of the cationic parent azolium ion by more electron-donating
substituents. As t-butyl and adamantyl substituents have
different F values yet very similar kDO values, this suggests
that steric effects are also important in these cases.
The 4,5-dihydroimidazolium salts 2H+b–d–Clꢀ have lower
kDO values, and higher pKa values, than analogous
imidazolium salts 1H+b–d–Clꢀ although the differences are
small. The largest effect of saturation of the C(4)–C(5) double
bond is an increase in pKa by 0.5 units for 2H+c compared to
1H+c.
be fully dissociated in water, this small counterion effect is
unsurprising.
In summary, we have determined pKas for the conjugate
acids of a large series of NHCs in water. The largest effects on
pKa are observed by varying N-substituent or ring size, while
the effects of C(4)–C(5) saturation are small.
The authors thank the EPSRC (UK) and UCD Dublin for
financial support. We gratefully acknowledge the NMR staff in
Durham University and UCD.
The kDO values for tetrahydropyrimidinium salts 3H+k–l–PF6
ꢀ
Notes and references
are substantially smaller than for 1H+ and 2H+.10 The kDO
values decrease by up to 108-fold, and the pKas increase by
8.4 units to 27.8–28.2 as a result of the one-caꢀrbon increase in
ring size. The increased pKas for 3H+k–l–PF6 correlate with
the greater N–C–N angles in these systems compared to the
five-membered ring series. Diaminocarbenes prefer unusually
small N–C–N angles, e.g. that in 1i is 102.2111 compared with
an N–C(H)–N angle of 109.61 in 1H+i.12 Any structural change
that enforces a larger angle will favour the protonated ion and
increase the pKa. In 3H+k–PF6ꢀ, the N–C(H)–N angle is 125.31,13
while the N–C–N angle in the KN(SiMe2)2 complex of 3l is
116.31.14
1 For reviews see: A. J. Arduengo, Acc. Chem. Res., 1999, 32,
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2 For reviews see: W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41,
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3 For reviews see: D. Enders, O. Niemeier and A. Henseler, Chem.
Rev., 2007, 107, 5606–5655; N. Marion, S. Diez-Gonzalez and
I. P. Nolan, Angew. Chem., Int. Ed., 2007, 46, 2988–3000.
4 For previous pKa determinations see: (a) R. W. Alder, P. R. Allen
and S. J. Williams, Chem. Commun., 1995, 1267–1268; (b) Y. J. Kim
and A. Streitwieser, J. Am. Chem. Soc., 2002, 124, 5757–5761;
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K. Toth, J. Am. Chem. Soc., 2004, 126, 4366–4374;
(d) A. M. Magill, K. J. Cavell and B. F. Yates, J. Am. Chem.
Soc., 2004, 126, 8717–8724; (e) Y. Chu, H. Deng and J. P. Cheng,
J. Org. Chem., 2007, 72, 7790–7793.
5 T. L. Amyes and J. P. Richard, J. Am. Chem. Soc., 1996, 118,
3129–3141; J. P. Richard, G. Williams and J. L. Gao, J. Am. Chem.
Soc., 1999, 121, 715–726; J. P. Richard, G. Williams,
A. C. O’Donoghue and T. L. Amyes, J. Am. Chem. Soc., 2002,
124, 2957–2968.
Methylene-linked bis-imidazolium ion 4H+m–2Iꢀ is signifi-
cantly more acidic than analogous dialkylimidazolium ion
1H+e–Clꢀ. The effect of a cationic imidazolium substituent
is a 240-fold increase in kDO and decrease in pKa by 3 units in a
manner similar to that of changing from two N-alkyl to N-aryl
substituents. This effect decreases as the length of the alkyl
linker increases. Bis-imidazolium salt 4H+p–2Iꢀ with an aryl
linker has a similar acidity to 4H+m–2Iꢀ.
Deuterium exchange of the C(2)–H of acyclic
formamidinium salt 6H+–PF6 does not compete with
ꢀ
hydrolysis. The second order rate constant for deuteroxide-
catalysed hydrolysis for acyclic salt 6H+–PF6 (kHYD = 3.02 ꢁ
ꢀ
6 V. Gold and S. Grist, J. Chem. Soc., Perkin Trans. 2, 1972, 89–95.
7 A. J. Kresge, R. A. More O’Ferrall and M. F. Powell, in Isotopes
in Organic Chemistry, ed. E. Buncel and C. C. Lee, Elsevier,
New York, 1987, vo1. 7.
10ꢀ4 Mꢀ1 ꢀ1) is similar to that for tetrahydropyrimidinium
s
ꢀ 10
For the latter, deuterium exchange
analogue 3H+l–PF6
.
8 U. Kaatze, R. Pottel and A. Schumacher, J. Phys. Chem., 1992, 96,
6017–6020.
competes with hydrolysis, thus the deuterium exchange
reaction for 6H+–PF6 must be significantly slower. On the
ꢀ
9 C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165–195.
10 Under our experimental conditions, competing deuteroxide-
catalyzed hydrolysis was observed for tetrahydropyrimidinium
ions 3H+ (kHYD = 2.83 ꢁ 10ꢀ2 and 2.21 ꢁ 10ꢀ4 Mꢀ1 sꢀ1 for
basis that we could detect Z 10% exchange in competition
with hydrolysis, a lower limit of pKa 4 29.9 may be established
for 6H+–PF6ꢀ. Acyclic carbene 6 has a N–C–N angle of
121.11,15 while the precursor 6H+–OTfꢀ has a N–C(H)–N
angle of 133.21.13 This much larger N–C–N angle favours
acyclic ion 6H+ resulting in a higher pKa value.
ꢀ
3H+k–PF6 and 3H+l–PF6ꢀ).
11 A. J. Arduengo, H. Bock, H. Chen, M. Denk, D. A. Dixon,
J. C. Green, W. A. Herrmann, N. L. Jones, M. Wagner and
R. West, J. Am. Chem. Soc., 1994, 116, 6641–6649.
12 P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed., 2008, 47,
7433–7437.
13 R. W. Alder, M. E. Blake, S. Bufali, C. P. Butts, A. G. Orpen,
J. Schutz and S. J. Williams, J. Chem. Soc., Perkin Trans. 1, 2001,
1586–1593.
14 R. W. Alder, M. E. Blake, C. Bortolotti, S. Bufali, C. P. Butts,
E. Linehan, J. M. Oliva, A. G. Orpen and M. J. Quayle, Chem.
Commun., 1999, 241–242.
15 R. W. Alder, P. R. Allen, M. Murray and A. G. Orpen, Angew.
Chem., Int. Ed. Engl., 1996, 35, 1121–1123.
Counterion effects on kDO and pKa values are negligible, as
clearly seen by comparing data for imidazolium ions 1H+f–h,
which have almost identical kDO and pKa values despite
having different counterions. As the azolium ion salts will
c
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Chem. Commun., 2011, 47, 1559–1561 1561