Synthesis of substituted imidazolidines: Base-stable precursors of 4,5-dihydro-1h-imidazol-3-ium salts and N-heterocyclic carbenes

Article abstract of DOI:10.1002/ejoc.201402364

The present work establishes a new synthetic route that leads to substituted azolium salts. The base stable 1-(4-bromo-2,6-diisopropylphenyl)-3- (2,6-diisopropylphenyl)imidazolidine and 1,3-bis(4-bromo-2,6-diisopropylphenyl) imidazolidine were synthesized and the 4-Br substituents converted into various functional groups through Br/Li exchange or Pd-catalyzed cross-coupling reactions (Suzuki, Sonogashira, and vinylation). The substituted imidazolidines were oxidized, by using chloranil or N-bromosuccinimide, to provide the respective azolium salts, which are convenient precursors to N-heterocyclic carbenes. Copyright

Full text of DOI:10.1002/ejoc.201402364

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FULL PAPER
DOI: 10.1002/ejoc.201402364
Synthesis of Substituted Imidazolidines. Base-Stable Precursors of
4,5-Dihydro-1H-imidazol-3-ium Salts and N-Heterocyclic Carbenes
Meike Egert,[a] Sebastian Walther,[a] and Herbert Plenio*[a]
Keywords: Synthetic methods / Nitrogen heterocycles / Carbene ligands
The present work establishes a new synthetic route that
leads to substituted azolium salts. The base stable 1-(4-
or Pd-catalyzed cross-coupling reactions (Suzuki, Sonoga-
shira, and vinylation). The substituted imidazolidines were
bromo-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid- oxidized, by using chloranil or N-bromosuccinimide, to pro-
azolidine and 1,3-bis(4-bromo-2,6-diisopropylphenyl)imid-
azolidine were synthesized and the 4-Br substituents con-
verted into various functional groups through Br/Li exchange
vide the respective azolium salts, which are convenient pre-
cursors to N-heterocyclic carbenes.
building blocks,[7] because it is difficult to derivatize the
formed carbene itself. Azolium salts, which are the most
Introduction
N-heterocyclic carbenes (NHC) constitute a very impor- common precursors to NHC ligands, with easily accessible
tant class of ligands for transition metals.[1] Consequently, functional groups are also rare.[5c,8] Modifications of such
a large number of different NHCs have been prepared[2] and azolium salts, is fraught with problems, because they are
the synthetic chemistry leading to NHCs from different pre- sensitive towards basic reagents owing to the acidic proton
cursors was summarized in a recent review.[3] Among the at the potential carbene carbon. Examples for the transfor-
various NHC ligands, the N,NЈ-diarylimidazol(idene)ylid- mations of substituted azolium salts under mildly basic re-
ene motif appears to be most prominent for catalytically action conditions are known, but conversions requiring
active transition-metal complexes.[1c,1d,4] Such NHC ligands strong bases are difficult, owing to competing deproton-
are typically synthesized through reactions of 1,2-diamines ation of the azolium salt.[9] All of this constitutes a signifi-
or 1,2-diimines with C1 sources such as triethylorthoformate, cant deficit in synthetic approaches leading to NHCs with
formaldehyde or other 1,1-dielectrophiles (Scheme 1).[5] Al- functional groups.
ternatively, the NCN unit is preformed in formamidines,
We want to present here a new approach that leads to
which are cyclized by using various 1,2-dielectrophiles NHCs with various functional groups, which is based on
(Scheme 1).[6] The formed azolium salts are deprotonated substituted imidazolidines as the key intermediates. This
next to obtain the respective free carbenes needed for the class of heterocycles has received little attention despite the
formation of metal carbene complexes.
fact that the basic chemistry, i.e. the synthesis of simple
The synthesis of NHC ligands bearing functional groups imidazolidines and their conversion into 4,5-dihydro-1H-
normally requires multi-step synthesis starting from simple imidazol-3-ium salts, was reported in the classic Wanzlick
Scheme 1. Established synthetic routes leading to N,NЈ-diarylimidazolideneylidenes.
[a] Organometallic Chemistry, Technical University Darmstadt,
and Arduengo publications.[10] Later Salerno et al. opti-
mized the reaction conditions that led to the azolium salts
by testing different oxidants.[11] To the best of our knowl-
edge, the synthesis of functionalized 4,5-dihydro-1H-imid-
Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
E-mail: plenio@tu-darmstadt.de
www.chemie.tu-darmstadt.de/plenio/akplenio/startseite
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402364.
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azol-3-ium salts through imidazolidines was not investi- were synthesized according to standard procedures.[7,14]
gated. The N,NЈ-diarylimidazolidenes are stable towards Symmetric heterocycles 2a, 2c and 2e obtained from cycli-
strong bases and at elevated temperatures, which distin- zation of the respective 1,2-diamines with formaldehyde
guishes them from related 2-R-substituted imidazolidines were formed in very good yields, 73–90%, whereas unsym-
(R = H,CCl3 or H,C6F5), liberate the carbene through 1,1- metrical imidazolidines 2b and 2d were obtained in modest
elimination on warming.[12] In a rare case, Peris et al. re- yields, 40%. All imidazolidines were purified by recrystalli-
cently demonstrated the direct conversion of unsubstituted zation from ethanol.
imidazolidines (2-R = H,H) into (NHC)Ir complexes.[13]
Therefore an alternative approach to lead to imidazolid-
We want to report here on the synthesis of new N,NЈ- ines, the LiAlH4 reduction of the respective azolium salts,
diarylimidazolidenes with bromo or iodo substituents, the was tested.[10c] Compounds 2a, 2b and 2d were formed in
conversion of the halogen into other functional groups, and yields of 70–86%. Based on these results it is concluded
the oxidation of the modified imidazolidines into the re- that for the synthesis of unsymmetrical imidazolidines the
spective azolium salts.
reductive synthesis is superior, whereas symmetric imid-
azolidines 2c and 2e are best prepared by reaction of the
respective diamines and formaldehyde.
Results and Discussion
In contrast to early literature reports on N,NЈ-dialkyl-
The synthetic strategy leading to azolium salts with func- imidazolidines,[15] our N,NЈ-diarylimidazolidines were
tional groups is depicted in Scheme 2. The desired imid- found to be sensitive towards acids. Consequently, at-
azolidines can be prepared in the reaction of N,NЈ-diaryl- tempted chromatographic purification on silica resulted in
ethylenediamines with formaldehyde (preferably under an the cleavage of the N,NЈ-diarylimidazolidines and forma-
inert atmosphere and in the absence of oxidants). Cyclic tion of the respective diamines for several of the imidazolid-
aminals – five-membered rings – are sensitive towards acid, ines tested even after adding basic Et3N to the eluent. For-
but immune towards strong bases. The 4-halogen substitu- tunately, chromatographic purification of imidazolidines
ents at the aryl ring provide a convenient handle for the with neutral or basic alumina as the stationary phase ap-
introduction of functional groups, either through Br/Li ex- pears to be a general approach for purification of the cyclic
change or through various cross-coupling reactions. Fol- aminals reported here.
lowing derivatization under basic reaction conditions, the
substituted imidazolidines can be oxidized to provide the imidazolidines 2 into functionalized imidazolidines 3, fol-
respective azolium salts as NHC precursors.[11]
lowed by synthesis of azolium salts 4 (see Schemes 4 and 5).
