. 2c: (b) Go´mez-Ben´ıtez, V.; Bal-
´
dovino-Pantaleo´n, O.; Herrera-Alvarez, C.; Toscano, R. A.; Morales-
Morales, D. Tetrahedron Lett. 2006, 47, 5059–5062.
(24) The tBu groups show some disorder. A disorder model is presented
for C10 and C12. See the Supporting Information for details.
(25) For direct observation of CdO insertion into a metal-hydrogen
bond, see the following examples. Ta-H: (a) Weinert, C. S.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 2005, 24, 5759–5766. W-H: (b) van der
Zeijden, A. A. H.; Berke, H. HelV. Chim. Acta 1992, 75, 513–522. (c) Furno,
F.; Fox, T.; Schmalle, H. W.; Berke, H. Organometallics 2000, 19, 3620–
3630. Mo-H: (d) Liang, F.; Jacobsen, H.; Schmalle, H. W.; Fox, T.; Berke,
H. Organometallics 2000, 19, 1950–1962. (e) Liang, F.; Schmalle, H. W.;
Fox, T.; Berke, H. Organometallics 2003, 22, 3382–3393. (f) Zhao, Y.;
Schmalle, H. W.; Fox, T.; Blacque, O.; Berke, H. Dalton Trans. 2006, 7,
3–85. (g) Cugny, J.; Schmalle, H. W.; Fox, T.; Blacque, O.; Alfonso, M.;
Berke, H. Eur. J. Inorg. Chem. 2006, 54, 0-552Re-H: (h) Du, G.; Fanwick,
P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007, 129, 5180–5187. Ru-
H: (i) Baratta, W.; Ballico, M.; Esposito, G.; Rigo, P. Chem. Eur. J. 2008,
14, 5588–5595. Pt-H: (j) van Leeuwen, P. W. N. M.; Roobeek, C. F.;
Orpen, A. G. Organometallics 1990, 9, 2179–2181.
(28) Partial hydrosilylation was seen with (EtO)3SiH (80% conversion
with 2.0 mol % catalyst) when the reaction was quenched after 2 h of stirring
at room temperature. Reaction with Et3SiH or PMHS under similar
conditions afforded no hydrosilylation product.
(29) When 0.2 mol % of 3b was used as the catalyst, 38 mol % of
PhCHO was converted after 2 h of reaction.
(30) (a) Boudjouk, P.; Choi, S.-B.; Hauck, B. J.; Rajkumar, A. B.
Tetrahedron Lett. 1998, 39, 3951–3952. (b) Kim, S. O.; Rhee, S.; Lee,
S. H. Bull. Korean Chem. Soc. 1999, 20, 773–774.
(31) Jurkauskas, V.; Sadighi, J. P.; Buchwald, S. L. Org. Lett. 2003, 5,
2417–2420.
(26) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163–1188.
(27) Experiments designed to elucidate the reversibility of aldehyde
insertion will be reported later.

Nickel PCP-Pincer Hydride Complexes
Organometallics, Vol. 28, No. 2, 2009 585
Table 1. Hydrosilylation of Aldehydes Catalyzed by Nickel Pincer Hydride
Scheme 5. Catalytic Cycle for the Hydrosilylation of
Aldehydes
pounds, where only the 1,4-addition products were isolated. The
high selectivity for CdO over CdC hydrosilylation was also
observed for aldehydes with isolated CdC bonds (entry 13). In
addition to substituted benzaldehydes, other aromatic (entries
6-8) and aliphatic (entries 11-13) aldehydes were viable
substrates for hydrosilylation. In each reaction studied, no side
products such as enoxysilanes and disiloxanes were observed,
making our nickel system superior to other nickel systems
reported in the literature.20a,b An aldehyde bearing a cyano group
on the ortho position (entry 10) gave a mixture of two products:
the expected 2-cyanobenzyl alcohol 5 and the lactone 6.
Compound 6 resulted from cyclization of 5 under the hydrolysis
conditions. Ketones were much less reactive catalytically. Only
partial hydrosilylation was observed for acetophenone (18%),
cyclohexanone (60%), and benzophenone (6%), even at an
elevated temperature (70 °C, 24 h) using a higher catalyst
loading (1 mol %).
