Selective C-H Activation of Haloalkanes
A R T I C L E S
minor products were observed in these reactions as described in
the text. For Tp’Rh(CNR)((CH2)2CH2Cl)(H) (6a), 1H NMR (C6D6):
δ Rh-H: -14.94 (d, JRh-H ) 24 Hz, 1 H); Tp′Me: 2.19 (s, 3 H),
2.20 (s, 3 H), 2.29 (s, 3 H), 2.35 (s, 3 H), 2.45 (s, 3 H), 2.53 (s, 3
H); Tp′-H: 5.62 (s, 1 H), 5.65 (s, 1 H), 5.82 (s, 1 H); CNR: 0.65
(s, 9 H), 2.63 (s, 2 H); CH2CH2CH2Cl: 3.00 (t, JH-H ) 7.3 Hz, 2
H, CH2CH2CH2Cl), 3.54 (m, 1 H, CH2CH2CH2Cl), 3.63 (m, 1 H,
pentane was condensed onto 1 in vacuo. The resulting dark yellow
solution was kept at -20 °C and photolyzed for 30 min. The resulting
pale yellow solution was allowed to warm to 0 °C under vacuum.
The remaining residue was dissolved in 0.6 mL of C6D6 and 0.5 µL
of an internal standard (hexamethyldisiloxane) added. The resulting
solution was placed in a 26 °C NMR probe and monitored by 1H NMR
spectroscopy over the course of 5 h. The spectra obtained showed a
mixture of activation products as described in the text. 1H NMR (C6D6):
δ Rh-H: -13.52 (d, JRh-H ) 11 Hz, Tp′Rh(CNR)(H)(Cl)); -14.67
(d, JRh-H ) 22 Hz, Tp′Rh(CNR)(c-CCl(CH2)4)(H)).
CH2CH2CH2Cl), 4.31 (m,
2
H, CH2CH2CH2Cl). For
1
Tp′Rh(CNR)((CH2)3CH2Cl)(H) (3a), H NMR (C6D6): δ Rh-H:
-14.94 (d, J ) 24 Hz, 1 H); Tp′Me: 2.21 (s, 6 H), 2.30 (s, 3 H),
2.37 (s, 3 H), 2.52 (s, 3 H), 2.56 (s, 3 H); Tp′-H: 5.66 (s, 2 H),
5.84 (s, 1 H); CNR: 0.67 (s, 9 H), 2.70 (s, 2 H); CH2Cl: 3.41 (m,
2 H). For Tp′Rh(CNR)((CH2)4CH2Cl)(H) (7a), 1H NMR (C6D6): δ
Rh-H: -14.93 (d, JRh-H ) 25 Hz, 1 H); Tp′Me: 2.21 (s, 3 H),
2.22 (s, 3 H), 2.31 (s, 3 H), 2.38 (s, 3 H), 2.55 (s, 3 H), 2.57 (s, 3
H); Tp′-H: 5.67 (s, 2 H), 5.85 (s, 1 H); CNR: 0.68 (s, 9 H), 2.68
(s, 2 H); CH2Cl: 3.24 (t, JH-H ) 7.4 Hz, 2 H). Any remaining
resonances were obscured. Treatment of a THF solution of
Tp′Rh(CNR)((CH2)2CH2Cl)(H) with an excess of carbon tetrachlo-
ride at -20 °C and allowing the resulting light yellow solution to
stand at low temperature for ∼2 h in the dark resulted in the
effective conversion to Tp′Rh(CNR)((CH2)2CH2Cl)(Cl). X-ray
quality crystals of Tp′Rh(CNR)((CH2)2CH2Cl)(Cl) · CH2Cl2 were
obtained by allowing a dichloromethane solution of the complex
to evaporate slowly at -20 °C for roughly 3 weeks.
