Synthesis of anovel sterically hindered imidazolinium chloride: An N-heterocyclic carbene precursor

Article abstract of DOI:10.3184/174751912X13332884511554

A hindered imidazolinium chloride which is a bulky, strongly donating N-heterocyclic carbene precursor has been synthesised from pinacolone in six steps. The chlorides can be easily converted to its corresponding hexafluorophosphate salt in near quantitative yield. The two new compounds were characterised by ¹H and ¹³C NMR, IR, and elemental analyses.

Full text of DOI:10.3184/174751912X13332884511554

288 RESEARCH PAPER
MAY, 288–289
JOURNAL OF CHEMICAL RESEARCH 2012
Synthesis of anovel sterically hindered imidazolinium chloride:
an N-heterocyclic carbene precursor
Xiang Liua, Kaiqi Gea, Pei Guana, Pang Hea, Yunfei Lia, Yanhui Shia,b* and Changsheng Caoa
aSchool of Chemistry and Chemical Engineering and Jiangsu Key laboratory of Green Synthetic Chemistry for Functional Materials, Xuzhou
Normal University, Xuzhou, Jiangsu 221116, P. R. China
bState Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, Jiangsu 210093, P. R. China
A hindered imidazolinium chloride which is a bulky, strongly donating N-heterocyclic carbene precursor has been
synthesised from pinacolone in six steps. The chlorides can be easily converted to its corresponding hexafluorophos-
phate salt in near quantitative yield. The two new compounds were characterised by 1H and 13C NMR, IR, and elemen-
tal analyses.
Keywords: N-heterocyclic carbine, steric hindrance, imidazolinium chloride, imidazolinium hexafluorophosphate
The chemistry of N-heterocyclic carbenes (NHCs) and their
complexes has been thoroughly investigated in recent years
and a large number of publications relating to NHC complexes
have been published.1–5 Palladium complexes with NHC
ligands have become important in Pd-catalysed cross-coupling
reactions,6,7 which are extremely useful tools in several areas
of chemistry. It is known that sterically hindered and strongly
electron-donating ligands can form more active catalysts.8
Hence there is a need to develop bulky, strongly donating
ligands.9–10 In order to explore new efficient NHC ligands for
Pd catalysis, we have prepared two sterically hindered, strongly
donating N-heterocyclic carbene precursors ( imidazolinium
salts 6 and 7).
The imidazolinium chloride (6) was prepared from
pinacolone in an overall 30% yield by six steps (Scheme 1).
Firstly, 2,3,3-trimethylbutan-2-ol (1) was prepared by Grignard
reaction of pinacolone with methylmagnesium iodide which
was generated in situ from methyl iodide with magnesium in
anhydrous diethyl ether. The alcohol was colourless liquid;
however, when heated in presence of water it was converted to
a white solid hydrate (1·0.5H2O). The Ritter reaction of 1 with
acetonitrile in a strongly acidic medium gave the acetamide 2.
The total yield for these two steps was 72%. The hydrolysis of
amide 2 in aqueous KOH solution gave amine 3 in 69% yield.
The Schiff reaction of amine 3 with a 30% glyoxal aqueous
solution generated diimine 4 in high yield. The reduction of 4
with NaBH4 gave diamine 5 in 82% yield. The cycloadditon
reaction of 5 with NH4Cl in triethyl orthoformate gave the
imidazolinium chloride 6. The imidazolinium chloride 6 was
converted quantitatively to imidazolinium hexafluorophos-
phate 7 with NH4PF6 in water. Compounds 1-3 are known
compounds. However, they are not fully characterised in the
literature11. Therefore, their data along with that for compounds
4–7 which are new compounds is given in the experimental.
In conclusion, we have demonstrated a method for prepar-
ing two sterically hindered, strongly donating N-heterocyclic
carbene precursors (6 and 7) starting from pinacoline in
several steps in a total 30% yield. Compounds 6 and 7 are
new and are fully characterised by NMR spectroscopy, IR and
elemental analyses.
Experimental
All reagents were commercially available and were used without
further purification. 1H NMR spectra were recorded on a Bruker DPX
Scheme 1
* Correspondent. E-mail: yhshi_2000@126.com

