A facile experimental method to determine the hydride affinity of polarized olefins in acetonitrile

Article abstract of DOI:10.1002/anie.200600536

(Chemical Equation Presented) Choosing a suitable hydride reducing agent and thermodynamic analysis of reduction mechanisms is facilitated by experimental hydride affinities ΔH_H-A, which are reported herein for 28 polarized olefins 1 in acetonitrile (see scheme). The method should also be applicable to ketones, aldehydes, and imines.

Full text of DOI:10.1002/anie.200600536

Communications
Hydride Affinities
DOI: 10.1002/anie.200600536
A Facile Experimental Method To Determine the
Hydride Affinity of Polarized Olefins in Aceto-
nitrile**
Xiao-Qing Zhu,* Min Zhang, Qiao-Yun Liu,
Xiao-Xiao Wang, Jian-Yu Zhang, and Jin-Pei Cheng*
Reductions of various unsaturated compounds, such as
olefins, alkynes, ketones, aldehydes, and imines in solution
by hydride reagents (NaBH4, LiAlH4, NADH, etc.) to the
corresponding saturated compounds are fundamental reac-
tions[1] that have been extensively applied in organic synthe-
ses.[2] The thermodynamics and kinetics of reduction of
unsaturated compounds are not only dependent on the
hydride-donating ability (hydricity) of the hydride reagents,
but also on the hydride-accepting ability (hydride affinity) of
the unsaturated compounds (hydride acceptors).Hence, the
hydride affinity of unsaturated compounds should be an
important clue to select suitable reducing agents for their
efficient reduction and to perform detailed thermodynamic
analyses on the reduction mechanisms.Examination of past
publications on this subject shows that, although much
attention has been paid to determining hydricity of reducing
agents (hydride donors) and the hydricity values of many
reducing agents have been accurately determined,[3] no
reports have been made on the determination of hydride
affinity of unsaturated organic compounds, especially in
solution.The main reason could be that the state free
energy or state enthalpy of the free hydride anion in solution
is not available, and this strictly limits the experimental
methods that can be efficiently applied for the direct
determination of the hydride affinities of unsaturated com-
pounds in solution.Several years ago, we embarked on a
major project to experimentally determine the hydride
affinity scale of various unsaturated compounds in organic
solvents.Herein we report a facile and efficient experimental
method to determine the hydride affinity of polarized olefins
1 in acetonitrile.
[*] Prof. Dr. X.-Q. Zhu, M. Zhang, Q.-Y. Liu, X.-X. Wang, J.-Y. Zhang,
Prof. Dr. J.-P. Cheng
Department of Chemistry and the
State Key Laboratory of Elemento-Organic Chemistry
Nankai University
Tianjin 300071 (P.R. China)
Fax: (+86)22-2350-2458
E-mail: xqzhu@nankai.edu.cn
[**] X.-Q.Z. is grateful for financial support from the Ministry of Science
and Technology of China (Grant No. 2004CB719905), the National
Natural Science Foundation of China (Grant Nos. 20272027,
20472038, and 20421202), and Natural Science Fund of Tianjin
(Grant No. 033804311).
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Table 1: Hydride and hydrogen affinities of olefins 1, as well as proton
The hydride affinity of olefins 1 [DHHꢀA(1)] is defined in
this work as the enthalpy change in the reaction of olefin 1
with free hydride ion to form carbanion 2 at 258C [Eq.(1)].
affinities and hydrogen affinities of the radical anions 1Cꢀ in acetoni-
trile [kcalmolꢀ1].[a]
G
DHHꢀA(1)[b]
DHHA(1)[c]
DHPA(1Cꢀ)[d]
DHHA(1Cꢀ)[e]
1a (X=CN, Y=CN)
CH3O
CH3
H
Cl
Br
ꢀ60.4
ꢀ60.6
ꢀ61.0
ꢀ61.6
ꢀ61.5
ꢀ62.6
ꢀ63.0
ꢀ42.6
ꢀ42.7
ꢀ42.6
ꢀ42.8
ꢀ42.7
ꢀ43.2
ꢀ43.0
ꢀ25.3
ꢀ24.1
ꢀ22.8
ꢀ20.4
ꢀ20.1
ꢀ16.6
ꢀ13.5
ꢀ70.1
ꢀ69.0
ꢀ68.1
ꢀ66.2
ꢀ65.9
ꢀ63.0
ꢀ60.5
CF3
NO2
According to the definition of hydride affinity of 1 [Eq.(2)], it
is clearly difficult to directly determine the hydride affinity of
the olefins [DHHꢀA(1)] in solution by an experimental method
because the enthalpy of the free hydride ion in solution is not
available.
