C-H activation of alkanes, alkenes, alkynes, arenes, and ethers using a stannylene/aryl halide mixture

Article abstract of DOI:10.1021/om100267a

The reaction of SnC(SiMe₃)₂CH₂CH ₂C(SiMe₃)₂/ArI (Ar = Ph, 2,4,6- triisopropylphenyl) with cyclohexane, diethyl ether, tetrahydrofuran (THF), toluene, and mesitylene yields C-H activation products in which a new Sn-C bond is formed at the location of the weakest C-H bond. The regioselectivity of this reaction with trans-4-methyl-2-pentene and 4-methyl-2-pentyne was explored as a function of aryl halide for PhI, 2,4,6-trimethyliodobenzene, 2,4,6-triisopropyliodobenzene, 4-iodoanisole, 4-iodobenzonitrile, and 2,6-mesityliodobenzene. A degree of regiochemical control could be obtained as highlighted by increased amounts of primary activation. The use of 2,4,6-tert-butyliodobenzene resulted in C-H activation of the ortho tert-butyl groups in all solvents tried. The primary kinetic isotope effect for the reaction of SnC(SiMe₃)₂CH₂CH ₂C(SiMe₃)₂/2,4,6-triisopropyliodobenzene with toluene/toluene-d₇ was found to be 4.9 ± 0.5.

Full text of DOI:10.1021/om100267a

Organometallics 2010, 29, 5033–5039 5033
DOI: 10.1021/om100267a
C-H Activation of Alkanes, Alkenes, Alkynes, Arenes, and Ethers
Using a Stannylene/Aryl Halide Mixture†
Ajdin Kavara, Thaddeus T. Boron III, Zubair S. Ahsan, and Mark M. Banaszak Holl*
Chemistry Department, University of Michigan, 930 N. University, Ann Arbor,
Michigan 48109-1055, United States
Received April 5, 2010
The reaction of SnC(SiMe3)2CH2CH2C(SiMe3)2/ArI (Ar = Ph, 2,4,6-triisopropylphenyl) with
cyclohexane, diethyl ether, tetrahydrofuran (THF), toluene, and mesitylene yields C-H activation
products in which a new Sn-C bond is formed at the location of the weakest C-H bond. The
regioselectivity of this reaction with trans-4-methyl-2-pentene and 4-methyl-2-pentyne was explored
as a function of aryl halide for PhI, 2,4,6-trimethyliodobenzene, 2,4,6-triisopropyliodobenzene,
4-iodoanisole, 4-iodobenzonitrile, and 2,6-mesityliodobenzene. A degree of regiochemical control
could be obtained as highlighted by increased amounts of primary activation. The use of 2,4,6-tert-
butyliodobenzene resulted in C-H activation of the ortho tert-butyl groups in all solvents tried. The
primary kinetic isotope effect for the reaction of SnC(SiMe3)2CH2CH2C(SiMe3)2/2,4,6-triisopropy-
liodobenzene with toluene/toluene-d7 was found to be 4.9 ( 0.5.
Introduction
treatment of radical methods is available elsewhere,10 notable
recent advances include an environmentally friendly variant of
the traditional bromination reaction and an enantioselective
variant of the Kharasch-Sosnovsky reaction.11,12 Generation
of methyl or ethyl radicals from Me2Zn/air or Et3B/air has been
used for hydrogen abstraction of C-H bonds R to the ethereal
oxygen for hydroxyalkylation of ethers and acetals.13
Our lab has published a series of papers on the C-H
activation of alkanes, ethers, alkenes, alkynes, and amines
utilizing stable, heavy, divalent group 14 EL2 species (E = Si,
Ge, Sn) in combination with an aryl halide.14-18 Involve-
ment of an EL2/PhX radical intermediate that abstracts the
homolytically weakest hydrogen from the substrate has been
implicated in the reaction manifold (Scheme 1).18 Oxidative
addition products, which can be minimized by carrying out
the reaction under high dilution conditions, are formed in
competition with C-H activation products. The effect of
C-H activation is one of the important goals of synthetic
and mechanistic chemists. These reactions promise increased
synthetic efficiency, waste reduction, and utilization of in-
expensive source materials such as petroleum or methane
gas.1,2 C-H activation reactions are performed for the
purposes of direct functionalization or carbon-carbon bond
formation.3 Designing reagents and conditions that exhibit
selective reactivity with only one type of C-H bond in an
organic compound is essential.4 Successful approaches in-
clude the use of directing groups, chelation assistance, and
exploitation of differential homolytic strengths of C-H
bonds.5,6 In the literature, a general distinction is made
between radical, σ bond metathesis, and electrophilic
pathways.7,8 Competition between radical and electrophilic
pathways has been observed.9 Although a comprehensive
€
(10) Gansauer, A. Radicals in Synthesis I; Springer-Verlag: Berlin,
2006.
(11) Podgorsek, A.; Stavber, S.; Zupan, M.; Iskra, J. Tetrahedron
† Part of the Dietmar Seyferth Festschrift. Dedicated to Dietmar Seyferth
with thanks for his many contributions to main group and organometallic
chemistry.
Lett. 2006, 47, 7245–7247.
*To whom correspondence should be addressed. E-mail: mbanasza@
(12) Eames, J.; Watkinson, M. Angew. Chem., Int. Ed. 2001, 40, 3567–
3571.
(13) Yoshimitsu, T.; Arano, Y.; Nagaoka, H. J. Org. Chem. 2005, 70,
2342–2345.
umich.edu.
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1998, 37, 2180–2192.
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174–238.
(4) Kakiuchi, F.; Kochi, T. Synthesis 2008, 2008, 3013–3039.
(5) Desai, L. V.; Stowers, K. J.; Sanford, M. S. J. Am. Chem. Soc.
2008, 130, 13285–13293.
(6) Davies, H. M. L.; Beckwith, R. E. J. Chem. Rev. 2003, 103, 2861–
2904.
(7) Bergman, R. G. Nature 2007, 446, 391–393.
(8) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879–2932.
(9) Lin, Y.-S.; Misawa, H.; Yamada, J.; Matsumoto, K. J. Am. Chem.
Soc. 2001, 123, 569–575.
(14) Miller, K. A.; Bartolin, J. M.; O’Neill, R. M.; Sweeder, R. D.;
Owens, T. M.; Kampf, J. W.; Banaszak Holl, M. M.; Wells, N. J. J. Am.
Chem. Soc. 2003, 125, 8986–8987.
