A metal-amide dependent, catalytic C-H functionalisation of triphenylphosphonium methylide

Article abstract of DOI:10.1039/c2sc21123h

We report the [Y{N(SiMe₃)₂}₃] catalysed dehydrocoupling of triphenylphosphonium methylide with phenylsilane to form the silylated ylide Ph₃PCHSiH₂Ph. Attempts to catalyse this reaction with the related group 2 hexamethyldisilazide base [Ca{N(SiMe ₃)₂}₂] led to the catalytic formation of the phosphine Ph₂PCHSiH₂Ph along with Ph₂PMe in low selectivity, while group 1 bases [M{N(SiMe₃)₂}] (M = Li, Na, K) proved ineffective for both transformations. The stoichiometric reactions of Ph₃PCH₂ with [M{N(SiMe₃)₂} _n] have been investigated and allowed the isolation and characterisation of a cyclometallated phosphonium methylide complex of yttrium. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2sc21123h

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A metal–amide dependent, catalytic C–H
functionalisation of triphenylphosphonium methylide†
Cite this: Chem. Sci., 2013, 4, 691
*
Adi E. Nako, Andrew J. P. White and Mark R. Crimmin
We report the [Y{N(SiMe3)2}3] catalysed dehydrocoupling of triphenylphosphonium methylide with
phenylsilane to form the silylated ylide Ph3PCHSiH2Ph. Attempts to catalyse this reaction with the
related group 2 hexamethyldisilazide base [Ca{N(SiMe3)2}2] led to the catalytic formation of the
phosphine Ph2PCHSiH2Ph along with Ph2PMe in low selectivity, while group 1 bases [M{N(SiMe3)2}]
(M ¼ Li, Na, K) proved ineffective for both transformations. The stoichiometric reactions of Ph3PCH2
with [M{N(SiMe3)2}n] have been investigated and allowed the isolation and characterisation of a
cyclometallated phosphonium methylide complex of yttrium.
Received 31st July 2012
Accepted 5th November 2012
DOI: 10.1039/c2sc21123h
www.rsc.org/chemicalscience
documented [(Cp*)2ScMe] as a catalyst for not only the hydro-
methylation of propene but also the dehydrocoupling of
methane and Ph2SiH2.9 A similar dehydrocoupling reaction was
discovered by Hou and co-workers and applied to ortho-selective
silylation of anisoles with arylsilanes catalysed by half-sandwich
scandium alkyl complexes.10
Onium ylides represent an extremely important class of
nucleophile for carbon–carbon bond forming reactions. Despite
a number of studies documenting the stoichiometric cyclo-
metallation of phosphonium ylides, the use of ylides as carbon-
based directing groups in C–H functionalisation chemistry has
limited precedent.1,11 We recently reported the coordination of
triphenylphosphonium methylide to a “Y(I){N(SiMe3)2}2” frag-
ment.12 Based on observations made during this study, we now
report the C–H activation and dehydrocoupling of triphenyl-
phosphonium methylide with phenylsilane catalysed by the
simple metal amide complex [Y{N(SiMe3)2}3].
Introduction
In 1981 Watson reported the degenerate exchange of 13CH4 with
[(Cp*)2Lu12CH3] to form [(Cp*)2Lu13CH3] and CH4.1 In the
12
following year Marks observed the C–H activation of benzene
and tetramethylsilane with the thoracyclobutane derived from
thermolysis of [(Cp*)2Th{CH2CHMe2}2].2 These reactions are
believed to proceed via s-bond metathesis and provide a mech-
anistic counterpoint to C–H bond activation involving 1,2-addi-
tion across M]X bonds (M ¼ NR, O), oxidative addition to late-
transition metals, electrophilic substitution at late-transition
metals, and metalloradical mediated hydrogen-atom abstrac-
tion.3,4 More recently, stoichiometric methods for the C–H bond
activation of hydrocarbons have given way to catalytic protocols
and a large number procedures have been developed for the
hydroalkylation and hydroarylation of substrates containing
unsaturated carbon–carbon and carbon–nitrogen bonds.5
Despite these efforts, examples of catalytic C–H activation
and functionalisation with d0 and d0fn-complexes remain
largely unexplored. For example, Jordan and co-workers repor-
ted the hydroarylation of alkenes with substituted pyridines
catalysed by the cationic zirconium complex [Cp2ZrMe(THF)]
[BPh4].6 The scope of this reaction has been improved by
employing half-sandwich complexes of scandium and yttrium
in the presence of B(C6F5)3,7 while a pincer-supported scandium
imide has been proposed as a catalyst for the hydroarylation of