The next steps are conversion of halogen-substituted
We tested reactions of symmetric and unsymmetrical The conversion of the bromo-substituted imidazolidines
N,NЈ-diarylethylenediamines with aq. formaldehyde leading under highly basic reaction conditions was tested. The Br/
to the formation of the expected imidazolidines (Scheme 3). Li exchange in mono- or disubstituted imidazolidine 2b and
Previously unknown N-(4-bromo-2,6-diisopropylphenyl)- 2c and the reaction with dimethylformamide (DMF) leads
NЈ-(2,6-diisopropylphenyl)ethane-1,2-diamine and N,NЈ- to formation of respective benzaldehydes 3b and 3g in good
bis(4-bromo-2,6-diisopropylphenyl)ethane-1,2-diamine yields. Clean conversion of –CHO-substituted imidazolid-
Scheme 2. Outline of the synthetic route leading to substituted azolium salts from N,NЈ-diarylethylenediamines.
Scheme 3. Synthesized halogenated imidazolidines.
2
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Synthesis of Substituted Imidazolidines
Scheme 4. Functionalization of 1,3-bis(4-bromo-2,6-diisopropylphenyl)imidazolidine. Reagents and conditions: (a) Pd(OAc)2, phosphine,
4-tolyl-B(OH)2, toluene, 80 °C, yield 96%; (b) nBuLi, DMF, THF, yield 39%; (c) HCCSiMe3, phosphine, Pd(OAc)2, CuI, iPr2NH, yield
81%; (d) nBu4N+F–, THF, yield 60%; (e) and (f) 1,2-dimethoxyethane, NBS, yield 4a 70%, 4b 80%; (g) toluene, chloranil, yield 63%.
Scheme 5. Functionalization of 1-(4-bromo-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydro-1H-imidazol-3-ium salts. Rea-
gents and conditions: (a) nBuLi, ClSiMe3, THF, yield 71%; (b) vinyl-BF3K, Na2PdCl4, PPh3, THF/water, yield 96%; (c) nBuLi, DMF,
THF, yield 75%; (d) HCCSiMe3, tBuPCy2, Na2PdCl4, CuI, iPr2NH, yield 70%; (e) nBu4N+F–, THF, yield 70%; (f) aerobic work-up,
yield 71%; (g) and (i) chloranil, toluene, yield 4f 63%; 4h 70%; (h) NBS, 1,2-dimethoxyethane, yield 76%.
ines into respective azolium salts 4b and 4g was achieved bic work-up is azolium salt 4e. Direct formation of the azol-
by using N-bromosuccinimide (NBS) as oxidant (see ium salt appears to be advantageous. However, one of the
Schemes 4 and 5). Reaction of imidazolidines 2b with main advantages of functionalization through imidazolid-
nBuLi and ClSiMe3 led to respective silylated heterocycle ines, is the facile purification of such heterocycles. Owing
3e, when the reaction work-up is done under anaerobic con- to the ionic nature the respective azolium salts purification
ditions. However, the main product formed during the aero- is more difficult.
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M. Egert, S. Walther, H. Plenio
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this solution was added dropwise to the oxalylchloride solution
cooled to 0 °C. The reaction was allowed to cool to room tempera-
ture and left stirring overnight. HCl (2 m, 100 mL) was added to
the stirred reaction mixture and the organic layer separated. The
organic layer was washed with 10% NaHCO3 solution, dried with
MgSO4, filtered and the filtrate evaporated. The light brownish res-
idue was treated with pentane to obtain a white solid, which was
filtered off and dried in vacuo (9.0 g, 62%). H NMR (300 MHz,
[D6]DMSO + CDCl3 3:2): δ = 1.15 (d, J = 6.9 Hz, 24 H, CH3),
3.03 (sept, 2 H, CH), 3.04 (sept, J = 6.9 Hz, 2 H, CH), 7.14–7.24
Pd-catalyzed cross-coupling reactions are powerful syn-
thetic tools for aryl halide functionalization.[1d,16] We there-
fore tested three such reactions (Sonogashira coupling, Su-
zuki coupling and vinylation) with the halogen-substituted
imidazolidines. Initially, we also tested iodinated imid-
azolidines 2d and 2e for the cross coupling reactions, how-
ever, the yields by using the brominated relatives are com-
parable and therefore the latter are the preferred reactants.
The Sonogashira reactions of HCCSiMe3 with 2b and 2c
1
were studied. In both reactions, respective coupled products (m, 3 H, CH), 7.27 (s, 2 H, CH), 10.21 (s, 1 H, NH), 10.29 (s, 1 H,
NH) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 22.96, 23.28,
28.22, 28.43, 121.33, 122.98, 126.15, 127.85, 131.42, 131.77, 145.54,
148.56, 160.06, 160.37 ppm. HRMS: calcd. for C26H35BrN2O2
486.1882; found 486.18907.
3c and 3h were isolated in very good yields (Scheme 3 and
Scheme 4). However, oxidation of 3c with NBS, as in the
previous examples, initially failed probably owing to the re-
action of NBS with the C–C triple bond. Instead, use of
N-(4-Bromo-2,6-diisopropylphenyl)-NЈ-(2,6-diisopropylphenyl)eth-
ane-1,2-diamine (1b): N-(4-Bromo-2,6-diisopropylphenyl)-NЈ-(2,6-
diisopropylphenyl)oxalamide (19.1 g, 39.18 mmol) was heated to
reflux in borane-tetrahydrofuran (THF) complex (1 m, 235 mL,
235 mmol) overnight. The reaction mixture was cooled to room
temperature and treated with methanol and conc. HCl to quench
excess borane. The volatiles were evaporated and the white residue
dissolved in diethyl ether. NaOH (1 m) was added to basify the
solution and the organic layers separated. The aq. phase was
washed twice with diethyl ether and the combined organic phases
were dried with MgSO4. After filtration, the filtrate was evaporated
in vacuo and dried overnight. The light green residue was dissolved
in diethyl ether and treated with HCl in dioxane to precipitate the
ammonium salt. The product was obtained as a white solid (10.0 g,
chloranil led to selective conversion of imidazolidine into
respective azolium salts 4c and 4h. To activate products 3c
and 3h for additional coupling reactions, deprotection of
the –CCSiMe3 unit under basic conditions was tested,
which led to the respective –CCH substituted imidazolid-
ines in good yields. Suzuki arylation of 2c with 4-tolyl-
boronic acid led to respective coupling product 3a, and
NBS oxidation gave azolium salt 4a. Vinylation of 2b with
vinyl-BF3K under Pd catalysis[17] afforded respective cross-
coupled products 3f in which chloranil is the preferred rea-
gent for oxidation, which leads to vinyl-substituted azolium
salt 4f.
1
48%). H NMR (300 MHz, [D6]DMSO): δ = 1.13 (d, 12 H, CH3),
1.20 (d, J = 6.9 Hz, 12 H, CH3), 3.37 (m, 8 H, CH and CH2), 7.25–
7.34 (m, 5 H, CH) ppm. 13C NMR (75 MHz, [D6]DMSO): δ =
24.02, 24.49, 27.26, 27.31, 47.31, 48.82, 118.17, 125.09, 125.48,
126.85, 129.14, 142.73, 142.77, 145.77 ppm. HRMS: calcd. for
C26H39BrN2 458.2297; found 458.23042.
Summary and Conclusions
It was demonstrated that new heterocycles 1-(4-bromo-
2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imidazol-
idine and 1,3-bis(2,6-diisopropylphenyl)imidazolidine are
convenient precursors for functionalized NHC ligands,
which are stable under highly basic reaction conditions. Br/
Li exchange of the bromine group allows the introduction
of various electrophiles, which are difficult to synthesize
through standard routes. Pd-catalyzed cross-coupling reac-
tions at the halide are facile and generate the respective
ethinylated, arylated and vinylated reaction products in ex-
cellent yields. The imidazolidines can be converted into the
respective 4,5-dihydro-1H-imidazol-3-ium (imidazolinium)
salts by using N-bromosuccinimide or chloranil. The syn-
thetic work reported here, opens the route for the synthesis
of various functionalized NHC-metal complexes.
N,NЈ-Bis(4-bromo-2,6-diisopropylphenyl)ethane-1,2-diamine (1c):
N,NЈ-(Ethane-1,2-diylidene)bis(4-bromo-2,6-diisopropylaniline)
(2.8 g, 5.24 mmol) was dissolved in THF (50 mL). LiAlH4
(2.5 equiv., 0.5 g, 0.013 mol) was added slowly under an argon at-
mosphere and the initially reddish suspension stirred overnight at
room temperature. The resulting greenish suspension is carefully
poured on ice with aq. HCl to quench excess LiAlH4, then basified
with sodium hydroxide and the organic product extracted with di-
ethyl ether. The organic phase was dried with magnesium sulfate,
filtered and the filtrate was evaporated to obtain a light yellow
1
product (2.27 g, 81%). H NMR (300 MHz, CDCl3): δ = 1.26 (d,
J = 6.8 Hz, 24 H, CH3), 3.13 (s, 4 H, CH2), 3.32 (sept, J = 6.8 Hz,
4 H, CH), 7.23 (s, 4 H, CH) ppm. 13C NMR (75 MHz, CDCl3): δ
= 24.18, 28.08, 52.16, 117.53, 126.94, 142.36, 145.07 ppm. HRMS:
calcd. for C26H38Br2N2 536.1402; found 536.13940.
General Procedure for the Synthesis of Imidazolidines from Di-
amines: To 1.0 equiv. of diamine (typically 0.01 mol) dissolved in
glacial acetic acid (10 mL) were added formaldehyde (2.5 equiv.,
37% aq. solution). After stirring the suspension for 12 h at room
temp., water (10 mL) and diethyl ether (20 mL) were added and the
acetic acid neutralized by slowly adding K2CO3. The formed solu-
tion is extracted with diethyl ether (2ϫ 100 mL), the combined
organic solutions are dried with MgSO4, filtered and the solvents
evaporated to dryness in vacuo. The residue was purified by
recrystallization from ethanol.
Experimental Section
For general experimental details see the Supporting Information.
N-(4-Bromo-2,6-diisopropylphenyl)-NЈ-(2,6-diisopropylphenyl)oxal-
amide: 2-[(2,6-Diisopropylphenyl)amino]-2-oxoacetic acid (7.48 g,
30 mmol) was dissolved in CH2Cl2 and oxalylchloride (1.1 equiv.,
2.83 mL, 33 mmol) added slowly. DMF (two drops) was added and
the solution stirred at room temperature for 4 h. The volatiles were
evaporated and the residue dissolved in CH2Cl2 (200 mL). In an-
other Schlenk flask 4-bromo-2,6-diisopropylaniline (1.2 equiv.,
9.22 g, 36 mmol) and triethylamine (1.4 equiv., 5.82 mL, 42 mmol)
were dissolved in CH2Cl2 (50 mL). After stirring for a few minutes,
1,3-Bis(2,6-diisopropylphenyl)imidazolidine (2a): N,NЈ-Bis(2,6-di-
isopropyl)ethylenediamine (3.83 g, 0.01 mol), formaldehyde (37%
4
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Synthesis of Substituted Imidazolidines
aq. solution, 1.86 mL, 0.025 mol), yield 2.86 g, 73%. 1H NMR
(300 MHz, CDCl3): δ = 7.25–7.29 (m, 1 H), 7.19 (d, J = 8.7 Hz, 2
H, HAr), 4.46 (s, 2 H, N-CH2-N), 3.64 (s, 4 H, CH2-CH2), 3.59
(sept, J = 7.0 Hz, 4 H, CH-iPr), 1.32 (d, J = 6.9 Hz, 24 H, CH3-
iPr) ppm. 13C NMR (75 MHz, [D8]toluene): δ = 150.28, 140.93,
1,3-Bis(4-bromo-2,6-diisopropylphenyl)imidazolidine (2c): Azolium
salt (1.0 g, 1.71 mmol). The imidazolidine was obtained as a white
1
solid (809 mg, 86%). H NMR (300 MHz, CDCl3): δ = 7.23 (s, 4
H, CH), 4.30 (s, 2 H, CH2), 3.53 (s, 4 H, CH2), 3.44 (sept, J =
7.0 Hz, 4 H, CH), 1.24 (d, J = 6.8 Hz, 24 H, CH3) ppm. 13C NMR
127.07, 124.31, 74.16, 53.45, 28.33, 24.66 ppm. HRMS: calcd. for (75 MHz, CDCl3): δ = 24.46, 28.47, 53.22, 73.80, 121.18, 127.69,
[M – H]+ C27H39N2 392.31114; found 391.31008.
139.77, 152.62 ppm.
1-(4-Iodo-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid-
azolidine (2d): Azolium salt (2.0 g, 3.62 mmol). The imidazolidine
was obtained as a white solid (1.3 g, 70%). 1H NMR (300 MHz,
CDCl3): δ = 7.54 (s, 2 H, CH), 7.26–7.18 (m, 3 H, CH), 4.38 (s, 2
H, CH2), 3.62–3.48 (sept, J = 4.0 Hz, 8 H, CH2 and CH), 1.28 (d,
J = 7.0 Hz, 24 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ =
152.99, 150.16, 140.97, 140.58, 133.86, 127.17, 124.34, 93.11, 73.99,
53.36, 53.31, 28.32, 28.29, 24.64, 24.49 ppm. HRMS: calcd. for
[M – H]+ C27H38IN2 517.2079; found 517.20345.
