FeII-catalysed insertion reaction of α-diazocarbonyls into X-H bonds (X = Si, S, N, and O) in dimethyl carbonate as a suitable solvent alternative

Article abstract of DOI:10.1039/c9ra07203a

The insertion reaction of a broad range of diazo compounds into Si-H bonds was found to be efficiently catalysed by Fe(OTf)₂ in an emerging green solvent i.e. dimethyl carbonate (DMC). The α-silylated products were obtained in good to excellent yields (up to 95%). Kinetic studies showed that the extrusion of N₂ to form an iron carbene intermediate is rate-limiting. The iron-catalysed insertion reaction of methyl α-phenyl-α-diazoacetate into polar X-H bonds (S-H, N-H, and O-H) was also established in DMC.

Full text of DOI:10.1039/c9ra07203a

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FeII-catalysed insertion reaction of a-diazocarbonyls
into X–H bonds (X ¼ Si, S, N, and O) in dimethyl
carbonate as a suitable solvent alternative†
Cite this: RSC Adv., 2019, 9, 31241
*
Nour Tanbouza,
Hoda Keipour
and Thierry Ollevier
The insertion reaction of a broad range of diazo compounds into Si–H bonds was found to be efficiently
catalysed by Fe(OTf)2 in an emerging green solvent i.e. dimethyl carbonate (DMC). The a-silylated
products were obtained in good to excellent yields (up to 95%). Kinetic studies showed that the
extrusion of N2 to form an iron carbene intermediate is rate-limiting. The iron-catalysed insertion
reaction of methyl a-phenyl-a-diazoacetate into polar X–H bonds (S–H, N–H, and O–H) was also
established in DMC.
Received 8th September 2019
Accepted 9th September 2019
DOI: 10.1039/c9ra07203a
rsc.li/rsc-advances
Dimethyl carbonate (DMC), the simplest among the dialkyl not. Organosilicon compounds are increasingly mainstream
carbonate family (DACs), is an emerging and interesting green within a variety of applications.12 They are pertinent in the
reagent and solvent alternative in organic synthesis due to its pharmaceutical sector where the integration of silicon motifs as
high biodegradability and low toxicity.1,2 It is exploited indus- carbon isosteres has allowed distinctive therapeutic potential in
trially for its efficiency as a solvent for numerous applications,3 a number of biologically active molecules.13 The transition
however it still remains underused as a solvent in homogeneous metal-catalysed insertion reaction into Si–H bonds is a facile
catalysis. Even though major contributions have been achieved and atom-economic method to produce organosilicons.14 This
to promote the twelve principles of green chemistry,4 searching reaction was rst described by Kramer and Wright in 1963.15
for efficient green solvent alternatives still remains a challenge Since then, pivotal breakthroughs have been made with Ru-,16
for chemists.5 Solvent selection guides are huge inuencers for Rh-,17 Ir-,18 Ag-,19 Cu-,20 and enzyme-catalysed21 insertion reac-
rendering the choice of an appropriate solvent less tedious in tions of diazo compounds into the Si–H bond. Iron catalysis
terms of life cycle evaluation.6
being one of our group's major areas of research,22 we developed
For this study, we were successful in establishing dimethyl a highly efficient iron-catalysed insertion reaction of a-diazo-
carbonate as an appropriate solvent for an iron-catalysed esters into Si–H bonds.23 Aerwards, an enantioselective iron-
insertion reaction of acceptor/donor diazo compounds into catalysed version was developed by Xie and Lin.24 The use of
Si–H bonds and polar X–H bonds (X ¼ S, N, and O). Diazo iron salts in transition metal catalysis is gaining traction due to
compounds are the most commonly used carbene precursors. their low toxicity, abundance, and environmental benignity.
They can be de-diazonized to obtain highly reactive free carbene Iron catalysts have proved on and on to be efficient and green
intermediates or metal carbene species. Then, varieties of substitutes of conventional noble metal catalysts.25
chemical transformations can occur, such as X–H (X ¼ C, S, N,
Even though iron-catalysed Si–H insertion reactions were
O, and Si) insertions,7 cyclopropanation,7b ylide formation,8 and successfully developed to render it more sustainable, they were
Wolff rearrangement.9,10 Notably, the transition metal-catalysed still conducted in dichloromethane (CH2Cl2), a solvent which is
insertion reaction of diazo compounds into various X–H bonds commonly used in homogeneous catalysis due to its good
represents a facile route towards different tailored functional- solubility of most organics, low polarity, and non-coordinating
ized compounds which are arduous to obtain through other properties. However, it is notorious for its toxicity, harmfulness
routes.11
to both human health and the environment, and costly
Special attention has been paid to the insertion reaction of disposal.26 Herein, we disclose the use of DMC as an appro-
diazo compounds into Si–H bonds, because even though silicon priate solvent alternative to CH2Cl2 in a ferrous system for the
is vastly abundant on earth, routes towards organosilicons are insertion reaction of a-diazocarbonyl compounds into Si–H
bonds. Kinetic studies are then employed to establish the rate
determining step of an iron-catalysed insertion of a-diazoesters
´
´
´
´
Departement de Chimie, Universite Laval, 1045 Avenue de la Medecine, Quebec, QC,
into Si–H bonds. This study is then further extended to cover
insertion reactions of a-diazoesters into different polar X–H
bonds (X ¼ O, N, and S).
G1V 0A6, Canada
† Electronic supplementary information (ESI) available: Experimental procedures
and copies of 1H NMR, 13C NMR, and 19F NMR spectra. See DOI:
10.1039/c9ra07203a
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A series of FeII and FeIII salts were selected for screening in
DMC with Et3SiH (Table 1).27 The following screening shows that
the nature of the catalyst greatly inuences the reactivity of the
diazo compound. Fe(OTf)2 was capable of affording the a-sily-
lated product 2a in a 95% yield within 6 h (entry 1).28 FeBr2 and
FeCl2 led to poor conversions aer 72 h (entries 2 and 3). The
diazo compound 1a was unreactive towards Fe(OAc)2 and was
recovered quantitatively (entry 4). Fe(BF4)2$6H2O resulted in
a low 6% yield which was due to insertion with H2O and
dimerization (entry 5). Interestingly, both Fe(OTf)3 and Fe(acac)3
led to the formation of the silyl enolate, which was attributed to
the strong Lewis acidity of FeIII salts (entries 6 and 7).
In order to conduct an appropriate screening of solvents, and
to gure out the correlation between the solvent's sol-
vatochromic properties and how it affects the course of this
reaction, a Kamlet–Ta plot of aprotic solvents was used to this
advantage (Fig. 1).29 The plot includes different common
organic solvents previously screened for this reaction in addi-
tion to a screening of different classical and less common green
solvent alternatives (see ESI† for individual yields and reaction
times). Solvents that produced very good yields of the insertion
product (>90%) are coloured green ( ), whilst those leading to
moderate yields (between 90% and 50%) and poor yields (<50%)
are identied in yellow ( ) and red ( ), respectively. As it can be
seen from the distribution of solvents on the Kamlet–Ta plot,
solvents with high basicity (b > 0.4) and low polarizability (p* <
0.5) resulted in moderate and low yields of the desired insertion
product. It can be deduced that the reaction prefers a reaction
media that is low in polarity and basicity which is delivered by
all of CH2Cl2, dichloroethane (DCE, (CH2Cl)2), benzene (PhH),
chlorobenzene (PhCl), bromobenzene (PhBr), and DMC. An
exception is observed with toluene (PhMe) which is reactive
towards metal carbenes. Another is with MeCN; this can be
attributed to the low solubility of the silane. Following the
solvent selection guides, DMC was chosen for this reaction.
Other organic carbonates, i.e. diethyl carbonate (DEC) and
Fig.
1
Kamlet–Taft plot of screened aprotic solvents. Reaction
conditions: 1a (0.25 mmol), Fe(OTf)2 (5 mol%), Et3SiH (1.25 mmol),
solvent (2 ml).
propylene carbonate (PC), were not as efficient as dimethyl
carbonate. Polar protic solvents, H2O and EtOH, were also
tested in this reaction where no sign of the insertion product
was observed, and only O–H insertion products were obtained
in high yields (see ESI†).