isonitriles with pyridine.8 In related studies, Tilley and Sadow
Results and discussion
C–H functionalisation of triphenylphosphonium methylide
Employing [Y{N(SiMe3)2}3] (10–20 mol%) as a pre-catalyst for the
reaction of Ph3PCH2 with PhSiH3 in C6D6 solution at 25 ꢀC
resulted in noticeable gas evolution and complete consumption
1
of the ylide. Monitoring the reaction by H and 31P NMR spec-
troscopy revealed the formation of a new product characterised
by a diagnostic doublet of doublets in the silyl hydride region of
1
2
the H NMR spectrum d ¼ 5.24 ppm (dd, J
¼ 9.0 Hz and
31P–1H
3J1H–1H ¼ 4.5 Hz) and a 31P resonance consistent with a new four-
coordinate phosphorus environment (d ¼ +23.8 ppm) reminis-
cent of that reported for Ph3PCHSiMe3 in d8-THF (d ¼ +21.9 ppm)
and Ph3PCH2 in C6D6 (d ¼ +22.0 ppm). These peaks are assigned
to the silylated ylide 1a formed in 88% yield (Scheme 1 and
Table 1) from the dehydrocoupling of Ph3PCH2 with PhSiH3.
While the by-product, H2, was observed at d ¼ 4.48 ppm in C6D6
Department of Chemistry, Imperial College London, Exhibition Road, South
Kensington, SW7 2AZ, UK. E-mail: m.crimmin@imperial.ac.uk; Tel: +44 (0)
2075942846
† Electronic supplementary information (ESI) available: Full experimental details
for catalytic procedures and the synthesis of 1a, 2a–c. CIF les for for 1a, 2a–c and
details of the DFT calculations. CCDC 894051, 894052, 894053 and 894054. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2sc21123h
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catalyst resulted in production 1a and 1c in a 1 : 1 ratio. To
investigate the possibility that the reaction is an example of
general base catalysis, alternative silazide bases were examined
as pre-catalysts (Table 1 and ESI Fig. S1†).
Group 1 metal amides, [M{N(SiMe3)2}] (M ¼ Li, N, K) inhibited
methylide/silane dehydrocoupling, producing mixtures of 1b–1c
in low yield (19–32% conversion). In contrast, [Ca{N(SiMe3)2}2]
formed 1b in high yields with modest selectivity. The C–H func-
tionalized product 1a was not formed under these reaction
conditions (Table 1). Monitoring reactions of Ph3PCH2 with
PhSiH3 and20mol%[Ca{N(SiMe3)2}2]inC6D6 by1HNMRrevealed
that the production of 1b was accompanied by C6H6 as judged by
anincreaseinthe intensity oftheresonanceat7.15ppmrelativeto
the internal standard.13 Exposing 1a to these reaction conditions
did not result in the formation of 1b, furthermore attempts to
effect the dehydrocoupling of MePPh2 with PhSiH3 to form 1b
failed. The former experiment demonstrates that 1a is not a
kinetic product on the pathway to the formation of 1b.
Scheme 1 The catalytic dehydrocoupling of Ph3PCH2 with PhSiH3 to form
Ph3CHSiH2Ph 1a.
Table 1 The catalytic dehydrocoupling of Ph3PCH2 with PhSiH3 with group 2
and 3 metal–amide catalysts
M
n
mol%
Time
Conversion
1a : 1b : 1c
Yield 1aa
—
Li
Na
K
—
1
1
1
2
—
20
20
20
20
20
30
20
1 h
95%
23%
19%
32%
89%
95%
97%
96%
1 : 0 : 1
47%b
0%
0%
0%
0%
88%
77%
57%
18 h
22 h
22 h
5.0 h
0.5 h
3.5 h
0.5 h
0 : 0.3 : 1
0 : 0.3 : 1
0 : 0.7 : 1
0 : 2.1 : 1
12 : 0 : 1
7.7 : 1 : 1
1.5 : 0 : 1
Stoichiometric reactions of group 1–3 metal–amides with
triphenylphosphonium methylide
Ca
Y
In order to understand these observations further, we investi-
gated the stoichiometric reactions of Ph3PCH2 with [Li
{N(SiMe3)2}], [Ca{N(SiMe3)2}2] and [Y{N(SiMe3)2}3]. The results
of these experiments are presented in Scheme 2. The reaction of
[Li{N(SiMe3)2}] or [Ca{N(SiMe3)2}2] with Ph3PCH2 both gave
clean formation of a new product within the rst 15 minutes of
mixing at 25 ꢀC in C6D6 solution as evidenced by 31P NMR
spectroscopy (2a d ¼ +27.9 ppm; 2b d ¼ +30.1 ppm). Following
preparative scale reactions, it was discovered that [Li
{N(SiMe3)2}] reacts with Ph3PCH2 in a 1 : 1 stoichiometry to give
2a in 49% isolated yield, while [Ca{N(SiMe3)2}2] reacts with
Ph3PCH2 in a 1 : 2 stoichiometry to give 2b in 60% isolated
yield. Heating of C6D6 solutions of [Li{N(SiMe3)2}] or [Ca
{N(SiMe3)2}2] with 1 equiv. of Ph3PCH2 to 80 ꢀC for 27 h did not
result in any further reaction, and C–H activation of the ylide
was not observed under these conditions. Furthermore heating
isolated samples of 2a and 2b for 80 ꢀC for 18 h resulted in no
changes to the 1H or 31P NMR spectra.