Activation/Elimination of 1,1-Dichlorocyclopentane. 1 (6 mg)
was placed in a resealable NMR tube and was treated with roughly
0.6 mL of a mixture consisting of three drops of 1,1-dichlorocy-
clopentane in cyclohexane. The resulting dark yellow solution was
kept at 10 °C and photolyzed for 20 min. The resulting mixture
was allowed to warm to RT under vacuum. The remaining residue
was dissolved in 0.6 mL of C6D6 and 0.5 µL of an internal standard
(hexamethyldisiloxane) added. The resulting solution was placed
1
in a 26 °C NMR probe and was monitored by H NMR spectros-
copy overnight. The spectra obtained showed a mixture of activation
products as described in the text. 1H NMR (C6D6): δ Rh-H: -13.46
(d, JRh-H ) 11 Hz, Tp′Rh(CNR)(H)(Cl)).
Competition Experiments of 1-Chloroalkane Substrates vs
Chloromethane. 1 (6 mg) was placed in a resealable NMR tube
and was treated with roughly 150-300 µL of a liquid substrate
(1-chloropropane, -butane, or -pentane). Chloromethane (charged
0.579 L volume with pressure appropriate to make a 1:1 molar
mixture upon condensation of the gas into the NMR tube) was
added to the tube. The resulting mixtures were analyzed by 1H NMR
spectroscopy at the temperature to be used for photolysis (typically
-50 °C), which established the ratio of substrates in solution. The
samples were irradiated at -50 °C for 45 min and were analyzed
Activation of N-Phenyl-N′-Neopentylcarbodiimide. 1 (6 mg)
was placed in a resealable NMR tube and was treated with roughly
0.5 mL of a mixture consisting of five drops of carbodiimide in
cyclohexane. The resulting mixture of light-colored solid in yellow
liquid was kept at 10 °C and was photolyzed for 20 min. The
resulting mixture was allowed to warm to RT under vacuum. The
remaining orange gel containing light-colored solid was treated with
0.4 mL of C6D6. The resulting solution was placed in a 26 °C NMR
probe and was monitored by 1H NMR spectroscopy over the course
of 3.5 h. The obtained spectra appeared to contain a mixture of
activation products, which were assigned to the three possible
aromatic activation products of the carbodiimide based on the
Rh-H chemical shift and coupling constant of the doublet
1
by H NMR spectroscopy immediately after excess substrate was
removed in vacuo and the remaining residue was dissolved in C6D6.
The ratios of the resulting hydrides from activation of chlo-
romethane (δ -14.05) vs the other substrate (δ -14.94) were
measured and corrected based on the molar ratio of substrates
present in the NMR spectrum measured before photolysis. Results
are presented in Scheme 2.
corresponding to Tp′Rh(CNR)(H)(C6H5).7c For 4a-c, H NMR
1
(C6D6): δ Rh-H: -13.48 (d, JRh-H ) 23 Hz), -13.76 (d, JRh-H
24 Hz), and -13.78 (d, JRh-H ) 24 Hz).
)
Activation of 1-Fluoropentane. 1 (30 mg) was dissolved in 5
mL of 1-fluoropentane. The solution was cooled to -20 °C and
irradiated for 15 min, during which time the bright yellow solution
became colorless. The solvent was removed immediately at low
temperature on the vacuum line. The resulting solid was dissolved
in C6D6 and a 1H NMR spectrum was taken immediately showing
Activation of 1,4-Dichlorobutane. 1 (5 mg) was placed in a
resealable NMR tube. To this 0.6 mL of 1,4-dichlorobutane was added.
