Direct synthesis of N-phosphanyl-heterocyclic carbenes

Article abstract of DOI:10.1016/j.tetlet.2011.11.054

A direct method for the synthesis of N-phosphanyl-heterocyclic carbenes is described. The method is based on the reaction of lithium imidazolides and benzimidazolides having a bulky alkyl group at the nitrogen atom with di(tert-butyl)chlorophosphine leadi

Full text of DOI:10.1016/j.tetlet.2011.11.054

Tetrahedron Letters 53 (2012) 494–496
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Direct synthesis of N-phosphanyl-heterocyclic carbenes
Anatolii P. Marchenko a, Heorgii N. Koidan a, Igor I. Pervak a, Anastasiia N. Huryeva a,
Evgeniy V. Zarudnitskii a, Andrei A. Tolmachev b,c, Aleksandr N. Kostyuk a,
⇑
a Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanska Str. 5, 02660 Kyiv-94, Ukraine
b Enamine Ltd, Vul. Oleksandra Matrosova 23, 01103 Kyiv, Ukraine
c Department of Chemistry, Kyiv National Taras Shevchenko University, Vul. Volodymyrska 64, 01033 Kyiv, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
A direct method for the synthesis of N-phosphanyl-heterocyclic carbenes is described. The method is
based on the reaction of lithium imidazolides and benzimidazolides having a bulky alkyl group at the
nitrogen atom with di(tert-butyl)chlorophosphine leading directly to the carbenes.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 1 September 2011
Revised 11 October 2011
Accepted 11 November 2011
Available online 22 November 2011
Keywords:
Lithium imidazolides
Lithium benzimidazolides
N-Phosphanyl-heterocyclic carbenes
Di(tert-butyl)chlorophosphine
Organolithiums prepared by direct deprotonation of acidic
hydrogen using strong bases are excellent reagents for introducing
a heterocyclic moiety into a complex structure.1,2 Lithium deriva-
tives of imidazole and benzimidazole play an important role in
the synthesis of their various phosphorylated derivatives.3 Also,
their structures have been the subject of numerous theoretical
and practical studies. It was found that lithium 1-methyl-(4-tert-
butyl)imidazolide has carbene rather than carbanion nature. The
13C NMR chemical shift of the C2 atom of lithium 1-methyl-(4-
tert-butyl)imidazolide is found downfield (13C d = 195) and the
X-ray structural study supports quite strongly the carbene nature
of lithium imidazolides.3,4
At the same time lithium N-methylimidazolide having an even
more pronounced downfield shift of the C2 atom (13C d = 201.6) re-
acts, for example, with t-Bu2PCl, similar to a carbanion affording 2-
phosphorylated imidazole,5 but not the corresponding carbene
which is a stable compound as described earlier by us.6
from the lithium atom and rendering more carbene character to
the molecule.
We found that lithium imidazolides 2a–d bearing a bulky
N-bound tert-butyl or adamantyl group reacted with di(tert-butyl)
chlorophosphine to afford the corresponding carbenes 3a–d
(Scheme 1).7
On going to less sterically hindered imidazole 1e bearing a 2,6-
diethylphenyl group the same reaction with di(tert-butyl)chloro-
phosphine led to phosphine 4e and not carbene 3e.8 This difference
in activity is not due to thermal instability of the carbene 3e, as the
latter is easily obtained by the traditional method, that is, the ac-
tion of sodium hexamethyldisilazide on the corresponding salt 5.6,9
In the 13C NMR spectra the chemical shifts of the carbene C2
atom occurred in the typical carbene range: d 205 (2b), 234 (3a),
234 (3b), 223 (3c), 222 (3d), and 225 (3e). Carbenes 3a–e were
crystalline solids, sensitive to air, and distillable (for 3b–d) in va-
cuo. Nevertheless, on heating over 150 °C they, like carbene 3
(R = Me, n = 0) prepared earlier by us,6 were transformed into
2-phosphorylated imidazoles 4a–e.10
It should be noted that the yields of the benzimidazol-2-yli-
denes 3a,b were in the range of 50% which is due to the poor
stability of lithium benzimidazolides and their proclivity to
ring-opening.4
In conclusion, we have reported a new method for the synthesis
of N-phosphorylated carbenes circumventing the laborious stage
of synthesis of the corresponding N-phosphorylated imidazolium
salts. The method is promising and requires further theoretical
and practical study.