3
2cc
Lu
3
a
Reactions conducted at 0.22 M concentration of starting materials,
yields recorded by 1H NMR using 1,2,4,5-tetramethylbenzene as an
internal standard. Reaction conducted in triplicate in pre-silylated
(HMDS) NMR tubes. Conversion based on Ph3PCH2.
b
c
solution by 1H NMR spectroscopy, the assignment of 1a was
conrmed by multinuclear NMR spectroscopy, single crystal
X-ray diffraction and high-resolution mass spectrometry,
following a preparative scale experiment (Fig. 1).
Although the major product from the [Y{N(SiMe3)2}3] cata-
lysed reaction of Ph3PCH2 with PhSiH3, 1a is not the sole
product and Ph2PMe (1c) along with trace amounts of
Ph2PCH2SiH2Ph (1b) are observed as minor by-products by 31P
NMR spectroscopy. A background reaction conducted without a
Fig. 1 The crystal structure of one (1a-A) of the four independent molecules
ꢀ
˚
present in the crystals of 1a. Selected bond lengths (A) and angles ( ) for the
four independent molecules (in the form 1a-A, 1a-B, 1a-C, 1a-D) are;
P(1)–C(2) 1.6771(19), 1.6790(19), 1.6757(19), 1.672(2); C(2)–Si(3) 1.797(2),
1.796(2), 1.804(2), 1.851(3); P(1)–C(2)–Si(3) 127.79(12), 126.70(12), 124.67(12),
123.84(15).
Scheme 2 Reactions of triphenylphosphonium methylide with [M{N(SiMe3)2}n]
(M ¼ Li, n ¼ 1; M ¼ Ca, n ¼ 2; M ¼ Y, n ¼ 3).
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In contrast, the reaction between [Y{N(SiMe3)2}3] and the calcium complex 2b demonstrates crystallographic C2
Ph3PCH2 in C6D6 solution at 80 ꢀC for 3 h resulted in clean symmetry. While calcium onium ylide adducts have little
conversion to the cyclometallated complex 2c along with precedent a number of alkyls have been crystallographically
HN(SiMe3)2. Although complex 2c (d ¼ +28.3 ppm, d, 2J
¼
characterised, and the Ca–C bond length of the coordinated
89Y–31
P
11.2 Hz) demonstrated 31P NMR spectroscopy data that is ylide (2.6413(14) A) is longer than that observed in the four-
˚
15
˚
consistent with 2a, 2b (see above) and the previously reported coordinate alkyl [Ca{CH(SiMe3)2}2(THF)2] (2.4930(18) A).