(15) Bartolin, J. M.; Kavara, A.; Kampf, J.; Banaszak Holl, M. M.
Organometallics 2006, 25, 4738–4740.
(16) Kavara, A.; Cousineau, K. D.; Rohr, A. D.; Kampf, J. W.;
Banaszak Holl, M. M. Organometallics 2008, 27, 1041–1043.
(17) Kavara, A.; Kampf, J. W.; Banaszak Holl, M. M. Organome-
tallics 2008, 27, 2896–2897.
(18) Walker, R. H.; Miller, K. A.; Scott, S. L.; Cygan, Z. T.; Bartolin,
J. M.; Kampf, J. W.; Banaszak Holl, M. M. Organometallics 2009, 28,
2744–2755.
r
2010 American Chemical Society
Published on Web 10/05/2010
pubs.acs.org/Organometallics

5034 Organometallics, Vol. 29, No. 21, 2010
Kavara et al.
Scheme 1. C-H Activation Reaction Is Believed to Occur via the Formation of an Active Intermediate Involving Two Equivalents of EL2
and One Equivalent of Aryl Halidea
a Electron transfer from EL2 to the aryl ring initiates the breaking of the Ar-I bond. Abstraction of the hydrogen atom and recombination of the
R0CH2• and L2IE• radicals occurs within the solvent cage.
Scheme 2. Steric Bulk at the ortho Position of the Aryl Halide Changes the Distribution of C-H Activation, Double-Bond Addition, and
Oxidative Addition Productsa
a The selectivities indicated were determined by 1H NMR spectroscopy.
steric bulk in the ortho positions of the aryl halide has not
been systematically explored to date. An interesting example
Experimental Section
Manipulations involving SnC(SiMe3)2CH2CH2C(SiMe3)2
(1) were performed using inert-atmosphere techniques. Solvents
were dried over sodium benzophenone ketyl and degassed. PhI
was purchased from Aldrich Chemicals and degassed. 2,4,6-Trii-
sopropyliodobenzene was synthesized according to literature
procedures.21 2,4,6-Tri-tert-butylbromobenzene was borrowed
from Arthur Ashe III lab, who prepared it by bromination of
2,4,6-tri-tert-butylbenzene.22 Compound 1 and Sn[N(SiMe3)2]2
(11) were synthesized according to literature procedures.23,24 1H,
13C, and 119Sn NMR spectra were acquired on a Varian 500 MHz
instrument (499.904, 125.714, and 186.417 MHz, respectively). 1H
and 13C were referenced according to residual proton (δ 7.15) and
solvent carbons (δ 128.0), respectively. 119Sn spectra were refer-
of such an effect is the reaction of SnC(SiMe3)2CH2CH2C-
(SiMe3)2 (1) with cyclopentene, which resulted in a mixture
of products including the C-H activation product (2), the
product derived from the addition of stannylene and aryl
halide across the double bond (3), and a trace amount of
oxidative addition product (4) (Scheme 2).16,19 When 2,4,6-
trimethyliodobenzene was used, pure C-H activation pro-
duct was obtained. We note that the use of high dilution to
obtain greater yields of the C-H activation products is
counterintuitive given a proposed intermediate containing
two molecules of EL2 and one molecule of aryl halide as
illustrated in Scheme 1.
Regioselective C-H activation of alkanes has been a long-
standing goal.20 In terms of regiochemical preference, transi-
tion metal mediated pathways generally give products of less
hindered functionalization, i.e., primary > secondary >
tertiary.1 To date, our chemistry has consistently yielded
products derived from C-H activation of the thermodyna-
mically weaker bonds, i.e., tertiary>secondary>primary or
activation next to an activating group such as an ether
oxygen.14,16,18 We now report how the product distribution
in C-H activation reactions with stannylene/aryl halide is
dependent on the electronic and steric substitution of the aryl
halide. The stereoelectronic properties of the aryl halide have
been used to control the amount of oxidativeadditionproduct
and to effect a degree of regioselectivityfor substrates contain-
ing different C-H bonds. For the first time, we have demon-
strated a degree of kinetic control of the C-H activation
reaction by manipulating the steric bulk of the aryl halide.
1
enced to the H signal of internal tetramethylsilane using a Ξ of
37.290632 for Me4Sn.25 All coupling constants listed are for 119Sn
satellites. Mass spectra were acquired on a VG (Micromass)
70-250-S magnetic sector mass spectrometer. IR spectra were
acquired on a Perkin-Elmer Spectrum BX. Reactions typically
were performed using a syringe pump (Razel R-99) inside an inert-
atmosphere box.
(C4H7O)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (5). A one-necked
100 mL flask fitted with a rubber septum was charged with 165
mg of 2,4,6-trimethyliodobenzene (0.67 mmol, 1.1 equiv) and
5 mL of THF. A solution containing 300 mg of 1 (0.65 mmol,
1 equiv) and 10 mL of THF was placed in a gastight syringe
equipped with a 20-gauge needle. A 5 mL amount of the red
stannylene solution was added to the iodomesitylene/THF
(21) Togo, H.; Nabana, T.; Yamaguchi, K. J. Org. Chem. 2000, 65,
8391–8394.
(22) Knorr, R.; Pires, C.; Behringer, C.; Menke, T.; Freudenreich, J.;
€
Rossmann, E. C.; Bohrer, P. J. Am. Chem. Soc. 2006, 128, 14845–14853.
(23) Kira, M.; Ishida, S.; Iwamoto, T.; Yauchibara, R.; Sakurai, H.
(19) Kira, M.; Yauchibara, R.; Hirano, R.; Kabuto, C.; Sakurai, H.
J. Am. Chem. Soc. 1991, 113, 7785–7787.
(20) Hartwig, J. F. In Activation and Functionalization of C-H Bonds;
Goldberg, K. I., Goldman, A. S., Eds.; American Chemical Society: Washington,
DC, 2004; Vol. 885.
J. Organomet. Chem. 2001, 636, 144–147.
(24) Lappert, M. F.; Misra, M. C.; Onyszchuk, M.; Rowe, R. S.;
Power, P. P.; Slade, M. J. J. Organomet. Chem. 1987, 330, 31–46.
(25) Harris, R. K.; Becker, E. D.; De Menezes, S. M. C.; Goodfellow,
R.; Granger, P. Pure Appl. Chem. 2001, 73, 1795–1818.