The difference in chemical shifts of the C2 carbon in the 13C
NMR spectra of the two lithium imidazolides—N-methylimidazo-
lide and N-methyl-4-(tert-butyl)imidazolide—is probably due to
the steric effect of the tert-butyl group which shields the adjacent
nitrogen atom from lithium and imparts more anionic character
to the molecule. It can be assumed that in N-tert-butylimidazolide
the same effect from the tert-butyl group is extended to the sec-
ond carbon atom, thus shielding the nitrogen to a greater extent
⇑
Corresponding author. Tel.: +380 44 5373218; fax: +380 44 5373253.
E-mail address: a.kostyuk@enamine.net (A.N. Kostyuk).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.054

A. P. Marchenko et al. / Tetrahedron Letters 53 (2012) 494–496
495
-
P(Bu-
Et
t
)
P(Bu-
t
)
2
Li
TfO
2
N
N
N
N
(CH)n
(t-Bu)2PCl
(CH)n
(CH)n
(Me3Si)2NNa
n-BuLi
+
:
:
0-5 °C
N
N
R
N
R
N
R
Et2O
Et
3a-e
2a-e
1a-e
for 2e
150 °C
(CH)n
5
(t-Bu)2PCl
4e
N
(a) R = Ad, n = 4
(b) R = t-Bu, n = 4
(c) R = Ad, n = 0
(d) R = t-Bu, n = 0
P(Bu- )
t
2
N
R
4a-e
(e) R = 2,6-diethylphenyl, n = 0
Scheme 1. Synthesis of carbenes 3.
72.79; H, 10.18; N, 8.08; P, 8.94]; 1H NMR (300 MHz, C6D6): d 7.07 (dd, 1H,
J1 = 1.5 Hz, J2 = 4.5 Hz, CHIm), 6.73 (m, 1H, CHIm), 2.14 (m, 6H, CH2-Ad), 1.98
(br s, 3H, CH-Ad), 1.56 (m, 6H, CH2-Ad), 1.37 (d, 18H, J = 12.3 Hz, t-Bu); 13C
NMR (125 MHz, C6D6): d 222.0, 126.4 (d, J = 33 Hz), 113.1 (d, J = 7.5 Hz), 55.9,
44.4, 36.2, 34.6 (d, J = 24 Hz), 29.9, 29.1 (d, J = 16 Hz); 31P NMR (81 MHz, C6D6):
d 97.9.
References and notes
1. (a) Wakefield, B. J. Organolithium Methods; Academic Press: London, 1988; (b)
Brandsma, L. Preparative Polar Organometallic Chemistry; Springer: Berlin, 1990;
(c) Rewcastle, G. W.; Katritzky, A. R. Adv. Heterocycl. Chem. 1993, 56, 155; (d)
Clayden, J. In Organolithiums: Selectivity for Synthesis, Tetrahedron Organic
Chemistry Series; Baldwin, J. E., Williams, R. M., Eds.; Pergamon: Oxford, 2002;
Vol. 23,.
2. (a) Snieckus, V. Chem. Rev. 1990, 90, 879; (b) Mongin, F.; Quéguiner, G.
Tetrahedron 2001, 57, 4059; (c) Turck, A.; Plé, N.; Monguin, F.; Quéguiner, G.
Tetrahedron 2001, 57, 4489.
3. Hilf, C.; Bosold, F.; Harms, K.; Marsch, M.; Boche, G. Chem. Ber. 1997, 130, 1213–
1221. and references cited therein.
4. Hilf, C.; Bosold, F.; Harms, K.; Lohrenz, J. C. W.; Marsch, M.; Schimeczek, M.;
Boche, G. Chem. Ber. 1997, 130, 1201–1212.
1-tert-Butyl-3-[di(tert-butyl)phosphanyl]imidazol-2-ylidene
(3d):
Yield
(1.15 g, 86%); bp 85 °C/0.05 Torr; [Found: C, 67.39; H, 10.1; N, 10.13; P,
11.27. C15H29N2P requires C, 67.13; H, 10.89; N, 10.44; P, 11.54]; 1H NMR
(500 MHz, C6D6): d 7.05 (dd, J1 = 1.5 Hz, J2 = 4.5 Hz, CH), 6.75 (d, J = 1.2 Hz, CH),
1.49 (s, 9H, t-Bu), 1.35 (d, 18H, J = 12.3 Hz, t-Bu); 13C NMR (125 MHz, C6D6): d
222.7 (br s), 126.8 (d, J = 32.7 Hz), 113.7 (d, J = 6.3 Hz), 55.6, 34.6 (d, J = 24 Hz),
31.0, 29.0 (d, J = 16 Hz); 31P NMR (81 MHz, C6D6): d 98.4.
8. 2-[Di(tert-butyl)phosphanyl]-1-(2,6-diethylphenyl)-1H-imidazole (4e). To
a
solution of 1-(2,6-diethylphenyl)-1H-imidazole (1.0 g, 5 mmol) in THF
(15 mL) at À45 °C, n-BuLi (2.5 M solution in hexane 2 mL, 5 mmol) was
added dropwise over 2 min. The reaction mixture was stirred for 30 min at
À40 °C then heated to room temperature and stirred for another 30 min. The
mixture was cooled to À60 °C and a solution of di(tert-butyl)chlorophosphine
(0.9 g, 5 mmol) in hexane (7 mL) was added. The mixture was allowed to warm
to rt and stirred for 30 min then H2O (2 mL) was added. The organic layer was
separated, and the aqueous layer extracted with benzene. The combined
organic layers were dried over Na2SO4 and evaporated in vacuo. The residue
was distilled, bp 130 °C/0.05 Torr. Yield (1.50 g, 88%), mp 71–72 °C (pentane).
[Found; C, 73.97; H, 9.31; N, 7.90; P, 9.43. C21H33N2P requires C, 73.22; H, 9.66;
N, 8.13; P, 8.99]; 1H NMR (300 MHz, C6D6): d 7.42 (s, 1H, CHIm), 7.36 (t, 1H,
J = 7.5 Hz, CHAr), 7.19 (d, 2H, J = 7.5 Hz, CHAr), 6.99–7.01 (m, 1H, CHIm), 2.37
(m, 2H, Et), 2.18 (m, 2H, Et), 1.19 (d, 18H, J = 15.3 Hz, t-Bu), 1.12 (t, 6H,
J = 7.5 Hz, Et); 13C NMR (125 MHz, C6D6): d 141.6, 136.1, 135.5 (d, J = 88 Hz),
129.3, 128.7 (d, J = 12.8 Hz), 127.0, 125.8, 40.7 (d, J = 35 Hz), 28.5 (d, J = 2.5 Hz),
24.7, 15.1; 31P NMR (81 MHz, C6D6): d 11.4.
5. Grotjahn, D. B.; Gong, Yi; Zakharov, L.; Golen, J. A.; Rheingold, A. L. J. Am. Chem.
Soc. 2006, 128, 438.
6. Marchenko, A. P.; Koidan, H. N.; Huryeva, A. N.; Zarudnitskii, E. V.; Yurchenko,
A. A.; Kostyuk, A. N. J. Org. Chem. 2010, 75, 7141–7145.
7. 1-(1-Adamantyl)-3-[di(tert-butyl)phosphanyl]benzimidazol-2-ylidene (3a): To
a
solution of 1-(tert-butyl)benzimidazole or 1-(1-adamantyl)benzimidazole
(1a) (5 mmol) in THF (15 mL) at À25 °C, n-BuLi (2.5 M solution in hexane 2 mL,
5 mmol) was added dropwise over 1 min. The reaction mixture was stirred for
1 h until the temperature had increased to À15 °C, then the brown solution was
cooled to À70 °C and
a solution of di(tert-butyl)chlorophosphine (0.9 g,
5 mmol) in hexane (7 mL) was added. After warming to 15 °C, the mixture
was stirred overnight (for 3a) or for 4 days (for 3b). The solvents were removed
in vacuo, the residue was rinsed with hot pentane (30 mL) and the solid residue
filtered and washed with pentane (2 Â 20 mL). The filtrate was concentrated in
vacuo, and the residue was distilled (for 3b). In the case of 3a the filtrate was
concentrated to 8 mL and cooled to À18 °C for 24 h to afford crystals. Yield
(1.05 g, 54%), mp 126–128 °C (pentane); [Found: C, 76.48; H, 9.12; N, 7.16; P,
7.65. C25H37N2P requires C, 75.72; H, 9.40; N, 7.06; P, 7.81]; 1H NMR (500 MHz,
C6D6): d 8.05 (d, 1H, J = 9 Hz, CH), 7.56 (d, 1H, J = 8.5 Hz, CH), 7.01 (t, 1H,
J = 7.5 Hz, CH), 7.02 (t, 1H, J = 7.5 Hz, CH), 2.48 (s, 6H, CH2-Ad), 1.62 (br s, 6H,
CH2-Ad), 1.83 (br s, 3H, CH-Ad), 1.36 (d, 18H, J = 12.5 Hz, t-Bu); 13C NMR
(125 MHz, C6D6): d 233.8 (d, J = 4 Hz), 143.9 (d, J = 24 Hz), 132.6 (d, J = 2.5 Hz),
128.2, 120.8 (d, J = 75.5 Hz), 113.6, 113.2 (d, J = 16 Hz), 58.3, 43.2, 36.5, 35.0 (d,
J = 24 Hz), 30.0, 29.1 (d, J = 16 Hz); 31P NMR (81 MHz, C6D6): d 77.1.