phosphonium methylide complex [Y(I)(CH2PPh3){N(SiMe3)2}2]
Similarly, a minor lengthening of the yttrium–carbon bond
(d ¼ +30.9 ppm), 13C NMR data were characteristic of C–H to the zwitterionic ligand in comparison to the anionic ligand is
activation of an ortho-position of a phenyl group and a diag- observed in complex 2c, the Y–Cylide and Y–Caryl bond lengths
˚
nostic resonance assigned to the carbon attached to yttrium was being 2.4835(17) and 2.4346(18) A respectively. Comparison of
observed at d ¼ 195.9 ppm. This value contrasts dramatically the C–P bond lengths and the M–C–P bond angles across the
with that assigned to the metal bound carbon in [Cp*2Lu series reveals that in each of the complexes the carbon–phos-
{CH2P(Ph)2(o-C6H4)}] (d ¼ 133.1 ppm) but is reminiscent of that phorus bond is elongated with respect to the free ylide (1a,
reported for [Cp*2Y{CH2P(Ph)2(o-C6H4)}] (d ¼ 208.00 ppm).11 1.672(2) to 1.6790(19) A; 2a, 1.698(4) to 1.707(4) A; 2b,
˚
˚
Examining 1H NMR data of the dehydrocoupling reaction to 1.7178(4) A; 2c, 1.7373(19) A). While the M–C–P bond angle is
produce 1a (Scheme 1) reveals that 2c is present in the reaction similar for complexes 2a (133.6(3) to 136.7(3)ꢀ) and 2b
˚
˚
mixture.
(134.26(7)ꢀ), constraining the ylide in a ve-membered met-
The structure of 2c was conrmed unambiguously by X-ray alocycle tightens this angle to 107.17(8)ꢀ. Complex 2c is kinet-
crystallography (Fig. 2c). While similar cyclometallated ically competent for the dehydrocoupling of Ph3PCH2 with
complexes are known, in all cases a rare-earth alkyl reagent is PhSiH3 producing 1a in 77% conversion employing 30 mol%
ꢀ
used to deprotonate the ylide. For example, [Cp*2LuMe] reacts catalyst aer 3.5 h at 25 C (Table 1).
with Ph3PCH2 to produce [Cp*2Lu{CH2P(Ph)2(o-C6H4)}].1
Possible mechanisms of C–H functionalisation, including
Comparison of solid-state data
isotope labelling and DFT studies
Examples of crystallographically characterised onium ylide At least four distinct mechanistic pathways for the C–H silyla-
adducts of group 1 and 2 are rare. In the solid-state, the lithium tion step can be envisioned, (i) direct addition of the Si–H bond
complex 2a exists with two molecules in the asymmetric unit, across the ylide in 2c (or a non-cyclometallated adduct) via a s-
these molecules differ only slightly from one another. Complex bond metathesis mechanism followed by decomposition of a
2a forms a dimer and the lithium centres are bridged by the zwitterionic intermediate with loss of H2, (ii) C–H silylation of
silazide ligands with additional coordination being provided by the aryl ring of 2c followed by 1,4-silyl migration to the meth-
the ylide The Li–N bond lengths in 2a are unremarkable while ylide position, (iii) generation of a transient yttrium alkylidene
˚
the short Li–C bond lengths (2.302(8) and 2.285(9) A) are from 2c (or a non-cyclometallated adduct) followed by 1,2-
consistent with that reported by Davidson and co-workers for addition of the Si–H bond across the M]CHPPh3 moiety, and
the only other known lithium adduct of a neutral phosphonium (iv) Si–H bond activation followed by a-elimination to form a
14
˚
ylide [Li{m-NBn2}(CH2PPh3)]2 (2.207(4) A). In the solid-state, silylene that inserts into the C–H bond of Ph3PCH2.
Fig. 2 (Left) The crystal structure of one (2a-A) of the two independent molecules present in the crystals of 2a. (Centre) The crystal structure of the C2 symmetric
complex 2b. Atoms labelled with an “A” after the number are related to their counterparts without the letter by the C2 axis that passes through the calcium centre and
ꢀ
˚
bisects the N(1)–Ca(1)–N(1A) and C(10)–Ca(1)–C(10A) angles. (Right) The crystal structure of 2c. Selected bond lengths (A) and angles ( ): For 2a (in the form 2a-A, 2a-
B), Li(1)–C(1) 2.302(8), 2.264(8); Li(2)–C(32) 2.285(9), 2.254(8); P(1)–C(1) 1.698(4), 1.701(4); P(2)–C(32) 1.705(4), 1.707(4); Li(1)–C(1)–P(1) 136.6(3), 135.0(3); Li(2)–
C(32)–P(2) 133.6(3),136.7(3). For 2b, Ca(1)–C(10) 2.6413(14), Ca(1)–N(1) 2.3596(11), Ca–C(10)–P(11) 1.7178(14). For 2c, Y–C(1) 2.4835(17), Y–C(4) 2.4346(18),
P(2)–C(1) 1.7373(19), Y–N(21) 2.2381(14), Y–N(31) 2.2454(15), Y–C(1)–P(2) 107.17(8), C(4)–Y–C(1) 78.80(6).
