Direct formation of propargyltin compounds via C-H activation

Article abstract of DOI:10.1021/om800132h

The mixed reagent SnC(SiMe₃)₂CH₂CH ₂C(siMe)₂]/ ArI reacts with alkynes to give primary and secondary propargylic C-H activation. Alkynes tested include 1-phenylpropyne, 1-phenylbutyne, 1-trimethylsily

Full text of DOI:10.1021/om800132h

2896
Organometallics 2008, 27, 2896–2897
Direct Formation of Propargyltin Compounds via C-H Activation
Ajdin Kavara, Jeff W. Kampf, and Mark M. Banaszak Holl*
Chemistry Department, UniVersity of Michigan, 930 N. UniVersity AVenue, Ann Arbor, Michigan
48109-1055
ReceiVed February 13, 2008
compounds that do not involve such starting materials was of
Summary: The mixed reagent SnC(SiMe3)2CH2CH2C(SiMe3)2]/
ArI reacts with alkynes to giVe primary and secondary prop-
argylic C-H actiVation. Alkynes tested include 1-phenylpro-
pyne, 1-phenylbutyne, 1-trimethylsily1hexyne, and 2-hexyne. Aryl
halides tested include iodobenzene and 2,4,6-triisopropyliodo-
benzene. An X-ray crystal structure is reported for the product
of the secondary propargylic actiVation of 2-hexyne.
interest. Direct C-H activation at the carbon atom alpha to the
triple bond, concomitant with tin-carbon bond formation, could,
in principle, provide a direct route to propargyltin compounds.
However, C-H activations involving alkynes are rare and have
not been used to yield a tin-carbon bond. Previously, we have
demonstrated that the mixed reagent Sn[N(SiMe3)3]2/PhI can
directly activate C-H bonds and form a tin-carbon bond in
substrates such as THF, pentane, cyclohexane, cyclopentane,
and 2-methoxy-2-methylpropane.10 Recently, we have also
shown that we can perform allylic C-H activation using the
SnC(SiMe3)2CH2CH2C(SiMe3)2 (1)/PhI combination.11 These
reactions were carried out at ambient temperature. On the basis
ofthesepreviousresults,weexploredthereactivityofSn[N(SiMe3)2]2/
PhI with alkynes, but no propargylic C-H activation was
observed. However, we discovered that 1 did not react with
any of the alkynes used in this study and that the 1/ArI
combination was effective in activating both primary and
secondary propargyl C-H bonds. These reactions could be
carried out at ambient temperature and did not require electron
donor solvents or Lewis acid additives. Thus, no problems with
isomerization to allenyl isomers were encountered.
A variety of synthetic methods have been developed for the
synthesis of propargylmetal compounds.1 Such compounds are
useful as coupling reagents for the formation of C-C bonds.2–4
Propargyltin compounds are especially useful due to their
functional group tolerance and air stability.5,6 One of the main
challenges in their synthesis is avoiding the formation of allenyl
isomers. This difficulty arises because both electron donor
solvents and Lewis acids catalyze the interconversion of
propargyl- and allenyltin isomers (eq 1).7
Propargyl chlorides react with hexamethylditin in the presence of
a Pd(II) catalyst that contains a pincer ligand. In optimal cases, 7:1 to
10:1 ratios of propargyl-/allenyltin isomers are obtained. However,
many propargylic chlorides react to give less than 50% of the propargyl
isomer, and others give only the allenyl isomer.8,9 Propargyl acetates,
carbonates, and phosphates can be stoichiometrically converted to
propargyltin complexes upon reaction with tributyltin chloride after
activation with a Ti(OiPr)4/2 iPrMgCl reagent. This method frequently