Article
Organometallics, Vol. 29, No. 21, 2010 5035
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 by 1H NMR spectroscopy. The volatiles were removed
in vacuo, resulting in a white powder (110 mg, 25.7% yield). 1H
NMR (C6D6): δ 4.23 (dd, 3JH-H = 2.00 Hz, 3JH-H = 7.21 Hz,
1H, Sn-CH), 3.74 (pseudoquartet, JH-H = 7.21 Hz, 1H, O-CH2),
3.46 (dt, 3JH-H = 8.0 Hz, 2JH-H = 4.8 Hz, 1H, O-CH2), 2.42 (m,
1H, Sn-CH-CH2), 2.26-1.80 (m, 4H, Sn-C(SiMe3)2-CH2-CH2),
2.09 (m, 1H, Sn-CH-CH2), 1.64 (m, 1H, O-CH2-CH2), 1.38 (m,
1H, O-CH2-CH2), 0.43 (s, 9H, Si(CH3)3), 0.38 (s, 9H, Si(CH3)3),
0.30 (s,9H, Si(CH3)3), 0.15 (s, 9H, Si(CH3)3). 13CNMR (C6D6):δ
81.95 (Sn-CH), 70.04 (O-CH2), 35.24 (Sn-C(SiMe3)2-CH2), 35.02
(Sn-C(SiMe3)2-CH2), 32.89 (Sn-CH-CH2), 26.62 (O-CH2-CH2),
4.70 (Si(CH3)3), 4.61 (Si(CH3)3), 4.52 (Si(CH3)3), 4.37(Si(CH3)3).
119Sn NMR (C6D6): δ 133.4. MS CI with methane m/z: 647.1
(M - CH3). IR (film) cm-1: ν 2920, 1462, 1377, 1251, 1036, 901,
848, 754, 653, 607
(C7H7)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (8). A one-necked
100 mL flask fitted with a rubber septum was charged with
157 mg of 2,4,6-triisopropyliodobenzene (0.48 mmol, 1.1 equiv)
and 5 mL of toluene. A solution containing 200 mg of 1 (0.43
mmol, 1 equiv) and 10 mL of toluene was placed in a gastight
syringe equipped with a 20-gauge needle. Then 5 mL of the red
stannylene solution was added to the 2,4,6-triisopropyliodoben-
zene/alkane 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 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 (120 mg, 41% yield).
1H NMR (C6D6): δ 7.38 (pseudo-d, 7.2 Hz, 3H, ortho-Ph and
para-Ph), 7.0 (pseudo-t, JH-H = 7.6 Hz, 2H, meta-Ph), 3.25 (s
with br Sn satellites, 2JSn-H = 22.8 Hz, 2H, Sn-CH2), 2.04 (m,
2H, SnC(Si(CH3)2CH2), 1.89 (m, 2H, SnC(Si(CH3)2CH2), 0.32
(s, 18H, Si(CH3)3), 0.13 (s, 18H, Si(CH3)3). 13C NMR (C6D6): δ
139.0 (ipso-Ph), 129.8 (ortho-Ph), 128.97 (meta-Ph), 126.32
(para-Ph), 34.90 (Sn-CH2), 34.21 (Sn-C(SiMe3)2-CH2-CH2),
4.60 (Si(CH3)3), 4.47 (Si(CH3)3). 119Sn NMR (C6D6): δ 118.86.
MS CI with methane m/z: 667.2 (M-CH3). IR (film) cm-1: ν
2921, 2854, 1461, 1377, 1260, 1251, 900, 847, 755, 721, 654. Anal.
Calcd for C23H47ISi4Sn C: 40.53; H: 6.95. Found: C: 40.84; H:
7.22.
(C6H11)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (6). A one-necked
100 mL flask fitted with a rubber septum was charged with 236 mg
of 2,4,6-triisopropyliodobenzene (0.71 mmol, 1 equiv), and 5 mL
of cyclohexane. A solution containing 300 mg of 1 (0.65 mmol,
1 equiv) and 10 mL of cyclohexane was placed in a gastight syringe
equipped with a 20-gauge needle. A 5 mL amount of the red
stannylene solution was added to the 2,4,6-triisopropyliodoben-
zene/alkane 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 activa-
(C9H11)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (9). A one-necked
100 mL flask fitted with a rubber septum was charged with 157
mg of 2,4,6-triisopropyliodobenzene (0.48 mmol, 1.1 equiv) and
5 mL of mesitylene. A solution containing 200 mg of 1 (0.43
mmol, 1 equiv) and 10 mL of toluene was placed in a gastight
syringe equipped with a 20-gauge needle. Then 5 mL of the red
stannylene solution was added to the 2,4,6-triisopropyliodoben-
zene/mesitylene 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
1
tion product by H NMR spectroscopy. The volatiles were re-
moved in vacuo, and column chromatography was performed to
give a white powder (398.2 mg, 91.3% yield). 1H NMR (C6D6): δ
2.14-1.92 (m, 8H, Sn-C(SiMe3)2-CH2-CH2 and SnCHCH2), 1.77
(tt, 3JH-H = 12.5 Hz, 3JH-H = 3.5 Hz, 1H, Sn-CH), 1.68 (m, 2H,
SnCHCH2CH2), 1.52 (m, 1H, SnCHCH2CH2CH2), 1.27 (m, 3H,
SnCHCH2CH2 and SnCHCH2CH2CH2), 0.39 (s, 18H, Si(CH3)3),
0.18 (s, 18H, Si(CH3)3). 13C NMR (C6D6) δ 42.19 (Sn-CH), 35.73
(Sn-C(SiMe3)2-CH2), 31.79 (SnCHCH2), 29.17 (SnCHCH2CH2),
26.91 (SnCHCH2CH2CH2), 4.55 (Si(CH3)3), 4.48 (Si(CH3)3).
119Sn (C6D6): δ 166.8. MS EI m/z: 659.4 (M-CH3). IR (film)
cm-1: ν 2920, 1458, 1377, 1251, 897, 848. Anal. Calc’d for
C22H51ISi4Sn: C: 39.23; H: 7.63. Found: C: 39.53; H: 7.84.
