Carbonyl and ester C-O bond hydrosilylation using κ4-diimine nickel catalysts

Article abstract of DOI:10.1039/c8dt01857j

The synthesis of alkylphosphine-substituted α-diimine (DI) ligands and their subsequent addition to Ni(COD)₂ allowed for the preparation of (^iPr2PPrDI)Ni and (^tBu2PPrDI)Ni. The solid state structures of both compounds were found to feature a distorted tetrahedral geometry that is largely consistent with the reported structure of the diphenylphosphine-substituted variant, (Ph₂PPr DI)Ni. To explore and optimize the synthetic utility of this catalyst class, all three compounds were screened for benzaldehyde hydrosilylation activity at 1.0 mol% loading over 3 h at 25 °C. Notably, (Ph₂PPr DI)Ni was found to be the most efficient catalyst while phenyl silane was the most effective reductant. A broad scope of aldehydes and ketones were then hydrosilylated, and the silyl ether products were hydrolyzed to afford alcohols in good yield. When attempts were made to explore ester reduction, inefficient dihydrosilylation was noted for ethyl acetate and no reaction was observed for several additional substrates. However, when an equimolar solution of allyl acetate and phenyl silane was added to 1.0 mol% (Ph₂PPr DI)Ni, complete ester C-O bond hydrosilylation was observed within 30 min at 25 °C to generate propylene and PhSi(OAc)₃. The scope of this reaction was expanded to include six additional allyl esters, and under neat conditions, turnover frequencies of up to 990 h^-1 were achieved. This activity is believed to be the highest reported for transition metal-catalyzed ester C-O bond hydrosilylation. Proposed mechanisms for (Ph₂PPr DI)Ni-mediated carbonyl and allyl ester C-O bond hydrosilylation are also discussed.

Full text of DOI:10.1039/c8dt01857j

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Carbonyl and Ester C-O Bond Hydrosilylation
Using κ4-Diimine Nickel Catalysts
Christopher L. Rock, Thomas L. Groy, Ryan J. Trovitch*
School of Molecular Sciences, Arizona State University, Tempe, AZ 85287
ABSTRACT
The synthesis of alkylphosphine-substituted α-diimine (DI) ligands and their subsequent
addition to Ni(COD)2 allowed for the preparation of (iPr2PPrDI)Ni and (tBu2PPrDI)Ni. The solid
state structures of both compounds were found to feature a distorted tetrahedral geometry that is
largely consistent with the reported structure of the diphenylphosphine-substituted variant,
(
Ph2PPrDI)Ni. To explore and optimize the synthetic utility of this catalyst class, all three
compounds were screened for benzaldehyde hydrosilylation activity at 1.0 mol% loading over 3
h at 25 °C. Notably, (Ph2PPrDI)Ni was found to be the most efficient catalyst while phenyl silane
was the most effective reductant. A broad scope of aldehydes and ketones were then
hydrosilylated, and the silyl ether products were hydrolyzed to afford alcohols in good yield.
When attempts were made to explore ester reduction, inefficient dihydrosilylation was noted for
ethyl acetate and no reaction was observed for several additional substrates. However, when an
equimolar solution of allyl acetate and phenyl silane was added to 1.0 mol% (Ph2PPrDI)Ni,
complete ester C-O bond hydrosilylation was observed within 30 min at 25 °C to generate
propylene and PhSi(OAc)3. The scope of this reaction was expanded to include six additional
allyl esters, and under neat conditions, turnover frequencies of up to 990 h-1 were achieved. This
activity is believed to be the highest reported for transition metal-catalyzed ester C-O bond
hydrosilylation. Proposed mechanisms for (Ph2PPrDI)Ni-mediated carbonyl and allyl ester C-O
bond hydrosilylation are also discussed.