1-(4-Bromo-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid-
azolidine (2b): N-(4-Bromo-2,6-diisopropylphenyl)-NЈ-(2,6-diiso-
propylphenyl)ethane-1,2-diamine (5.0 g, 0.011 mol), formaldehyde
(2.5 equiv., 35% aq. solution, 2.16 mL, 0.0275 mol). The obtained
sticky brown residue was treated with cold ethanol to precipitate
the product as a white solid, yield 1.9 g, 37%. 1H NMR (300 MHz,
CDCl3): δ = 7.23–7.21 (m, 3 H, Ar), 7.16–7.13 (m, 2 H, Ar), 4.36
(s, 2 H, CH2), 3.57 (m, 4 H, CH2), 3.49 (m, J = 7.0 Hz, 4 H, CH),
1.27 (d, J = 6.4 Hz, 12 H, CH3), 1.28 (d, J = 6.5 Hz, 12 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 152.73, 150.17, 140.59, 140.10,
127.66, 127.16, 124.34, 121.06, 73.98, 53.36, 53.30, 28.45, 28.32,
24.63, 24.48 ppm. HRMS: calcd. for [M – H]+ C27H39BrN2
469.2219; found 469.21960.
Functionalization of Bromo and Iodo-Substituted Imidazolidines
1,3-Bis(3,5-diisopropyl-4Ј-methyl-[1,1Ј-biphenyl]-4-yl)imidazolidine
(3a): 1,3-Bis(4-bromo-2,6-diisopropylphenyl)imidazolidine (2c;
200 mg, 0.363 mmol), dicyclohexyl(9-ethyl-9H-fluorene-9-yl)-
1,3-Bis(4-bromo-2,6-diisopropylphenyl)imidazolidine (2c): N,NЈ- phosphine·HBF4 (6.9 mg, 0.0145 mmol) and Pd(OAc)2
Bis(4-bromo-2,6-diisopropylphenyl)ethane-1,2-diamine (5.38 g,
0.010 mol), formaldehyde (2.5 equiv., 37% aq. solution, 1.86 mL,
(0.0036 mmol, 0.8 mg) were degassed in a dry Schlenk flask. Et3N
(5 mL) and toluene (5 mL) were added and the solution was de-
0.025 mol), yield 4.93 g, 90%. 1H NMR (300 MHz, CDCl3): δ = gassed twice. After letting the reaction heat up to room temperature
7.24 (s, 4 H, HAr), 4.31 (s, 2 H, N-CH2-N), 3.54 (s, 4 H, CH2-CH2),
3.45 (sept, J = 6.9 Hz, 4 H, CH-iPr), 1.25 (d, J = 6.8 Hz, 24 H, was dissolved in Et3N and the solution was added to the reaction
CH3-iPr) ppm. 13C NMR (75 MHz, CDCl3): δ = 152.64, 139.78,
mixture of 2c at 40 °C. The reaction mixture was heated to 80 °C
it was warmed to 40 °C. Tolylboronic acid (99 mg, 0.727 mmol)
127.71, 121.20, 73.80, 53.22, 28.49, 24.47 ppm. HRMS: calcd. for for 12 h. The mixture was cooled to room temperature, the formed
[M – H]+ C27H37Br2N2 547.1323; found 547.13651.
precipitate was filtered off and washed with Et3N. The filtrate was
evaporated. To the residue was added diethyl ether. The organic
solution was twice washed with water. The organic layer was sepa-
rated, dried with MgSO4, filtered and the volatiles removed in
vacuo. The product was obtained as a greenish solid (200 mg,
96%). 1H NMR (300 MHz, CDCl3): δ = 1.34 (d, J = 6.8 Hz, 24 H,
CH3), 2.41 (s, 6 H, CH3), 3.60 (sept, J = 6.8 Hz, 4 H, CH), 3.65 (s,
4 H, CH2), 4.47 (s, 2 H, CH2), 7.26 (d, J = 8.4 Hz, 4 H, CH), 7.35
(s, 4 H, CH), 7.51 (d, J = 8.1 Hz, 4 H, CH) ppm. 13C NMR
(75 MHz, CDCl3): δ = 21.26, 24.71, 28.50, 53.53, 74.17, 123.20,
127.21, 129.50, 136.86, 139.10, 139.78, 140.15, 150.43 ppm.
HRMS: calcd. for [M – H]+ C41H51N2 571.4053; found 571.40874.
1-(4-Iodo-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid-
azolidine (2d): N-(2,6-Diisopropylphenyl)-N-(4-iodo-2,6-diisoprop-
ylphenyl)ethane-1,2-diamine[18] (5.06 g, 0.01 mol), formaldehyde
(2.5 equiv., 37 % aq. solution, 1.86 mL, 0.025 mol), yield 2.06 g,
1
40%. H NMR (300 MHz, CDCl3): δ = 7.43 (s, 2 H, HAr), 7.13–
7.24 (m, 3 H, HAr), 4.35 (s, 2 H, CH2-N-CH2), 3.40–3.60 (m, 8 H,
CH-iPr + CH2-CH2), 1.26 (d, J = 7.0 Hz, 12 H, CH3-iPr), 1.24 (d,
J = 7.0 Hz, 12 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 152.98,
150.16, 140.97, 140.58, 133.85, 127.16, 124.34, 93.19, 73.98, 53.35,
28.31, 28.28, 24.63, 24.48 ppm. HRMS: calcd. for [M – H]+
C27H38IN2 517.2079; found 517.20415.
1,3-Bis(4-formyl-2,6-diisopropylphenyl)imidazolidine (3b): 1,3-Bis(4-
bromo-2,6-diisopropylphenyl)imidazolidine (2c; 1.0 g, 1.82 mmol)
was suspended in THF (50 mL) and cooled to –78 °C. nBuLi solu-
tion (2.18 mL, 2.5 m in hexanes, 5.45 mmol) was added and the
reaction mixture was stirred for 2 h at –78 °C. DMF (0.42 mL,
5.45 mmol) was added and the solution was stirred at room tem-
perature for 12 h. The reaction mixture was filtered and the filtrate
treated with aq. NH4Cl solution. After extraction with diethyl
ether, the phases were separated and the organic phase was dried
with MgSO4. The volatiles were removed in vacuo. The obtained
solid was washed with pentane (10 mL) to obtain the crude product
as a light yellow solid. Column chromatography was performed
with ICN Alumina B Super I and cyclohexane/ethyl acetate (80:1)
to obtain the product as a white solid (335 mg, 41%). 1H NMR
(CDCl3, 300 MHz): δ = 1.31 (d, J = 6.9 Hz, 24 H, CH3), 3.53 (sept,
J = 6.9 Hz, 4 H, CH), 3.63 (s, 4 H, CH2), 4.42 (s, 2 H, CH2), 7.69 (s,
4 H, CH), 9.99 (s, 2 H, CHO) ppm. 13C NMR: (CDCl3, 75 MHz): δ
= 24.49, 28.51, 53.23, 73.77, 126.08, 135.17, 147.08, 151.40,
192.41 ppm. HRMS: calcd. for C29H40N2O2 447.3012; found
447.30388. IR: 1695 cm–1 (aldehyde).
1,3-Bis(4-iodo-2,6-diisopropylphenyl)imidazolidine (2e): N,NЈ-Bis(4-
iodo-2,6-diisopropylphenyl)ethane-1,2-diamine[19] (6.32 g,
0.01 mol), formaldehyde (2.5 equiv., 37 % aq. solution, 1.86 mL,
0.025 mol), yield 5.47 g, 85%. 1H NMR (500 MHz, [D8]toluene): δ
= 7.51 (s, 4 H, HAr), 4.30 (s, 2 H, N-CH2-N), 3.37 (s, 4 H, C2H4),
3.36 (sept, J = 7.0 Hz, 4 H, CH-iPr), 1.08 (d, J = 6.9 Hz, 24 H,
CH3-iPr) ppm. 13C NMR (126 MHz, [D8]toluene): δ = 152.86,
140.64, 134.15, 93.91, 74.10, 53.32, 28.38, 24.23 ppm. HRMS:
calcd. for [M – H]+ C27H37I2N2 643.1045; found 643.09791.
