PtCl2/XPhos: A highly efficient and readily available catalyst for the hydrosilylation of propargylic alcohols

Article abstract of DOI:10.1039/c1cc14433b

A highly regioselective hydrosilylation of propargylic alcohols has been developed using an in situ prepared PtCl₂/XPhos catalyst system. The reaction is tolerant of many functional groups and exhibits excellent regio and geometric selectivity.

Full text of DOI:10.1039/c1cc14433b

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COMMUNICATION
PtCl2/XPhos: A highly efficient and readily available catalyst for the
hydrosilylation of propargylic alcoholsw
Mark G. McLaughlin and Matthew J. Cook*
Received 21st July 2011, Accepted 24th August 2011
DOI: 10.1039/c1cc14433b
A highly regioselective hydrosilylation of propargylic alcohols
has been developed using an in situ prepared PtCl2/XPhos
catalyst system. The reaction is tolerant of many functional
groups and exhibits excellent regio and geometric selectivity.
We began by examining but-3-yn-2-ol 1a (Table 1), the reaction
proceeded well with 5 mol% PtCl2 and 10 mol% XPhos,
analogous to the conditions reported by Alami,8 producing a
single regio and geometric isomer in excellent yield. In the
absence of XPhos the regioselectivity was diminished with
only a 9 : 1 selectivity being observed. Further optimisation
lead to the catalyst loading being reduced with excellent yields
still being produced at just 1 mol% catalyst, however below
this the reaction becomes sluggish and lower yields are observed.
The amount of silane used could also be lowered but again, a
pronounced slowing of the reaction is observed.
The hydrosilylation of alkynes is a powerful reaction resulting in a
stereodefined alkene with a useful functional handle for further
synthetic elaboration. There are many catalysts that promote this
reaction and late transition metal catalysts can be especially
efficient.1,2 In particular, platinum complexes such as Speier’s
catalyst [H2PtCl6] and Karstedt’s catalyst [Pt2(dvs)3]3 can produce
high catalyst turnovers, however the selectivity can be poor.4,5
A solution to this problem was found through the use of bulky
trialkylphosphine ligands6 and more recently the use of platinum
N-heterocyclic carbene complexes.7 These sterically encumbered
catalysts can impart high levels of selectivity on the reactions. Of
note, was the development of the (dvs)Pt–Pt–Bu3 catalyst which
exhibits high activity with a high level of selectivity.6a The
drawbacks to these methods are the cost of the Pt(0) precatalysts
and the pyrophoric nature of the phosphines required.
The substrate scope for secondary propargylic alcohols is
examined in Table 2. The reaction is tolerant of alkyl, branched
alkyl and aryl groups 2a–h with typical yields 80–91% using just
1 mol% PtCl2 and 2 mol% XPhos.
More functionalised substrates could also be incorporated
with heteroaromatics and alkenes being well tolerated with
good yields and selectivities shown for the alkyne.10 Propargyl
alcohol itself 1b is a poor substrate for this reaction with low
yields being obtained.
Alami reported that a combination of Pt(II) salts and bulky
dialkylarylphosphines, such as XPhos, catalysed the hydro-
silylation of alkynes.8 The reaction proceeded in good yields
and excellent selectivities, however a large catalyst loading is
required, typically 5 mol% Pt and 10 mol% phosphine.
We required a rapid and reliable method for the synthesis of
b-silyl alyllic alcohols 2 that was tolerant of significant steric
encumbrance. Panek previously reported the use of the highly
reactive (dvs)Pt–Pt–Bu3 catalyst for the regioselective hydro-
silylation of propargylic alcohols,9 however no examples of
sterically congested propargylic alcohols were reported. We
speculated that we could modify the reaction reported by
Alami to combine the catalytic activity of the bulky Pt(0)
catalyst with the selectivity and operational ease of the air
stable Pt(II) system. Herein we report the use of air and
moisture stable Pt(II) salts and Buchwald phosphines to yield
the requisite vinyl silanes using a wide range of propargyl
alcohols exclusively as the b-isomer with the E-geometry.