1b (X=CN, Y=CO2Et)
CH3O
CH3
H
Cl
Br
ꢀ58.1
ꢀ58.4
ꢀ59.0
ꢀ59.4
ꢀ59.5
ꢀ60.3
ꢀ60.8
ꢀ40.9
ꢀ41.0
ꢀ41.3
ꢀ41.2
ꢀ41.3
ꢀ41.5
ꢀ41.6
ꢀ27.4
ꢀ26.5
ꢀ24.9
ꢀ22.6
ꢀ22.7
ꢀ19.0
ꢀ15.8
ꢀ71.6
ꢀ70.8
ꢀ69.6
ꢀ67.8
ꢀ67.8
ꢀ64.8
ꢀ61.9
DHHꢀAð1Þ ¼ Hð2Þꢀ½HðHꢀÞ þ Hð1Þꢁ
ð2Þ
CF3
NO2
However, examining Equation (1) shows that the hydride
ꢀ
affinity of olefins is simply equal to the heterolytic Cb H bond
1c (X=CN, Y=CONH2)
dissociation energy of the corresponding carbanion 2 of 1
[DHhet(2)] by switching the sign [Eq.(5)].This grants efficient
and facile access to determining the hydride affinity of olefins
1 in solution according to the enthalpy change in the reaction
of the corresponding carbanion 2 with the strong hydride
acceptor N-methylacridinium [AcrH+; Eqs.(3) and (4)]. In
CH3O
CH3
H
Cl
Br
ꢀ55.1
ꢀ55.3
ꢀ55.7
ꢀ56.6
ꢀ56.7
ꢀ57.2
ꢀ57.7
ꢀ38.6
ꢀ38.6
ꢀ38.7
ꢀ39.2
ꢀ39.2
ꢀ39.1
ꢀ39.1
ꢀ30.2
ꢀ28.9
ꢀ26.9
ꢀ24.4
ꢀ24.5
ꢀ20.8
ꢀ17.7
ꢀ73.7
ꢀ72.5
ꢀ70.8
ꢀ68.8
ꢀ68.9
ꢀ65.7
ꢀ63.2
CF3
NO2
1d (X=CO2Et, Y=CO2Et)
CH3O
CH3
H
Cl
Br
ꢀ52.8
ꢀ53.1
ꢀ53.7
ꢀ54.2
ꢀ54.3
ꢀ55.0
ꢀ55.7
ꢀ36.9
ꢀ37.0
ꢀ37.3
ꢀ37.4
ꢀ37.5
ꢀ37.6
ꢀ37.8
ꢀ31.6
ꢀ30.7
ꢀ29.1
ꢀ27.0
ꢀ27.8
ꢀ23.8
ꢀ21.6
ꢀ74.4
ꢀ73.8
ꢀ72.5
ꢀ70.8
ꢀ71.0
ꢀ68.2
ꢀ66.4
CF3
NO2
[a] Relative uncertainties were estimated to be smaller than or close to
1 kcalmolꢀ1 in each case. [b] DHHꢀA(1) was estimated from Equation (5),
taking DHhet(AcrH2)=81.1 kcalmolꢀ1 from reference [5]. [c] Estimated
from Equation (6) with Eo(H0/ꢀ)=ꢀ1.137 V vs. Fc+/Fc0 (Fc=ferrocen).[9]
[d] Estimated from Equation (7) with Eo(H+/0)=ꢀ2.307 V vs. Fc+/Fc0.[9]
[e] Estimated from Equation (8) with Eo(H+/0)=ꢀ2.307 V vs. Fc+/Fc0.
Equation (5), DHr is the enthalpy change of the reaction
[Eq.(3)], which can be obtained by titration calorimetry; [5]
ꢀ
DHhet(AcrH2) is the heterolytic C9 H bond dissociation
energy of 9,10-dihydroacridine (AcrH2) in solution,
which we have determined previously.[5]
DHr ¼ DHhetð2ÞꢀDHhetðAcrH2Þ
ð4Þ
ð5Þ
DHHꢀAð1Þ ¼ ꢀDHhetð2Þ ¼ ꢀ½DHr þ DHhetðAcrH2Þꢁ
Herein, the hydride affinity scales of four types of
polarized olefins 1a–d, which have been extensively
applied in organic syntheses,[6] were examined in acetoni-
trile solution.The hydride affinities of 1a–d are summar-
ized in Table 1.The enthalpy changes of the reactions of
2a–d with AcrH+ [Eq.(3)] and the standard redox
potentials of the relevant species are listed in the
Supporting Information.