Synthesis of Imidazolidines through Reduction of Azolium Salts
General Procedure for Reduction of Azolium Salts: Azolium salt
(1 equiv.) was dissolved in THF, LiAlH4 (2.6 equiv.) added and the
reaction mixture stirred at room temperature for 12 h and filtered.
The filtrate was carefully added to ice water to hydrolyze excess
LiAlH4. The reaction mixture was extracted with diethyl ether and
the organic solution dried with MgSO4 and filtered. The volatiles
were removed in vacuo.
1-(4-Bromo-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid-
azolidine (2b): Azolium salt (2.0 g, 3.95 mmol). The imidazolidine
was obtained as a white solid (1.5 g, 81%).
1,3-Bis{2,6-diisopropyl-4-[(trimethylsilyl)ethynyl]phenyl}imid-
azolidine (3c): 1,3-Bis(4-bromo-2,6-diisopropylphenyl)imidazolid-
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5

Job/Unit: O42364
/KAP1
Date: 28-05-14 19:03:48
Pages: 9
M. Egert, S. Walther, H. Plenio
FULL PAPER
ine (2c; 200 mg, 0.363 mmol) was stirred in diisopropylamine
(20 mL) for 15 min. Catalyst stock solution (5.45 mL, see below)
was added and the reaction mixture was degassed twice. The solu-
tion was heated to 80 °C for 15 min, cooled to 60 °C and trimethyl-
silylacetylene (1.6 mmol, 157 mg) was added. The formation of pre-
cipitate indicates the initiation of the reaction. The mixture was
stirred at 60 °C overnight. The precipitate was filtered off and
washed with diisopropylamine. The combined iPr2NH solutions
were evaporated. Diethyl ether was added and the solution was
extracted twice with water. The separated organic solution was
dried with MgSO4, filtered and the volatiles evaporated. The resi-
evaporating the solvent the product was obtained as a light brown
solid (170 mg, 96%). 1H NMR (300 MHz, CDCl3): δ = 1.28 (m,
24 H, CH3), 3.53 (sept, J = 6.8 Hz, 2 H, CH), 3.54 (sept, J =
6.8 Hz, 2 H, CH), 3.59 (m, 4 H, CH2), 4.40 (s, 2 H, CH2), 5.23 (dd,
J = 1.0, J = 10.9 Hz, 1 H, CH2), 5.72 (dd, J = 1.0, J = 17.6 Hz, 1
H, CH2), 6.72 (dd, J = 17.6, J = 10.9 Hz, 1 H, CH), 7.14–7.24 (m,
5 H, CH) ppm. 13C NMR (75 MHz,CDCl3): δ = 24.61, 24.65,
28.31, 53.43, 74.10, 133.37, 122.27, 124.31, 127.07, 136.04, 137.21,
140.86, 140.90, 150.24, 150.30 ppm. HRMS: calcd. for C29H42N2
418.3348; found 418.33105.
1-(4-Formyl-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid-
azolidine (3g): The bromine substituted imidazolidine (2b, 1.0 g,
2.12 mmol) was dissolved in THF and cooled to –78 °C. nBuLi
(6.36 mmol, 2.5 m in hexane, 2.5 mL) was added and the solution
stirred at this temperature for 2 h. Dry DMF (0.5 mL, 6.36 mmol)
was added and the reaction mixture allowed to room temperature.
After stirring for another 2 h, the yellowish solution was treated
with aq. NH4Cl solution and extracted with diethyl ether. The or-
ganic layer was separated, dried with MgSO4 and filtered. The vol-
atiles were evaporated in vacuo to obtain the crude product. Col-
umn chromatography was performed with ICN Alumina B with
cyclohexane/ethyl acetate (80:1) to obtain the product as a white
1
due was recrystallized from ethanol, yield 172 mg, 81%. H NMR
(300 MHz, CDCl3): δ = 7.24 (s, 4 H, HAr), 4.32 (s, 2 H, N-CH2-
N), 3.54 (s, 4 H, CH2-CH2), 3.46 (sept, J = 6.9 Hz, 4 H, CH-iPr),
1.25 (d, J = 6.9 Hz, 24 H, CH3-iPr), 0.27 (s, 18 H, CH3-Si) ppm.
13C NMR (75 MHz, CDCl3): δ = 150.31, 141.54, 128.23, 121.57,
105.95, 93.33, 73.95, 53.30, 28.26, 24.48, 0.24 ppm. HRMS: calcd.
for [M – H]+ C37H55N2Si2 583.3904; found 583.39165.
1,3-Bis(4-ethynyl-2,6-diisopropylphenyl)imidazolidine (3d): Pro-
tected imidazolidine 3c (1.0 g, 1.71 mmol) was dissolved in THF.
N(nBu)4F·3H2O (1.29 g, 4.09 mmol) was added and the resulting
solution was stirred for 2 h at room temperature and then deminer-
alized water and diethyl ether was added. The reaction mixture was
extracted with diethyl ether, the layers were separated and the or-
ganic phase washed first with NaHCO3 solution and then with
saturated NaCl solution. The organic layer was separated, filtered
and dried with MgSO4. The volatiles were removed in vacuo to
obtain the product as a brown solid (452 mg, 60 %). 1H NMR
(300 MHz, CDCl3): δ = 1.25 (d, J = 6.9 Hz, 24 H, CH3), 3.06 (s, 2
H, CH), 3.47 (sept, J = 6.9 Hz, 4 H, CH), 3.56 (s, 4 H, CH2), 4.34
(s, 2 H, CH2), 7.28 (s, 4 H, CH) ppm. 13C NMR (75 MHz, CDCl3):
δ = 24.47, 28.24, 53.30, 73.92, 76.58, 84.38, 120.56, 128.39, 141.80,
150.48 ppm. HRMS: calcd. for C31H40N2 440.3192; found
440.31539.
1
solid (669 mg, 75%). H NMR (300 MHz, CDCl3): δ = 10.02 (s, 1
H, CHO), 7.72 (s, 2 H, CH), 7.18 (m, 3 H, CH), 4.45 (s, 2 H, CH2),
3.63–3.46 (m, 8 H, CH and CH2), 1.35–1.25 (m, 24 H, CH3) ppm.
13C NMR (75 MHz, CDCl3): δ = 24.50, 24.64, 28.38, 28.47, 53.33,
53.44, 73.96, 124.38, 126.04, 127.27, 135.10, 140.38, 147.71, 150.16,
151.53, 192.41 ppm. HRMS: calcd. for C28H40N2O 419.3063;
found 419.30795.