JOURNAL OF CHEMICAL RESEARCH 2012 289
400 MHz spectrometer at room temperature and referenced to the
residual H signals of the solvent. IR spectra were recorded on KBr
which time, water (100 mL) was added to quench the reaction. The
organic layer was separated and the water layer was extracted with
DCM (3×15 mL). The combined organic phase was dried over MgSO4,
and the solvent was removed to give the product as a white solid in
82% yield (1.65 g). m.p. 68–69 ºC. IR (KBr): 3763, 3447, 2973, 1637,
1
pellets on a FTIR-Tensor 27 spectrometer. Melting points were
detected by microscope melting point apparatus. Elemental analyses
were performed on a EuroVektor Euro EA-300 elemental analyser.
2,3,3-Trimethylbutan-2-ol (1): Mg (41.1 mmol, 1.0 g) and anhy-
drous diethyl ether (20 mL) were added to a three-necked flash
equipped with a condenser and dropping funnel under argon. Methyl
iodide (2.6 mL, 41.1 mmol) was added dropwise to the reaction
mixture. The reaction was stirred at room temperature for 1 h, after
which time, pinacolone (34.3 mmol, 4.3 mL) was added dropwise.
The reaction was stirred at room temperature for another 5 h and
quenched with 1 N HCl. The organic phase was separated and the
water phase was extracted with ether three times (3 × 15 mL). The
combined organic phase was dried over MgSO4. The volatile solvent
was removed with a rotatory evaporator to give the product. The
product was used in the next step directly without further purification.
Its hydrate compound 1·0.5H2O could be sublimed to give the pure
compound as a white solid (3.73 g, 87%). Data for 1·0.5H2O: m.p.
53–54 °C. IR (KBr): 3444, 2982, 1637, 1558, 1474, 1367, 1131, 1022,
947, 885 cm–1. 1H NMR (400 MHz, CDCl3): 1.82 (s, 1H), 1.30 (s, 1H),
1.18 (s, 6H), 0.94 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 75.0, 37.4,
25.3, 25.2. Anal. Calcd for C7H17O1.5 (125.21 g mol–1): C, 67.15; H,
13.69; N, 19.17. Found: C, 66.86; H, 13.45; N, 19.42%.
N-(2,3,3-Trimethylbutan-2-yl)acetamide (2): Acetic acid (3 mL)
and concentrated sulfuric acid (1 mL) were added to an acetonitrile
(25 mL) solution of 1 (3.48 g , 30 mmol) in a 50 mL round bottom
flask. The reaction was stirred at room temperature for 12 h, after
which time, 10% aqueous KOH was added to the reaction mixture
until the pH reached 7–8. The organic phase was separated and the
water layer was extracted with ether (3 × 15 mL). The combined
organic phase was dried over MgSO4 and the volatile solvent was
removed. The crude white solid which was obtained, was washed with
petroleum ether to give the product in 83% yield (3.92 g). m.p.
115–116 °C [lit.11 115–116 °C]. IR (KBr): 3446, 2201, 3106, 3017,
2976, 1655, 1572, 1474, 1400, 1371, 1305, 1238, 1180, 1158, 1032,
998, 968, 758, 607 cm–1. 1H NMR (400 MHz, CDCl3): 5.31 (s,
1H), 1.90 (s, 3H), 1.34 (s, 6H), 0.92 (s, 9H). 13C NMR (100 MHz,
CDCl3): δ 169.4, 59.1, 37.5, 25.3, 25.1, 21.9. Anal. Calcd for C9H19NO
(157.25 g mol–1): C, 68.74; H, 12.18; N, 8.91. Found: C, 67.47; H,
11.97; N, 9.20%.
1
1559, 1485, 1394, 1362, 1176, 738, 672 cm–1. H NMR (400 MHz,