The reaction is even more efficient when tertiary propargylic
alcohols are used (Table 3). The catalyst loading may be reduced
further to just 0.5 mol% and excellent yields of the corresponding
vinyl silanes produced. Dialkyl 4a–c and diaryl groups 4d are
tolerated well as are a combination of the two 4e. Cyclic
propargylic alcohols 4f–h are also excellent substrates and
even those bearing basic heteroatoms 4g–h are well tolerated.
We also examined the use of different silanes in the reaction,
as phenyldimethylsilane was used throughout (Table 4). Other
Table 1 Optimisation Studies
Entry mol% PtCl2/Xphos Eq. R3SiH b : a Ratioa Yield (%)b
1
2
3
4
5
6
5/10
5/0
2/4
1/2
0.5/1
1/2
1.6
1.6
1.5
1.5
1.5
1.1
>99 : 1
>91 : 9
>99 : 1
>99 : 1
>99 : 1
>99 : 1
93
72
93
91
74
83
School of Chemistry and Chemical Engineering,
Queen’s University Belfast, Belfast, BT9 5AG, Northern Ireland.
E-mail: m.cook@qub.ac.uk; Fax: +44 (0)28 90976524;
Tel: +44 (0)28 90974682
a
1
b
w Electronic supplementary information (ESI) available: Full experimental
procedures and spectra are provided. See DOI: 10.1039/c1cc14433b
Determined by H NMR. Isolated yield of pure b-isomer.
c
11104 Chem. Commun., 2011, 47, 11104–11106
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Table 2 Secondary Propargylic Alcohol Scope
Table 4 Alternative Silane Scope
a0.5 Equiv. Ph2SiH2 used; b0.3 Equiv. PhSiH3.
The hydrosilylation of 3a with 1,1,3,3-tetramethylsiloxane
proceeded smoothly to form the intermediate bis-hydrosilylated
product,14 which was treated with TBAF and iodobenzene in
the presence of Pd2dba3 as a catalyst to afford the cross-coupled
product 6 in 71% yield (eqn (1)).
Table 3 Tertiary Propargylic Alcohol Scope
ð1Þ
Aside from hydrosilylation, other hydrometallations can be
effieicently catalyzed by this system. Pinacol borane can perform
a hydroboronation on TMS protected propargylic alcohol in
good yields (eqn (2)).15 Although the reaction to form 8 does
proceeds uncatalysed, much higher yields are obtained, with
faster reaction times than previously reported using less pinacol
borane.16
ð2Þ
trialkyl silanes were attempted and triethylsilane was a very
efficient coupling partner. Triiso-propylsilane was presumably
too hindered as no reaction was observed and starting material
was recovered. Triethoxysilane coupled efficiently providing
access to vinyl trialcoxysilanes 8c which can be used in Hiyama
cross-coupling chemistry.11 Aryl silanes undergo the reaction
to form triaryl vinyl silanes. Interestingly silanes containing
multiple hydrides can undergo multiple couplings. Diphenyl-
silane can couple with two equivalents of alkyne to afford
divinyl silanes 8e and phenylsilane can couple with three
equivalents to provides trivinyl silanes 8f.12
We also examined internal alkynes to investigate whether
the Lewis basic oxygen atom was co-ordinating to the metal
and directing the addition17 or whether the regioselectivity was
based purely on steric factors.18 The first substrate attempted
was but-2-yn-1-ol 9, which gave a 1 : 1 mixture of regioisomers.
This clearly disproves that the oxygen is directing the addition
and that pure sterics govern the regioselectivity. When a
cyclohexyl group is introduced at the propargylic position 11
an increase in b-selectivity with a 3.2 : 1 ratio of 12a : 12b being
observed (eqn (4)), which is in line with what one would expect
based on the steric environment of the alkyne.