Chalk-Harrod mechanism33 for alkene hydrosilylation, and it
has been proposed for Rh-catalyzed hydrosilylation of carbonyl
substrates.34 In addition, oxidative addition of silanes or Si-H
σ-bond coordination to a nickel center has been reported.15h,35
In view of the stoichiometric experiments shown in Scheme
2, a catalytic cycle was proposed for the current hydrosilylation
system (Scheme 5). A similar mechanism has also been
proposed by Nolan in his Cu(I)/carbene-catalyzed hydrosilyla-
tion reactions.32 Alternatively, nickel hydrides 3a,b might
activate the silanes prior to their interaction with the carbonyl
groups. Such a process would resemble the well-known
(32) (a) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P. Organome-
tallics 2004, 23, 1157–1160. (b) D´ıez-Gonza´lez, S.; Kaur, H.; Zinn, F. K.;
Stevens, E. D.; Nolan, S. P. J. Org. Chem. 2005, 70, 4784–4796. (c) D´ıez-
Gonza´lez, S.; Scott, N. M.; Nolan, S. P. Organometallics 2006, 25, 2355–
2358. (d) D´ıez-Gonza´lez, S.; Stevens, E. D.; Scott, N. M.; Petersen, J. L.;
Nolan, S. P. Chem. Eur. J. 2008, 14, 158–168. (e) D´ıez-Gonza´lez, S.; Nolan,
S. P. Acc. Chem. Res. 2008, 41, 349–358.
(33) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16–21.

586 Organometallics, Vol. 28, No. 2, 2009
Chakraborty et al.
was evaporated under vacuum, the desired hydride 3a was isolated
as an orange-yellow crystalline solid (405 mg, 88% yield). 1H NMR
(400 MHz, toluene-d8): δ -7.90 (t, NiH, JP-H ) 55.2 Hz, 1H),
1.08-1.17 (m, PCH(CH3)2, 24H), 2.05-2.11 (m, PCH(CH3)2, 4H),
6.74 (d, Ar, JH-H ) 8.0 Hz, 2H), 6.97 (t, Ar, JH-H ) 8.0 Hz, 1H).
31P{1H} NMR (162 MHz, toluene-d8): δ 206.56 (s). Anal. Calcd
for C18H32O2P2Ni: C, 53.90; H, 8.04. Found: C, 53.88; H, 8.17.
[2,6-(tBu2PO)2C6H3]NiH (3b). This compound was prepared in
80% yield by a procedure similar to that used for 3a. X-ray-quality
crystals were grown by layering CH3OH on a saturated THF
solution of the hydride at -35 °C and slowly allowing it to diffuse.
1H NMR (400 MHz, C6D6): δ -7.96 (t, NiH, JP-H ) 53.2 Hz,
1H), 1.30 (virtual triplet, PC(CH3)3, JP-H ) 6.8 Hz, 36H), 6.85 (d,
Ar, JH-H ) 7.6 Hz, 2H), 7.02 (t, Ar, JH-H ) 7.6 Hz, 1H). 31P{1H}
However, the mechanism involving initial silane activation is
less likely to be operating in our nickel catalytic system. We
found no reaction when 3a was treated with PhSiH3 in toluene-
d8 at room temperature or 60 °C for 24 h.
In conclusion, we have disclosed a nickel hydride system
where the insertion of carbonyl groups into nickel-hydrogen
bonds has been directly observed. The resulting nickel alkoxides
react with silanes to release the silyl ether products and re-
form the nickel hydrides. More detailed mechanistic studies
pertaining to the carbonyl insertion step, electronic influence
on the turnover-limiting step of the catalytic cycle, and the
development of more reactive nickel catalysts for ketone
hydrosilylation are the subjects of ongoing research in our
laboratory.
NMR (162 MHz, C6D6):
δ 219.35 (s). Anal. Calcd for
C22H40O2P2Ni: C, 57.80; H, 8.82. Found: C, 57.65; H, 8.85.
Synthesis of [2,6-(iPr2PO)2C6H3]NiOCH2Ph (4a). Method
A. To a solution of 3a (40 mg, 0.10 mmol) in 5 mL of toluene was
added degassed benzaldehyde (10 µL, 0.10 mmol) under an argon
atmosphere, and the resulting mixture was stirred at room temper-
ature for 1 h. Evaporating the solvent under vacuum yielded an
orange oil, and the NMR spectra of the crude product were recorded
Experimental Section
All the air-sensitive compounds were prepared and handled under
an argon atmosphere using standard Schlenk and inert-atmosphere
box techniques. All aldehyde and ketone substrates were purchased
from commercial sources and were used without further purification.
Dry and oxygen-free solvents were collected from an Innovative
Technology solvent purification system and used throughout all the
experiments. Toluene-d8 and C6D6 were distilled from Na and
benzophenone under argon. Both 1H NMR and 31P{1H} NMR
spectra were recorded on a Bruker Avance-400 MHz NMR
spectrometer. Chemical shift values in 1H NMR spectra were
referenced internally to the residual solvent resonances (δ 7.26 for
CDCl3, δ 7.15 for C6D6, and δ 2.09 for toluene-d8). The 31P{1H}
NMR spectra were referenced to an external 85% H3PO4 sample
(δ 0). Column chromatography was performed with silica gel and
solvents of commercial grade. All isolated alcohol products were
1
(see the Supporting Information). H NMR (400 MHz, toluene-
d8): δ 1.18-1.29 (m, PCH(CH3)2, 12H), 1.32-1.41 (m, PCH(CH3)2,
12H), 2.09-2.15 (m, PCH(CH3)2, 4H), 4.88 (s, OCH2Ph, 2H), 6.50
(d, Ar, JH-H ) 8.0 Hz, 2H), 6.84 (t, Ar, JH-H ) 8.0 Hz, 1H),
7.00-7.56 (m, CH2Ph, 5H). 31P{1H} NMR (162 MHz, toluene-
d8): δ 176.59 (s). Attempts to further purify 4a via recystallization
led to decomposition of the product.