CDCl3): δ 2.62 (s, 4H), 0.98 (s, 12H), 0.91 (s, 18H). 13C NMR (100 MHz,
CDCl3): δ 56.4, 43.0, 37.4, 25.5, 21.2. Anal. Calcd for C16H36N2
(256.47 g mol–1): C, 74.93; H, 14.15; N, 10.92. Found: C, 75.25; H,
14.02; N, 10.79%.
1,3-Bis(2,3,3-trimethylbutan-2-yl)-4,5-dihydro-1H-imidazol-3-ium
chloride (6): Compound 5 (2.0 g, 7.8 mmol) and ammonium chloride
(0.42 g, 7.8 mmol) were added to a 50 mL round-bottomed flask with
triethyl orthoformate (25 mL). The reaction mixture was heated at
130ºC for 12 h, whilst a large amount of white solid appeared.
The solid was filtered and washed with ether to give the of product
(2.0 g) 85% yield. IR (KBr): 3736, 3677, 3630, 2970, 1635, 1457,
1
1385, 1365, 1304, 1232, 1163, 1023, 670 cm–1. H NMR (400 MHz,
CDCl3) 8.31 (s, 1H), 4.16 (s, 4H), 1.51 (s, 12H), 0.97 (s, 18H). 13C
NMR (100 MHz, CDCl3) 156.7, 65.2, 49.2, 38.6, 26.2, 23.6. Anal.
Calcd for C17H35ClN2 (302.93 g mol–1): C, 67.40; H, 11.65; N, 9.25.
Found: C, 67.03; H, 11.46; N, 9.51%.
1,3-Bis(2,3,3-trimethylbutan-2-yl)-4,5-dihydro-1H-imidazol-3-ium
hexafluorophosphate (7): Compound 6 (1.03 g, 2.0 mmol ) dissolved
into water (6 mL) was added drop-wise into a saturated NH4PF6
(30 mL) aqueous solution in a 50 mL beaker. A large amount of a
white solid appeared. The solid was filtered and washed with water to
give the product as a white solid in 99% yield (1.46 g). IR (KBr):
3763, 3691, 3677, 3650, 3630, 3112, 2975, 1619, 1542, 1508, 1475,
1
1390, 1284, 1205, 1154, 859, 833, 670 cm–1. H NMR (400 MHz,
CDCl3) 7.49 (s, 1H), 3.94 (s, 4H), 1.16 (s, 12H), 1.01 (s, 18H). 13C
NMR (100 MHz, CDCl3): δ 144.5, 65.0, 48.9, 38.7, 26.1, 22.5; Anal.
Calcd for C17H35F6N2P (412.44 g mol–1): C, 49.51; H, 8.55; N, 6.79.
Found: C, 49.21; H, 8.38; N, 6.90%.
We gratefully acknowledge Qing Lan Project of Jiangsu
Education Committee (08QLT001 and 08QLD006), Scientific
Research Foundation (SRF) for the Returned Overseas
Chinese Scholars (ROCS), State Education Ministry (SEM),
the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD), 333 project for the
cultivation of high-level talents (33GC10002) and National
Natural Science Foundation of China (NSFC) (21071121 and
21172188) for financial support of this work.
2,3,3-Trimethylbutan-2-amine (3): Compound 2 (1.0 g, 6.4 mmol)
in a 23% aqueous KOH solution (20 mL) was heated in an autoclave
at 200 ºC for 24 h. After the reaction the mixture was cooled to room
temperature and extracted with ether (3 × 15 mL). The product was
obtained as a light yellow oil by evaporation of the solvent in 67%
yield (0.49 g). IR (KBr): 3799, 3742, 3732, 3686, 3668, 3646, 3626,
3363, 2972, 1645, 1602, 1557, 1471, 1372, 1232, 1158, 1093, 1051,
Received 3 February 2012; accepted 23 February 2012
Paper 1201145 doi: 10.3184/174751912X13332884511554
Published online: 10 May 2012
1
982, 876, 828, 792, 651 cm –1. H NMR (400 MHz, CDCl3): 1.56 (s,
2H), 1.03 (s, 6H), 0.89 (s, 9H). 13C NMR (100 MHz, CDCl3): δ 53.8,
36.7, 26.2, 25.4. Anal. Calcd for C7H17N (115.22 g mol–1): C, 72.97;
H, 14.87; N, 12.16. Found: C, 73.31; H, 14.63; N, 12.28%.
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