1-{2,6-Diisopropyl-4-[(trimethylsilyl)ethynyl]phenyl}-3-[(2,6-diiso-
propylphenyl)imidazolidine] (3h): The bromine-substituted imid-
azolidine (2b, 220 mg, 0.467 mmol) was dissolved in iPr2NH. Cata-
lyst stock solution (3.75 mL, see below) was added and the reaction
mixture was degassed twice. The solution was heated to 80 °C for
15 min, cooled to 60 °C and trimethylsilylacetylene (68.6 μL,
0.486 mmol) added. The formation of precipitate indicates the initi-
ation of the reaction. The mixture was stirred at 60 °C for 12 h. The
precipitate was filtered off and washed with iPr2NH. The combined
iPr2NH solutions were evaporated. The crude product was dis-
solved in diethyl ether and stored for 12 h at –30 °C. The volatiles
were removed in vacuo. The obtained solid was dissolved in diethyl
ether and washed with water. The phases were separated and the
organic phase dried with MgSO4 and filtered. The volatiles were
removed in vacuo to obtain the brown product (160 mg, 70%). The
crude product was recrystallized from ethanol.
1-[2,6-Diisopropyl-4-(trimethylsilyl)phenyl]-3-(2,6-diisopropylphen-
yl)imidazolidine (3e): 1-(4-Bromo-2,6-diisopropylphenyl)-3-(2,6-di-
isopropylphenyl)imidazolidine (2b, 200 mg, 0.424 mmol) was dis-
solved in dry THF (15 mL) and cooled to –78 °C. nBuLi was added
(0.54 mL, 2.5 m in hexane, 1.36 mmol) carefully and the yellowish
solution was stirred for 2 h at –78 °C. Fresh distilled ClSiMe3
(0.174 mL, 1.36 mmol) was added, and the mixture stirred at room
temperature overnight. The beige solution was treated with pent-
ane, filtered, and the filtrate was evaporated to obtain the product
as a light beige solid. Overall yield 150 mg (71 %). 1H NMR:
(CDCl3, 300 MHz): δ = 0.29 (s, 9 H, CH3), 1.28 (m, 24 H, CH3),
3.54 (sept, J = 6.5 Hz, 4 H, CH), 3.59 (s, 4 H, CH2), 4.41 (s, 2 H,
CH2), 7.15 (d, J = 6.8 Hz, 2 H, CH), 7.23 (t, J = 6.4 Hz, 1 H, CH),
7.31 (s, 2 H, CH) ppm. 13C NMR: (CDCl3, 300 MHz): δ = –0.77,
24.68, 28.30, 53.49, 74.12, 124.30, 127.05, 129.40, 138.45, 140.94,
141.83, 149.05, 150.25 ppm. HRMS: calcd. for C30H48N2Si
464.3587; found 464.35698.
Catalyst stock solution: In a Schlenk flask Na2PdCl4 (17.7 mg,
0.060 mmol), CuI (8.6 mg, 0.045 mmol) and tBuPCy2·HBF4
(41.1 mg, 0.12 mmol) was added and the flask was degassed three
times. Dry iPr2NH (15 mL) was added and the solution was stirred
for 1 h at 40 °C.
1-(2,6-Diisopropyl-4-vinylphenyl)-3-(2,6-diisopropylphenyl)imid- 1H NMR (300 MHz, CDCl3): δ = 7.28–7.23 (m, 3 H, CH), 7.17–
azolidine (3f): Potassium vinyltrifluoroborate (57 mg, 0.425 mmol), 7.15 (m, 2 H, CH), 4.38 (s, 2 H, CH2), 3.60–3.46 (m, 8 H, CH2 and
Pd(OAc)2 (2.0 mg, 8.9 μmol), PPh3 (6.7 mg, 0.0254 mmol), Cs2CO3 CH), 1.28 (m, 24 H, CH3), 0.28 (s, 9 H, CH3) ppm. 13C NMR
(414 mg, 1.27 mmol) and 1-(4-bromo-2,6-diisopropylphenyl)-3-
(2,6-diisopropylphenyl)imidazolidine (2b, 200 mg, 0.424 mmol)
were stirred overnight in THF/H2O (9:1, 4 mL) at 85 °C. After
cooling to room temperature water (3 mL) was added and the mix-
ture was extracted with CH2Cl2 (3ϫ 5 mL). The combined organic
phases were evaporated to obtain a dark brown residue as crude
product. The product was filtered through a basic aluminum oxide
plug to separate the catalyst (cyclohexane/ethyl acetate, 80:1). After
(75 MHz, CDCl3): δ = 150.39, 150.23, 141.79, 140.70, 128.23,
127.13, 124.32, 121.52, 106.02, 93.28, 74.06, 53.37, 28.33, 28.26,
24.64, 24.49, 0.25 ppm. 1H NMR (300 MHz, [D8]toluene): δ = 7.48
(s, 2 H, HAr), 6.97–7.18 (m, ArH), 4.44 (s, 2 H, N-CH2-N), 3.62–
3.44 (m, 8 H, CH-iPr + CH2-CH2), 1.25–1.30 (m, 24 H, CH3-iPr),
0.29 (s, 9 H, CH3-Si) ppm. 13C NMR (75 MHz, [D8]toluene): δ =
150.49, 150.00, 141.88, 140.67, 128.60, 124.50, 122.56, 106.88,
93.40, 74.50, 53.59, 28.49, 28.40, 24.63, 24.32, 1.35, 0.15 ppm.
6
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O42364
/KAP1
Date: 28-05-14 19:03:48
Pages: 9
Synthesis of Substituted Imidazolidines
HRMS: calcd. for [M – H]+ C32H47N2Si 488.3509; found
487.35523.
1,3-Bis{2,6-diisopropyl-4-[(trimethylsilyl)ethynyl]phenyl}-4,5-di-
hydro-1H-imidazol-3-ium 2,3,5,6-Tetrachloro-4-hydroxyphenolate
(4c): In a round-bottom flask 1,3-bis{2,6-diisopropyl-4-[(trimethyl-
silyl)ethynyl]phenyl}imidazolidine (3c; 100 mg, 0.171 mmol) was
dissolved in toluene (4 mL) and chloranil (1.5 equiv., 63 mg,
0.256 mmol) added. The reaction mixture was stirred overnight,
pentane added and the mixture cooled to –35 °C in a freezer. The
formed precipitate was filtered off, washed with cold pentane and
dried in vacuo, yield 81 mg, 57% 1H NMR (4c) (500 MHz, [D6]-
DMSO): δ = 9.51 (s, 1 H, N-CH=N), 7.46 (s, 4 H, HAr), 4.55 (s, 4
H, CH2-CH2), 3.05 (s, 4 H, CH-iPr), 1.35 (s, 12 H, CH3-iPr), 1.18
(s, 12 H, CH-iPr), 0.25 (s, 18 H, CH3-Si) ppm. 13C NMR
(126 MHz, [D6]DMSO): δ = 160.12, 146.65, 143.91, 129.98, 127.80,
125.11, 103.71, 95.78, 53.62, 28.12, 24.52, 22.99, 0.34 ppm. HRMS:
calcd. for [M+] C37H55N2Si2 583.3904; found 583.38816.
