N-Heterocyclic Carbene Complexes
1256 1266
red oil remained. The product was purified by column chromatography
on silica gel 60 with diethyl ether/pentane (1/2) as eluent (Rf =0.3). All
product fractions were pooled and evaporated to dryness to give the
product as an off-white powder (3.307 g, 10.40 mmol, 51%). Alternative-
ly, the product can be purified by repeated recrystallization from pentane
and diethyl ether/pentane. FTIR (ATR mode): n˜ =3358 (w), 2960 (w)
2912 (m), 2848 (m), 1610 (w), 1590 (w), 1480 (s), 1434 (s), 851 (s), 726
1,3-Bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium bromide (6): Com-
pound 5 (1.190 g, 6.986 mmol) was dissolved in absolute 1,2-dimethoxy-
ethane and treated with NBS (1.243 g, 6.986 mmol). The reaction mixture
was stirred at room temperature for 2 h, during which a yellow precipi-
tate formed. The volatile compounds were removed in vacuo and a
brown, oily residue remained. The pure product was obtained after
column chromatography on silica gel 60 with dichlormethane/ethanol (6/
1) as eluent. The product eluted in the second fraction (Rf =0.2). The
product fractions were combined and evaporated to dryness to yield
1.480 g of 6 as a brown powder (5.938 mmol, 85%). FTIR (ATR mode):
n˜ =3405 (w), 2964 (m), 2882 (m), 1671 (s), 1324 (m), 1123 cmÀ1 (m);
NMR (CDCl3): d=9.40 (s, 1H, N+=CH), 4.36 (hept, 2H, NCH,
1
(s), 689 cmÀ1 (s); H NMR (CDCl3): d=6.82 (s, 4H, aromatic H), 3.05 (t,
3
4H, 2îNCH2, J(H,H)=6.78 Hz), 3.0 2.8 (brs, 2H, NH), 2.26 (s, 12H, o-
CH3 of Mes), 2.23 (s, 6H, p-CH3 of Mes), 1.89 ppm (quint, 2H, CH2,
3J(H,H)=6.78 Hz); 13C NMR (CDCl3): d=143.5, 131.4, 129.7, 129.4 (all
aromatic C), 47.0 (2îNCH2), 32.5 (CH2), 20.5, 18.3 ppm (both CH3 of
Mes); GC-MS (EI) calcd for C21H30N2: m/z: 310.24; found: 310.25; ele-
mental analysis (%) calcd for C21H30N2: C 81.24, H 9.74, N 9.02; found:
C 81.14, H 9.69, N 9.01.
3
3J(H,H)=6.39), 3.31 (t, 4H, NCH2, J(H,H)=5.94), 2.05 (quint, 2H, CH2,
3J(H,H)=5.94 Hz), 1.27 ppm (d, 12H, CH3); 13C NMR (CDCl3): d=151.5
(N+=C), 56.0 (NCH), 38.2 (NCH2), 20.1 (CH3), 19.1 ppm (CH2).
Bromo(h4-1,5-cyclooctadiene){1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimi-
din-2-ylidene}rhodium (7): [{Rh(cod)Cl}2] (120 mg, 244 mmol) was dis-
solved in THF (5 mL), and lithium tert-butoxide (47 mg, 590 mmol) was
added with vigorous stirring. The mixture was stirred for a further 30 min
at room temperature, and then 6 (134 mg, 536 mmol) was added. The re-
action mixture was stirred overnight at 558C, after which time TLC
showed no further conversion. The solvent was removed in vacuo. The
product was purified by column chromatography on silica gel 60 with di-
chloromethane/ethanol (250/4) as mobile phase. The product eluted as a
yellow band in the second fraction (Rf =0.6). The product fractions were
1,3-Dimesitylhexahydropyrimidine
(2):
Compound
1
(1.308 g,
4.211 mmol) was dissolved in methanol and treated with 36.5% aqueous
formaldehyde solution (0.386 g, 4.63 mmol formaldehyde) diluted with
methanol (2 mL). The reaction mixture was stirred at 458C. A white pre-
cipitate formed during the reaction. Conversion was complete after 16 h,
as indicated by GC-MS. The precipitate was filtered off. Recrystallization
of the crude product from ethanol yielded 1.231 g of colorless crystals
(3.832 mmol, 91%). FTIR (ATR mode): n˜ =2944 (w), 2913 (w), 2822
(w), 1478 (s), 1393 (m), 1320 (m), 1248 (m), 1205 (s), 849 cmÀ1 (s);
1H NMR (CDCl3): d=6.81 (s, 4H, aromatic H), 4.23 (s, 2H, NCH2N),
3.21 (t, 4H, NCH2), 2.35 (s, 12H, o-CH3 of Mes), 2.23 (s, 6H, p-CH3 of
Mes), 1.86 ppm (m, 2H, CH2); 13C NMR (CDCl3): d=145.2, 136.8, 134.1,
129.4 (all aromatic C), 69.7 (NCH2N), 49.8 (NCH2), 27.9 (CH2), 20.6,
19.8 ppm (both CH3 of Mes); GC-MS (EI) calcd for C22H30N2: m/z:
322.24; found: 322.25.