Method B. Under an argon atmosphere, a suspension of 2a (87
mg, 0.2 mmol) and sodium benzyloxide (39 mg, 0.3 mmol) in 10
mL of toluene was stirred at room temperature for 24 h. The
resulting mixture was passed through a short plug of Celite to
remove NaCl, and the filtrate was concentrated under vacuum.
Again, an orange oil was obtained and its NMR spectra matched
the data above, although further purification of 4a led to decom-
position of the product.
1
known compounds and characterized by H NMR spectroscopy.
The NMR data obtained for all compounds were consistent with
the literature values. 1,3-(tBu2PO)2C6H4 (1b),22 [2,6-(iPr2PO)2-
C6H3]NiCl (2a),23a and [2,6-(Ph2PO)2C6H3]NiCl (2c)23b were
prepared as described in the literature.
Synthesis of [2,6-(tBu2PO)2C6H3]NiCl (2b). Under an argon
atmosphere 50 mL of toluene was added to a mixture of 1,3-
(tBu2PO)2C6H4 (1.20 g, 3.0 mmol) and anhydrous NiCl2 (389 mg,
3.0 mmol), giving an orange suspension. While the solution was
being boiled for 18 h, a brown precipitate formed, which was
removed by filtration after the mixture was cooled to room
temperature. The volume of the orange filtrate was reduced to 5
mL, and then Et2O was added to cause precipitation. The product
was collected by filtration, washed with Et2O, and dried under
vacuum to give an orange-green powder of 2b (725 mg, 49% yield).
X-ray-quality crystals were grown by allowing a layer of pentane
to slowly diffuse into a saturated CH2Cl2 solution of the nickel
chloride. 1H NMR (400 MHz, CDCl3): δ 1.49 (virtual triplet,
PC(CH3)3, JP-H ) 6.8 Hz, 36H), 6.38 (d, Ar, JH-H ) 8.0 Hz, 2H),
6.92 (t, Ar, JH-H ) 8.0 Hz, 1H). 31P{1H} NMR (162 MHz, CDCl3):
δ 187.20 (s). Anal. Calcd for C22H39ClO2P2Ni: C, 53.75; H, 8.00;
Cl, 7.21. Found: C, 54.11; H, 8.09; Cl, 7.25.
General Procedures for Hydrosilylation. To a flame-dried
Schlenk flask was added a solution of nickel 3a (8.0 mg, 20 µmol)
in toluene (6 mL) and an aldehyde substrate (10 mmol) under an
argon atmosphere. The resulting mixture was stirred at room
temperature for 5-10 min, after which PhSiH3 (1.48 mL, 12 mmol)
was added via a gastight syringe. The reaction mixture was stirred
at room temperature or at a higher temperature until there was no
aldehyde left (monitored by withdrawing aliquots and analyzing
their 1H NMR spectra). The reaction was then quenched by a 10%
aqueous solution of NaOH (about 10 mL) with vigorous stirring
for more than 12 h. The solution containing the alcohol product
was extracted with diethyl ether three times, dried over anhydrous
Na2SO4, and concentrated under vacuum. The desired alcohol was
further purified by flash column chromatography.
Acknowledgment. We are grateful for financial support
from the University of Cincinnati. X-ray data were collected
on a Bruker SMART6000 diffractometer which was funded
by an NSF-MRI grant (No. CHE-0215950).
Synthesis of [2,6-(iPr2PO)2C6H3]NiH (3a). Under an argon
atmosphere the suspension of LiAlH4 (872 mg, 23 mmol) and 2a
(500 mg, 1.15 mmol) in 60 mL of toluene was stirred at room
temperature for 24 h. The resulting mixture was filtered through a
short plug of Celite to give a yellow solution. After the solvent
Supporting Information Available: Spectroscopic characteriza-
tion data for compound 4a and the alcohol products shown in Table
1, as well as crystallographic data (CIF and PDF) for 2b and 3b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(34) Ojima, I.; Kogure, T.; Kumagai, M.; Horiuchi, S.; Sato, T. J.
Organomet. Chem. 1976, 122, 83–97.
(35) Steinke, T.; Gemel, C.; Cokoja, M.; Winter, M.; Fischer, R. A.
Angew. Chem., Int. Ed. 2004, 43, 2299–2302.
OM800948F