1-(4-Ethynyl-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)imid-
azolidine (3i): The protected imidazolidine (3h, 452 mg,
0.925 mmol) was dissolved in THF (10 mL). N(nBu)4F·3H2O
(350 mg, 1.11 mmol) was added and the resulting solution was
stirred for 2 h at room temperature. Then demineralized water
(20 mL) and diethyl ether (20 mL) were added. The reaction mix-
ture was extracted, the phases were separated and the organic phase
washed with NaHCO3 solution. The organic phase was washed
with saturated NaCl solution, dried with MgSO4 and filtered. The
volatiles were removed in vacuo to obtain the product as brown
solid (283 mg, 74%). 1H NMR (300 MHz, CDCl3): δ = 1.26 (m,
24 H, CH3), 3.06 (s, 1 H, CH), 3.45–3.60 (m, 8 H, CH and CH2),
4.38 (s, 2 H, CH2), 7.15 (d, J = 27.28 Hz, 2 H, CH), 7.24 (t, J =
27.28 Hz, 1 H, CH), 7.28 (s, 2 H, CH) ppm. 13C NMR (75 MHz,
CDCl3): δ = 150.54, 150.18, 142.08, 140.64, 128.37, 127.14, 124.32,
120.45, 84.43, 76.51, 74.04, 53.44, 53.37, 28.30, 28.22, 24.64,
24.48 ppm. HRMS: calcd. for C29H40N2 416.3192; found 416.3164.
1-[2,6-Diisopropyl-4-(trimethylsilyl)phenyl]-3-(2,6-diisopropyl-
phenyl)-4,5-dihydro-1H-imidazol-3-ium Chloride (4e): Under an at-
mosphere of nitrogen 1-(4-bromo-2,6-diisopropylphenyl)-3-(2,6-di-
isopropylphenyl)imidazolidine (3e; 100 mg, 0.212 mmol) was dis-
solved in dry THF (10 mL) and cooled to –78 °C. nBuLi (2.5 m in
hexane, 0.679 mmol, 0.27 mL) was added and the mixture was
stirred for 2 h at –78 °C. To the yellowish solution distilled Me3SiCl
(0.679 mmol, 86.8 μL) was added and the solution was stirred at
room temperature overnight under ambient atmosphere. The vola-
tiles were evaporated and pentane was added. The mixture was
stored in the fridge overnight. The white precipitate was filtered
off, washed with pentane and dried in vacuo to obtain 80 mg (81%)
of product. 1H NMR: ([D6]DMSO, 500 MHz): δ = 0.21 (s, 9 H,
CH3), 1.13 (t, J = 7.1 Hz, 12 H, CH3), 1.28 (t, J = 7.5 Hz, 12 H,
CH3), 3.02 (sept, J = 6.8 Hz, 4 H, CH), 4.50 (s, 4 H, CH2), 7.33
(d, J = 7.8 Hz, 2 H, CH), 7.40 (s, 2 H, CH), 7.48 (t, J = 7.8 Hz, 1 H,
CH), 9.76 (s, 1 H, CH) ppm. 13C NMR: ([D6]DMSO, 500 MHz): δ
= –1.04, 23.49, 23.51, 25.16, 25.20, 28.36, 28.41, 53.91, 124.87,
129.26, 130.09, 130.95, 131.17, 143.36, 145.07, 146.21, 160.26 ppm.
HRMS: calcd. for [M+] C30H47N2Si 463.3509; found 463.34884.
Oxidation of Imidazolidines to the Respective Azolium Salts
1,3-Bis(3,5-diisopropyl-4Ј-methyl-[1,1Ј-biphenyl]-4-yl)-4,5-dihydro-
1H-imidazol-3-ium Bromide (4a): To a solution of 1,3-bis(3,5-di-
isopropyl-4Ј-methyl-[1,1Ј-biphenyl]-4-yl)imidazolidine (100 mg,
0.175 mmol) in 1,2-dimethoxyethane (3 mL) was slowly added a
solution of NBS (62.14 mg, 0.349 mmol) in 1,2-dimethoxyethane
(3 mL) with exclusion of light and the mixture was stirred overnight
at room temperature. The formed precipitate was filtered off and
washed with 1,2-dimethoxyethane (4 mL), pentane (5 mL) and di-
ethyl ether (5 mL) to afford the product as a white solid (80 mg,
70%). 1H NMR (300 MHz, [D6]DMSO): δ = 1.32 (d, J = 6.71 Hz,
12 H, CH3), 1.42 (d, J = 6.52 Hz, 12 H, CH3), 2.37 (s, 6 H, CH3),
3.02 (sept, J = 6.70 Hz, 4 H, CH), 4.68 (s, 4 H, CH2), 7.32 (d, J =
8.11 Hz, 4 H, CH), 7.67 (m, 8 H, CH) ppm. 13C NMR (75 MHz,
[D6]DMSO): δ = 21.19, 24.33, 24.82, 29.23, 54.27, 123.93, 127.53,
129.73, 130.03, 136.66, 138.18, 143.86, 146.97, 154.62 ppm.
HRMS: calcd. for C41H51N2 571.4053; found 571.4019. Because
the azolium–H resonance in the 1H NMR spectrum was not ob-
served of the bromide counterion, oxidation of imidazolidine with
chloranil was done and NMR spectra of the product recorded.
With the 2,3,5,6-tetrachloro-4-hydroxyphenolate counterion, the
azolium–H resonance was observed at δ = 9.52 ppm. All other pro-
ton resonances are observed at almost the same shifts in both spec-
3-(2,6-Diisopropyl-4-vinylphenyl)-1-(2,6-diisopropylphenyl)-4,5-di-
hydro-1H-imidazol-3-ium 2,3,5,6-Tetrachloro-4-hydroxyphenolate
(4f): In a round-bottom flask 1-(2,6-diisopropyl-4-vinylphenyl)-3-
(2,6-diisopropylphenyl)imidazolidine (3f; 165 mg, 0.394 mmol) was
dissolved in toluene (5 mL) and chloranil (1.5 equiv., 145 mg,
0.591 mmol) added. The reaction mixture is stirred overnight, pent-
ane added and cooled to –35 °C in a freezer. The formed precipitate
was filtered off, washed with cold pentane and dried in vacuo, yield
1
tra. H NMR (300 MHz, [D6]DMSO): δ = 1.27 (d, J = 6.7 Hz, 12
1
165 mg, 63%. H NMR (300 MHz, [D6]DMSO + CDCl3 4:1): δ =
H, CH3), 1.43 (d, J = 6.6 Hz, 12 H, CH3), 2.37 (s, 6 H, CH3), 3.14
(sept, J = 6.6 Hz, 4 H, CH), 4.59 (s, 4 H, CH2), 7.32 (d, J = 7.9 Hz,
4 H, CH), 7.63 (m, 8 H, CH), 9.52 (s, 1 H, CH), 10.27 (s, 1 H,
OH) ppm.