1
the C-H activation product by H NMR spectroscopy. The
volatiles were removed and heated to 80 °C in vacuo to remove
the impurities. The remaining white solid was elementally pure
1
(390 mg, 84.9% yield). H NMR (C6D6): δ 7.14 (s, 2H, ortho-
Ph), 6.69 (s, 1H, para-Ph), 3.33 (s with Sn satellites, JSn-H =
2
30.0 Hz, 2H, Sn-CH2), 2.21 (s, 6H, ArCH3), 2.07 (m, 2H,
Sn-C(SiMe3)2-CH2), 1.90 (m, 2H, Sn-C(SiMe3)2-CH2), 0.33 (s,
18H, Si(CH3)3), 0.14 (s, 18H, Si(CH3)3). 13C NMR (C6D6): δ
138.78 (ipso-Ph), 137.77 (meta-Ph), 127.40 (ortho-Ph), 114.94
(para-Ph), 36.01 (Sn-CH2), 34.64 (Sn-C(SiMe3)2-CH2-CH2),
21.37 (ArCH3), 4.14 (Si(CH3)3), 4.04 (Si(CH3)3). 119Sn NMR
(C6D6): δ 96.59. MS EI m/z: 695.5 (M - CH3). IR (film) cm-1: ν
2925, 1598, 1463, 1377, 1245, 902, 844. Anal. Calcd for C25H51-
ISi4Sn: C: 42.31; H: 7.24. Found: C: 42.31; H: 7.17.
(C4H9O)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (7). A one-necked
100 mL flask fitted with a rubber septum was charged with 175.2
mg of 2,4,6-trimethyliodobenzene (0.71 mmol, 1.1 equiv) and 5
mL of diethyl ether. A solution containing 300 mg of 1 (0.65
mmol, 1 equiv) and 10 mL of diethyl ether was placed in a
gastight syringe equipped with a 20-gauge needle. A 5 mL
amount of the red stannylene solution was added to the iodo-
mesitylene/diethyl ether 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
(C15H23)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (10). A one-necked
100 mL flask fitted with a rubber septum was charged with 156.7
mg (0.47 mmol, 1.1 equiv) of 2,4,6-tri-isopropyliodobenzene and
10 mL of benzene. Stannylene (1) (200 mg, 0.43 mmol, 1 equiv) was
added in one portion, and the red solution was stirred overnight
until the red color disappeared completely. Volatiles were removed
in vacuo, and the flask was heated to 80 °C via a Kugel-Rohr
apparatus to obtain a white solid. (340 mg, 99.3% yield). 1H NMR
(C6D6): δ 7.17 (s, 2H, meta-Ph), 3.05 (m, 2H, ortho-CH(CH3)2,
2.73 (septet, 3JH-H = 7.0 Hz, para-CH(CH3)2), 2.23-2.00 (m, 4H,
1
only the C-H activation product by H NMR spectroscopy.
The volatiles were removed in vacuo, and the resulting solid was
column chromatographed to give elementally pure material.
(200 mg, 46.6% yield). 1H NMR (C6D6): δ 4.12 (q, 3JH-H = 6.8
Hz, 1H, Sn-CH), 3.41 (pseudo-p, JH-H = 6.8 Hz, 1H, O-CH2-
CH3), 2.98 (pseudo-p, JH-H = 6.8 Hz, 1H, O-CH2-CH3),
3
2.21-1.83 (m, 4H, Sn-C(SiMe3)2-CH2-CH2), 1.72 (d, JH-H
= 6.8 Hz, 3H, Sn-CH-CH3), 1.03 (t, JH-H = 7.2 Hz, 3H,
3
O-CH2-CH3), 0.40 (s, 9H, Si(CH3)3), 0.39 (s, 9H, Si(CH3)3), 0.25
(s, 9H, Si(CH3)3), 0.18 (s, 9H, Si(CH3)3). 13C NMR (C6D6): δ
81.90 (Sn-CH), 65.9 (O-CH2-CH3), 35.25 (Sn-C(SiMe3)2-CH2),
35.10 (Sn-C(SiMe3)2-CH2), 19.66 (Sn-CHCH3), 15.62 (O-CH2-
CH3), 4.78 (Si(CH3)3), 4.60 (Si(CH3)3), 4.57 (Si(CH3)3), 4.49
(Si(CH3)3). 119Sn NMR (C6D6): δ 117.68. MS CI with methane
m/z: 649.4 (M - CH3). IR (film) cm-1: ν 2919, 1250, 899, 843
Anal. Calcd for C20H49IOSi4Sn: C: 36.20; H: 7.44. Found: C:
37.10; H: 7.64.
3
SnC(Si(CH3)2CH2-CH2), 1.42 (d, JH-H = 6.5 Hz, 6H, para-
CH(CH3)2), 1.15 (d, 3JH-H = 7.0 Hz, 12H, ortho-CH(CH3)2, 0.50
(s, 18H, Si(CH3)3), 0.20 (s, 18H, Si(CH3)3). 13C NMR (C6D6): δ
153.8 (ipso-Ph), 151.0 (ortho-Ph), 150.2 (meta-Ph), 123.6 (para-
Ph), 38.56 (ortho-CH(CH3)2), 36.83 (SnC(Si(CH3)2CH2-CH2),
34.53 (para-CH(CH3)2), 26.20 (para-CH(CH3)2), 24.33 (ortho-
CH(CH3)2), 5.16 (Si(CH3)3), 4.40 (Si(CH3)3). 119Sn NMR
(C6D6): δ -28.67. MS EI m/z: 779.8 (M - CH3). IR (film)

5036 Organometallics, Vol. 29, No. 21, 2010
Kavara et al.
cm-1: ν 2920, 1461, 1377, 1249, 896, 845. Anal. Calcd for
C31H63ISi4Sn: C: 46.91; H: 8.00. Found: C: 46.88; H: 8.01.
