Seeking Heteroatom-Rich Compounds: Synthetic and Mechanistic Studies into Iron Catalyzed Dehydrocoupling of Silanes

Article abstract of DOI:10.1021/acscatal.0c01440

A detailed synthetic investigation into the dehydrocoupling of silanes with amines, phosphines, and alcohols using an iron precatalyst (1) is presented. We have furnished over 30 examples of aminosilane synthesis along with kinetic studies using MeBnNH and MePhSiH2 as coupling partners. The kinetic studies suggest a reversible reaction with silane which generates aminosilane and an Fe-hydride dimer that undergoes rate-limiting protonolysis with amine with N-H bond cleavage in the transition state, consistent with a primary KIE of 2.42(3). The presence of dimers as on-cycle intermediates was analyzed in depth. Beyond this we have explored the substrate scope of phosphinosilane formation which shows a preferential heterodehydrocoupling to give the phosphinosilane with primary and secondary silanes. Silylethers can also be prepared and alcohols that contain alkene functionality do not show any tendency to reduce the double bond.

Full text of DOI:10.1021/acscatal.0c01440

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Article
Seeking Heteroatom-Rich Compounds: Synthetic and Mechanistic
Studies into Iron Catalyzed Dehydrocoupling of Silanes
Danila Gasperini, Andrew King, Nathan T. Coles, Mary F. Mahon, and Ruth L Webster
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c01440 • Publication Date (Web): 06 May 2020
Downloaded from pubs.acs.org on May 7, 2020
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Seeking Heteroatom-Rich Compounds: Synthetic
and Mechanistic Studies into Iron Catalyzed
Dehydrocoupling of Silanes
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†
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†
Danila Gasperini, Andrew K. King, Nathan T. Coles, Mary F. Mahon, Ruth L.
Webster†*
†Department of Chemistry, University of Bath, Claverton Down, Bath, United Kingdom,
BA2 7AY.
Abstract
A detailed synthetic investigation into the dehydrocoupling of silanes with amines,
phosphines and alcohols using an iron pre-catalyst (1) is presented. We have furnished
over 30 examples of aminosilane synthesis along with kinetic studies using MeBnNH and
MePhSiH as coupling partners. The kinetic studies suggest a reversible reaction with
2
silane which generates aminosilane and an Fe-hydride dimer that undergoes rate-limiting
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protonolysis with amine with N–H bond cleavage in the transition state, consistent with a
primary KIE of 2.42(3). The presence of dimers as on-cycle intermediates were analyzed
in depth. Beyond this we have explored substrate scope of phosphinosilane formation
which shows a preferential heterodehydrocoupling to give the phosphinosilane with
primary and secondary silanes. Silylethers can also be prepared and alcohols that contain
alkene functionality do not show any tendency to reduce the double bond.
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Keywords
Dehydrocoupling, homogeneous catalysis, reaction mechanisms, iron, silanes
Introduction
Dehydrocoupling of p-block compounds is an attractive method with which to generate
hydrogen gas. There has been a great deal of focus in the literature, using a wide range
of metal pre-catalysts, on heterodehydrocoupling of amine-boranes, most notably
ammonia-borane (H N·BH ) because of the high weight% of H that can be released from
3
3
2
1
this simple molecule. Although there has been focus on dehydropolymerization of amine-
and phosphine-boranes for the preparation of materials with novel properties, there are
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limited examples of dehydrocoupling and dehydropolymerization of silanes with protic
substrates such as amines, phosphines and alcohols.
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Much of the seminal research into homogeneous catalysis for amino- and
2
phosphinosilane formation via dehydrocoupling was carried out by Harrod, covering
several elegant examples using titanocene catalysts,3, 4 and by Eisen using uranium
5
amides for N–Si bond formation. Other examples of aminosilane formation by
dehydrocoupling include work from Hill,6, 7 Sarazin8-11 and Nembenna12 on Group II
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15
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catalysis, Crimmin and Guan using Ca and Y, Sadow exploring Mg and a range of
lanthanides,17 Tsuchimoto with Zn18 and Wright using Al.19 In general, catalytic
aminosilane formation is facile, with most examples in the literature being performed at
room temperature in a few hours, with the exception of particularly sterically hindered
reagents, which often need heating. In contrast the formation of phosphinosilanes tends
to require more forcing conditions and there are far fewer examples in the literature.
Recent work from Manners has demonstrated that tris(pentafluorophenyl)borane (BCF)
can be used in conjunction with tertiary silanes at 100-130 °C to prepare
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phosphinosilanes along with elegant examples of cyclic phosphinosilanes when a primary
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or secondary silane is employed. Waterman has used La[N(SiMe ) ] and a Zr catalyst
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for heterodehydrocoupling along with silylene elimination from silazanes and Stephan
has undertaken P–P bond activation to furnish phosphinosilanes using a Rh catalyst.20, 24
There are few examples of transition metal catalyzed dehydrocoupling methods for
silylether formation, with examples in the literature being limited to late transition metals
(likely due to oxophilicity in the early to mid-transition metals). Ito and Sawamura have
reported using Cu25 and Au26, 27 while Yamada has tested a range of acetylacetonate
(
acac) salts, with Cu(acac) giving the highest yields (important to note is the very low
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29
reactivity obtained with Fe(acac) ). Sadow has developed a Zn system and Grubbs
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has reported an attractive NaOH catalyzed method, but most examples are transition
metal-free and follow on from Piers’ early report of high levels of reactivity using tertiary
silanes and a range of alcohols in the presence of BCF catalyst.31-33
We saw iron catalyzed heterodehydrocoupling as a remarkably simple route to prepare
highly functionalized silane-protected molecules that have a range of applications. In
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terms of small molecule chemistry, aminosilanes have been used as masked ammonia
reagents in Pd-catalyzed aryl-N bond formation, and undergo facile deprotection to reveal
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the desilylated aniline product and are postulated intermediates in catalytic Staudinger
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amidation reactions. Phosphinosilanes have been used in Pd-catalyzed cross-coupling
by Stille to prepare tertiary phosphines,36 by Clarke37 and Yang and Sun38, 39 in Ni-
catalyzed cross-coupling and for C–CN bond activation. Gudat undertook an
organocatalytic approach in cross-coupling to prepare P–C bonds from dihalide reagents
40
and TMS–PPh . More recently phosphinosilanes have been used by Hirano and Miura
2
for the preparation of 1,2-diphosphines41, 42 and by Daugulis to functionalize benzynes,
43
utilizing both Si and P components of the reagent. Oro has also used both components
of the phosphinosilane for the Ir-catalyzed functionalization of CO2.44
45
Silylethers are ubiquitous in protecting group and traceless directing group strategies in
organic synthesis. R HSi-protected alcohols can be easily deprotected using standard
2
acidic work-up procedures as demonstrated by Brunner,46 Nile47 and Lavastre and
48
Morken. Hartwig used dehydrocoupling to prepare silylethers using an Ir catalyst and
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the silylether product was then used as a directing group in C–H borylation,49 Tamao-
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Fleming oxidation or Hiyama coupling of benzoxasiloles, which is also formed using an
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Ir catalyst. Herzon has also recently reported elegant studies into C–H functionalization
of (+)-Pleuromutilin51 where the resulting tertiary siloxane was used to undertake an
iridium catalyzed C–H bond silylation and subsequent Tamao-Fleming oxidation of the
silylated carbon.
Beyond the heterodehydrocoupling of silanes to prepare silyl-protected organic motifs,
polysilazanes are used as coatings and precursors to ceramics52, 53 while polysiloxanes
(or silicones) are widely used materials with applications that include elastomers,
lubricants, foams and adhesives.54, 55
The chemistry that has already been developed using the iron(II) pre-catalyst 1
demonstrates the versatility of this complex, particularly in main group bond
transformations that appear to be redox neutral at iron.56-62 However, we are continually
seeking to understand the fundamental bond making and breaking processes involved in
catalysis with this complex in order to try to develop new modes of reactivity. We herein
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present our synthetic and mechanistic investigations into dehydrocoupling and
dehydropolymerization to prepare aminosilanes, phosphinosilanes and silylethers
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(Scheme 1). Only during the preparation of this manuscript have these classes of
transformations been reported using an iron catalyst.63
HSiR3
Fast
R E SiR
2
3
iPr
iPr
iPr
N
N
H
Fe CH TMS
Fe ER2
2
Fe
Fe
Mild conditions
RT to 80 °C
iPr
H
Over 50 examples
E = N, P, O
Slow
1
Mechanistic studies
H2
R EH
2
Scheme 1. The work presented includes a wide substrate scope with over 50 examples
of amino-and phosphinosilanes and silyl ethers being presented, along with mechanistic
studies.
Results and Discussion
2.1 Aminosilane substrate scope
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C6D6
R1
R2
R1
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H
HSi R2
N
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R3
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H
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H2
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H
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Si
nBu
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nBu
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Ph
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H
N
H
Ph
N
Me
N
Me
N
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Ph
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Ph
H
H
H
H
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88%
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>99%
2c
>99%, 2d
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>99%
2e
>99%
2f
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Ph
H
Ph
Me
Si
Ph
Ph
Ph
Ph
H
N
H
H
Ph
Ph
H
H
Me
Si
H
tBu
Si
Si
Si
N
Me
Me
N
N
H
N
H
N
H
Me
Me
H
Me
Si
OMe
H
Ph
>99%
>99%
2h
50%, 50 °C
74%, 80 °C
98%, 80 °C
84, 80 °C
2g
2k
2i
2
j
Ph
Ph
Ph
Ph
H
H
H
H
N
H
Si
Si
Si
Ph
Si
Ph
H
N
H
H
H
N
H
Ph
Ph
N
H
N
H
Ph
N
H
N
H
H
H
H
>
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>99%, 48 h
53%, 2n
65:35a
>99%, 2o
0:30a
2m
7
Me
Ph
Ph
Ph
Ph
H
H
H
H
H
H
Si
H
Si
H
Si
Si
Me
N
H
N
H
Me
N
H
Me
N
H
Me
H
Ph Ph
H
8
H
H
3%, 2p
0:20a
>99%
2q
>99%
2r
82%
2s
8
2
t, 80 °C
N
H
Mn 669 gmol-1
H
H
N
Mw 855 gmol-1
Si
n
PDI 1.3
Ph
a
Scheme 2. Varying the primary amine. mono : di.
A range of different primary amines undergo dehydrocoupling with secondary silanes
at room temperature (Scheme 2). The reaction tolerates alkyl and aryl amines and works
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well with diphenyl- and methylphenyl silanes (2a to 2k). Sterically hindered amines can
be used, for example o-methoxyanisole gives quantitative yield of 2g while tert-butylamine
and tritylamine, although requiring heating to 80 °C, do give excellent yield of product (2j
and 2k). Methylphenylsilane is completely consumed, quantitatively forming the
disilylated product of 1,4-xylylenediamine (2i), while phenylsilane undergoes double
amination with aniline (2m) in contrast benzylamine, which gives a 70:30 mixture of dimer
to monomer (2l). No reaction is observed with tertiary silanes such as triphenylsilane and
dimethylphenylsilane in conjunction with aniline or morpholine. Interestingly, substrates
that contain a double bond also undergo alkene reduction (2n to 2s), which we presume
is a consequence of transfer hydrogenation (TH) analogous to our previous report of TH
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with amines and boranes. We were also able to furnish oligo(silazane) 2t as an oil. GPC
-1
analysis of this species gives a modest M of 855 gmol . Unfortunately attempts to
w
increase the molecular weight of the polymer, by using very forcing conditions
synonymous with condensation polymerization, or dilute conditions associated with step-
growth polymerization, did not increase the chain length. Reducing the catalyst loading
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to 1 mol% and increasing the reaction temperature to 80 °C only led to a modest increase
in chain length.64
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(5 mol%)
C6D6
R2
+ HSi R3
R4
R2
R3
R
H
N
Si
R
R4
N
R1
R1
RT, 24 h
- H2
3
Ph
Et
Et
Et
Ph
H
H
H
H
H
Si
Si
Si
Et
Si
Si
N
Ph
N
N
Et
N
Ph
N
Ph
Ph
Et
Et
O
O
Me
Me
Me
>
99%
>99%
>99%a
>99%a
92%a
3a
3b
3c
3d
3e
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
Si
Si
Et
Si
nBu
Si
Ph
Si
Ph
N
N
N
N
N
Me
Me
Me
Me
Me
O
Me
Me
Me
Me
>
99%
3f
>99% (78%), 3g
Scale up: 5 mmol
>99%, TON 200
84%
3h
>99%
3i
80%a
3j
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
Si
Si
Si
Si
Si
N
N
N
N
N
N
Ph
Me
N
Me
Ph
Me
Me Me
O
O
>
99%
>
99%a
95%a
>99%, 3n
70:30b
>99%,
3o
80:20b
3k
3l
3m
a
b
Scheme 3. Varying the secondary amine. 10 mol% 1, 80 °C, 48 h; di (shown) : mono
29
1
by Si{ H} NMR, 50 °C.