We also examined whether we could perform a one-pot
hydrosilylation/Hiyama-Denmark cross-coupling reaction.13
When the terminal alkyne position of propargyl alcohol
was substituted with a phenyl group 13, modest selectivity was
c
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Chem. Commun., 2011, 47, 11104–11106 11105

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always obtained. The regioselectivity of internal alkynes was
also probed and we discovered that a combination of steric
and electronic factors governed the regioselectivity.
ð3Þ
ð4Þ
ð5Þ
We thank QUB, DEL and the EPSRC for finacial support.
Notes and references
1 For reviews on hydrosilylation see: (a) B. M. Trost and Z. T. Ball,
Synthesis, 2005, 853; (b) in Hydrosilylation, ed. B. Marciniec,
Advances in Silicon Science, Springer Science, 2009.
2 For mechanism of metal catalysed hydrosilylation see: (a) A. J. Chalk
and J. F. Harrod, J. Am. Chem. Soc., 1965, 87, 16; (b) S. B. Duckett
and R. H. Perutz, Organometallics, 1992, 11, 90.
3 dvs = 1,3-divinyl-1,1,3,3-tetramethyldisiloxane.
4 (a) L. N. Lewis, K. G. Sy, G. L. Bryant and P. E. Donahue,
Organometallics, 1991, 10, 3750; (b) M. G. Voronkov,
V. B. Pukhnarevich, I. I. Tsykhanskaya, N. I. Ushakova,
Y. L. Gaft and I. A. Zakharova, Inorg. Chim. Acta, 1983, 68, 103.
5 (a) B. D. Karstedt, U.S. Patent, 3,775,452, 1973; (b) J. Stein,
L. N. Lewis, Y. Gao and R. A. Scott, J. Am. Chem. Soc., 1999,
121, 3693.
6 (a) G. Chandra, P. Y. Lo, P. B. Hitchcock and M. F. Lappert,
Organometallics, 1987, 6, 191; (b) P. J. Murphy, J. L. Spencer and
G. Procter, Tetrahedron Lett., 1990, 31, 1051.
ð6Þ
ð7Þ
7 Reviews of NHC-Pt catalyzed hydrosilylations see: (a) G. Berthon-
Gelloz and I. E. Mark6o
ed. S. P. Nolan, Wiley-VCH Verlag GmbH & Co. KgaA, 2006;
(b) S. Dıez-Gonzalez, N. Marion and S. P. Nolan, Chem. Rev.,
´
, in N-Heterocyclic Carbenes in Synthesis,
´
´
2009, 109, 3612; For a simplyfied access to these complexes,
including electronically switchable NHCs, see: (c) A. S.
K. Hashmi, C. Lothschutz, C. Bohling, T. Hengst, C. Hubbert
¨
¨
and F. Rominger, Adv. Synth. Catal., 2010, 352, 3001; (d) A. S.
K. Hashmi, C. Lothschutz, K. Graf, T. Haffner, A. Schuster and
¨
¨
F. Rominger, Adv. Synth. Catal., 2011, 354, 1407.
8 (a) A. Hamze, O. Provot, J.-D. Brion and M. Alami, Tetrahedron
Lett., 2008, 49, 2429–2431; (b) A. Hamze, O. Provot, J.-D. Brion
and M. Alami, J. Organomet. Chem., 2008, 693, 2789.
9 (a) C. E. Masse, L. A. Dakin, B. S. Knight and J. S. Panek, J. Org.
Chem., 1997, 62, 9335; (b) R. T. Beresis, J. S. Solomon,
M. G. Yang, N. F. Jain and J. S. Panek, Org. Synth., 1998,
75, 78, Coll. Vol. 2004, 10, 531.
10 An isomerisation of the pure b-isomer can occur to afford a mixture
of a and b isomers if the reaction is left to stand at room temp.
