Table 2 Synthesis of chiral imidazolinium saltsa
Scheme 3 Regioselective methylation of the urea complex 6.
crowded environment. The presumption is supported by our
observation that N,N0-dimesitylformamidine 1a is far more
active than N,N0-bis(2-iPrphenyl)formamidine 1b at 25 1C
(Table 1, entries 1 and 2). In the case of 1g having two
disubstituted arene rings, the N atom near the less hindered
mesityl ring attacked the (R)-styrene oxide, offering the major
product (S)-4d-OTf, while the minor product (S)-4e-OTf
resulted from the nucleophilic attack of the N atom near the
more hindered diisopropylphenyl ring. The regiochemistry
was confirmed by the X-ray crystal structure of (S)-4d-I
(Fig. S4, see ESIw), obtained by treatment of (S)-4d-OTf with
excess NaI (p. S41, see ESIw).
In conclusion, we have established a facile and versatile method
for the preparation of various imidazolinium salts. Noticeably, this
novel synthetic strategy for the preparation of chiral backbone-
monosubstituted imidazolinium salts is more straightforward, and
does not involve any transition-metal catalyzed reaction compared
to the previously reported methods.
N,N0-
Entry Diarylformamidine
Yieldb
[%]
Imidazolinium salts
1
83
2
3
45c
43c
51
4
11
Financial support from the Shanghai Municipal Committee of
Science and Technology (08dj1400100-2), National Basic Research
Program of China (973)-2010CB833302, Shanghai Pujiang
Talent Program (11PJ1402500), and the National Natural Science
Foundation of China for financial support (21171056, 21072206,
20472096, 20872162, 20672127, 20821002 and 20732008) is greatly
acknowledged.
a
Reaction conditions. Step 1: 1 (2.4 mmol), epoxide (2 mmol), NaH
(3.0 mmol), DMF (15 mL), 25 1C, 10 h. Step 2: Tf2O (2.2 mmol), Et3N
b
(2.2 mmol), DCM (10 mL), 25 1C, 5 h. Isolated yield of the desired
imidazolinium salts. 70 1C (step 1).
c
alcohols 5a and 5b by an alternative method (Scheme 2). The ring
opening products 2a and 2b from 1a and 1b (Scheme S2,
see ESIw) have been identified as the same as 5a and 5b,
respectively, by NMR spectroscopy, indicating that the selective
nucleophilic attack occurred at the less hindered carbon of
styrene oxide. Thus we postulated that the regioselective products
(S)-4b-OTf and (S)-4c-OTf probably resulted from the selective
attack of the nitrogen atom near the disubstituted arene ring at
the less hindered carbon of (R)-styrene oxide in the first step
and the subsequent cyclization. The regiochemistry observed
here is quite similar to that in the methylation of the urea
complex 6 (Scheme 3). The treatment of 6 with methyl iodide
resulted in regioselective methylation with the nitrogen carrying
disubstituted arene ring to afford 7.11 The authors presumed that
the twist in the Ar–N bond imposed by the steric requirements of
two substituents in the ortho positions prevents delocalization of
the anion into the more hindered ring, increasing electron density
and reactivity at the nearer nitrogen atom, despite its more
Notes and references
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Scheme 2 Alternative synthesis of the alcohols 5a and 5b.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12541–12543 12543