N-Methylbenzylamine and morpholine undergo dehydrocoupling with diphenyl-, diethyl-
and methylphenyl silane equally well under our standard reaction conditions, giving
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quantitative yield of product within 24 h (3a to 3d, 3f and 3g, Scheme 3). To demonstrate
scalability 5 mmol of morpholine and methylphenylsilane with 2 mol% 1 went to
completion after 48h (73% isolated yield). To demonstrate both robustness and
scalability, we carried out a standard reaction of morpholine and methylphenylsilane, and
after 24 h further portions of amine and silane were added via syringe. This iterative
addition process was repeated until 5 mmol of each reagent was added over the course
of 10 days: the catalyst is clearly still active in solution at this time giving complete
conversion to 3g with only 5 mol% 1 after 10 days. Quantitative yield is obtained when N-
methylbutylamine is coupled to methylphenylsilane (3i). Heterocyclic amines piperidine,
indoline and tetrahydroquinoline give quantitative yield of the methylphenylsilyl adducts
1
1
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1
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(3k to 3m). Phenylsilane undergoes double amination with both methylbenzylamine and
morpholine (3n and 3o).
2.2 Aminosilane mechanistic investigations
Intrigued by whether our catalyst would show any similarities to other detailed amine-
silane dehydrocoupling kinetics studies (for example Sarazin and Sadow both report ʋ =
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k [amine] [silane][catalyst] (Ba and Mg respectively8, 16) whereas Hill reports ʋ = k
0
0
2
6
[
amine][silane] [catalyst] with Mg/Ca and ʋ = k [amine][silane][catalyst] with Ba ), we
0
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proceeded to undertake in situ NMR monitoring studies. We opted to use the reaction of
MeBnNH with MePhSiH . Important to note is that the reaction using 3 mol% 1 is 92%
2
2
complete after around 5 hours at room temperature. Clearly the reaction is facile and,
although all amine-silane dehydrocoupling reactions were allowed to react for 24 h, very
high levels of conversion (>90%) can be achieved within a few hours, but extended
reaction times are needed to achieve quantitative (>99%) conversion to product in most
cases (Schemes 2 and 3). The data obtained suggest that the reaction has a first order
dependence on the Fe-center when using 1 as reaction pre-catalyst (Figure 1a). We then
studied the transformation by varying the concentration of silane. As can be seen, the
reaction is independent of silane concentration (Figure 1b). As expected, with 0.5
equivalents of silane around 50% yield of product is obtained (yellow squares). In
contrast, a doubling of amine loading results in an approximate doubling of the reaction
rate (Figure 1c) and so a first order dependence in amine is observed.
a)
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b)
c)
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0
Figure 1. a) Plot showing first order relationship for formation of 3f at different pre-catalyst
loadings (6 mol%, 5 mol%, 4 mol%, 3 mol%). Conditions: 0.5 mmol silane, 0.5 mmol
amine, 0.5 mL C D , RT. b) Reaction profiles showing the formation of MePhSi–NMeBn,
6
6
3f, at different silane loadings (○ 2 equiv., ● 1.5 equiv., ■ 1 equiv., □ 0.5 equiv.)
Conditions: 3 mol% 1, 0.5 mmol amine, 0.5 mL C D , RT. c) Plot showing first order
6
6
relationship for formation of 3f at different amine loadings (2 equiv., 1.5 equiv., 1 equiv.,
.5 equiv.). Conditions: 3 mol% 1, 0.5 mmol silane, 0.5 mL C D , RT.
