Abnormal C4-Bonding in N-Heterocyclic Carbene Complexes
FULL PAPER
Synthesis 4a: Pd
(OAc)2 (0.19 g, 0.83 mmol) and excess of KCl (0.31 g,
Celite and the volatiles were removed in vacuo to give 7c as a red oil
(1.57 g, 85%). Microanalytically pure material formed upon crystalliza-
tion of 7c from MeOH/Et2O. 1H NMR (360 MHz, [D6]DMSO): d=7.52
4.15 mmol) was added to a solution of 3c (0.34 g, 0.83 mmol) in DMSO
(6 mL). The reaction mixture was heated at 508C for 2 h and then at
1208C for 3 h. After the mixture had been cooled to room temperature,
CH2Cl2 (20 mL) and Et2O (80 mL) were added. A precipitate appeared
which was collected and washed repeatedly by adding CH2Cl2 (20 mL)
followed by precipitation with Et2O (80 mL). After drying in vacuo, 4a
was obtained as an off-white powder (0.28 g, 83%). An analytically pure
sample was obtained by recrystallization of 4a from CH3NO2/Et2O.
1H NMR (500 MHz, [D6]DMSO): d=6.04, 5.85 (2ꢅAB, 2JHH =13.8 Hz,
2H, CH2), 3.78 (s, 6H, NCH3), 2.23, 2.09 ppm (2ꢅs, 12H, Cimi-CH3);
13C{1H} NMR (125 MHz, [D6]DMSO): d=155.8 (C-Pd), 125.9, 124.3 (2ꢅ
Cimi), 57.1 (CH2), 40.4 (NCH3), 8.4, 8.2 ppm (2ꢅCimi-CH3); elemental
analysis calcd (%) for C13H20Cl2N4Pd (409.65): C 38.12, H 4.92, N 13.68;
found: C: 38.08, H 4.93, N 13.54.
(s, 2H, Himi), 6.50 (s, 2H, CH2), 3.59 (s, 6H, NCH3), 2.71 (s, 6H, Cimi
-
CH3), 2.24 ppm (s, 6H, Cimi-CH3); 13C{1H} NMR (125 MHz, [D6]DMSO):
d=145.7, 130.9 (2ꢅCimi), 117.4 (Cimi-H), 56.2 (CH2), 31.9 (NCH3), 10.2,
8.9 ppm (2ꢅCimi-CH3); elemental analysis calcd (%) for C13H22B2F8N4-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
tion of 7a (0.56 g, 1.84 mmol) in DMSO (6 mL). The reaction mixture
was stirred at 508C for 2 h and then at 1208C for 3 h. After the mixture
had been cooled to room temperature, CH2Cl2 (20 mL) and Et2O
(80 mL) were added to give a precipitate which was collected and
washed repeatedly by adding CH2Cl2 (20 mL) followed by precipitation
with Et2O (80 mL) and dried in vacuo. This afforded 8a as a yellow
powder (0.49 g, 65%). An analytically pure sample of 8a was obtained
by recrystallization from CH3NO2/Et2O. 1H NMR (500 MHz,
Synthesis of 4b: The method was analogous to the one described for the
synthesis of 4a except for the KCl, which was omitted. Starting from Pd-
2
A
[D6]DMSO): d=6.33, 5.93 (2ꢅAB, JHH =12.9 Hz, 2H, CH2), 3.50 (s, 6H,
an off-white solid (0.64 g 55%). An analytically pure sample was ob-
NCH3), 2.63 (s, 6H, Cimi-CH3), 2.31 (s, 3H, Cimi-CH3), 2.21 ppm (s, 3H,
tained by recrystallization of 4b from DMSO/Et2O. 1H NMR (500 MHz,
C
imi-CH3); 13C{1H} NMR (125 MHz, [D6]DMSO): d=145.6, 130.7 (2ꢅ
2
[D6]DMSO): d=6.06, 5.91 (2ꢅAB, JHH =14.2 Hz, 2H, CH2), 3.69 (s, 6H,
Cimi), 117.4 (C-Pd), 56.3 (CH2), 32.2 (NCH3), 10.8, 9.1 ppm (2ꢅCimi-CH3);
elemental analysis calcd (%) for C13H20Cl2N4Pd(409.65)·H2O·CH3NO2: C
NCH3), 2.24, 2.09ppm (2ꢅs, 12H, Cimi-CH3); 13C{1H} NMR (125 MHz,
[D6]DMSO): d=161.1 (C-Pd), 126.0, 124.7 (2ꢅCimi), 57.3 (CH2), 40.4
(NCH3), 8.5, 8.2 ppm (2ꢅCimi-CH3); elemental analysis calcd (%) for
C13H20I2N4Pd (592.55)·0.25 Et2O: C 28.51, H 4.20, N 8.99; found: C:
28.20, H 3.93, N 8.81.
ACHTUNGTRENNUNG
35.39, H 5.17, N 13.76; found: C: 35.14, H 4.91, N 13.51.
Synthesis of 8b: Excess NBu4I (0.52 g, 1.41 mmol) was added to a solu-
tion of 12 (0.21 g, 0.35 mmol) in MeCN (20 mL) and the mixture was
stirred at room temperature for 1 h. Addition of Et2O (80 mL) induced
the formation of a red precipitate, which was washed repeatedly by
adding MeCN (20 mL) followed by precipitation with Et2O (80 mL).
After drying in vacuo, 8b was obtained as a reddish solid (0.16 g, 77%).
An analytically pure sample was obtained by recrystallization of 8b from
DMSO/Et2O. 1H NMR (500 MHz, [D6]DMSO): d=6.32, 5.96 (2ꢅAB,
2JHH =13.5 Hz, 2H, CH2), 3.46 (s, 6H, NCH3), 2.60, 2.19 ppm (2ꢅs, 6H,
Cimi-CH3); 13C{1H} NMR (125 MHz, [D6]DMSO): d=145.3, 131.6 (2ꢅ
Cimi), 123.3 (C-Pd), 60.3 (CH2), 32.1 (NCH3), 15.6 (C5imi-CH3), 11.6
(C5’imi-CH3), 10.5 ppm (Cimi-CH3); the two signals at dC =15.6 and
11.6 ppm coalesce to a singlet at 338 K (dC =13.0 ppm); elemental analy-
Synthesis of 6: A mixture of acetic anhydride (2.5 g, 25 mmol) and 2,4 di-
methylimidazole 5 (0.93 g, 10 mmol) in benzene (10 mL) was stirred at
808C for 2 h. Then MeI (0.6 mL, 9.6 mmol) in benzene (6 mL) was added
and the mixture was stirred at 1408C in a sealed tube. After 16 h, the re-
action was quenched with water and aqueous KOH (1m, 100 mL) was
added until pH
> 10. The product was extracted with CH2Cl2 (3ꢅ
100 mL). The combined organic phases were washed with aqueous KOH
(1m, 100 mL), H2O (2ꢅ100 mL) and brine (2ꢅ100 mL), dried over
Na2SO4, and evaporated to dryness, thus affording 6 as a colorless oil
(0.70 g, 64%). An analytically pure sample was obtained from the corre-
sponding HCl salt upon recrystallization from MeOH/Et2O. 1H NMR
(500 MHz, CDCl3): d=6.60 (s, 1H, Himi), 3.38 (s, 3H, NCH3), 2.34,
2.14 ppm (2ꢅs, 6H, Cimi-CH3); 13C{1H} NMR (125 MHz, CDCl3): d144.3,
sis calcd (%) for C13H20I2N4Pd
8.87; found: C: 26.30, H 3.66, N 8.67.
Synthesis of 9: Pd(OAc)2 (0.50 g, 2.23 mmol) was added to a solution of
ACHTUNGTRNE(NUNG 592.55)·0.5 DMSO: C 26.62, H 3.67, N
AHCTUNGTRENNUNG
127.4 (2ꢅCimi), 116.7 (Cimi-H), 29.8 (NCH3), 13.63, 9.92 ppm (2ꢅCimi
-
7c (0.91 g, 2.23 mmol) and KCl (0.83 g, 11.1 mmol) in DMSO (6 mL),
and the reaction mixture was stirred at 508C for 2 h and then at 1208C
for 3 h. After the mixture had been cooled to room temperature, CH2Cl2
(20 mL) and Et2O (80 mL) were added and the formed precipitate was
collected. Repeated precipitation from CH2Cl2 (20 mL) and Et2O
(80 mL) and subsequent drying yielded 9 as an off-white powder (0.73 g,
89%). Analytically pure material was obtained by recrystallization from
CH3NO2/Et2O. 1H NMR (500 MHz, [D6]DMSO): d=6.33, 5.94 (2ꢅAB,
2JHH =13.2 Hz, 2H, CH2), 3.50 (s, 6H, NCH3), 2.63 (s, 6H, Cimi-CH3), 2.31
(s, 3H, Cimi-CH3), 2.21 ppm (s, 3H, Cimi-CH3); 13C{1H} NMR (100 MHz,
[D6]DMSO): d=141.8, 141.7, 131.2, 130.5, 129.5 (5ꢅCimi), 126.8 (br, Cimi),
59.6 (CH2), 31.5 (NCH3), 11.2, 10.0 ppm (2ꢅCimi-CH3); elemental analysis
CH3); elemental analysis calcd (%) for C6H11ClN2 (146.62)·0.5 H2O: C
46.31, H 7.77, N 18.00; found: C: 46.26, H 7.13, N 17.29.