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INTRODUCTION
Homogeneous hydrosilylation catalysts have a rich history of development and are widely
used by the chemical industry for the preparation of value-added products and materials.1 In the
1950s, researchers at Dow Corning discovered that H2PtCl6·6H2O mediates the hydrosilylation
of olefins2 and Pt catalysts are still used to prepare silicone-based coatings, sealants, and
consumer products due to their exceptional activity.3 Approximately 15 years later, Ojima and
co-workers found that (Ph3P)3RhCl (Wilkinson’s Catalyst)4 exhibits impressive activity for a
related transformation, the hydrosilylation of carbonyl-containing compounds.5 Although
carbonyl hydrosilylation has only recently been used to prepare silicones,6 chiral Rh catalysts
have been extensively utilized to generate alcohols from ketones in an asymmetric fashion.7
Although Pt- and Rh-based hydrosilylation catalysts remain popular, the cost8 and toxicity9
associated with these metals are inherently disadvantageous. For this reason, recent efforts to
develop carbonyl hydrosilylation catalysts have focused on the utilization of Earth-abundant
metals including Mn,10 Fe,11 Co,12 Cu,13 and Zn.14 Homogenous compounds of Ni are known to
mediate alkene hydrosilylation;15 however, their propensity to catalyze carbonyl hydrosilylation
has remained relatively understudied. In 2009, Guan and co-workers reported that [2,6-
(iPr2PO)2C6H3]NiH (Figure 1, A) catalyzes aldehyde hydrosilylation with turnover frequencies
(TOFs) of 250 h-1.16 Later that year, Mindiola and co-workers achieved aldehyde and ketone
hydrosilylation TOFs of 287 h-1 and 14 h-1, respectively, upon heating solutions of substrate,
HSiEt3, [(PNiPr3)Ni(µ2-Br)]2 (Figure 1, B), and KOtBu to 100 °C.17 The leading example of Ni-
catalyzed aldehyde hydrosilylation in terms of ambient temperature activity was published by
Postigo and Royo in 2012, whereby TOFs of up to 2304 h-1 were achieved using PhSiH3 and
(Cp*-NHCMe)Ni(OtBu) (Figure 1, C).18 At 60 °C, a half-sandwich Ni complex (Figure 1, D)
developed by Albrecht and co-workers was found to hydrosilylate aldehydes with initial TOFs of
up to 23,000 h-1.19 Compounds of this type have also been employed to reduce aldehydes and
ketones in the presence of Ph2SiH2.20 A (PBP)Ni borane complex developed by the Peters Group
has been used to reduce benzaldehydes in the presence of PhSiH3 and extensive mechanistic
studies suggest that the ligand is chemically non-innocent.21 Most recently, Schmidt and co-
workers reported a cationic (κ2-PN)Ni(allyl) precatalyst for carbonyl hydrosilylation.22
2

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Figure 1. A selection of leading Ni catalysts for carbonyl hydrosilylation.
In 2013, our group synthesized the tetradentate α-diimine (DI) Ni compound, (Ph2PPrDI)Ni (1,
Scheme 1), which features a propylene bridge between the imine and phosphine donor groups.23
At 5.0 mol% loading, 1 was found to catalyze the hydrosilylation of cyclohexanone and
diisopropyl ketone to yield a mixture of silyl ethers over the course of 24 h at 25 °C.23 Herein,
we report the synthesis and characterization of two alkylphosphine variants and compare their
carbonyl hydrosilylation activity to 1. Following optimization, a broad scope of aldehyde and
ketone hydrosilylation was achieved with modest TOFs of 41 h-1 and 4 h-1, respectively.
Attempts to hydrosilylate esters allowed for the dihydrosilylation of ethyl acetate at 60 °C to
yield a mixture of (EtO)PhSiH2 and (EtO)2PhSiH. While many other esters were unaffected,
unsubstituted allyl esters were selectively cleaved to produce propylene and
tricarboxyphenylsilanes via ester C-O bond hydrosilylation with unprecedented activity.
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RESULTS AND DISCUSSION:
Synthesis and Characterization. We have previously shown that the addition of Ph2PPrDI to
Ni(COD)2 results in displacement of both COD ligands and the formation of 1.23 Knowing that 1
is capable of catalyzing carbonyl hydrosilylation at a relatively high catalyst loading (5 mol%),
we desired to screen alkylphosphine-substituted catalysts and a variety of silanes while
optimizing the conditions of this transformation. First, iPr2PPrDI and tBu2PPrDI were prepared via
Schiff-base condensation of 2,3-butanedione with the respective (3-aminopropyl)phosphine (for
details see experimental section). Chelate addition to a solution of Ni(COD)2 in toluene allowed
for the formation of the respective tetradentate Ni complex, (iPr2PPrDI)Ni (2) and (tBu2PPrDI)Ni
(3), after 18 h at 25 °C (Scheme 1). Analysis by 31P NMR spectroscopy revealed that 2 and 3
each contain a single phosphorus environment, with resonances at 52.33 and 73.59 ppm,
respectively (Figures S8 and S11 of the Supporting Information).
Scheme 1. Synthesis of compounds 1-3.
Crystals of 2 and 3 were grown from saturated diethyl ether solutions at -35 °C and analyzed
by single crystal X-ray diffraction. The solid state geometry of 2 can be described as distorted
tetrahedral (Figure 2), and it is nearly identical to that reported for 1 as judged by the P1-Ni1-P2
angles of 113.58(2) (for 1) and 113.80(3)° (for 2). In contrast, the sterically-demanding tert-butyl
groups of 3 repel one another, leading to a wider P1-Ni1-P2 angle of 131.35°. As observed for 1,
inspection of the ligand bond lengths determined for 2 and 3 reveal shortened C2-C3/C2A
contacts of 1.420(3) and 1.423(4) Å, along with elongated C-N distances of 1.350(3) and
1.339(3) Å, respectively (Table 1). These metrical parameters are consistent with single electron
DI chelate reduction,24 indicating that the electronic structures of 2 and 3 are likely best
described as having a Ni(I) center that is antiferromagnetically coupled to a DI radical anion
(Scheme 1).23 Comparable electronic structure assignments have been reported for low-valent
Fe25 and Co26 compounds featuring Ph2PPrDI.
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Figure 2. Molecular structures of 2 (top) and 3 (bottom) drawn with 30% probability ellipsoids.
Hydrogen atoms removed for clarity. Atom labels ending with “A” are symmetry generated.
Table 1. Bond Lengths (Å) and Angles (˚) determined for 1,23 2, and 3.
123
2
3
Ni1-N1
Ni1-N2/N1A
Ni1-P1
Ni1-P2/P1A
N1-C2
N2/N1A-C3/C2A
C2-C3/C2A
1.9369(17)
1.9250(18)
2.1343(6)
2.1345(6)
1.340(3)
1.933(2)
1.931(2)
2.1455(7)
2.1544(7)
1.350(3)
1.350(3)
1.420(4)
1.9582(17)
1.9582(17)
2.2262(5)
2.2262(5)
1.339(3)
1.341(3)
1.414(3)
1.339(3)
1.423(4)
N1-Ni1-N2/N1A
N1-Ni1-P1
N2/N1A-Ni1-P2/P1A
P1-Ni1-P2/P1A
81.88(7)
99.09(5)
94.14(5)
113.58(2)
82.46(9)
97.54(7)
96.12(6)
113.80(3)
80.53(10)
94.34(5)
94.34(5)
131.35(3)
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Carbonyl Hydrosilylation. With 1-3 in hand, the ambient temperature benzaldehyde
hydrosilylation activity of each compound was evaluated in the presence of PhSiH3. Using 1.0
mol% 1 in benzene-d6 solution, complete conversion to a mixture of silyl ethers was observed
within 3 h at 25 °C by 1H NMR spectroscopy. Under the same conditions, catalysts 2 and 3 were
found to achieve considerably lower conversion percentages of 8% and 67%, respectively (vide
infra). Using 1, attempts to broaden the silane scope beyond PhSiH3 revealed only 14%
benzaldehyde hydrosilylation when Ph2SiH2 was used as the reductant, while no conversion was
observed in the presence of Et2SiH2, iPr2SiH2, tBu2SiH2, Et3SiH, Me2PhSiH, or Ph3SiH.
Although the limited silane scope and ineffectiveness of 2 and 3 should be considered
disappointing, we proceeded to perform a substrate scope and functional group tolerance study
using 1. Adding 0.1 mol% 1 to a neat solution of benzaldehyde and PhSiH3 resulted in a mixture
of silyl ethers after 24 h at 25 °C. Treatment of the products with 10% aqueous NaOH allowed
for the isolation of pure benzyl alcohol in 81% yield. Under these optimized conditions, 11
additional aldehydes were screened (Table 2), with 1 showing tolerance for fluoro (b), chloro (c),
and ether (f,h) functional groups. Compound 1 also exhibits chemoselectivity for the
hydrosilylation of carbonyls over nitriles (g) and olefins (j), even though it is known to mediate
alkyne hydrosilylation.23 Interestingly, upon adding 1 to an equimolar mixture of 4-
bromobenzaldehyde and PhSiH3, the solution immediately turned blue in color (from
characteristic red) and no conversion of the substrate was observed (d). This observation is likely
due to C-Br bond oxidative addition; however, attempts to characterize the paramagnetic solid
resulting from 4-bromobenzaldehyde addition to 1 have proven unsuccessful. For the other
entries in Table 2, aldehyde hydrosilylation TOFs of 41 h-1 were achieved.
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Table 2. Hydrosilylation of aldehydes using 0.1 mol% 1 and PhSiH3 at 25 °C.a,b
aPercent conversion determined by 1H NMR spectroscopy (TOF = 41 h-1 for all substrates except d).
bIsolated yields of the corresponding alcohol in parentheses (spectroscopic analysis provided in the SI).
cApproximately 0.5 mL of benzene was added to aid solubility.
When acetophenone was combined with PhSiH3 and 1.0 mol% 1, reduced activity was
observed and heating to 60 °C was required to achieve >99% conversion to silyl ethers. As with
aldehydes, treatment of the acetophenone-derived products with 10% aqueous NaOH allowed for
isolation of 1-phenylethanol in 68% yield (Table 3, a). Acetophenone substitution did not
influence conversion (b-e) and 1 was found capable of reducing sterically hindered ketones
including 2’,4’,6’-trimethylacetophenone (d), dicyclohexylketone (f), and diisopropylketone (h)
under these conditions. For each substrate in Table 3, modest TOFs of 4 h-1 were observed at 60
°C. In general, it can be stated that the carbonyl hydrosilylation activity of 1 is lower than what
has been observed using other well-defined Ni catalysts.16,18,19
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Table 3. Hydrosilylation of ketones using 1.0 mol% 1 and PhSiH3 at 60 °C.a,b
aPercent conversion determined by 1H NMR spectroscopy (TOF = 4 h-1 for all substrates). bIsolated yields
of the corresponding alcohol in parentheses. cThis substrate was previously converted to a mixture of silyl
ethers using 5.0 mol% 1 and PhSiH3 (see reference 23).