0
6
6
1
Monitoring the paramagnetic region of a H NMR spectrum for a catalytic reaction
shows that iron amido complex 4 is observed throughout. Interestingly, 1 is also observed
throughout the reaction. Starting from 1, using an analogous method to that reported by
Holland,65 4 was prepared and isolated. Use of 4 in catalysis shows that the lengthy
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kinetic induction period that is observed with 1 is circumvented and 92% conversion to
product is obtained after 4 h, compared to 7 h when using 1 (there is 50% conversion
after 90 minutes using 4 and 50% conversion after 150 minutes with 1). This also
suggests that 4 is likely to be an on-cycle species in catalysis. The reaction is first order
in 4, which we attribute to this species only contributing one Fe-center to the key
intermediate (5/5’ vide infra).64 Holland and co-workers have shown previously that in
both the solid state and in solution iron hydride species with a methyl backbone β-
diketiminate ancillary ligand exists as the dimeric hydride complex 5.66 At 298 K this
1
1
1
1
1
1
1
1
1
1
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0
1
67
dimeric species is observed by H NMR in our reaction. Preparation, isolation and
64
kinetics studies with 5 give a first order fit. This was unexpected, but similar results have
68
been reported by Holland when investigating B–C bond cleavage mediated by 5. This
first order fit is attributed to the presence of the open isomer (see 5’, Scheme 4), which is
in rapid equilibrium with 5. If the amine and the iron hydride dimer (5) are involved in the
rate-limiting step with only one Fe–H bond being broken during this step then only one
equivalent of amine should react and therefore we would expect a first order dependence
64
in HNMeBn when 5 is employed directly in catalysis, which is the case. We therefore
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5
5
6
postulate that the rate limiting transition state involves the formation of a cyclic species,
possibly of the form 5’ , followed by release of hydrogen gas and formation of 4 and 5.
TS
0
1
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Collapse of 5’TS might proceed via a mixed hydride-amido dimer such as 6. We have
n
evidence for the ability for such species to form; reacting 5 with H N Bu and MePhSiH 6
2
2
1
is immediately observed by H NMR and X-ray quality crystals were readily obtained
(Figure 2).
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n
Figure 2. Mixed hydride-amido dimer (6) isolated from reaction of BuNH , PhMeSiH
2
2
64
and 5. Ellipsoids are represented at 50%. Except for the hydride ligand and N-H, all
hydrogen atoms, solvent, disorder and the 2,6-diisopropyl groups have been omitted for
clarity.
1
1
1
1
1
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0
In order to probe the nuclearity of the species present in solution, DOSY experiments
69
n
were performed on a mixture of 5 and BuNH in C D ; disappearance of 5 is observed
2
6
6
-10
2 -1
with H release and formation of dimers (D = 5.04-5.48 × 10 m s ), primarily consisting
2
of complex 6 but formation of other mixed dimeric species cannot be ruled out (rsolution
=
70
n
8.2 Å). Pleasingly when reacting pre-catalyst 1 with a mixture of amine ( BuNH ) and
2
PhMeSiH in C D multiple species were observed; the slower peaks were analyzed and
2
6
6
-10
2 -1
showed similar diffusion coefficients (D = 4.50-5.95 × 10 m s ) to the experiment above
and a large solution hydrodynamic radius (rsolution = 8.5 Å) synonymous with dimers. These
1
results are in line with the appearance of dimer 5 during kinetic H NMR analysis and the
order in 5 which suggests it exists as an active dimer throughout catalysis. Furthermore,
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5
5
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1
calculation of the hydrodynamic radius of 6 using single crystal data is very similar to
the solution data obtained (rsolid = 9.2 Å)
0
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Combined, these data have allowed us to postulate a preliminary catalytic cycle
Scheme 4). The steps in the catalytic cycle are in-line with reports from Sadow,16
(
8
6
Sarazin and Hill in that catalyst activation forms an initial metal-amido species which
then undergoes metal-hydride formation and release of the aminosilane product. Unlike
the report from Sarazin, where hydride transfer is rate limiting, or Sadow where attack of
silane by an Mg-amido species is rate limiting, the subsequent proton transfer between
N-methylbenzylamine and iron-hydride is probably the slow step in our reaction.
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1
Me
Ph
N
H
SiMe4
iPr
1
1
1
1
1
1
1
1
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1
2
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2
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iPr N
N
iPr Fe i
Pr
H SiMePh
2
N
Me
Ar
Ar
Fe
Ar
Fast
Ph
Me
N
N
H
N
N
H
4
Fe
Si
Ph
N
Ph
N
R
2
Me
Ar
6'
iPr
iPr N
H2
N
iPr Fe i
iPr
iPr
Pr
i
i
H
Pr Pr
N
N
N
H
Fe
Fe
H
Pr Pr
N
i
i
iPr
iPr
Ar
H
H
H
5
Ph
N
Me
N
Fe
N
N
Ar
Fe
N
Ar
Fast
Ar
Ar
H
N
N
Ar
Fe
Slow
5'TS
N
iPr
H
Fe
iPr N
iPr
H
N
iPr
5'
Ph
Me
Scheme 4. Postulated mechanism based on NMR studies.