gives excellent propargyl/allenyl ratios of >40:1 with both primary
and secondary propargyl carbonates.6 In addition, this method has been
extended to the formation of chiral propargyltin compounds by starting
with chiral, secondary propargyl phosphates.5
Activation of the propargylic C-H bond was achieved when 1
equiv of 1 dissolved in 10 mL of benzene and 15 mL of alkyne was
added to 1.1 equiv of aryl iodide dissolved in 5 mL of 1-phenylpro-
pyne, 1-phenylbutyne, or 2-hexyne.12 When employing phenyl iodide,
simple mixing of the reagents also gave oxidative-addition product
PhISnC(SiMe3)2CH2CH2C(SiMe3)2 (2).11 The amount of 2 formed
could be minimized and/or eliminated by employing syringe pump
addition of PhI and/or the use of bulkier aryl iodides such as 2,4,6-
trimethyliodobenzene or 2,4,6-triisopropyliodobenzene. Addition rates
were chosen such that a substantial degree of red color did not build
up during the addition of the intensely red stannylene solution to the
aryl iodide solution. For the cases of 1-phenylpropyne, 1-phenylbutyne,
and 2-hexyne, employing the syringe pump techniques and using 2,4,6-
triisopropyliodobenzene resulted in quantitative formation of propar-
gylic C-H activation products 3, 4, and 6, respectively. For the case
All of these methods for the synthesis of propargyl tin
compounds require the propargyl acetate, carbonate, halide, or
phosphate species. Many of these species are not commercially
available, and examination of alternative routes to propargyl
* Corresponding author. E-mail: mbanasza@umich.edu.
(1) Ma, S.; Wang, L. J. Org. Chem. 1998, 63, 3497–3498.
(2) Ma, S.; Zhang, A.; Yu, Y.; Xia, W. J. Org. Chem. 2000, 65, 2287–
2291.
(10) Bartolin, J. M.; Kavara, A.; Kampf, J.; Banaszak Holl, M. M.
Organometallics 2006, 25, 4738–4740.
(11) Kavara, A.; Cousineau, K. D.; Rohr, A. D.; Kampf, J. W.; Banaszak
Holl, M. M. Organometallics 2008, 27, 1041–1043.
(3) Acharya, H. P.; Miyoshi, K.; Kobayashi, Y. Org. Lett. 2007, 9, 3535–
3538.
(12) A one-necked 100 mL flask fitted with a rubber septum was charged
with 315 mg of 2,4,6-triisopropyliodobenzene (0.95 mmol, 1 equiv) and 5
mL of 1-phenylpropyne. A solution containing 400 mg of 1 (0.86 mmol,
0.9 equiv), 15 mL of 1-phenylpropyne, and 10 mL of benzene was placed
in a gastight syringe equipped with a 20-gauge needle. Then 25 mL of the
red stannylene solution was added to the 2,4,6-triisopropyliodbenzene/alkyne
solution at a rate of 8 mL/h using a syringe pump. The last 5 mL was
added at 2 mL/h. The solution was stirred for 8 h yielding a cloudy, white
solution containing only the C-H activation product as indicated by 1H
NMR spectroscopy. The volatiles were removed in vacuo, and the resulting
solid was recrystallized from pentane at-78 °C to give a white powder
(402 mg, 67.4% yield). Full spectroscopic details are provided in the
Supporting Information.
(4) Corey, E. J.; Rucker, C. Tetrahedron Lett. 1982, 23, 719–722.
(5) Okamoto, S.; Matsuda, S.; An, D. K.; Sato, F. Tetrahedron Lett.
2001, 42, 6323–6326.
(6) An, D. K.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1998, 39, 4861–
4864.
(7) Guillerm, G.; Meganem, F.; Lequan, M.; Brower, K. R. J. Orga-
nomet. Chem. 1974, 67, 43–52.
(8) Kjellgren, J.; Sunden, H.; Szabo´, K. J. J. Am. Chem. Soc. 2004, 126,
474–475.
(9) Kjellgren, J.; Sunden, H.; Szabo´, K. J. J. Am. Chem. Soc. 2005, 127,
1787–1796.
10.1021/om800132h CCC: $40.75
2008 American Chemical Society
Publication on Web 06/05/2008