Carboxylate Hydrosilylation. To further expand the scope of 1-mediated hydrosilylation, the
reduction of esters was also investigated. Ethyl acetate was combined with PhSiH3 and 1.0 mol%
1 and heated at 60 °C for 24 h. Analysis via 1H NMR spectroscopy revealed 46% ethyl acetate
reduction, with 99% consumption of PhSiH3, to yield a mixture of (EtO)PhSiH2 and
(EtO)2PhSiH in a 1:4 ratio. Since the reductant was prematurely consumed under these
conditions, this experiment was repeated over 24 h using 3 equivalents of PhSiH3 and 80% ethyl
acetate dihydrosilylation was noted (Scheme 2). Disappointingly, many other esters (methyl
benzoate, ethyl benzoate, ethyl 4-fluorobenzoate, ethyl 4-methoxybenzoate, ethyl cinnamate, and
phenyl acetate) were not reduced at temperatures as high as 90 °C.
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Scheme 2. Ethyl acetate dihydrosilylation using 1.0 mol% 1 at 60 °C.
However, when allyl acetate was combined with PhSiH3 and 1.0 mol% 1 in a J. Young tube, a
color change from red to pale yellow occurred almost immediately, in conjunction with vigorous
bubbling. Analysis by 1H NMR spectroscopy revealed the evolution of propylene (indicating
cleavage of the ester C-O bond) and complete consumption of allyl acetate was observed after 30
min at 25 °C. Upon precipitating the catalyst with I2, filtering through a glass frit, and removing
excess PhSiH3, a single silyl ester product, PhSi(OAc)3, was isolated in 77% yield (Table 4, a).
Attempts to achieve this transformation using Ni(COD)2 in place of 1 revealed no conversion,
while 2 and 3 afforded 8% and 0% conversion, respectively. These observations highlight the
importance of the Ph2PPrDI ligand, and more specifically, the role of phosphine substitution as it
relates to catalytic C-O cleavage. Using 1, six additional unsubstituted allyl esters were screened
for ester C-O bond hydrosilylation, with allyl benzoate (b), allyl 2-phenylacetate (c), and allyl 2-
phenoxyacetate (d) all reaching completion within 30 min (TOF = 3.3 min-1). Allyl hexanoate
(e), allyl cyclohexylpropionoate (f), and allyl cinnamate (g) were partially reduced after 30 min
and required 3 h to reach completion (TOF = 33 h-1). The entries in Table 4 are believed to be the
first known examples of tricarboxysilane synthesis via ester C-O bond hydrosilylation.27
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Table 4. Hydrosilylation of allyl esters using 1.0 mol% 1 and PhSiH3 at 25 °C.a,b
aPercent conversion determined by 1H NMR spectroscopy. bYield in parenthesis of isolated
tricarboxyphenylsilane. cDetermined after 3 h.
Efforts were also made to maximize the efficiency of allyl acetate C-O bond hydrosilylation.
Adding a 3:1 mixture of allyl acetate and PhSiH3 to 1.0 mol% 1 in benzene-d6 solution allowed
for complete Si-H bond utilization to generate PhSi(OAc)3 within 1 h at 25 °C. To maximize the
TOF of this transformation, a neat solution of allyl acetate and PhSiH3 was added to 0.1 mol% 1.
Rapid gas evolution was observed along with a color change from red to pale yellow. After 1 h,
the reaction turned red in color (indicating that the precatalyst had been regenerated following
substrate consumption) and analysis by 1H NMR spectroscopy revealed >99% conversion to a
mixture of silyl esters (TOF = 990 h-1). No conversion was observed in the absence of 1.
The selectivity and activity observed for this transformation are noteworthy. As observed in
Scheme 2, it is very common for esters to undergo dihydrosilylation to yield a mixture of silyl
ethers.10f,n,m,11f,14a,28 Moreover, it has been shown that ester reduction in the presence of silane
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reductant can result in silyl acetal formation via carbonyl hydrosilylation29 or ether formation
following deoxygenation.30 Selective ester C-O bond hydrosilylation to form carboxysilane has
been observed using 4 mol% Co2(CO)8; however, this transformation required heating to 200 °C
over the course of 6 h (maximum TOF = 4 h-1).31 Examples of catalytic allyl ester C-O bond
hydrosilylation are very rare and are limited to Pt-catalyzed allyl acetate hydrosilylation.32 In
each of these examples, a mixture of alkene hydrosilylation and C-O cleavage products was
observed due to poor catalyst selectivity.33 The maximum ester C-O bond hydrosilylation TOF of
990 h-1 achieved using 1 is believed to be the highest reported for any known catalyst.