Deuterium labelling using d -aniline as an easily accessible substrate gives a primary
2
64
KIE for the formation of 2f of 2.42(3), which may indicate proton transfer is occurring
through a non-linear transition state. This also supports our proposed catalytic cycle
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5
5
5
5
5
6
where the reaction is first order in amine. We have also undertaken Arrhenius and Eyring
-1
‡
-1
analysis of catalysis mediated by 1: Ea = 54.59 kJ mol ± 0.03; ΔH = 12.43 kcal mol ±
0
1
2
3
4
5
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0
‡
-1
-1
‡
-1
0
.02; ΔS = -5.13 cal mol K ± 0.01; ΔG = 13.95 kcal mol ± 0.2. This is in line with
6
-1
8
-1
reports using Mg (13.9 kcal mol ) and Ba (15.6 kcal mol ) systems. The latter study
‡
‡
from Sarazin also reports similar Ea and ΔG to our system. The importance of ΔH to
the activation energy barrier for our process is highlighted by the primary KIE value. We
‡
can rationalize the low ΔS obtained by looking at the process of 5/5’ and thus onward
formation of 5’ . In these steps an Fe–H bond is broken and H–H bond begins to form.
TS
Akin to work from Holland on the likelihood of 5’ to be a reaction intermediate, this
‘interchange mechanism’ is often linked to very low ΔS‡.67
On closer inspection of the deuterium labelling study we can report another intriguing
result. Reaction of d -aniline with methylphenylsilane gives a mixture of PhDN–SiHMePh
2
(
d -2f) and PhDN–SiDMePh (d -2f) in a 77:23 ratio (Scheme 5a). In the absence of
1
2
catalyst there is no H/D exchange and no dehydrocoupling (to give product(s) of the form
2
f, Scheme 5b). To determine whether the HD released from reaction of d -aniline with
2
methylphenylsilane is responsible for silane deuteration, presumably due to HD activation
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by iron catalyst, we performed the standard reaction under an atmosphere of D (Scheme
2
5
c). Reaction in a J-Young NMR tube gives a small amount of Si–D product (d -Si-2f), but
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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4
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4
5
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5
5
5
5
5
5
5
6
0
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9
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9
0
no deuterium incorporation at nitrogen takes place (in the aniline starting material or
aminosilane product). A standard reaction, of the form presented in Scheme 5a, will
generate an estimated 6 bar of H /HD/D pressure in a J-Young NMR tube (based on the
2
2
Ideal Gas Law), whereas the reactions in Scheme 5c are back-pressurized with 1 bar D2,
hence the difference in J-Young NMR tube deuterium incorporation results. The same
reaction in a Schlenk tube with a large headspace back-filled with 1 atm. D gives 82%
2
d -Si-2f. These data indicate that a greater number of equivalents of D leads to more D-
1
2
incorporation: the same reaction under a D atmosphere was performed in the same scale
2
in a sealed Schlenk (25 mL) without stirring and it showed 87% deuterium incorporation,
suggesting that mass transfer is not influencing the reaction outcome. N–H bond
activation is likely more challenging than other steps in the catalytic cycle; in-keeping with
our postulated catalytic cycle. 2f does not show any deuteration when exposed to 1 atm.
D (Scheme 5d): the iron catalyst is not able to further functionalize 2f. Repeating this
2
reaction of 2f in the presence of one equivalent of d -aniline, 1 atm. D and 5 mol% 1
2
2
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2
2
2
2
2
2
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4
4
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5
5
5
5
5
5
5
5
6
gives a 1:1 mixture of 2f and d -N-2f, therefore indicating that N-Si bond formation is
1
reversible (Scheme 5e). Furthermore H/D exchange is not observed when 2f is mixed
0
1
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with d -aniline. Si–H bond deuteration is not observed under these conditions indicating
2
that silazane formation is fast and reversible, precluding any deuteration of free silane
released in the reverse reaction. This also suggests that silane deuteration occurs via a
different catalytic cycle. Taking 1, H NPh (to allow catalyst activation) and
2
methylphenylsilane under an atmosphere of D does not show deuterium-incorporation
2
at nitrogen, indicating that direct formation of iron(II)-deuteride from iron(II)-amido does
not take place. Reaction of methylphenylsilane, D and 1 (5 mol%) gives 9% MePhSiHD
2
further indicating that H/D exchange via Si–H activation is feasible in the presence of 1
and this process is currently under investigation.