1-tert-Butyl-3-[di(tert-butyl)phosphanyl]benzimidazol-2-ylidene (3b): Yield
(920 mg, 57%); bp 110 °C/0.05 Torr; mp 62–64 °C (pentane); [Found: C,
72.43; H, 9.03; N, 8.47; P, 9.37. C19H31N2P requires C, 71.66; H, 9.81; N, 8.80;
P, 9.73]; 1H NMR (300 MHz, C6D6): d 8.02 (d, 1H, J = 7.5 Hz, CHAr), 7.36 (d, 1H,
J = 7.8 Hz, CHAr), 7.08 (m, 1H, CHAr), 6.99 (d, 1H, CHAr), 1.66 (s, 9H, t-Bu), 1.34
(d, 18H, J = 12.6 Hz, t-Bu); 13C NMR (125 MHz, C6D6): 234.1 (d, J = 4 Hz), 143.9
(d, J = 24 Hz), 132.7 (d, J = 2.5 Hz), 121.1, 120.7, 113.2, 113.1 (d, J = 17.6 Hz),
57.3, 34.9 (d, J = 24 Hz), 30.2, 29.1 (d, J = 15 Hz); d 31P NMR (81 MHz, C6D6): d
77.9.
9. 3-[Di(tert-butyl)phosphanyl]-1-(2,6-diethylphenyl)-1H-imidazol-3-ium
triflate (5). To a mixture of 1-(2,6-diethylphenyl)imidazole (1.09 g, 5.45 mmol)
and sodium trifluoromethansulfonate (dried at 200 °C at 0.05 Torr for 2 h)
(2.46 g, 10.9 mmol),
a solution of di(tert-butyl)bromophosphine (1.23 g,
5.45 mmol) in THF (50 mL) was added. The reaction mixture was stirred for
12 h at room temperature (18 °C). The solvent was removed in vacuo (on
heating to 30–35 °C), and the residue was washed with pentane (2 Â 20 mL)
and filtered. The solid residue was dried at 30–35 °C in vacuo (0.05 Torr) and
then rinsed with CH2Cl2. The filtrate was concentrated in vacuo, the residue
rinsed again with Et2O, and the solid residue dried at 30–35 °C in vacuo
(0.05 Torr). Yield (1.27 g, 47%), mp 125–127 °C. [Found: C, 54.1; H, 6.62; N,
5.36; P, 6.75. C22H34F3N2O3PS requires C, 53.43; H, 6.93; N, 5.66; P, 6.26]; 1H
NMR (300 MHz, CDCl3): d 8.52 (br s, 1H), 8.07 (br s, 1H), 7.67 (br s, 1H), 7.44 (t,
1H, J = 7.8 Hz, CHAr), 7.22 (d, 2H, J = 7.8 Hz, CHAr), 2.21–2.29 (m, 4H, Et), 1.28
(d, 18H, J = 13.5 Hz), 1.08 (t, 6H, J = 7.5 Hz); 13C NMR (125 MHz, CDCl3): d 142.0
(d, J = 24 Hz), 140.5, 131.7, 131.5, 127.9, 127.4, 126.8, 123.3 (q, J = 319 Hz, CF3),
35.3 (d, J = 30 Hz), 28.4 (d, J = 15 Hz), 24.0, 14.7; 31P NMR (81 MHz, CDCl3): d
121.9.