To bridge the primary findings of this study, an attempt was made to reduce and cleave the
ester C-O bond of 5-(acetoxymethyl)furfural (Scheme 3, a). Notably, this substrate is a
derivative of 5-(hydroxymethyl)furfural, which has been identified as a promising biomass-
derived platform chemical.34 In the presence of 1.0 mol% 1 and two equivalents of PhSiH3,
complete hydrosilylation of the aldehyde functionality was achieved after 3 h at ambient
temperature (Figure S69). However, cleavage of the allyl ester C-O bond to form the silyl ether
of 5-methylfurfuryl alcohol was not observed, even upon heating to 90 °C for 24 h. To further
demonstrate that the transformation described in Table 4 is limited to α-allyls, D-glucal
triacetate, cinnamyl acetate, and prenyl acetate (Scheme 3, b-d) were screened for 1-mediated
ester C-O bond hydrosilylation. In each case, no conversion was noted after heating to 90 °C for
24 h.
Scheme 3. Attempts to hydrosilylate the ester C-O bond of substituted allyl acetates using 1.
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Mechanistic Clues and Relevance to Prior Work. Throughout our study of 1-mediated
carbonyl and carboxylate hydrosilylation, efforts have been made to collect mechanistic
information. When aldehydes or ketones are independently added to 1, no reaction is observed
over the range of temperatures employed for catalysis. However, upon adding 100 equivalents of
PhSiH3 to 1, slow silane coupling to form PhH2SiSiH2Ph35 and (PhH2Si)2SiH(Ph)36 occurs at
ambient temperature (~35% conversion after 24 h, see Figures S70 and S71). Although these
silane coupled products are also generated during catalysis, no change to the catalyst is
observable by 31P NMR spectroscopy. This experiment indicates that Si-H oxidative addition is
accessible, even though Ni(II) intermediates are not persistent. Once formed, carbonyl
hydrosilylation is likely to proceed following substrate insertion into the newly formed Ni-H
bond and Si-O reductive elimination (Scheme 4).
Scheme 4. Proposed modified Ojima mechanism for (DI)Ni-mediated carbonyl hydrosilylation.
Experiments have also been conducted to determine why 1 outperforms precatalysts 2 and 3 for
carbonyl and ester C-O bond hydrosilylation. Repeating 1-mediated benzaldehyde
hydrosilylation (Table 2, a) in the presence of 20 equivalents of PMe3 (relative to catalyst)
resulted in only 3% percent conversion after 3 h (Figure S72) and 31P NMR spectroscopy
revealed the formation of new Ni compounds including Ni(PMe3)4 (-21.81 ppm, Figure S73). To
gain additional insight, 2 equivalents of PMe3 were added directly to 1. After 1 h at 25 °C, 31P
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NMR spectroscopy revealed near complete PMe3 consumption and partial conversion of 1 to two
Ni compounds: Ni(PMe3)4 and a second possessing resonances at 25.88 (q) and -24.69 (d) ppm
[proposed to be (κ1-P-Ph2PPrDI)Ni(PMe3)3] (Figure S74). Given that the diphenylphosphine
substituents of Ph2PPrDI are readily displaced from Ni by a stronger σ-donating phosphine such as
PMe3 (even though the chelate effect is in place), it can be proposed that the trialkylphosphine
substituents of 2 and 3 are less likely to be substituted by silane or substrate during catalysis.
The trialkylphosphine substituents of 2 and 3 render these compounds particularly poor
catalysts for ester C-O bond hydrosilylation. When 20 equivalents of PMe3 relative to 1 were
added prior to allyl acetate hydrosilylation (Table 4, a), only 12% conversion was noted after 30
min at 25 °C. Moreover, only free ligand, Ni(PMe3)4, and free PMe3 were observed by 31P NMR
spectroscopy. Upon adding each substrate in Table 4 to a benzene-d6 solution of 1 and PhSiH3,
the solution immediately changes from red to yellow in color. When a stoichiometric quantity of
allyl benzoate was added to 1 in the absence of PhSiH3, a similar color change was noted over
the course of 24 h at 25 °C and analysis indicated that the resulting product is NMR silent (no
new 31P resonances were observed). Although our attempts to isolate and characterize an allyl
ester addition product of this type have proven unsuccessful, seminal work by Yamamoto and
co-workers to monitor the oxidative addition of esters at low-valent Ni offers valuable insight.37
In 1976, they identified two distinct pathways for ester cleavage; acyl C-O bond cleavage (as
observed in Scheme 2) and the ester C-O bond cleavage pathway utilized in Table 4.38
Furthermore, in the absence of reductant, it was found that allyl acetate cleavage occurred
exclusively via ester C-O bond activation (as we have found in Table 4). Therefore, it is believed
that 1-mediated allyl ester cleavage is likely to occur following olefin coordination and allylic C-
O bond oxidative addition,39 as shown in Scheme 5. The trialkylphosphine substituents in 2 and 3
appear to inhibit olefin coordination and subsequent catalysis.