1.20 (m, 12 H, CH3), 1.36 (m, 12 H, CH3), 3.07 (sept, J = 4.1 Hz,
4 H, CH), 4.52 (s, 4 H, CH2), 5.37 (d, J = 11.1 Hz, 1 H, CH2), 5.98
(d, J = 17.9 Hz, 1 H, CH2), 6.78 (dd, J = 11.1, 17.9 Hz, 1 H, CH),
7.38–7.40 (m, 2 H, CH), 7.45 (s, 2 H, CH), 7.51–7.56 (m, 1 H, CH),
9.46 (s, 1 H, CH), 10.10 (s, 1 H, OHAn.) ppm. 13C NMR (300 MHz,
[D6]DMSO + CDCl3 4:1): δ = 23.07, 23.15, 24.59, 24.66, 28.21,
53.46, 115.93, 120.25, 121.96, 124.35, 127.71, 128.45, 130.73,
135.34, 139.50, 143.74, 145.70, 145.95, 160.30 ppm. HRMS: calcd.
for [M+] C29H41N2 417.327; found 417.32266.
1,3-Bis(4-formyl-2,6-diisopropylphenyl)-4,5-dihydro-1H-imidazol-3-
ium Bromide (4b): To a solution of imidazolidine aldehyde (3b;
295 mg, 0.658 mmol), 3a in 1,2-dimethoxyethane (≈ 6 mL) was
slowly added a solution of NBS (234 mg, 1.32 mmol) in 1,2-dime-
thoxyethane (4 mL) with exclusion of light, and the mixture stirred
overnight. A precipitate formed, which was filtered off and washed
with 1,2-dimethoxyethane (2 mL) to obtain the product as a light
yellow solid (276 mg, 80 %). 1H NMR ([D6]DMSO, 500 MHz,
343 K): δ = 1.32 (d, J = 6.8 Hz, 12 H, CH3), 1.42 (d, J = 6.7 Hz,
1-(4-Formyl-2,6-diisopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-di-
hydro-1H-imidazol-3-ium Bromide (4g): To a solution of aldehyde-
substituted imidazolidine (3g; 450 mg, 1.07 mmol) in 1,2-dimeth-
12 H, CH3), 3.08 (sept, J = 6.8 Hz, 4 H, CH), 4.76 (s, 4 H, CH2), oxyethane (6 mL) was slowly added a solution of NBS (380.8 mg,
8.03 (s, 4 H, CH), 10.15 (s, 2 H, CHO) ppm. 13C NMR ([D6]-
DMSO, 500 MHz, 343 K): δ = 23.31, 23.92, 28.46, 53.69, 125.91,
134.25, 138.33, 147.26, 153.42, 192.00 ppm. IR: 1694 cm–1 (alde-
2.14 mmol) in 1,2-dimethoxyethane (6 mL) with exclusion of light
and the mixture was stirred for 4 h at room temperature. The
formed precipitate was filtered off and washed with 1,2-dimethoxy-
hyde). HRMS: calcd. for C29H39N2O2Br 525.2116; found ethane (4 mL), pentane (5 mL) and diethyl ether (5 mL) to afford
1
525.20692.
the product as a white solid (406 mg, 76%). H NMR (500 MHz,
Eur. J. Org. Chem. 0000, 0–0
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Job/Unit: O42364
/KAP1
Date: 28-05-14 19:03:48
Pages: 9
M. Egert, S. Walther, H. Plenio
FULL PAPER
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[D6]DMSO + CDCl3 5:1): δ = 10.11 (s, 1 H, CHO), 8.01 (s, 2 H,
CH), 7.60 (t, J = 7.78, 7.87 Hz, 1 H, CH), 7.46 (d, J = 7.7 Hz, 2
H, CH), 4.69 (s, 4 H, CH2), 3.05 (sept, J = 6.8 Hz, 2 H, CH), 2.97
(sept, J = 6.8 Hz, 2 H, CH), 1.38 (dd, J = 6.8, 22.6 Hz, 12 H, CH3),
1.27 (dd, J = 6.8, 21.6 Hz, 12 H, CH3) ppm. 13C NMR (500 MHz,
[D6]DMSO/CDCl3): δ = 23.62; 23.80, 24.17, 24.33, 28.45, 28.62,
53.63, 53.89, 125.31, 126.19, 129.88, 131.65, 134.53, 138.32, 145.73,
147.44, 153.58, 192.29 ppm. HRMS: calcd. for [M+] C28H39BrN2O
497.2168; found 497.21976.
3-{2,6-Diisopropyl-4-[(trimethylsilyl)ethynyl]phenyl}-1-(2,6-diiso-
propylphenyl)-4,5-dihydro-1H-imidazol-3-ium 2,3,5,6-Tetrachloro-4-
hydroxyphenolate (4h): In a round-bottom flask 1 equiv. of the re-
spective imidazolidine is dissolved in toluene and 1 equiv. of
chloranil was added. The reaction mixture was stirred overnight,
pentane added cooled to –35 °C in a freezer. The formed precipitate
was filtered, washed with cold pentane and dried in vacuo. 1-{2,6-
diisopropyl-4-[(trimethylsilyl)ethynyl]phenyl}-3-(2,6-diisopropyl-
phenyl)imidazolidine (3h; 30 mg, 0.0614 mmol), chloranil
(1.5 equiv., 22.6 mg, 0.0921 mmol), yield 32 mg, 71%. 1H NMR
(500 MHz, [D6]DMSO): δ = 0.25 (s, 9 H, CH3), 1.19 (d, J = 6.7 Hz,
12 H, CH3), 1.34 (d, J = 6.6 Hz, 12 H, CH3), 3.06 (sept, J = 6.7 Hz,
4 H, CH), 4.53 (s, 4 H, CH2), 7.42 (d, J = 7.8 Hz, 2 H, CH), 7.47
(s, 2 H, CH), 7.55 (t, J = 7.8 Hz, 1 H, CH), 9.47 (s, 1 H, CH),
10.20 (s, 1 H, OH) ppm. Detection of the 13C NMR signal shifts
was through HSQC. 13C NMR (75 MHz, 300 MHz, [D6]DMSO)
= –0.58, 24.40, 22.78, 28.04, 53.34, 124.50, 127.64, 130.91, 159.62.
Detection of quaternary carbons was through HMBC. 13C NMR
(75 MHz, 300 MHz, [D6]DMSO) 95.83, 103.97, 120.77, 128.04,
129.72, 130.17, 145.88, 147.07 ppm. HRMS: calcd. for [M+]
C32H46N2Si 486.343; found 486.34348.
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Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) through grant Pl 178/13-2.
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Received: April 7, 2014
Published Online: ᭿
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O42364
/KAP1
Date: 28-05-14 19:03:48
Pages: 9
Synthesis of Substituted Imidazolidines
NHC Precursors
M. Egert, S. Walther, H. Plenio* ..... 1–9
Synthesis of Substituted Imidazolid-
ines. Base-Stable Precursors of 4,5-Di-
hydro-1H-imidazol-3-ium Salts and N-Het-
erocyclic Carbenes
The facile synthesis of functionalized 4,5-
dihydro-1H-imidazol-3-ium salts, through
functionalized imidazolidines is reported,
which are useful precursors for substituted
N-heterocyclic carbenes.
Keywords: Synthetic methods / Nitrogen
heterocycles / Carbene ligands
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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