1-(1-Adamantyl)-3-[di(tert-butyl)phosphanyl]imidazol-2-ylidene (3c): To
a
3-[Di(tert-butyl)phosphanyl]-1-(2,6-diethylphenyl)imidazol-2-ylidene (3e). To
a suspension of salt 5 (1.18 g, 2.4 mmol) in Et2O (7 mL) at 0 °C was added
solution of 1-(tert-butyl)imidazole or 1-(1-adamantyl)imidazole (5 mmol) in
THF (10 mL) at À45 °C, n-BuLi (2.5 M solution in hexane 2 mL, 5 mmol) was
added dropwise over 2 min. The reaction mixture was stirred until the
temperature had increased to À40 °C, then the light yellow solution was
dropwise over 2 min
a solution of sodium hexamethyldisilazide (430 mg,
2.3 mmol) in Et2O (7 mL). After 30 min the solvent was removed in vacuo, the
residue was extracted with degassed pentane (15 mL), the solid was removed
by filtration, rinsed with pentane (2 Â 10 mL), and the filtrate evaporated to
5 mL. Crystals precipitated after freezing at 18 °C were collected. Yield (450 mg,
57%), mp 96–98 °C. [Found: C, 73.41; H, 9.15; N, 8.05; P, 8.81. C21H33N2P
requires C, 73.22; H, 9.66; N, 8.13; P, 8.99]; 1H NMR (300 MHz, C6D6): d 7.12–
7.15 (m, 1H + C6D6, CHAr), 7.01–7.06 (m, 3H, 2CHAr + CHIm), 6.41 (s, 1H,
CHIm), 2.38 (m, 4H, Et), 1.36 (d, 18H, J = 12.6 Hz, t-Bu), 1.08 (t, 6H, J = 7.5 Hz,
Et); 13C NMR (125 MHz, C6D6): 225.1, 141.4, 140.0, 129.4 (d, J = 102 Hz), 128.1,
cooled to À60 °C and
a solution of di(tert-butyl)chlorophosphine (0.9 g,
5 mmol) in hexane (7 mL) was added. After warming to room temperature
the mixture was stirred overnight. The solvents were removed in vacuo, the
residue was rinsed with pentane (40 mL), the solid residue filtered and washed
again with pentane (2 Â 20 mL). The filtrate was concentrated in vacuo and the
residue distilled. Yield (1.17 g, 68%), bp 140–145 °C/0.05 Torr; mp 40–42 °C
(pentane); [Found: C, 73.42; H, 9.86; N, 7.91; P, 8.57. C21H35N2P requires C,

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A. P. Marchenko et al. / Tetrahedron Letters 53 (2012) 494–496
126.6, 119.5 (d, J = 7.5 Hz), 34.6 (d, J = 25 Hz), 28.9 (d, J = 16 Hz), 24.8, 15.3; d
31P NMR (81 MHz, C6D6): d 99.4.
121.2, 121.0, 114.8, 59.2 (d, J = 1.25 Hz), 34.6 (d, J = 23 Hz), 32.9 (d, J = 14 Hz),
30.6 (d, J = 16 Hz); 31P NMR (81 MHz, C6D6): d 23.0. 1H NMR (500 MHz, CDCl3):
d 7.86–7.79 (m, 2H, CHAr), 7.25–7.23 (m, 2H, CHAr), 2.03 (s, 9H, t-Bu), 1.32 (d,
18H, J = 12.0 Hz, t-Bu); 13C NMR (125 MHz, CDCl3): d 155.3 (d, J = 39 Hz), 144.6
(d, J = 1.25 Hz), 135.1 (d, J = 1.25 Hz), 121.9, 120.9, 120.8, 114.9, 59.8 (d,
J = 1.25 Hz), 34.8 (d, J = 23 Hz), 33.4 (d, J = 12.5 Hz), 30.7 (d, J = 15 Hz); 31P NMR
(81 MHz, CDCl3): d 23.7.
10. Thermal isomerization of carbenes 3a–e. A solution of carbene (100 mg) in
benzene-d6 (0.7 mL) was heated in a sealed tube at 150 °C. For 4a—the reaction
was complete in 1 h 20 min, for 4b—2 h, for 4c—30 min, for 4d—1 h, for 4e—3 h.