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Scheme 5. Proposed mechanism for (DI)Ni-mediated allyl ester C-O bond hydrosilylation.
Finally, it should be noted that the Ni-catalyzed reductive cleavage of allyl esters was recently
demonstrated by Jiang and co-workers, who used NiCl2 and NaBH4 to prepare partially
deoxygenated glycals within 5 min at room temperature (TOF = 100 min-1 based on Ni).40,41
When considered alongside our results, it can be said that Ni catalysts are particularly effective at
cleaving α-allyl ester C-O bonds under reducing conditions.
CONCLUSIONS
The synthesis and characterization of two trialkylphosphine-substituted (κ4-DI)Ni compounds
has been described. The solid state structures of (iPr2PPrDI)Ni and (tBu2PPrDI)Ni suggest that both
compounds feature a singly-reduced DI chelate that is antiferromegnetically coupled to Ni(I), as
reported for (Ph2PPrDI)Ni. Although these three compounds were found to be structurally similar,
(
Ph2PPrDI)Ni exhibited considerably higher activity for the hydrosilylation of benzaldehyde and
allyl acetate ester C-O bond hydrosilylation. Seven α-allyl esters were cleaved to yield
tricarboxyphenylsilanes and optimization of this transformation has allowed for leading TOFs of
up to 990 h-1. Mechanistic experiments suggest that the trialkylphosphine substituents of
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(
iPr2PPrDI)Ni and (tBu2PPrDI)Ni inhibit carbonyl and ester C-O bond hydrosilylation by
preventing Si-H oxidative addition and olefin coordination, respectively.
EXPERIMENTAL SECTION
General Considerations: All reactions were performed inside an MBraun glovebox under an
atmosphere of purified nitrogen. Toluene, tetrahydrofuran, diethyl ether, and pentane were
purchased from Sigma-Aldrich, purified using a Pure Process Technology solvent system, and
stored in the glovebox over activated 4Å molecular sieves and sodium before use. Benzene-d6
was purchased from Cambridge Isotope Laboratories or Oakwood Chemicals and dried over 4Å
molecular sieves and potassium. Celite was obtained from Acros Organics. Bis(1,5-
cyclooctadiene) nickel was purchased from Strem. Benzaldehyde, p-tolualdehyde, p-
methoxybenzaldehyde, furfural, cyclohexanecarboxaldehyde, p-chloroacetophenone, diisopropyl
ketone, cyclohexanone, 2-hexanone, and allyl cyclohexyl propionoate were sourced from Sigma
Aldrich. p-Chlorobenzaldehyde, hexanal, decanal, cyclohex-3-enylcarbaldehyde, acetophenone,
2,4,6-trimethylacetophenone, p-methoxyacetophenone, dicyclohexylketone, allyl phenyl acetate,
allyl phenoxyacetate, allyl hexanoate, ethyl cinnamate, phenyl acetate, methyl benzoate, ethyl
benzoate, ethyl 4-methoxybenzoate, 5-(acetoxymethyl)furfural, and 2,3-butanedione were
purchased from TCI America. p-Bromobenzaldehyde, p-cyanobenzaldehyde, p-
fluoroacetophenone, allyl acetate, allyl cinnamate, ethyl 4-fluorobenzoate, cinnamyl acetate, D-
glucal triacetate, and phenyl silane were purchased from Oakwood Chemicals. p-
Fluorobenzaldehyde was obtained from Acros. Allyl benzoate and prenyl acetate were purchased
from Combi Blocks. Ethyl acetate was purchased from Mallinckrodt Chemicals. All liquid
substrates were dried over 4Å molecular sieves prior to use. All solid substrates were
recrystallized from diethyl ether prior to use. 3-(Di-i-propylphosphino)-propylamine,42 3-(di-t-
butylphosphino)-propylamine,42 3-(diphenylphosphino)propylamine,43 Ph2PPrDI,23 and
(
Ph2PPrDI)Ni23 were synthesized according to literature procedure.
Solution nuclear magnetic resonance (NMR) spectra were recorded at room temperature on
either a Varian 400 MHz, Bruker 400 MHz, or Varian 500 MHz NMR spectrometer. All 1H
NMR and 13C NMR chemical shifts (ppm) are reported relative to Si(Me)4 using 1H (residual)
and 13C chemical shifts of the solvent as secondary standards. 31P NMR chemical shifts (ppm)
are reported relative to phosphoric acid.