CH2), 2.13-1.85 (m, 4H, SnC(Si(CH3)2CH2-CH2), 1.18 (d,
3JH-H = 6.8 Hz, 6H, CH(CH3)2), 0.39 (s, 18H, Si(CH3)3),
0.20 (s, 18H, Si(CH3)3). 13C NMR (C6D6): δ 88.8 (Sn-CH2-
CtC), 77.9 (Sn-CH2-CtC), 34.8 (Sn-C(SiMe3)2-CH2-CH2),
23.9 (Sn-CH2), 21.9 (CH(CH3)2), 21.6 (CH(CH3)2), 4.50
(Si(CH3)3), 4.33 (Si(CH3)3). 119Sn NMR (C6D6): δ 104.0. IR
of the mixture (film) cm-1: ν 2951, 1941, 1445, 1360, 1320, 1248,
958, 857, 756, 680, 654. Anal. Calcd for mixture of primary and
tertiary product for C22H49ISi4Sn: C: 39.35 ; H: 7.35. Found: C:
39.53; H: 7.52. MS CI of 17 and 18 mixture with methane m/z:
658 (M - CH3)
(C6H11)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (15 and 16). A one-
necked 100 mL flask fitted with a rubber septum was charged
with 157 mg of 2,4,6-triisopropyliodobenzene (0.48 mmol, 1.1
equiv) and 5 mL of trans-4-methyl-2-pentene. A solution con-
taining 200 mg of 1 (0.43 mmol, 1 equiv) and 10 mL of trans-4-
methyl-2-pentene was placed in a gastight syringe equipped with
a 20-gauge needle. Then 5 mL of the red stannylene solution was
added to the 2,4,6-triisopropyliodobenzene/trans-4-methyl-2-
pentene 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. The volatiles were
removed in vacuo, and the 1H NMR of the crude mixture
indicated 71% primary carbon C-H activation product, 24%
tertiary C-H activation product, and 5% oxidative addition
product. The resulting solid was recrystallized from pentane at
-78 °C to give a white powder (220 mg, 76% yield). 15 1H NMR
(C6D6): δ 5.82 (pseudo-dt, 2H, JH-H = 11.5 Hz, JH-H = 1.6
Hz, HCdCH), 2.81 (m, 3H, CH3CHdCH), 2.19 (m, 2H, SnC-
(Si(CH3)2CH2), 1.82 (m, 2H, SnC(Si(CH3)2CH2), 1.65 (s with br
(C18H29)BrSnC(SiMe3)2CH2CH2C(SiMe3)2 (19).
A one-
necked 100 mL flask fitted with a rubber septum was charged
with 154.4 mg (0.47 mmol, 1.1 equiv) of 2,4,6-tri-tert-butylbromo-
benzene and 40 mL of pentane. Stannylene (1) (200 mg, 0.43 mmol,
1 equiv) was added in one portion, and the red solution was stirred
for three days until the red color disappeared completely. Volatiles
were removed, and a white solid was recrystallized from pentane at
-78 °C (250 mg, 69% yield). 1HNMR(C6D6):δ7.50 (s, 2H, ortho-
Ar), 7.38 (s, 1H, para-Ar), 2.44 (s with br Sn satellites, 3JSn-H
=
15.6 Hz, 2H, Sn-CH2), 2.12 (m, 2H, SnC(Si(CH3)2CH2), 1.92 (m,
2H, SnC(Si(CH3)2CH2), 1.02 (s, 6H, Sn-CH2-C-(CH3)2), 1.36 (s,
18H, Ar-(C(CH3)3)2, 0.36 (s, 18H, Si(CH3)3), 0.18 (s, 18H, Si-
(CH3)3). 13C NMR (C6D6): δ 151.70 (ipso-Ar), 150.84 (meta-Ar),
120.6 (ortho-Ar), 120.3 (para-Ar), 44.40 (Sn-CH2), 40.68 (Sn-CH2-
C(CH3)2Ar), 35.61 (Ar-(C(CH3)3)2), 34.95 (Sn-C(SiMe3)2-CH2-
CH2), 32.54 (Sn-CH2-C(CH3)2Ar), 32.22 (Ar-(C(CH3)3)2), 4.69
(Si(CH3)3), 4.61 (Si(CH3)3). 119Sn NMR (C6D6): δ 175.7. MS CI
with methane m/z: 775.5 (M - CH3).
3
Sn satellites, JSn-H = 9.19 Hz, 6H, Sn-C(CH3)2), 0.40 (9H,
Si(CH3)3), 0.39 (9H, Si(CH3)3), 0.21 (9H, Si(CH3)3), 0.13 (9H,
Si(CH3)3). 13C NMR (C6D6): δ 128.94 (HCdCH), 35.78 (Sn-
C(SiMe3)2-CH2), 35.74 (CH3-HCdC), 34.8 ((Sn-C(CH3)2), 26.2
(Sn-C(CH3)2) 4.78 (Si(CH3)3), 4.37 (Si(CH3)3), 4.28 (Si(CH3)3),
4.25 (Si(CH3)3). 119Sn NMR (C6D6): δ 169.2. MS EI m/z: 659.1
(M - CH3). 16 1H NMR (C6D6): δ 5.77 (m, 1H, HCdC), 5.56
3
(m, 1H, CdCH), 2.58 (d with Sn satellites, JH-H = 8.4 Hz,
2JSn-H = 23.6 Hz, 2H, Sn-CH2), 2.32 (septet, 3JH-H = 6.8 Hz,
(CH3)2-CH), 2.10 (m, 2H, SnC(Si(CH3)2CH2), 1.86 (m, 2H,
SnC(Si(CH3)2CH2), 1.1 (d, 3JH-H = 6.8 Hz, 6H, (CH3)2-CH),
0.37 (s, 18H, Si(CH3)3), 0.15 (s, 18H, Si(CH3)3). 13C NMR
(C6D6): δ 139.6, 123.5 (HCdCH), 35.0 (Sn-C(SiMe3)2-CH2-
CH2), 32.1 (Sn-CH2), 23.9 ((CH3)CH), 23.2 ((CH3)CH), 4.51
(Si(CH3)3), 4.49 (Si(CH3)3) 119Sn NMR (C6D6): δ 132.0. MS EI
m/z of mixture: 659.1 (M - CH3). IR (film) of mixture cm-1: ν
2954, 2866, 1464, 1406, 1251, 901, 843. Anal. Calcd for the
mixture of C22H51ISi4Sn: C: 39.23; H: 7.63. Found: C: 38.97; H:
7.74.
(C6H7O)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (20 and 21). A
one-necked 100 mL flask fitted with a rubber septum was
charged with 79 mg (0.24 mmol, 1.1 equiv) of 2,4,6-tri-isopro-
pyliodobenzene and 3 mL of 2,3-dimethylfuran. Stannylene (1)
(100 mg, 0.22 mmol, 1 equiv) dissolved in 2 mL of 2,3-dimethyl-
furan was added slowly with a syringe, and the red solution was
stirred overnight until the red color disappeared completely.
Volatiles were removed in vacuo, and the flask was heated to
80 °C via a Kugel-Rohr apparatus to obtain a white solid
(130 mg, 87% yield) composed of 2-methyl and 3-methyl
C-H activation products and 5% oxidative addition product.
Attempts to purify the C-H activation products via crystal-
lization or column chromatography failed.