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a)
b)
c)
D
D
N
ND2
1 (5 mol%)
N
Ph
Ph
Si
+
Si
H
D
H SiMePh (1 eq)
2
Me
Me
C D , RT, 24 h
6
6
7
7%
d1-N-2f
23%
d2-N,Si-2f
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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2
2
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3
3
3
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ND2
ND2
+
H SiMePh
2
H SiMePh (1 eq)
2
1
: 1
C D , RT, 24 h
6
6
H
N
H
N
NH2
1 (5 mol%)
Ph
Ph
Si
+
Si
H
D
H SiMePh (1 eq)
2
Me
Me
D (1 atm)
2
9
1
2
1%
8%
9%
82%
d1-Si-2f
J-Young NMR
Schlenk tube
C D , RT, 24 h
6
6
f
d)
e)
H
H
1
(5 mol%)
N
Ph
N
Ph
Si
Si
H
H
D (1 atm)
Me
2
Me
>99%
C D , RT, 24 h
6
6
2f
H
N
D
N
H
N
1
(5 mol%)
Ph
Ph
Ph
Si
+
Si
Si
H
H
H
Me
Me
D NPh
2
Me
D (1 atm)
2
1
:
:
1
C D , RT, 24 h
6
6
2f
d1-N-2f
Scheme 5. Studies into deuterium incorporation into 2f.
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2
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Based on the combined experimental and literature data an updated catalytic cycle for
reaction of PhNH with MePhSiH in the presence of D and incorporating H/D exchange
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
can be proposed (Scheme 6). The open-isomer was invoked as possible mechanistic
65
scenario for hydride ligand exchange with D from 5 to 5’-D by Holland and co-workers.
2
The catalytic cycle for Si–D bond formation could then proceed in a similar manner to that
of Si-N bond formation, in that there is a fast reversible equilibrium between 5’-D and the
open isomer, reaction with amine releases H or HD and forms 4, it would be at this point
2
that fast reversible equilibrium between silane and 5’-D allows deuteration of the silane.
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Page 25 of 53
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9
1
Me
Ph
N
H
SiMe4
iPr
iPr N
N
Fe i
H SiMePh
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
iPr
Pr
2
N
[Fe]
Me
HDSi
MePh
Ar
Ar
Fe
Ar
Fast
Me
H(D)
Ph
N
N
N
H
N
N
Si
Me
Bn
4
Ph
N
Ph
H2 / HD
Slow
Me
Fe
Fast
(
H)
N
Me
H
D
R2
N
Ph
[
Fe]
[Fe]
Ar
N
R2
H
D
6'
iPr
iPr
[Fe]
[Fe]
H2
N
N
Fe i
D
iPr
iPr
iPr
iPr
[Fe]
[Fe]
Pr
H
i
i
H
Pr Pr
Fast
N
N
N
H
H
5
'-D
Fe
Fe
N
iPr iPr
-
HD
iPr
Ar
H
H
Ph
Me
5
H
N
D
H
Fe
N
D
[
Fe]
N
Ar
H
Fe
N
N
[Fe]
Ar
Fast
Ar
Ar
H
N
N
Ar
Fe
H
Slow
N
5'TS
D2
iPr H
Fe
iPr N
D D
[Fe]
iPr
H
[Fe]
H
iPr
5'
N
Ph
Me
Scheme 6. Revised catalyst cycle for dehydrocoupling demonstrating that, starting from
a common intermediate (5’) a second cycle that could account for Si–D bond formation
due to H/D exchange in the presence of D2.