Communications
Organometallics, Vol. 27, No. 13, 2008 2897
Scheme 1. Reactions of SnC(SiMe3)2CH2CH2C(SiMe3)2/ArI
with Alkynes
Figure 1. ORTEP diagram of (C6H9)ISnC(SiMe3)2CH2CH2C(SiMe3)2
(6) (50% thermal ellipsoids). Selected bond lengths (Å) and angles (deg):
Sn-C1, 2.1841(14); Sn-I, 2.73712(19); C1-C2, 1.455(2); C2-C3,
1.193(2); C3-C4, 1.460(2); C1-C5, 1.533(2); C5-C6, 1.525(2); Sn-C10,
2.1841(14); Sn-C7, 2.1832(14); Sn-C1-C2, 107.86(10); Sn-C1-C5,
111.73(9); C2-C1-C5, 112.97(12);C1-C2-C3, 178.89(17); C2-C3-C4,
178.18(18); C7-Sn-C1, 121.32(5); C10-Sn-C1,122.18(5); C7-Sn-I,
113.24(4); C10-Sn-I, 111.63(4); I-Sn-C1, 96.61(4).
(67% isolated yield) with no formation of the allenyl isomers.
Attempts to use hexamethylditin and the Pd(II) pincer catalyst
to give secondary propargylic products resulted in purely allenyl
products. No propargylic isomer could be detected by 1H NMR
spectroscopy for other substrates such as (1-chloroprop-2-
ynyl)benzene, (2-chlorobut-3-ynyl)benzene, and (3-chloroprop-
1-yne-1,3-diyl)dibenzene. In contrast, secondary C-H act-
ivations proceeded quantitatively employing the 1/2,4,6-
triisopropyliodobenzene combination to give compounds 4 and
6. Using the 1/PhI reagent, 5 was obtained with 95% conversion,
of 1-trimethylsily1hexyne, 1 equiv of 1 dissolved in 15 mL of alkyne
was added to 1.1 equiv of iodobenzene dissolved in 5 mL of alkyne.
1H NMR spectroscopy of the reaction mixture indicated a 95%
conversion to propargylic C-H activation product 5 along with 5%
conversion to oxidative-addition product 2. The use of alkyne as
solvent dramatically reduces the amount of 2 formed. When stoichio-
metric amounts of alkyne are employed, 2 is the predominant product.
Even when employing alkyne as solvent, it is necessary to employ
high-dilution techniques to minimize formation of 2 and give high
yields of the desired C-H activation product.
1
as indicated by H NMR spectroscopy (45% isolated yield).
Sato et al. previously have reported effective methods for the
synthesis of secondary propargylic tin compounds. For example,
ethyl non-2-yn-4-ylcarbonate was converted to tributyl(non-2-
yn-4-yl)tin in 72% yield with a 31:1 propargylic/allenyl product
ratio using the mixed Ti(OiPr)4/2iPrMgCl reagent.6
All three of the synthetic methods published to date have
their advantages and drawbacks. Our approach requires the
synthesis and use of air-sensitive stannylene 1, the use of a
syringe pump and/or a bulky aryl iodide, and the use of alkyne
as solvent. The primary advantages are that it does not require
the preparation of alkynes with leaving groups and leads to no
detectable amount of isomerization to allenyl products. The
method of Szabo´ et al. requires the synthesis of the palladium
pincer catalysts, the use of hexamethylditin, and the use of
alkynes with a leaving group (typically Cl) at the appropriate
position but can be employed with functional groups (COOEt,
OH, NAc) not tolerated by the stannylene. The method of Sato
et al. requires preparation of carbonate-functionalized alkynes
but can be employed to make stereospecific propargyltin
compounds.
All compounds were characterized by a combination of 1H, 13C,
and 119Sn NMR spectroscopy, infrared spectroscopy, mass spectrom-
etry, and elemental analysis. Compound 3, the result of the C-H
activation of a primary C-H bond, exhibits two -SiMe3 resonances
in the 1H NMR spectrum. Compounds 4, 5, and 6, the result of the
C-H activation of a secondary C-H bond, all exhibit four -SiMe3
resonances. In the case of compound 5, an additional -SiMe3
resonance is present from the 1-trimethylsily1hexyne substrate. The
119Sn NMR spectrum also provided a means of differentiating the
primary versus secondary C-H activation products, as the resonance
of 3 occurred at 96.59 ppm, whereas the resonances of 4, 5, and 6
were observed at 142.76, 130.55, and 132.72 ppm, respectively. The
assignment of a 5JSn-H coupling constant of 36.49 Hz for compound
6 was surprising. To confirm this spectroscopic assignment, an X-ray
crystallographic study of 6 was undertaken using a colorless crystal
grown from pentane at 25 °C (Figure 1). The bond connectivity
assigned spectroscopically was confirmed including the propargylic
C-H activation and the presence of the triple bond.
Acknowledgment. This work was supported by National
Science Foundation grant CHE-0453849.
Supporting Information Available: Text giving complete
experimental preparation details is available for compounds 3, 4,
5, and 6. A CIF file giving crystallographic data for compound 6
is available. This material is available free of charge via the Internet
at http://pubs.acs.org.
Comparing this approach to the previously published methods
of Szabo´ et al.8,9 the use of hexamethylditin/Pd(II) pincer
catalyst/(3-chloroprop-1-ynl)benzene resulted in an 87% isolated
yield of trimethyl(3-phenylprop-2-ynyl)tin with an 8:1 propar-
gylic/allenyl product ratio. The reaction of 1/2,4,6-triisopropy-
liodobenzene with 1-phenylpropyne proceeded quantitatively
OM800132H