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Precedent exists for each of the proposed C–H silylation dependence of the reaction (Table 1), we favour a mechanism in
mechanisms. Hence, a-elimination reactions of uoroalkyl and which the Si–H bond undergoes a s-bond metathesis reaction
stannyl d0fn-complexes are known to generate carbene and with the M–C bond in 2c, or a non-cyclometallated analogue.
stannylene reactive intermediates respectively,16 while the Subsequent decomposition of the corresponding zwitterionic
insertion of a base-stabilised silyene into a C–H bond of a ylide intermediate would liberate H2, ultimately yielding 1a and
has been observed.17
allowing catalytic turnover.
The addition of chlorosilanes across N-heterocyclic carbene
adducts of the rare-earth metals to form zwitterionic organo-
metallic complexes highlights the possibility of a direct func-
Conclusions
tionalisation of the ylide.18 Furthermore, C–H bond activation In summary, we have discovered that the C–H activation and
by 1,2-addition of the C–H bond across a Y]C moiety is dehydrocoupling of triphenylphosphonium methylide with
known.19 Finally, recent studies have shown that ortho-silylated phenylsilane occurs selectively in the presence of [Y
phosphonium methylides are kinetically labile and undergo 1,4- {N(SiMe3)2}3]. Initial studies suggest that the reaction is metal
silyl migration at temperatures as low as –78 ꢀC to form the dependent and while [Li{N(SiMe3)2}] and [Ca{N(SiMe3)2}2] both
more stable silylated ylides.20 DFT studies conducted with the form coordination complexes with Ph3PCH2 these compounds
B3LYP functional and 6,31G+(d,p) basis-set implemented in do not undergo C–H functionalisation under the conditions
Gaussian09 imply a thermodynamically favourable process, with observed for [Y{N(SiMe3)2}3]. We are continuing to explore the
DrGo ¼ ꢁ12.3 kcal molꢁ1 for the isomerisation of A to 1a application of d0-organometallic complexes to C–H bond
(Scheme 3) (see ESI† for further details).
functionalization.
In an attempt to shed light on the possible mechanism of
C–Si bond formation, a series of isotopic labelling experiments
were conducted. The reaction of Ph3PCD2 with PhSiH3 catalyzed
Acknowledgements
by [Y{N(SiMe3)2}3] in benzene was monitꢀored by 1H, 2H and 31
P
We are grateful to the Royal Society for the provision of a
NMR spectroscopy. Following 1 h at 25 C Ph3PCDSiH2Ph was University Research Fellowship (MRC) and Imperial College for
present in 77% yield. Examination of the 1H and 2H NMR data the funding of a PhD studentship (AEN). Peter Haycock is
revealed no detectable D-incorporation into either the ortho acknowledged for variable temperature and multinuclear NMR
sp2C–H of the triphenylphosphonium moiety or the silyl experiments. We are grateful to the reviewers for insightful
hydrides of the product. These data argue against mechanisms comments.
(ii) and (iv).
Reaction of Ph3PCD2 with Ph3PCHSiH2Ph in the presence of
20 mol% [Y{N(SiMe3)2}3] for 1 week at 80 C did not result in
Notes and references
ꢀ
H/D exchange of the Si–H positions and also militates against a
reversible silylene insertion mechanism. The fate of the isotopic
label was determined by conducting a transfer hydrogenation
experiment. Hence, the reaction was repeated in a closed
double-chamber system with a benzene solution of Ph3PCD2
and PhSiH3 in one vial and a benzene solution of 5 mol%
[RhCl(PPh3)3] and 1 equiv. styrene in a second vial (see ESI† for
Experimental details). Consistent with the formation of H–D
from the reaction of Ph3PCD2 with PhSiH3, non-selective
D-incorporation into both the methylene and methyl positions
of, the styrene hydrogenation product, ethyl benzene was
1 (a) P. L. Watson, J. Chem. Soc., Chem. Commun., 1983, 276; (b)
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G. B. Shul’pin, Activation of Saturated Hydrocarbons by
1
observed by H and 2H NMR spectroscopy.
Transition
Metal
Complexes,
Reidel,
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While the complexity of this system does not allow unam-
biguous deconvolution of the C–Si bond forming step, based
upon the aforementioned experiments and the metal–amide
Netherlands, 1984; (b) B. L. Conely, W. J. Tenn III,
K. J. H. Young, S. Ganesh, V. Z. Meier, O. Mironov,
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Scheme 3 1,4-silyl migration of ortho-silylated triphenylphosphonium ylides.
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