(C6H9)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (17 and 18). A one-
necked 100 mL flask fitted with a rubber septum was charged
with 235.1 mg of 2,4,6-triisopropyliodobenzene (0.71 mmol, 1.1
equiv) and 2 mL of 4-methyl-2-butyne. A solution containing
300 mg of 1 (0.43 mmol, 1 equiv) and 10 mL of 4-methyl-2-
butyne was placed in a gastight syringe equipped with a 20-
gauge needle. Then 5 mL of the red stannylene solution was
added to the 2,4,6-triisopropyliodobenzene/mesitylene 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
products. The volatiles were removed and heated to 80 °C
in vacuo to remove the impurities. The remaining white solid
was elementally pure (420 mg, 96.6% yield). Column chromato-
graphy was performed with hexane to yield pure tertiary product
(140 mg, 32.2% yield). The primary C-H activation product
decomposed on the column. 17 1H NMR (C6D6): δ 2.20 (s with
br Sn satellites, 2JSn-H = 29.6 Hz, 3H, CtC-CH3), 2.11 (m, 2H,
SnC(Si(CH3)2CH2), 1.97 (m, 2H, SnC(Si(CH3)2CH2), 1.59 (s
with br Sn satellites, 3JSn-H = 17.6 Hz, 6H, Sn-C-(CH3)2), 0.42
(s, 18H, Si(CH3)3), 0.22 (s, 18H, Si(CH3)3). 13C NMR (C6D6):
δ 98.1 (Sn-C(CH3)2-CtC), 91.7 (Sn-C-CtC), 35.0 (Sn-C-
(SiMe3)2-CH2-CH2), 22.2 (Sn-C(CH3)2), 21.6 (CtCCH3), 21.1
((CH3)2C-CtC), 4.49 (Si(CH3)3), 4.28 (Si(CH3)3). 119Sn NMR
(C6D6): δ 57.9. MS EI m/z: 658.0 (M - CH3). Anal. Calcd for
C22H49ISi4Sn: C: 39.35 ; H: 7.35. Found: C: 39.32; H: 7.44.
18 1H NMR (C6D6): δ 2.56 (m, 1H, (CH3)2-CH-CtC), 2.44
(d with br Sn satellites, 2JH-H = 2.0 Hz, 2JSn-H = 23.2 Hz, 2H,
20 1H NMR (C6D6): δ 7.07 (d, 3JH-H = 2.0 Hz, 1H, O-CH),
3
6.02 (d, JH-H = 2.0 Hz, 1H, O-CH-CH), 3.05 (s with br Sn
satellites, 2JSn-H = 16 Hz, 2H, Sn-CH2), 2.08-1.90 (m, 4H, Sn-
C(SiMe3)2-CH2-CH2), 0.37 (s, 18H, Si(CH3)3, 0.22 (s, 18H,
Si(CH3)3). 13C NMR (C6D6): δ 149. 3, 140.2 (OCdC), 116.3,
114.0 (OCdC), 35.01 (Sn-C(SiMe3)2-CH2-CH2), 24.87 (Sn-
CH2), 4.50 (Si(CH3)3), 4.46 (Si(CH3)3). 119Sn NMR (C6D6): δ
115.0. IR of the mixture (film) cm-1: ν 2855, 1458, 1377, 1261,
1250, 900, 848. MS of 20 and 21 mixture (CI with methane) m/z:
671.5.
21 1H NMR (C6D6): δ 7.08 (d, 3JH-H = 2 Hz, 1H, O-CH),
3
6.63 (d, JH-H = 2 Hz, 1H, O-CH-CH), 2.89 (s with br Sn
2
satellites, JSn-H = 18 Hz, 2H, Sn-CH2), 2.13-1.85 (m, 4H,
SnC(Si(CH3)2CH2-CH2), 0.35 (s, 18H, Si(CH3)3, 0.13 (s, 18H,
Si(CH3)3. 13C NMR (C6D6): δ 148.4, 140.8 (OCdC), 117.1,
112.7 (OCdC), 33.7 (Sn-C(SiMe3)2-CH2-CH2), 24.6 (Sn-CH2),
4.58 (Si(CH3)3), 4.56 (Si(CH3)3). 119Sn NMR (C6D6): 125.1.
(C9H11)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (22). A one-necked
100 mL flask fitted with a rubber septum was charged with 124
mg (0.51 mmol, 1.1 equiv) of 2,4,6-trimethyliodobenzene and
20 mL of benzene. Stannylene (1) (233 mg, 0.50 mmol, 1 equiv)
was added in one portion, and the red solution was stirred
overnight until the red color disappeared completely. Volatiles
were removed in vacuo, and the flask was heated to 80 °C via a
Kugel-Rohr apparatus to obtain a white solid. (333 mg, 93%

Article
Organometallics, Vol. 29, No. 21, 2010 5037
1
yield). H NMR (C6D6): δ 6.67 (s, 2H, meta-Ph), 2.60 (s, 6H,
ortho-CH3), 2.21-1.99 (m, 4H, SnC(Si(CH3)2CH2-CH2), 2.07
(s, 3H, para-CH3), 0.49 (s, 18H, Si(CH3)3), 0.19 (s, 18H,
Si(CH3)3). 13C NMR (C6D6): δ 149.8 (ipso-Ar), 142.1 (Ar),
139.7 (Ar), 128.9 (meta-Ar), 36.49 (SnC(Si(CH3)2CH2-CH2),
29.55 (ortho-CH3), 21.27 (para-CH3), 5.08 (Si(CH3)3), 4.02
(Si(CH3)3). 119Sn NMR (C6D6): δ -3.50. MS EI m/z: 695.1
(M - CH3). IR (film) cm-1: ν 2918, 1458, 1377, 1251, 1071, 894,
847, 756.
Scheme 3. C-H Activation of Alkanes, Arenes, and Ethers
Proceeds Quantitatively with 1 and 2,4,6-Triisopropylbenzene
As Measured by 1H NMR Spectroscopya
(C7H7O)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (23). A one-necked
50 mL flask fitted with a rubber septum was charged with 28 mg
(0.12 mmol, 1.1 equiv) of 4-iodoanisole and 15 mL of benzene.
Stannylene (1) (50 mg, 0.11 mmol, 1 equiv) was added in one
portion, and the red solution was stirred overnight until the red
color disappeared completely. Volatiles were evaporated, and 1H
and 13C NMR spectra of pure oxidative product were obtained.