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X-ray Crystallography: Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in the glovebox and transferred to glass fiber with Apiezon N grease, which
was then mounted on the goniometer head of a Bruker APEX Diffractometer equipped with Mo
Kα radiation (Arizona State University). A hemisphere routine was used for data collection and
determination of the lattice constants. The space group was identified and the data was processed
using the Bruker SAINT+ program and corrected for absorption using SADABS. The structures
were solved using direct methods (SHELXS) completed by subsequent Fourier synthesis and
refined by full-matrix, least-squares procedures on [F2] (SHELXL). The crystallographic data
collected for compounds 2 and 3 has been deposited with The Cambridge Crystallographic Data
Centre (CCDC) and assigned the numbers 1586336 and 1586337, respectively. Crystallographic
parameters can be found in Table S1.
Preparation of tBu2PPrDI. In a glove box, a 100 mL thick-walled glass bomb was charged with
2,3-butanedione (98.9 mg, 1.15 mmol), p-toluenesulfonic acid (4 mg, 0.029 mmol), and 5 mL of
toluene. After stirring for 5 min, 3-(di-t-butylphosphino)propylamine (465.0 mg, 2.31 mmol) in 5
mL toluene and 4 Å molecular sieves were added. The vessel was sealed and stirred at 90 °C for
4 days. The reaction was subsequently cooled to room temperature, filtered through a bed of
Celite, and the solvent removed in vacuo. The resulting yellow oil was dissolved in a minimal
mixture of diethyl ether and pentane and cooled to -35 °C. White crystals identified as tBu2PPrDI
were isolated in 26.2% yield (136.6 mg, 0.302 mmol). Analysis for C26H54N2P2: Calc. C, 68.38%
H, 11.92%, N, 6.13% Found C, 68.36% H, 11.99% N, 6.01%. 1H NMR (benzene-d6): 3.39 (t, J =
6.4 Hz, 4H), 2.09 (s, 6H), 2.02 (dd, J = 14.8, 7.0 Hz, 4H), 1.55 (m, 4H), 1.13 (d, J = 10.9 Hz,
36H). 13C NMR (benzene-d6): 168.23, 53.68 (d, J = 14.2 Hz), 32.40 (d, J = 25.8 Hz), 31.74 (d, J
= 22.7 Hz), 30.25 (d, J = 13.8 Hz), 19.75 (d, J = 21.8 Hz). 31P NMR (benzene-d6): 26.58 (s).
Preparation of (iPr2PPrDI)Ni (2). In a glove box, a 100 mL thick-walled glass bomb was charged
with 2,3-butanedione (90.0 mg, 1.05 mmol), p-toluenesulfonic acid (4 mg, 0.029 mmol), and 5
mL of toluene. After stirring for 5 min, 3-(di-i-propylphosphino)propyl amine (388.0 mg, 2.21
mmol) in 5 mL toluene and 4 Å molecular sieves were added. The vessel was sealed and stirred
at 70 °C for 5 d. The reaction was subsequently cooled to room temperature, filtered through a
bed of Celite, and the solvent was removed in vacuo. The resulting yellow oil was dissolved in a
minimal amount of toluene and added to Ni(COD)2 (178.0 mg, 0.65 mmol) dissolved in 10 mL
toluene in a 20 mL scintillation vial. The resulting red solution was stirred overnight, filtered
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through a bed of Celite, and the solvent was removed in vacuo. The red solid was dissolved in a
minimal amount of diethyl ether and cooled to -35 °C. A red crystalline solid identified as 2 was
isolated in 84% yield relative to Ni(COD)2 (249.0 mg, 0.54 mmol). Analysis for C22H46N2P2Ni:
Calc. C, 57.54% H, 10.10%, N, 6.10% Found C, 57.66% H, 10.45% N, 5.99%. 1H NMR
(benzene-d6): 2.90 (m, 1H), 2.79 (m, 1H), 2.59 (m, 1H), 2.01 (m, 1H), 1.85 (m, 2H), 1.45 (t, J =
6.2 Hz, 3H), 1.39 (dd, J = 14.6, 6.7 Hz, 3H), 1.14 (dd, J = 13.5, 7.2 Hz, 3H), 0.89 (t, J = 7.4 Hz,
3H), 0.50 (dd, J = 15.1, 6.7 Hz, 3H), 0.41 (m, 1H). 13C NMR (benzene-d6): 140.97, 57.01, 30.10
(d, J = 5.2 Hz), 30.03 (d, J = 5.7 Hz), 29.66 (d, J = 3.2 Hz), 29.56 (d, J = 3.8 Hz), 22.97 (d, J =
5.7 Hz), 22.88 (d, J = 5.4 Hz), 19.87 (t, J = 3.7 Hz), 19.63 (t, J = 4.9 Hz), 18.52 (t, J = 5.3 Hz),
17.02 (t, J = 2.7 Hz), 15.98 (t, J = 3.7 Hz), 15.35 (d, J = 5.6 Hz), 15.26 (d, J = 5.1 Hz). 31P NMR
(benzene-d6): 52.33 (s).