The solvent was removed in vacuo and the residue was distilled or
recrystallized to give the target compound. 1-(1-Adamantyl)-2-[di(tert-
butyl)phosphanyl]-1H-benzimidazole (4a). Yield (91%), mp 171–172 °C
(hexane). [Found: C, 74.96; H, 9.11; N, 7.01; P, 7.74. C25H37N2P requires C,
75.72; H, 9.40; N, 7.06; P, 7.81]; 1H NMR (300 MHz, CDCl3): d 7.87–7.78 (m, 2H,
CHAr), 7.20–7.17 (m, 2H, CHAr), 2.71 (s, 6H, CH2-Ad), 2.31 (br s, 3H, CH-Ad),
1.85 (d, 6H, J = 12.3 Hz, CH2-Ad), 1.26 (d, 18H, J = 12.0 Hz, t-Bu); 13C NMR
(125 MHz, CDCl3): d 155.2 (d, J = 42 Hz), 144.7, 134.3 (d, J = 2.5 Hz), 121.5,
120.83, 120.79, 115.5, 61.8 (d, J = 2.5 Hz), 44.9 (d, J = 12.5 Hz), 36.2, 34.9 (d,
J = 24 Hz), 30.7 (d, J = 15 Hz), 30.3; 31P NMR (81 MHz, C6D6): d 25.8. 31P NMR
(81 MHz, CDCl3): d 26.1.
1-(1-Adamantyl)-2-[di(tert-butyl)phosphanyl]-1H-imidazole (4c). Yield 90%,
mp 91–92 °C (pentane, at À18 °C). [Found: C, 73.24; H, 9.78; N, 7.90; P, 9.11.
C
21H35N2P requires C, 72.79; H, 10.18; N, 8.08; P, 8.94]; 1H NMR (300 MHz,
C6D6): d 7.19 (s, 2H, CHIm), 2.46 (br s, 6H, CH2-Ad), 2.22 (br s, 3H, CH-Ad), 1.75
(d, 6H, CH2-Ad), 1.22 (d, 18H, J = 12.3 Hz, t-Bu); 13C NMR (125 MHz, C6D6): d
145.8 (d, J = 29 Hz), 127.7, 118.1 (d, J = 2.5 Hz), 58.3 (d, J = 1.25 Hz), 44.3 (d,
J = 11 Hz), 36.0, 34.6 (d, J = 16 Hz), 30.8 (d, J = 14 Hz), 30.2; 31P NMR (81 MHz,
C6D6):
Yield 98%, bp 90 °C/0.05 Torr. [Found: C, 68.14; H 10.12; N, 10.12; P, 12.07.
C
15H29N2P requires C, 67.13; H 10.89; N, 10.44; P, 11.54]; 1H NMR (500 MHz,
d 13.8. 1-tert-Butyl-2-[di(tert-butyl)phosphanyl]-1H-imidazole (4d).
1-tert-Butyl-2-[di(tert-butyl)phosphanyl]-1H-benzimidazole (4b). Yield 95%,
bp 110 °C/0.05 Torr, mp 103–104 °C (pentane). [Found: C, 71.22; H, 9.41; N,
8.75; P, 9.87. C19H31N2P requires C, 71.66; H 9.81; N, 8.80; P, 9.73%]; 1H NMR
(300 MHz, C6D6): d 8.05–8.02 (m, 1H, CHAr), 7.59–7.56 (m, 1H, CHAr), 7.19–
7.11 (m, 2H, CHAr), 1.76 (s, 9H, t-Bu), 1.37 (d, 18H, J = 12.0 Hz); 13C NMR
(125 MHz, C6D6): d 155.0 (d, J = 38 Hz), 145.0, 135.2 (d, J = 1.25 Hz), 122.1,
C6D6): d 7.19 (s, 1H, CHIm), 6.84 (s, 1H, CHIm), 1.51 (s, 9H, t-Bu), 1.29 (d, 18H,
J = 12.0 Hz, t-Bu); 13C NMR (125 MHz, C6D6): d 145.6 (d, J = 25 Hz), 128.0, 118.8,
56.6, 34.5 (d, J = 19 Hz), 32.0 (d, J = 10 Hz), 30.8 (d, J = 14 Hz); 31P NMR
(81 MHz, C6D6):
d 11.5. 2-[Di(tert-butyl)phosphanyl]-1-(2,6-diethylphenyl)-
1H-imidazole (4e): Yield 90%, bp 130 °C/ 0.05 Torr, mp 69–71 °C (pentane).