pooled and evaporated to dryness to yield
a yellow solid (0.189 g,
411 mmol, 84%). Yellow crystals suitable for X-ray analysis were ob-
tained by layering pentane over a dilute solution of 7 in dichloromethane
at À368C. FTIR (ATR mode): n˜ =2963 (m), 2928 (m), 2873 (m), 2832
(m), 1506 (s), 1449 (m), 1363 (m), 1304 (s), 1162 (s), 1078 cmÀ1 (m);
1H NMR (CDCl3): d=6.35 (hept, 2H, NCH), 4.84 (m, 2H, cod CH), 3.37
(m, 2H, cod CH), 3.05 2.85 (m, 4H, NCH2), 2.26 (m, 4H, cod CH2),
1.90 1.65 (m, 6H, CH2 +cod CH2), 1.29 ppm (d, 12H, CH3); 13C NMR
(CDCl3): d=203.7 (d, NCN, 1J(103Rh,13C)=45 Hz,), 94.4 (d, cod CH,
1J(103Rh,13C)=6.5 Hz), 69.0 (d, cod CH, 1J(103Rh,13C)=15.5 Hz), 57.1,
37.8, 32.3, 29.0, 21.0, 20.3, 19.7 ppm (cod CH2, NCH2, CH2, NCH, CH3);
elemental analysis (%) calcd for C18H32BrN2Rh: C 47.07, H 7.02, N 6.10;
found: C 47.39, H 7.00, N 6.01.
Bromo(h4-1,5-cyclooctadiene)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-
2-ylidene)rhodium (8): [{Rh(cod)Cl}2] (120 mg, 244 mmol) was dissolved
in THF (4 mL), and lithium tert-butoxide (47 mg, 590 mmol) was added
with vigorous stirring. The mixture was stirred for a further 30 min at
room temperature, and then 3 (215 mg, 536 mmol) was added. After a
few minutes a clear, yellow solution formed, which was stirred for 5 h at
room temperature. After this time, TLC showed no further conversion.
The solvent was removed in vacuo. The product was purified by column
chromatography on silica gel 60 with dichloromethane/ethanol (250/8) as
mobile phase. 8 eluted as a yellow band in the second fraction (Rf =0.3).
The product fractions were pooled and evaporated to dryness to yield a
yellow solid in 73% yield. (0.219 g, 356 mmol). Yellow crystals suitable
for X-ray analysis were obtained by layering pentane over a dilute solu-
tion of 8 in dichloromethane at À368C. FTIR (ATR mode): n˜ =2879 (m),
1477 (s), 1433 (m), 1298 (s), 1198 (m), 851 cmÀ1 (m); 1H NMR (CDCl3):
d=6.97 (m, 4H, aromatic H), 4.32, 4.20 (2îbrs, 2H, cod CH), 3.3 3.1
(m, 6H, NCH2 +cod CH), 2.7 2.1 (m, 20H, CH2 +CH3 of Mes), 1.5
1.2 ppm (m, 8H, cod CH2); 13C NMR (CDCl3): d=211.1 (d, NCN,
1J(103Rh,13C)=45 Hz,), 141.8, 141.7, 137.3 (all aromatic C), 93.8 (d, cod
CH, 1J(103Rh,13C)=7.5 Hz), 93.4 (d, cod CH, 1J(103Rh,13C)=6.8 Hz), 67.7
(d, cod CH, 1J(103Rh,13C)=15.1 Hz), 66.9 (d, cod CH, 1J(103Rh,13C)=
15.1 Hz), 47.9, 47.6, 32.2, 32.2, 27.6, 27.4, 21.1, 21.0, 20.9 ppm (cod CH2,
NCH2, CH2, CH3 of Mes).