A broad range of acceptor/donor a-diazo compounds and
silanes was examined to demonstrate the tolerance of this cata-
lytic system (Table 2). Electron-donating groups in the p-position
of the aryl group were tolerated as seen for cases 2b and 2c. Also,
a-diazoesters with electron-withdrawing groups in the o-, p-, and
m-positions of the aromatic ring (2d–h) allowed the reaction to
proceed with good to excellent yields. Different ester functional-
ities were found to be well-tolerated (2i–k). Interestingly, the a-
diazophosphonate 1l underwent complete conversion aer 72 h
to yield 48% of the corresponding insertion product 2l. The
diazotriuoromethyl substrates 1m and 1n efficiently underwent
the insertion reaction to give rise to the a-silylated products in
75% and 90% yields, respectively. The Si–H insertion reaction of
methyl a-phenyl-a-diazoacetate 1a was then conducted with
different silanes. An insertion with PhMe2SiH led to an 82% yield
of 2o with completion in 12 h. The reaction of the a-diazoester 1a
with (2-naphthyl)Me2SiH achieved completion in 6 h with the
corresponding product 2p in 76%. When tuning the aryl group of
an ArMe2SiH series, it was observed that a p-OMe substituent
resulted in an extended reaction time of 24 h with the corre-
sponding silylated product in a 20% yield. An (o,o0-F)C6H3
substituted silane produced the insertion product 2r in 28%. A
(m,m0-CF3)C6H3 substituted silane resulted in a moderate yield
(50% of 2s) with completion in 6 h. Bulkier silanes, such as
Ph2MeSiH and Ph3SiH, were tougher to insert where yields
dramatically dropped to 36% and 30%, respectively, in addition
to extended reaction times (up to 48 h). For alkyl silanes, the
Table 1 Screening of different metal salts for the insertion of methyl
a-phenyl-a-diazoacetate into Si–H bondsa
Entry
MXn
Time (h)
Yield (%)
1
2
3
4
5
6
7
Fe(OTf)2
FeBr2
FeCl2
Fe(OAc)2
Fe(BF4)2$6H2O
Fe(OTf)3
Fe(acac)3
6
72
72
48
18
48
48
95
4b
45b
0c
6d
—
e
e
—
a
1a (0.25 mmol), catalyst (5 mol%), Et3SiH (1.25 mmol), DMC (2 ml).
b
c
Incomplete conversion. No conversion and recovery of a-diazoester
d
e
1a. Insertion with H2O and dimerization as major pathways.
mixture of O-silyl enolate and dimerization product was obtained.
A
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Table 2 Screening of various acceptor/donor diazo compounds and silanes for the insertion reaction into Si–H bondsa
a
Reaction conditions: diazo compound (0.25 mmol), Fe(OTf)2 (5 mol%), silane (1.25 mmol), DMC (2 ml), individual reaction time (see ESI). b 10
equiv. of Et3SiH were used. c 25 mg of 4 A MS were used. Reaction was conducted at rt.30 Yield calculated by H NMR.
d
e
1
˚
reaction of methyl a-phenyl-a-diazoacetate with (t-Bu)Me2SiH a substrate.32 Given the fact that the extrusion of N2 is irre-
and (Hex)3SiH afforded the insertion products 2v and 2w in versible and that it is not involved in any subsequent step, the
moderate yields (66% and 60%, respectively) previous attempts to measured nitrogen ratios are only relevant to the carbene
establish the rate-determining step of an iron catalysed Si–H formation step.
insertion reaction have been inconclusive.23,24 Unlike Si–H
insertions with other metal catalysts, no H/D kinetic isotope
effect was observed aer conducting a competition experiment
with a D-silane. The competition experiment between Et3Si–H
and Et3Si–D was run again (Fig. 2, eqn (1)), but this time in DMC,
using equimolar amounts of both silanes under the previously
established optimum conditions. This experiment did not show
any sign that the activation of the Si–H bond is rate-determining,
where a value of 1.04 was obtained. Thus, we hypothesized that
the extrusion of nitrogen to form the iron carbene is most likely
the rate-determining step and that the activation of Si–H bond
occurs quickly aer the formation of the metal carbene.
So, the kinetics of this reaction were placed under scrutiny in
order to validate the hypothesis of a rate-determining step
governed by the formation of the iron carbene species. In
a study by Wu, a nitrogen kinetic isotope effect showed that the
formation of a Rh carbene is rate-limiting for a Si–H insertion
(Fig. 2, eqn (2)).31 Such a study was done by using isotope ratio
mass spectrometry (IRMS) for the determination of a nitrogen
kinetic isotope effect at natural abundance. The ratio of 14N/15N
resulting from the natural 15N-enrichment of unreacted a-
diazoester 1a during the progression of the insertion reaction
was measured by IRMS. This ratio can be used to calculate the
kinetic isotope effect from enrichment in the heavy isotope in
Fig. 2 Kinetic studies for the determination of the rate-limiting step.
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Table 3 Polar X–H (X ¼ O, N, and S) insertion reactions with methyl a-
phenyl-a-diazoacetatea
Here, a large and normal heavy KIE value of 1.022 ꢀ 0.007
was obtained, which supports the hypothesis that the formation
of the iron carbene intermediate is rate-determining.33 Also, the
insertion reaction was run under pseudo-rst order conditions
under the optimum conditions with different concentrations of
the a-diazoester 1a (Fig. 2, eqn (3)). The formation of 2a was
monitored by GC analysis and initial rates were found to be
rst-order with regard to the concentration of methyl a-phenyl-
a-diazoacetate 1a. A linear variation was observed between the
initial rate and the concentration of 1a, with [1a] ¼ 0.125, 0.25,
and 0.5 M, resulted in initial rates of 5.55, 10.48, and 15.16 mM
hꢁ1, respectively. Such results are in accordance with those
obtained from a study of an iron-catalysed insertion reaction of
diazo compounds into C–H bonds.34
With these results in hand, a mechanism can be therefore
drawn where the coordination of the diazo species to the iron
centre enriches it thus allowing p-back bonding to extrude
nitrogen gas (I, Fig. 3). The formation of the Fe carbene II is rate-
determining which subsequently reacts with the activated
silane in a one-step manner to yield the insertion product 2a
and regenerate the iron catalyst. An alternative mechanism
involving the coordination of the terminal nitrogen to the metal
centre has thus been refuted.35
a
Reaction conditions: methyl a-phenyl-a-diazoacetate (0.25 mmol),
b
Fe(OTf)2 (5 mol%), X–H (0.25 mmol), DMC (2 ml). 5 equiv. of the
c
amine were used and reaction was conducted at 80 ꢂC. The reaction
d
˚
was concentrated to 0.5 M. 4 equiv. of the thiol and 25 mg of 4 A
MS were used at 80 ꢂC.
Polar X–H insertion reactions of methyl a-phenyl-a-diazo-
acetate 1a was then examined (Table 3). Insertions into O–H
bonds were conducted in DMC where alcohol 3a and ethers 3b
and 3c were obtained in very good yields with completion in
6 h.36 HOAc has been tested for O–H insertion and proceeds to
give the corresponding ester 3d in an 88% yield. N–H insertions,
however, were strongly dependent on the nature of the amine
used. Secondary amines 3e and 3f were afforded in moderate
yields of 57% and 68%, respectively, with prolonged reaction
the corresponding product 3g in 84% yield but completion
could only be reached aer 72 h. Also, when using o-anisidine
and m-anisidine, corresponding amines were obtained in 81%
and 74%, respectively (Table 3, 3h and 3i). It is important to
mention that in all of the three cases using primary amines, the
reaction was selective toward a mono-insertion with no sign of
a double insertion product. S–H insertions were mediated by an
ꢂ
times (72 h) and even required increased temperatures (80 C)
˚
to achieve completion. Improved yields were obtained with
primary aromatic amines, where an insertion with aniline led to
excess of the aryl thiol substrate and using 4 A MS as an additive
along with heating to reux of DMC. The insertion reaction with
thiophenol proceeded in a modest yield of 47% (Table 3, 3j). A
higher yield was obtained when employing a p-OMe or a p-Br
substituted thiophenol affording the corresponding products
3k and 3l in 93% and 98%, respectively. The reaction of 2a with
benzyl thiol resulted in 3m in an 88% yield.
In conclusion, we have successfully realized an iron-
catalysed insertion reaction of a-diazo compounds into X–H
bonds in DMC as a suitable solvent alternative. A wide range of
a-silylesters were obtained in good to excellent yields. The
mechanism of the insertion reaction into Si–H bonds was
studied while showing that the formation of the iron carbene
intermediate is rate-limiting. This work demonstrates the
efficiency of iron carbenes used in insertion reactions and also
shows that chlorinated solvents can be advantageously
replaced by greener alternatives in diazo chemistry. Further
developments in the use of iron carbenes will be reported in
due course.
Conflicts of interest
Fig. 3 Plausible mechanism for the iron-catalysed insertion reaction
of methyl a-phenyl-a-diazoacetate into Si–H bonds.
There are no conicts to declare.
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This work was supported by the Natural Sciences and Engi-
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RGPIN-2017-04272, the FRQNT Centre in Green Chemistry and
Catalysis (CGCC) Strategic Cluster FRQNT-2020-RS4-265155-
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28 In a control experiment involving Ph2MeSiH (1 equiv.) in the
presence of the iron catalyst Fe(OTf)2 (5 mol%) in DMC (2
ml) at 40 ꢂC for 24 h, the silane was found intact and 34 H. M. Mbuvi and L. K. Woo, Organometallics, 2008, 27, 637–
completely recovered. This shows that there is no reaction
occurring between the silane and DMC.
645.
35 The mechanism is similar to that obtained with a Rh
catalyst. See ref. 31.
36 Phenol was tested as a substrate for O–H insertion with
diazo compound 1a where the crude showed complex
mixtures with the corresponding ether in low yields (19%).
29 P. G. Jessop, D. A. Jessop, D. Fu and L. Phan, Green Chem.,
2012, 14, 1245–1259.
ꢂ
30 Diazo compounds 1l and 1m decompose at 40 C.
31246 | RSC Adv., 2019, 9, 31241–31246
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