Scheme 1. Possible pathways of the reduction of 1 by hydride.
As the reductions of olefins 1 by hydride ions could
involve multistep hydride-transfer mechanism, such as
eꢀ–H+–eꢀ, eꢀ–H, H–eꢀ, and so on (Scheme 1),[7] it is evident
that the hydrogen affinity of olefins 1, DHHA(1), as well as
proton affinity and hydrogen affinity of the corresponding
radical carbanion of the olefins 1Cꢀ, DHPA(1Cꢀ) and DHHA(1Cꢀ),
are also very important for elucidating the reduction mech-
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Communications
anism.In this work, their values were also estimated accord-
ing to Equations (6)–(8), which were derived from appropri-
ketones and aldehydes should all be close to those of the
corresponding olefins 1 in the same solvent.
In Table 1, the hydrogen affinities of olefins 1 in aceto-
nitrile range from ꢀ42.6 to ꢀ43.0 kcalmolꢀ1 for 1a, from
ꢀ40.9 to ꢀ41.6 kcalmolꢀ1 for 1b, from ꢀ38.6 to ꢀ39.1 kcal
molꢀ1 for 1c and from ꢀ36.9 to ꢀ37.8 kcalmolꢀ1 for 1d.These
values are all close to or slightly larger than the average
hydrogen affinities of typical olefins in the gas phase
(ꢀ35.2 kcalmolꢀ1),[13] which means that the hydrogen affin-
ities of olefins 1 determined in this work should be reasonable
and reliable, and the same should hold true for their hydride
affinities in acetonitrile.The hydrogen affinities of olefins 1
are smaller than the corresponding hydride affinities by about
18 kcalmolꢀ1, that is, olefins 1 should prefer hydride to the
neutral hydrogen atom.
Proton and hydrogen affinities of the radical anions of
olefins (1Cꢀ) in acetonitrile range from ꢀ25.3 to ꢀ13.5 and
from ꢀ70.1 to ꢀ60.5 kcalmolꢀ1 for 1Cꢀ, from ꢀ27.4 to ꢀ15.8
and from ꢀ71.6 to ꢀ61.9 kcalmolꢀ1 for 1bCꢀ, from ꢀ30.2 to
ꢀ17.7 and from ꢀ73.7 to ꢀ63.2 kcalmolꢀ1 for 1cCꢀ, and from
ꢀ31.6 to ꢀ21.6 and from ꢀ74.4 to ꢀ66.4 kcalmolꢀ1 for 1dCꢀ,
respectively (Table 1).Thus, the proton affinities of radical
anions 1Cꢀ are much smaller than the corresponding hydrogen
affinities, by about 45 kcalmolꢀ1, which suggests that these
radical anions are strong acceptors of the neutral hydrogen
atom but weak proton acceptors.Conceivably, if reduction of
olefins 1 by a hydride donor were initiated by single-electron
transfer, hydrogen-atom transfer in the second step to the
formed 1Cꢀ should be much easier than proton transfer to the
same radical anion.Since the hydrogen-atom affinities of 1Cꢀ
all are large (ꢀ60.5 to ꢀ74.4 kcalmolꢀ1), hydrogen-atom
transfer could be extremely fast and possibly diffusion-
controlled.We conclude that the e ꢀ–HC sequence of hydride
transfer should be most likely among the possible multistep
mechanisms for reduction of olefins 1 by hydride if the
reactions were initiated by single-electron transfer.
DHHAð1Þ ¼ DHHꢀAð1ÞꢀF½EoðH0=ꢀÞꢀEoð20=ꢀÞꢁ
ð6Þ
ð7Þ
ð8Þ
DHPAð1ꢀCÞ ¼DHHꢀAð1Þ þ F½Eoð10=ꢀÞꢀEoðH0=ꢀÞꢁ
ꢀF½EoðHþ=0ÞꢀEoð20=ꢀÞꢁ
DHHAð1ꢀCÞ ¼ DHHꢀAð1ÞꢀF½EoðH0=ꢀÞꢀEoð10=ꢀÞꢁ
ate thermodynamic cycles (see Supporting Information).[8]
The results are also summarized in Table 1.
Table 1 shows that the hydride affinities of olefins 1 in
acetonitrile range from ꢀ60.4 to ꢀ63.0 kcalmolꢀ1 for 1a, from
ꢀ58.1 to ꢀ60.8 kcalmolꢀ1 for 1b, from ꢀ55.1 to ꢀ57.7 kcal
molꢀ1 for 1c, and from ꢀ52.8 to ꢀ55.7 kcalmolꢀ1 for 1d.The
hydride affinities of olefins 1 decrease in the order 1a > 1b >
1c > 1d.[10] Among the 28 olefins, 1a (G = NO2) is the
strongest oxidant (DHHꢀA = ꢀ63.0 kcalmolꢀ1) and the easiest
to reduce; 1d (G = CH3O) should be the weakest oxidant
(DHHꢀA = ꢀ52.8 kcalmolꢀ1) and the most difficult to reduce.
The hydride affinities of olefins 1[10] are much smaller than
those of the corresponding primary benzylic carbonium ions
in acetonitrile solution (e.g., ꢀ106, ꢀ112, ꢀ118, and
ꢀ121 kcalmolꢀ1 for 4-CH3OC6H4CH2 , 4-MeC6H4CH2 ,
+
+
C6H5CH2 , and 4-ClC6H4CH2 , respectively),[11] but slightly
smaller than or close to that of NAD+ models (NADH =
nicotinamide adenine dinucleotide) in acetonitrile (e.g.,
ꢀ64.2 kcalmolꢀ1 for BNA+ and ꢀ66.9 to ꢀ72.6 kcalmolꢀ1
for (G)PNA+; G = CH3O, CH3, H, Cl, Br, and CF3),[4] which
indicates that olefins 1, like the NAD+ models, are not strong
hydride acceptors.Thus, if these olefins are to be efficiently
+
+
reduced, only stronger reducing agents such as NaBH3(CN)
ꢀ1
(DHhet(B H) = 45.0 kcalmol in acetonitrile)[12] should be
ꢀ
chosen.Weak reducing agents, such as N-benzyl-1,4-dihydro-
nicotinamide (BNAH) and N-phenyl-1,4-dihydronicotin-
amide (PNAH), can not be used to reduce them in
nonprotic solvents, as is well supported by experimental
results.
The effect of the a substituents X and Y on the hydride
affinity DHHꢀA(1) (slope of ꢀ17.7, see Supporting Informa-
tion) is much larger than that of the remote substituent G
(slope of ꢀ2.56). Electron-withdrawing groups increase the
hydride affinity, while electron-donating groups decrease it,
that is, the hydride affinities of olefins 1 are mainly dependent
on the stability of carbanions 2, since a-Y(X) is closer to the
center of negative charge than G.By using a similar method,
the effects of a-substituents X and Y and remote substituent
G on DHHA(1), DHPA(1Cꢀ), and DHHA(1Cꢀ) can also easily be
elucidated (see Supporting Information).
Examining reaction (1) shows that, unlike the reduction of
benzylic carbonium ions by hydride, the reduction of olefins 1
ꢀ
by hydride does not only involve the formation of a new C H
=
s bond to release energy, but also dissociation of one C C p
bond to consume energy.Hence, the magnitude of the olefin
hydride affinities should be equal to the heterolytic dissoci-
ation energy of the newly formed C H s bond minus the
ꢀ
=
heterolytic dissociation energy of the broken C C p bond.
Thus, it is easy to understand why the hydride affinity scale of
olefins 1 is much smaller than that of the corresponding
primary benzylic carbonium ions, since the hydride affinities
of the primary benzylic carbonium ions do not involve the
In summary, the hydride affinities of 28 polarized olefins 1
in acetonitrile were determined by an experimental method,
while the hydrogen-atom affinities of 1 and the proton affinity
and hydrogen-atom affinity of the radical anions 1Cꢀ were
estimated by using thermodynamic cycles according to Hessꢀ
law.These thermodynamic data should be very valuable when
choosing suitable reducing agents for olefins and carrying out
detailed thermodynamic analysis of the reduction mechanism.
This paper not only reports the hydride affinities of polarized
olefins in solution for the first time, but also offers a facile and
efficient experimental method to determine the hydride
affinities of unsaturated organic compounds such as ketones,
=
heterolytic dissociation energy of the broken C C p bond.
From the same viewpoint, we also can explain why the
hydride affinity scale of olefins 1 is close to or slightly smaller
than that of NAD+ models in the same solvent: The reason is
that the reduction of the NAD+ models by hydride, like the
reduction of the olefins, also involves dissociation of one p
bond in the pyridine ring.This leads to the valuable prediction
that the hydride affinities of carbonyl compounds such as
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Chemie
aldehydes, and imines in solution, which have also not yet
been reported.The detailed hydride affinities of some imines
will be disclosed in a forthcoming paper.
Received: February 9, 2006
Published online: May 3, 2006
Keywords: alkenes · calorimetry · hydrides · radical anions ·
.
thermodynamics
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1
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