1,3-Dimesityl-3,4,5,6-tetrahydropyrimidin-1-ium bromide (3): Compound
2 (0.823 g, 2.56 mmol) was dissolved in absolute dimethoxyethane and
treated with N-bromosuccinimide (NBS, 0.455 g, 2.56 mmol). The reac-
tion mixture was stirred at room temperature for 3 h. A yellow precipi-
tate formed, which was filtered off, washed with pentane, and dried in
vacuo to yield 0.881 g (2.20 mmol, 86%) of 3 as a yellow powder. FTIR
(ATR mode): n˜ =2976 (w), 2914 (w), 1648 (s), 1479 (w), 1312 (s),
1209 cmÀ1 (s); 1H NMR (CDCl3): d=7.54 (s, 1H, N+=CH), 6.91 (s, 4H,
aromatic H), 4.18 (t, 4H, NCH2), 2.57 (m, 2H, CH2),), 2.39 (s, 12H, o-
CH3 of Mes), 2.24 ppm (s, 6H, p-CH3 of Mes); 13C NMR (CDCl3): d=
153.6 (N+=CH), 140.5, 136.4, 134.6, 130.1 (all aromatic C), 47.0 (NCH2),
21.0 (CH3 of Mes), 19.6 (CH2), 18.0 ppm (CH3 of Mes); elemental analy-
sis (%) calcd for C22H29BrN2: C 65.83, H 7.28, N 6.98; found: C 65.52, H
7.62, N 6.90.
1,3-Dimesityl-3,4,5,6-tetrahydropyrimidin-1-ium tetrafluoroborate (4):
Compound 3 (1.500 g, 3.737 mmol) was dissolved in ethanol and treated
with a solution of silver tetrafluoroborate (0.728 g, 3.737 mmol) in etha-
nol. Immediately, a yellow precipitate of silver bromide formed. After
stirring for a few minutes, the precipitate was filtered off and washed
with ethanol. Finally, the solution was concentrated to a few milliliters
and the product crystallized as 1.450 g of white crystals. (3.550 mmol,
95%). FTIR (ATR mode): n˜ =1657 (s), 1478 (w), 1316 (s), 1093 (s), 1051
(s); 1H NMR (CDCl3): d=7.48 (s, 1H, N+=CH), 6.93 (s, 4H, aromatic
H), 3.88 (t, 4H, NCH2), 2.54 (m, 2H, CH2), 2.28 (s, 12H, o-CH3 of Mes),
2.25 ppm (s, 6H, p-CH3 of Mes); 13C NMR (CDCl3): d=153.9 (N+=CH),
140.5, 136.3, 134.3, 130.1 (all aromatic C), 46.3 (NCH2), 20.9 (CH3 of
Mes), 19.2 (CH2), 17.4 ppm (CH3 of Mes); elemental analysis (%) calcd
for C22H29BF4N2: C 64.72, H 7.16, N 6.86; found: C 64.52, H 7.39, N 6.81.
1,3-Bis(2-propyl)hexahydropyrimidine (5): 1,3-Bis(2-propyl)propane-1,3-
diamine (1.162 g, 7.338 mmol) was dissolved in absolute methanol
(3 mL). 36.5% aqueous formaldehyde solution (0.734 g, 8.81 mmol form-
aldehyde) diluted with methanol (2 mL) was added, and the mixture was
stirred at 478C for 2 h. After that time the reaction was complete, as indi-
cated by GC-MS. The product was dried in vacuo to give a colorless
liquid in 95% yield (1.190 g, 6.71 mmol). 1H NMR (CDCl3): d=3.23 (s,
2H, NCH2N), 2.68 (hept, 2H, NCH, 3J(H,H)=6.84 Hz), 2.47 (t, 4H,
NCH2, 3J(H,H)=5.49 Hz), 1.60 (quint, 2H, CH2, 3J(H,H)=5.49),
0.98 ppm (d, 12H, CH3, 3J(H,H)=6.84 Hz); 13C NMR (CDCl3): d=70.4
(NCH2N), 52.3 (NCH2), 47.7 (NCH), 24.7 (CH2), 19.0 ppm (CH3); GC-
MS (EI) calcd for C10H22N2: m/z: 170.18, found: 170.20.
Chloro(h4-1,5-cyclooctadiene)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-
2-ylidene)rhodium (9): [{Rh(cod)Cl}2] (120 mg, 244 mmol) was dissolved
in THF (4 mL), and then lithium tert-butoxide (47 mg, 590 mmol) was
added with stirring. The mixture was stirred for a further 30 min at room
temperature, and then 4 (219 mg, 536 mmol) was added. A clear, yellow
solution formed immediately. The reaction mixture was stirred for 1 h at
538C, then the solvent was removed in vacuo. The product was obtained
after column chromatography on silica gel 60 with dichloromethane/
THF/ethanol (400/17/3) as solvent. The product eluted as a yellow band
in the second fraction (Rf =0.4). The product fractions were pooled and
1263
Chem. Eur. J. 2004, 10, 1256 1266
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim