Azodicarboxylates
FULL PAPER
solvent on E when applying Equation (1) on reactions of 1
with nucleophiles in dichloromethane.
by Equation (1). This agreement is surprising: Since E pa-
rameters are generally derived from reactions with a series
of C-centered nucleophiles (E of 1 from reactions with en-
amines, Table 2, Figure 2) and N and s parameters are gen-
erally derived from the rates of reactions with a series of C-
centered electrophiles (N and s of 4 from reactions with
benzhydrylium ions), Equation (1) can only be expected to
Reactions with triarylphosphines: To check whether the
electrophilicity parameters E of 1a–d (Figure 3) derived
from the rates of their reactions with enamines 2 (carbon
nucleophiles) are also suitable for the prediction of the rates
of the reactions of azodicarboxylates with heteronucleo-
philes, we studied the kinetics of the reactions of 1a–d with
the triarylphosphines 4 and the amines 5, the N and s pa-
rameters of which have previously been derived from the
rates of their reactions with benzhydrylium ions.[15,16]
ꢁ
hold for the formation of C X bonds, that is, for reactions
in which at least one of the reaction centers in the electro-
phile or nucleophile is carbon.[15] The unexpected observa-
ꢁ
tion that Equation (1) also holds for the formation of an N
P bond reminds one of the observation that Equation (1), as
well as the Ritchie Equation, were also found to work for
the combinations of diazonium ions with several heteronu-
Nucleophilic additions of the triarylphosphines 4 to di-
ACHTUNGTRENNUNGalkyl azodicarboxylates 1, which correspond to the first step
cleophiles.[10,18] The comparable magnitudes of the P N
(290 kJmol ) and C N (305 kJmol ) bond energies may
ꢁ
of the Mitsunobu reaction, yield the so-called Huisgen zwit-
terions, as depicted in Scheme 2.[17]
ꢁ1
ꢁ1
ꢁ
account for this finding.[19]
Reactions with amines: The reactions of azodicarboxylates 1
with amines 5 have been reported to yield triazanes 6 in pe-
troleum ether.[20] Scheme 3 shows that triazanes 6 are also
formed in acetonitrile, that is, the solvent in which the kinet-
ꢁ
ic investigations were performed. Though N N single bonds
Scheme 2. Reactions of 1 with phosphines 4.
are generally rather weak, the triazanes 6 are stabilized by
two electron-withdrawing ester groups and can be isolated
without problem.[20a]
The kinetics of the reactions of 1 with triarylphosphines
in CH2Cl2 were determined photometrically, as described
above for the reactions of 1 with enamines 2. Linear plots of
kobs versus [PAr3] confirmed second-order rate laws for
these reactions, and the resulting second-order rate con-
stants for the attack of PAr3 at the N=N unit are summar-
ized in Table 4.
As shown in the last column of Table 4, the experimental
rate constants for the reactions of 1 with the phosphines 4
generally agree within a factor of 10 with those calculated
Scheme 3. Reactions of 1c and 1d with amines 5d–f.
Table 4. Experimental and calculated second-order rate constants for the
reactions of the azodicarboxylates 1 with the phosphines 4 in dichlorome-
thane at 208C.
Nevertheless, the thermodynamic driving force for these
additions seems to be rather low. As shown below, the rela-
tive reactivities of the azodicarboxylates 1 toward amines
are the same as toward enamines 2 and phosphines 4. For
that reason, one can expect that 1a should even react faster
with amines than the azodicarboxylates 1b–d. The fact, that
no conversion was observed when 1a was combined with
morpholine 5d and pyrrolidine 5 f must, therefore, be due to
unfavorable thermodynamics (fast reverse reactions).
The reactions of 1 with the secondary amines 5a–f were
studied in CH3CN by following the decay of the electro-
philesꢄ absorbances. When the amines were used in large
excess, first-order kinetics were observed with exponential
decays of the absorbances of the azodicarboxylates 1. How-
ever, linearity of the kobs versus [5] plots was only observed
for the reactions of 1c with 5c and 5e. For all other reac-
tions of the azodicarboxylates 1 with the secondary amines
5a–f, the plots of the kobs values versus the amine concentra-
tions showed concave curvatures (Figure 4), indicating that
exptl
calcd
calcd
1
Phosphine 4 (N/s)[a]
k2
[mꢁ1 sꢁ1
k2
[mꢁ1 sꢁ1
]
k2
k2
/
[b]
exptl
G
]
U
1a 4a ((4-ClC6H4)3P) (12.58/0.65)
1a 4b (Ph3P) (14.33/0.65)
6.41ꢃ101
4.83ꢃ102
2.42ꢃ103
2.50ꢃ102
3.44ꢃ103
1.56ꢃ104
3.26ꢃ104
5.21ꢃ102
2.43ꢃ103
5.40ꢃ103
1.64ꢃ101
2.25ꢃ102
1.06ꢃ103
2.43ꢃ103
3.9
7.1
6.4
3.0
3.6
3.8
2.3
3.8
5.0
4.9
3.3
3.5
11
1a 4c ((4-MeC6H4)3P) (15.44/0.64)
1a 4d ((4-OMeC6H4)3P) (16.17/0.62) 1.07ꢃ104
1b 4b (14.33/0.65)
1b 4c (15.44/0.64)
1b 4d (16.17/0.62)
1c 4a (12.58/0.65)
1c 4b (14.33/0.65)
1c 4c (15.44/0.64)
1c 4d (16.17/0.62)
1d 4a (12.58/0.65)
1d 4b (14.33/0.65)
1d 4c (15.44/0.64)
1.45ꢃ102
6.13ꢃ102
2.36ꢃ103
4.28
4.51ꢃ101
2.16ꢃ102
7.25ꢃ102
4.83ꢃ10ꢁ1 1.69
2.14
2.32ꢃ101
1.13ꢃ102
1.11ꢃ101
10
[a] N/s-parameters from reference [15]. [b] Calculated by using Equa-
tion (1), N and s parameters from reference [15], and the E parameters
from Table 2.
Chem. Eur. J. 2010, 16, 11670 – 11677
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11673