3
1H NMR (C6D6): δ 7.87 (d with Sn satellites, JH-H = 8.5 Hz,
2JSn-H = 34.0 Hz, 2H, ortho-Ph), 6.74 (d, 3JH-H = 8.5 Hz, 2H,
meta-Ph), 3.19 (s, 3H, para-OCH3), 2.19-2.16 (m, 2H, SnC(Si-
(CH3)2CH2-CH2), 2.04-2.00 (m, 2H, SnC(Si(CH3)2CH2-CH2),
0.47 (s, 18H, Si(CH3)3), 0.20 (s, 18H, Si(CH3)3). 13C NMR (C6D6):
δ161.24 (Ph), 138.11 (Ph), 137.42 (Ph), 114.99 (Ph), 54.90 (OCH3),
35.14 (SnC(Si(CH3)2CH2-CH2), 4.70 (Si(CH3)3).
a When the reaction is performed in benzene, oxidative addition
occurs. Isolated yields are indicated for each product.
(C7H4N)ISnC(SiMe3)2CH2CH2C(SiMe3)2 (24). A one-necked
10 mL flask fitted with a rubber septum was charged with 28 mg
(0.12 mmol, 1.1 equiv) of 4-iodobenzonitrile and 20 mL of
benzene. Stannylene (1) (50 mg, 0.11 mmol, 1 equiv) was added
in one portion, and the red solution was stirred overnight until the
red color disappeared completely. Volatiles were evaporated and
1H and 13C spectra of pure oxidative product were obtained. 1H
function of increased aryl halide steric bulk (Scheme 4). The
statistically corrected relative rates of 3°/1° activation varied
from 4.7 for iodobenzene and 4-iodoanisole, which contain H
at the ortho position, to 1.1 when isopropyl groups were
present in the ortho position. The C-H activation of 4-meth-
yl-2-pentyne followed a similar trend for an increase in
primary activation and a decrease in oxidative-addition pro-
duct as a function of increasing aryl halide steric bulk (Scheme
5). In this case, the statistically correct relative rates for 3°/1°
activation ranged from 15 to 4. In an effort to further increase
the amount of primary activation, 2,6-dimesityliodobenzene
was synthesized;26 however, this aryl halide actually gave the
largest amount of tertiary activation observed and little im-
provement in the amount of primary activation. It was effec-
tive at eliminating the formation of the oxidative-addition
product. Possible reasons for this aryl halide not following the
anticipated trend in steric bulk may be the ability the mesityl
rings to rotate out of the way and/or the electronic effect of
attaching the two mesityl rings to the parent aryl halide ring.
Finally, 2,4,6-tri-tert-butyliodobenzene was also employed in
an attempt to maximize primary activation. For this aryl
3
NMR (C6D6): δ 7.68 (d with Sn satellites, JH-H = 7.5 Hz,
2JSn-H = 30.5 Hz, 2H, ortho-Ph), 6.85 (d, 3JH-H = 8.0 Hz, 2H,
meta-Ph), 2.11-2.06 (m, 2H, SnC(Si(CH3)2CH2-CH2), 1.96-1.9
(m, 2H, SnC(Si(CH3)2CH2-CH2), 0.39 (s, 18H, Si(CH3)3), 0.07 (s,
18H, Si(CH3)3). 13C NMR (C6D6): δ 152.96 (CN), 136.91 (ipso-
Ph), 131.86 (para-Ph), 118.52 (ortho-Ph), 114.09 (meta-Ph), 35.0
(SnC(Si(CH3)2CH2-CH2), 4.64 (Si(CH3)3), 4.49 (Si(CH3)3).
Results and Discussion
C-H Activation of Alkanes, Arenes, and Ethers. The reac-
tion of 1/2,4,6-triisopropyliodobenzene with cyclohexane,
diethyl ether, tetrahydrofuran (THF), toluene, and mesity-
lene yielded C-H activation products as illustrated in
Scheme 3. The high degree of solubility of many of the pro-
ducts contributed to lower isolated yields (26-99%) despite
1
the reactions proceeding quantitatively, as indicated by H
t
NMR spectroscopy. The reactivity observed for cyclohexane
and the regioselective activation of the weaker C-H bonds R
to ether oxygen for 1 was consistent with the reactivity pre-
viously observed for Sn[N(SiMe3)2]2 (11), Ge[CH(SiMe3)2]2
halide, only C-H activation of the ortho Bu groups on
the aryl ring was observed regardless of substrate choice
(Scheme 6).
As a more stringent test of regioselectivity, 2,3-dimethyl-
furan was employed as a substrate. C-H activation routes
employing this substrate could offer an alternative path to
natural products such as rose furan.27 In this case, employing
aryl halides of increasing bulk served to minimize formation
of the oxidative-addition product and give up to a factor of
2.6 preference for the methyl group attached to the carbon R
to the ether oxygen. Unfortunately, our attempts to cross-
couple this tin compound to form rose furan were not
successful (Scheme 7).
(12), Ge[N(SiMe3)2]2 (13), and SiN(tBu)CH2CH2N(tBu)
(14).14,15,18 The observation of clean C-H activation for
toluene and mesitylene had not previously been observed for
this chemistry. Typically, oxidative addition of the aryl halide
to the germylene or silylene was the primary or only product
observed for these substrates. The observation of arene C-H
activation products using using 1/2,4,6-triisopropyliodoben-
zene appears to result from slowing the formation of the
oxidative-addition product when employing this very sterically
encumbered aryl halide coupled with the differing stereoelec-
tronic properties of 1 as compared to 11-14.
Effects of Aryl Halide Substitution on the Hydrogen Atom
Transfer Reaction. In reactions involving hydrogen atom
transfer, the factors that control tertiary/secondary/primary
The reaction of trans-4-methyl-2-pentene with 1/aryl halide
showed a preference for primary over tertiary C-H activa-
tion, concomitant with a decrease in oxidative addition, as a
(26) Schiemenz, B.; Power, P. P. Organometallics 1996, 15, 958–964.
(27) Marshall, J. A.; DuBay, W. J. J. Org. Chem. 1993, 58, 3602–3603.

5038 Organometallics, Vol. 29, No. 21, 2010
Kavara et al.
Scheme 4. A Degree of Regiochemical Control Can Be Obtain for the Reaction with trans-4-Methyl-2-pentene When the Steric Bulk of
the Aryl Halide Is Varieda
a The selectivities indicated were determined by 1H NMR spectroscopy.
Scheme 5. A Degree of Regiochemical Control Can Be Obtain for the Reaction with 4-Methyl-2-pentyne When the Steric Bulk of the Aryl
Halide Is Varieda
a The selectivities indicated were determined by 1H NMR spectroscopy.