Synthesis of 7a: A solution of 6 (1.1 g, 10.0 mmol) in CH2Cl2 (5 mL) was
stirred in a sealed tube at 1508C under microwave irradiation for 30 min.
A red oil formed, which was separated by precipitation with MeOH
(20 mL) and Et2O (80 mL). The residue was washed once more with
MeOH (20 mL) and Et2O (80 mL) to give 7a (1.15 g, 75%) as a highly
viscous oil. Attempts to crystallize 7a under various conditions for micro-
analysis have failed thus far. 1H NMR (500 MHz, [D6]DMSO): d=7.85
(s, 2H, Himi), 6.90 (s, 2H, CH2), 3.66 (s, 6H, NCH3), 2.83, 2.25 ppm (2ꢅs,
12H, Cimi-CH3); 13C{1H} NMR (125 MHz, [D6]DMSO): d=145.8, 130.7
(2ꢅCimi), 117.6 (Cimi-H), 56.4 (CH2), 32.2 (NCH3), 10.9, 9.2 ppm (2ꢅCimi
-
calcd (%) for C26H48B2Cl2F8N8Pd2ACTHNGUTRENNU(G 922.00)·0.25 Et2O: C 34.48, H 4.55, N
CH3).
11.91; found: C: 34.30, H 4.91, N 11.72.
Synthesis of 7b: Neat CH2I2 (1.9 g, 7.2 mmol) was added to 6 (1.40 g,
12.7 mmol) and the mixture was stirred at 1408C for 16 h. The formed
brown oil was dissolved in MeOH (20 mL). Upon addition of Et2O
(80 mL), crude 7b separated as an reddish oil, which was collected by de-
cantation and dried in vacuo (2.2 g, 63%). Recrystallization from
MeOH/Et2O gave 7b as an analytically pure solid. 1H NMR (500 MHz,
[D6]DMSO): d=7.61 (s, 2H, Himi), 6.57 (s, 2H, CH2), 3.66 (s, 6H, NCH3),
Synthesis of 10: A suspension of 4b (0.10 g, 0.17 mmol) and AgBF4
(0.099 g, 0.51 mmol) in MeCN (20 mL) was stirred for 18 h in the absence
of light. The suspension was filtered through Celite and the volatiles
were evaporated, yielding 10 as an orange solid (0.10 g, 99%). Recrystal-
lization of 10 from MeCN/Et2O gave an analytically pure sample.
1H NMR (500 MHz, [D6]DMSO): d=6.24, 5.92 (2ꢅAB, 2JHH =14.0 Hz,
2H, CH2), 3.70 (s, 6H, NCH3), 2.26, 2.12 ppm (2ꢅs, 6H, Cimi-CH3);
13C{1H} NMR (100 MHz, [D6]DMSO): d=144.5 (C-Pd), 127.0, 126.0 (2ꢅ
Cimi), 57.0 (CH2), 34.5 (NCH3), 8.3, 8.3 ppm (2ꢅCimi-CH3); elemental
2.76, 2.26 ppm (2ꢅs, 12H,
C
imi-CH3); 13C{1H} NMR (125 MHz,
[D6]DMSO): d=145.6, 130.7 (2ꢅCimi), 117.4 (Cimi-H), 56.3 (CH2), 32.2
(NCH3), 10.8, 9.1 ppm (2ꢅCimi-CH3); elemental analysis calcd (%) for
C13H22I2N4 (488.15): C 31.99, H 4.54, N 11.48; found: C: 32.00, H 4.60, N
11.48.
analysis calcd (%) for C17H26B2F8N6PdACHTNUTRGNEUNG(594.46)·0.25 Et2O: C 35.27, H
4.69, N 13.71; found: C: 35.07, H 4.69, N 13.43.
Synthesis of 11: A suspension of 8a (0.15 g, 0.36 mmol) and AgBF4
(0.24 g, 1.10 mmol) in MeCN (20 mL) was stirred for 18 h at room tem-
perature in the dark. The suspension was filtered through Celite and the
volatiles were removed to leave 11 as an orange oil (0.31 g, 94%), which
Synthesis of 7c: A suspension of 7a (2.21 g, 4.53 mmol) and AgBF4
(1.86 g, 9.55 mmol) in MeCN (20 mL) was stirred at room temperature
for 16 h in the absence of light. The suspension was then filtered through
Chem. Eur. J. 2009, 15, 9375 – 9386
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9383