Zhu et al.
preparation, reactions, and applications of allenes.7-14 Rather
scant attention has been paid to the study of the fundamental
thermodynamics of reactions of allenes by using experimental
methods.15 Since in the reactions of allenes, the most are all
involved in electrophilic additions or reductions of the successive
double bonds,16 the hydride affinity of allenes should be the
most intrinsic thermodynamic scale to measure their chemical
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FIGURE 1. Isothermal titration calorimetry (ITC) for the reaction heat
of 1aH- (G ) H) with 9-phenylxanthylium cation (PhXn+ClO4-) in
acetonitrile at 298 K. Titration was conducted by adding 8 µL of
PhXn+ClO4 (3.86 mM) every 400 s into the acetonitrile containing
-
the 1aH- (G ) H) (ca.10.0 mM), which was obtained in situ from the
reactions of the corresponding saturated neutral compounds (1aH2) with
KH.
activities and reactivities. It is evident that the terrible lack of
knowledge about the hydride affinity of allenes in solution must
bring a lot of difficulties to understand the chemistry of allenes
and to further develop the application of allenes. Several years
ago we embarked on a major project to experimentally determine
the hydride affinity scale of various unsaturated organic com-
pounds in organic solvent.17 Herein we wish to offer the hydride
affinities of phenyl sulfone substituted allenes in acetonitrile as
the first report about the hydride affinities of allenes in solution.
In addition to phenyl sulfone substituted allenes (1a), the
corresponding isolated dienes (1b) and the corresponding
conjugated dienes (1c) were also investigated in this work for
a comparison to search the structural origins of allenes resulting
in their special chemical reactivity.
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Results and Discussion
The hydride affinity of dienes (1) is defined in this work as
the enthalpy change of the reaction of 1 with a free hydride ion
in acetonitrile to form the corresponding carbanion 1H- at 298
K (eqs 1 and 2). Since the free hydride anion in acetonitrile is
not available, it is clearly difficult to directly determine the
hydride affinity of the allenes in acetonitrile by experimental
methods. However, the hydride affinity of 1 in solution
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-
[∆HH A(1)] can be easily obtained from the reaction enthalpy
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18
change of the corresponding carbanions 1H- with a strong
hydride acceptor, such as 9-phenylxanthylium cation (PhXn+)
(eqs 3 and 4). In eq 4, ∆Hr is the enthalpy change of the reaction
(eq 3) in acetonitrile, which can be determined by using titration
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calorimetry (Figure 1); ∆HH A(PhXn+) is the hydride affinity
-
of PhXn+ in acetonitrile (-96.8 kcal mol-1), which can be
(17) (a) Zhu, X.-Q.; Zhang, M.; Liu, Q.-Y.; Wang, X.-X.; Zhang, J.-Y.;
Cheng, J.-P. Angew. Chem., Int. Ed. 2006, 45, 3954. (b) Zhu, X.-Q.; Li, H.-R.;
Li, Q.; Ai, T.; Lu, J.-Y.; Yang, Y.; Cheng, J.-P. Chem. -Eur. J. 2003, 9, 871.
(18) 1H- was prepared by deprotonation of the parent compound 1H2 with
KH in acetonitrile. Reaction of 1H- with PhXn+ in acetonitrile gives the
corresponding dienes 1 and PhXnH quantitatively by hydride transfer, which
were identified by MS and 1H NMR spectroscopy. Since the hydride transfer
from 1H- to PhXn+ in acetonitrile is not only fast but also quantitative, the
reaction enthalpy change ∆Hr can be directly determined by titration calorimetry.
8404 J. Org. Chem. Vol. 73, No. 21, 2008