Preparation of (tBu2PPrDI)Ni (3). In a glove box, a 20 mL scintiallation vial was charged with
18.9 mg of Ni(COD)2 (0.0689 mmol) and 10 mL of toluene. Recrystallized tBu2PPrDI (31.2 mg,
0.0689 mmol) in 5 mL toluene was slowly added. The solution immediately turned red and was
stirred for 24 h, followed by filtration through Celite and removal of solvent in vacuo. The
material was dissolved in a minimal quantity of diethyl ether and cooled to -35 °C. A red
crystalline solid identified as 3 was isolated in 74% yield (26.3 mg, 0.0512 mmol). Analysis for
C26H54N2P2Ni: Calc. C, 60.60% H, 10.56%, N, 5.43% Found C, 59.64% H, 10.32% N, 5.25%.
1H NMR (benzene-d6): 2.78 (m, 2H), 2.62 (m, 2H), 2.22 (m, 2H), 1.96 (m, 2H), 1.30 (d, J = 9.9
Hz, 18H), 1.21 (d, J = 9.9 Hz, 18H), 0.68 (t, J = 4.8 Hz, 6H), 0.43 (t, J = 12.0 Hz, 2H). 13C NMR
(benzene-d6): 140.64 (t, J = 4.1 Hz), 55.20 (s), 30.82 (bs), 30.72 (bs), 26.12 (t, J = 7.3 Hz), 17.70
(t, J = 1.8 Hz), 16.28 (t, J = 5.8 Hz). 31P NMR (benzene-d6): 73.59 (s).
General Procedure for Hydrosilylation of Aldehydes with 0.1 mol% 1: Under inert
atmosphere, a 20 mL scintillation vial was charged with approximately 0.0030 g of 1 (0.00504
mmol). Aldehyde (approx. 5.04 mmol) and PhSiH3 (approx. 5.04 mmol) were combined and
added to the catalyst. The resulting solution was stirred at room temperature for 24 h. Using 1H
NMR spectroscopy, >99% conversion was observed after 2 h (except for Table 2, entry d). The
solution was then hydrolyzed with 2 mL of 10% aqueous NaOH and the organic product was
extracted with diethyl ether (3x2 mL). The combined organic layers were dried with Na2SO4 and
the solvent was removed in vacuo to isolate the alcohol.
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General Procedure for Hydrosilylation of Ketones with 1.0 mol% 1: In a glove box, ketone
(approx. 0.7 mmol) and PhSiH3 (approx. 0.7 mmol) were added sequentially to a 20 mL
scintillation vial containing 1 (approx. 4.0 mg, 0.007 mmol). The resulting red solution was
dissolved in benzene-d6, transferred into a J. Young NMR tube, and heated at 60 °C for 24 h.
Conversion of >99% was observed by 1H NMR spectroscopy. The solution was hydrolyzed with
1 mL of 10% NaOH(aq) and the organic product was extracted using Et2O and dried over Na2SO4.
The solvent was removed in vacuo and the alcohol product was isolated.
General Procedure for the Hydrosilylation of Allyl Esters with 1.0 mol% 1: Under an inert
atmosphere, allyl ester (0.554 mmol) and PhSiH3 (0.554 mmol) were combined in a 20 mL
scintillation vial and then transferred into a vial containing 1 (approx. 3.3 mg, 0.00554 mmol) in
0.5 mL benzene-d6. The red solution was transferred into a J. Young NMR tube and sealed. A
color change to pale yellow was quickly observed. Greater than 99% conversion was observed
via 1H NMR spectroscopy, after which 1 equivalent of I2 in benzene (relative to Ni, approx. 20
μL of a 0.248 M solution) was added. The solution was allowed to sit for 1 h, filtered, and the
volatile components were removed under reduced pressure to isolate the respective
tricarboxyphenylsilane.
CONFLICTS OF INTEREST
The authors declare the following competing financial interest(s): R. J. T. retains rights to 1
through US20160176908 and WO2014201082. This catalyst has been commercialized by
Sigma-Aldrich Corporation (a subsidiary of Merck KGaA).
ACKNOWLEDGMENTS
This material is based upon work supported by the National Science Foundation under Grant No.
1651686.
NOTES
Corresponding Author
*(R. J. T.) Phone: 480-727-8930. Email: ryan.trovitch@asu.edu
Author Contributions
18

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The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Electronic Supplementary Information
Electronic Supplementary Information (ESI) available: metrical parameters for 2 and 3, catalytic
procedures, and NMR spectroscopic identification of compounds and organic products. See DOI:
10.1039/x0xx00000x
Accession Codes
CCDC 1586336 and 1586337 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +441223336033.
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Table of Contents Graphic
Table of Contents Synopsis
Ph2PPrDI)Ni chemoselectively catalyzes α-allyl ester C-O bond hydrosilylation to prepare silyl
esters with turnover frequencies of up to 990 h-1.
(