Scheme 6. 2,4,6-tert-Butylbromobenzene Undergoes Intermolecular C-H Activation Regardless of the Choice of Solvent
C-H bond selectivity are the amount of hydrogen atom
transfer in the transition state, charge separation in the
transition state, and co-linearity of the incipient radical
with the π system.28 The first two of these effects work to
increase the selectivity toward the tertiary C-H bond. When
a significant amount of hydrogen atom transfer is involved in
the transition state, the bond dissociation energies of C-H
bonds become important. If some charge separation occurs in
the transition state, the polar effects favor the partial formation
of a tertiary carbocation over a secondary or primary carboca-
tion. Previous studies have determined that the co-linearity
condition becomes difficult to satisfy when the steric bulk of the
aryl halide is increased. In this case the steric effects influence
tertiary/secondary/primary selectivity in a direction opposite
that predicted by thermodynamic and polar effects.28
The statistically averaged tertiary versus primary selec-
tivity observed in reactions of the phenyl radical precursor
phenylazotriphenylmethane (PAT) with toluene/triisopro-
pylbenzene was determined to be 9.7.28 Our results in the
reaction with trans-4-methyl-2-pentene and 4-methyl-2-pen-
tyne and iodobenzene/I, Schemes 4 and 5, do not grossly vary
(28) Baciocchi, E.; D’Acunzo, F.; Galli, C.; Lanzalunga, O. J. Chem.
Soc., Perkin Trans. 2 1996, 133–140.

Article
Organometallics, Vol. 29, No. 21, 2010 5039
Scheme 7. Regiochemistry of C-H Activation of 2,3-Dimethyl Furan Can Be Controlled by Varying the Bulk of the Aryl Halidea
a Utilization of this C-H activation product in cross-coupling reactions was not successful. The selectivities indicated were determined by 1H NMR
spectroscopy.
from the PAT values. The trans-4-methyl-2-pentene tertiary/
primary selectivity (4.7) with iodobenzene/I is lower than the
PAT value, while the value for 4-methyl-2-pentyne (15)
selectivity with iodobenzene/I is higher.
(13) with THF/THF-d8 was found to be 5.0 ( 0.2 and 4.1 (
0.2, respectively.14 On the basis of the value found for phenyl
radical generated by PAT THF/THF-d8 (4.2 ( 0.2), we have
argued against formation of a common intermediate in these
reactions, including the phenyl radical. The primary KIE
reported herein for the reaction of tri-isopropyliodobenzene
and 1 in toluene/toluene-d7 was found to be 4.9 ( 0.5, as
measured by 1H NMR spectroscopy. This is consistent with
the previously reported values. The range of KIE values
indicates that the EL2/PhX radical intermediate involved is
dependent on the exact nature of the group 14 reagent, the
aryl halide, and the solvent. This is logical since polar effects
and co-linearity conditions work in the opposite direction to
increase or decrease tertiary/primary selectivity, respec-
tively. With trans-4-methyl-2-pentene and 4-methyl-2-pen-
tyne, co-linearity with the π system becomes important, as
evidenced by switch in the regiochemistry when the steric
bulk of the aryl halide ortho substituents is increased.
When the ortho substituents are increased from hydrogen
to methyl, there is a decrease in tertiary/primary selectivity
by a factor of 2.3 for trans-4-methyl-2-pentene and 3-fold for
4-methyl-2-pentyne, Schemes 4 and 5, respectively. With
isopropyl substitution there is 4.2-fold decrease in tertiary/
primary selectivity with the alkene and a 3.7-fold decrease for
the alkyne. It should be noted that with the increase of steric
bulk in ortho positions there is a drop in the amount of
oxidative addition product concomitant with the increase of
primary/tertiary selectivity.
The energy of formation for the tertiary versus primary
radicals for trans-4-methyl-2-pentene and 4-methyl-2-pen-
tyne was calculated using density functional theory (B3LYP,
6-31G*). The difference in bond dissociation energies was
calculated to be 6.7 kcal/mol for 4-methyl-2-pentyne. The
value for trans-4-methyl-2-pentene with the same method
was calculated to be 6.0 kcal/mol. This translates into a
roughly 3-fold increase for tertiary/primary selectivity with
4-methyl-2-penyne as compared to trans-4-methyl-2-pen-
tene, as calculated using the Arrhenius equation. This agrees
with our experimental results discussed above, 15 for 4-meth-
yl-2-pentyne versus 4.7 for trans-4-methyl-2-pentene.
Despite this good agreement with experimental ratios
when considering the delta in bond dissociation energy
values between 4-methyl-2-pentyne and trans-4-methyl-2-
pentene, when considering the relative selectivity between
competing sites within a molecule good agreement with
calculated values is not obtained. The 6.7 kcal/mol difference
should translate into a ∼8 ꢁ 104 rate difference for the
tertiary versus primary C-H site. By way of comparison
with other systems, the tertiary/primary selectivities for NBS
and Br2/hν are 58 and 37, respectively. The EL2/PhX radical
species involved in the C-H activation reaction are highly
reactive and more comparable to tBuO radical or autoxida-
tion by O2, which have tertiary/primary selectivities of 6.8
and 13, respectively.28 This suggests that there is relatively
little C-H bond breakage in the transition state in C-H
activation with iodoarene/1. Our general conclusion is that
the EL2/PhX radical species involved here are not very
sensitive to bond dissociation energies, although the differ-
ential reactivity between different molecules does track well
with the bond dissociation energy differences.
Conclusion
We have demonstrated C-H activation of simple alkanes,
ethers, and aromatics with 1 and aryl halides in excellent
yield. The product distribution and regiochemistry of C-H
activation can be controlled to an extent with ortho substitu-
tion on the aryl halide. Oxidative addition product can be
decreased by using more sterically bulky/electron-donating
substituents. In substrates with alkenes and alkyne function-
alities possessing primary and tertiary C-H bonds, the yield
of primary C-H activation can be increased by increasing
the steric bulk on the aryl halide. Reactions employing 2,4,6-
tri-tert-butyliodobenzene result only in the CH activation of
the ortho tert-butyl groups.
Previously, we measured the primary isotope effects of the
C-H activation reaction with different divalent group 14/
iodoarene species. The primary kinetic isotope effect (KIE)
for the reaction of SiN(tBu)CH2CH2N(tBu) (14) with Et2O/
Et2O-d10 was found to be 5.7 ( 0.1.18 The primary KIE for
the reaction of Ge[CH(SiMe3)2]2 (12) and Ge[N(SiMe3)2]2