11 Review of silicon based cross-couplings in total synthesis see:
S. E. Denmark and J. H.-C. Liu, Angew. Chem., Int. Ed., 2010,
49, 2978.
12 For other report of multiple hydrosilylations of Ph2SiH2 and
PhSiH3 see: H. Sasabe, N. Kihara, K. Mizuno, A. Ogawa and
T. Takata, Tetrahedron Lett., 2005, 46, 3851.
13 (a) S. E. Denmark and Z. Wang, Org. Lett., 2001, 3, 1073;
(b) S. E. Denmark and Z. Wang, Org. Synth., 2005, 81, 54–62,
Coll. Vol. 2009, 11, 367.
observed with the b-isomer 14a prevailing in a 4.1 : 1 ratio
(eqn (5)). This is contrary to the steric environment around
the alkyne and is clearly showing preference for hydride attack
at the more electrophilic terminus of the alkyne.19 Indeed,
Alami observed this effect when applied to the palladium
catalyzed hydrostannylation of 13, with 4 : 1 regioselectivity
for b-substitution being observed.20 They also showed that
the regioselectivity in these systems directly correlated to
the Hammett s-value for substituted benzene rings with the
b-isomer favoured in all cases. If the steric environment at the
b-position is increased greatly by installing a tert-butyl group
15, the regioselectivity of the reaction can be switched to give
solely the a-isomer. No traces of the b-isomer 16a was
observed with only the a-isomer 16b being formed in good
yield. A single regioisomer could again be obtained when the
propargylic position is was highly substituted compared to the
b-position 17. In this instance only the b-isomer 18a was
observed again in good yield (eqn (7)). The results in eqn (6)
and (7) demonstrate that the steric environment around the
alkyne plays a pivotal role in determining the selectivity of the
products. However, when the alkyne is polarised an electronic
bias is present, such as eqn (5), the regiochemistry will be
dictated by the attack of the hydridic hydrogen at the more
electron deficient terminus.
14 We isolated the intermediate bis(vinylsiloxane), however this was
unstable to silica gel with a 32% yield being obtained.
15 When the unprotected alcohol was used only trace amounts of
product 8 were detected.
´
16 F. Berree, N. Gernigon, A. Hercouet, C. H. Line and B. Carboni,
Eur. J. Org. Chem., 2009, 329.
17 For discussion on the co-ordination of the oxygen to the Pt see:
G. Berthon-Gelloz, J.-M. Schumers, G. De Bo and I. E. Marko
´
,
J. Org. Chem., 2008, 73, 4190.
18 The internal alkynes were either purchased from Sigma-Aldrich or
were synthesised by the following procedures: (a) W. C. Agosta,
R. A. Caldwell, J. Jay, L. J. Johnston and B. R. Venepalli, J. Am.
Chem. Soc., 1987, 109, 3050; (b) J. Suffert and D. Toussaint,
J. Org. Chem., 1995, 60, 3550.
19 For hydrosilylations governed by electronic see: (a) A. Hamze,
O. Provot, J.-D. Brion and M. Alami, Org. Lett., 2005, 7, 5625;
(b) B. M. Trost and Z. T. Ball, J. Am. Chem. Soc., 2005, 127, 17644;
(c) T. Konno, K.-i. Taku, S. Yamada, K. Moriyasu and T. Ishihara,
Org. Biomol. Chem., 2009, 7, 1167; (d) D. A. Rooke and
E. M. Ferriera, J. Am. Chem. Soc., 2010, 132, 11926.
In conclusion we have demonstrated the power of the PtCl2/
XPhos catalyst system to perform hydrometallations across
alkynes. The reaction works particularly well for sterically
encumbed tertiary propargylic alcohol and a single regioisomer is
20 F. Liron, P. La Garrec and M. Alami, Synlett, 1999, 246.
c
11106 Chem. Commun., 2011, 47, 11104–11106
This journal is The Royal Society of Chemistry 2011