2.3 Phosphinosilane substrate scope
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
(5 mol%)
C6H6
R2
R2
R
R3
R
H
+ HSi R3
Si
R
P
P
R1
R
+
R4
P
R
P
R4
70 °C, 24 h
R
R
-
H2
Ph
Ph
Ph
Ph
H
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
H
H
Cy
Cy
Si
Ph
Si
Si
Ph
Si
P
P
P
P
Me
Ph
Ph
Me
Ph
Cy
Ph
Cy
>
99%, 7a
85:15
>99%, 7b
>99%, 7c
81:19
>99%, 7d
Ratio P-Si : P-P
99:1
99:1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
Ph
Ph
Ph
Cy
Cy
Cy
Si
Ph
Si
Si
Ph
Si
Si
Ph
Si
P
P
P
P
P
P
Me
H
Ph
Ph
Me
Cy
Ph
Cy
Ph
Cy
Ph
>
99%, 7e
>99%, 7f
>99%, 7g
0:100
trace, 7h
0:100
>99%, 7i
trace, 7j
0:100
80:20
92:8
0:100
Scheme 7. Scope of phosphinosilane reactivity.
Given the reaction scope achieved with amines, we expected to achieve good reactivity
with phosphines. When we began our optimization studies it was immediately clear that
phosphine-silane dehydrocoupling is a more challenging target for pre-catalyst 1: in order
to obtain high conversion to the dehydrocoupled product heating to 70 °C is necessary
(
Scheme 7). For comparison Waterman has heated to 90 °C and Manners to 100 °C with
2
2
20
their Zr and BCF catalyzed processes. Also apparent in our catalysis is the potential
for competitive phosphine homodehydrocoupling to take place; this is not unexpected as
we have previously reported diphosphane synthesis via dehydrocoupling using
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Page 27 of 53
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5
7
comparable reaction conditions. Homodehydrocoupling becomes competitive with the
desired heterodehydrocoupling reaction when tertiary silanes (triphenyl- and
methyldiphenyl-) are employed (7g to 7j) and in the case of diphenylphosphine, complete
conversion to tetraphenyldiphosphane (Ph P–PPh ) is observed (7g and 7i). When
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
dicyclohexylphosphine is reacted with tertiary silanes, although the transformation is
completely selective for homodehydrocoupling, only trace amount of product is observed.
This is in-line with our previous report on phosphine dehydrocoupling i.e. we do not
believe that the silane plays an active role in assisting homodehydrocoupling. Moreover,
under the catalytic reaction conditions, homodehydrocoupled product (Ph P) does not
2
2
react with Ph SiH even after 3 days at 80 °C: our system does not proceed via activation
2
2
20
24
of the P–P bond (as achieved by Manners and Stephan ) but rather through activation
of the P–H bond. In contrast, when secondary silanes are employed,
dicyclohexylphosphine
gives
much
better
selectivity
for
the
desired
heterodehydrocoupled product. 99:1 Hetero:homodehydrocoupled product is obtained
with dicyclohexylphosphine with both diphenylsilane and methylphenyl silane (7b and 7d),
whereas this ratio drops to 85:15 and 81:19 respectively when HPPh is employed (7a
2
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and 7c). Again, this is a direct reflection of the ease with which the phosphines
homodehydrocouple. Similar to Harrod’s report using titanocenes, phenylsilane only
reacts once with HPPh and HPCy yielding R PSiPhH selectively.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
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9
0
1
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3
4
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9
0
2
2
2
2
2.4 Silylether substrate scope
The inherent oxophilicity of Fe(II) does not appear to be limiting when alcohols are
employed as coupling partners. However, the nature of the alcohol appears to be
important and there is an interesting divergence in reactivity whereby phenol only gives
a trace amount of product (8a, Scheme 8) whereas aliphatic alcohols give almost
quantitative yield of siloxane (8b to 8g, with the exception of methanol which only gives
35% 8c). If the silane is changed from methylphenylsilane to phenylsilane then good
levels of mono-silylated phenolic product is achieved (8m). Returning to focus on
reactivity achieved with methylphenylsilane, 2-phenylglycinol shows interesting selectivity
with the alcohol reacting preferentially (8h). Elaborate organic motifs can undergo
heterodehydrocoupling to generate the silylated product including (−)-menthol (8g),
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cholesterol (8i), linalool (8j), β-citronellol (8k) with no evidence for double bond
functionalization of isomerization. Phenylsilane reacts with benzylalcohol and benzhydrol
quantitatively (8n and 8o) and with pinacol alcohol to give the dioxasilolane (8p).
Oligosiloxane, 8q, derived from phenylsilane and diethylene glycol was also prepared as
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
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4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a solid with very narrow PDI and DP of 4.5. DSC shows two T peaks (245.7 °C and
m
256.2 °C) which may indicate the presence of both linear and cyclic oligomers.
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