The solution was cooled to -20 °C and photolyzed for 15 min. The
volatile materials were removed at 0 °C under vacuum and analyzed
by GCMS showing the formation of 4-chloro-1-butene as a result of
1
a single hydride. For 11, H NMR (C6D6): δ Rh-H: -14.91 (d,
1
ꢀ-chloride elimination. A H NMR spectrum of the resulting pale
JRh-H ) 24 Hz); Tp′methine: 5.653 (s, 1H), 5.662 (s, 1H), 5.841
(s, 1H); Tp′methyl: 2.196 (s, 3H), 2.212 (s, 3H), 2.296 (s, 3H),
2.377 (s, 3H), 2.549 (s, 3H), 2.568 (s, 3H); CNCH2C(CH3)3: 0.655
(s, 9H), 2.650 (s, 2H); CH2(CH2)3CH2F: 4.28 (m, 2H), 2.00
(complex. m, 1H), 1.86 (complex m, 1H), 1.71 (complex m, 1H),
the remaining alkyl resonances are highly split and obscured.
Activation and Quench of 1-Fluoropentane. A solution of 11
was prepared as described above. The resulting bleached solution
was quenched with carbon tetrachloride and allowed to stand at
-20 °C for 2 h. The solvents were removed resulting in a brown
solid. A 1H NMR spectrum showed the clean formation of
Tp′Rh(L)(CH2(CH2)3CH2F)Cl (11-Cl). The solid was recrystal-
lized from THF/hexanes mixture at -20 °C, yielding a light
yellow solid. 1H NMR (C6D6): δ Tp′methyl: 2.089 (s, 3H), 2.154
(s, 3H), 2.221 (s, 3H), 2.341 (s, 3H), 2.783 (s, 3H), 2.960 (s,
3H); Tp′methine: 5.571 (s, 1H), 5.622 (s, 1H), 5.705 (s, 1H);
CNCH2C(CH3)3: 0.708 (s, 9H), 2.616 (s, 2H); CH2(CH2)3CH2F:
4.202 (m, 1H), 4.09 (m, 1H), 3.376 (m, 1H), 3.216 (m, 1H),
2.04 (m, 1H), 1.93 (m, 1H), 1.39 (m, 2H), the remaining alkyl
resonances are highly split and obscured. 13C{1H} NMR (C6D6):
δ 12.27, 12.74, 12.97, 14.55, 14.61, 14.63 (s, pzCH3), 18.00 (d,
J ) 19 Hz, CH2(CH2)3CH2F), 26.60 (s, CNCH2C(CH3)3), 28.34
(d, J ) 4.6, CH2CH2CH2CH2CH2F), 30.69 (d, J ) 19 Hz,
yellow solid showed a single product previously characterized as 5.
1H NMR for 5 (C6D6): δ Rh-H: -13.40 (d, J ) 11.5 Hz, 1 H);
Tp′methyl: 2.074 (s, 3H), 2.120 (s, 3H), 2.207 (s, 3H), 2.286 (s, 3H),
2.862 (s, 3H), 2.869 (s, 3H); Tp′methine: 5.499 (s, 2H), 5.803 (s, 1H);
CNCH2C(CH3)3: 0.655 (s, 9H), 2.554 (s, 2H).
Activation of Neopentyl Chloride. 1 (6 mg) was placed in a
resealable NMR tube and treated with roughly 0.5 mL of
neopentyl chloride. The resulting dark yellow solution was kept
at -20 °C and photolyzed for 30 min. The resulting pale yellow
solution was allowed to warm to 0 °C under vacuum. The
remaining residue was treated with 0.6 mL of C6D6 and 0.5 µL
of an internal standard (hexamethyldisiloxane). The resulting
solution was placed in a 26 °C NMR probe and monitored by
1H NMR spectroscopy overnight. The spectra obtained contain
a mixture of activation products as described in the text, the
dominant species assigned to the product of primary C-H
activation, Tp′Rh(CNR)((CH2)C(CH3)2CH2Cl)(H) (8a). 1H NMR
(C6D6): δ Rh-H: -14.94 (d, JRh-H ) 24 Hz). This product was
observed to convert to Tp′Rh(CNR)((CHCl)C(CH3)3)(H) (8b)
over the course of several days while heating to 80 °C.
Activation/Elimination of 1-Chlorocyclopentane. 1 (6 mg) was
placed in a resealable NMR tube. Roughly 0.3 mL of chlorocyclo-
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10751