Mendeleev Commun., 2020, 30, 38–39
Table 1 Experimentaldataforthe acidichydrolysisof dialkylarylphospho-
nates 1a–f.
Regarding the second (2 ® 3) step, the order of reactivity is
somewhat different to that of the first (1 ® 2) step:
R/Y Bn/H >> Pri/H > Me/H > Et/MeC(O) ∼ Et/H > Et/Me.
tc
/min
Final
product
Entry Reactant k1/h–1
k2/h–1 tcompl/h
Yield (%)
max2
At the same time, the gross reactivity coinsided with that
observed for the hydrolysis of the second P–O–R moiety of the
phosphonate. It means that the second step is the rate determining
one. Thus, this is the first case when the two step acidic hydrolysis
of phosphonates has been evaluated in detail and characterized
quantitatively.
In summary, the course and the substituent dependence for the
HCl-catalyzed two step hydrolysis of dialkyl arylphosphonates
has been evaluated, and the two steps have been characterized by
pseudo first order rate constants. It has been substantiated that in
the case of hydrolysis of the isopropyl ester, the AAl1 mechanism
predominates over the AAc2 route, and that the gross reactivity
follows the trend observed for the hydrolysis of the second
P–O–R moiety of dialkyl arylphosphonates.
1
2
3
4
5
6
a
1a
1b
1c
1d
1e
1f
2.67
40
0.70
0.27a
1.33
5.5
9.5
4.5
3a
3a
3a
3a
3b
3c
95
90
99
80
87
86
0.88 120
2.08
23.8
35
5
9.36 0.75
0.16 17.5
0.86 180
0.90 105
0.35
8.5
Independent experiment PhP(O)(OEt)(OH) ® PhP(O)(OH)2 resulted in
k2 = 0.29 h–1
.
The gross reactivity of arylphosphonates 1a–f in the 1 ® 3
transformation changes as follows:
R/Y Bn/H >> Pri/H > Me/H > Et/MeC(O) > Et/H >> Et/Me.
∼
∼
The reactivity of phosphonates 1a–f in the first (1 ® 2) step of
the hydrolysis is the following:
This work was supported by the National Research, Develop-
ment and Innovation Office (K119202) and by the Ministry of
Human Capacities of Hungary (grant BME FIKP-BIO). N. Z. K.
is grateful to the János Bolyai Research Scholarship of the
Hungarian Academy of Sciences (BO/00130/19/7).
R/Y Bn/H >> Me/H > Pri/H > Et/MeC(O) ∼ Et/H ∼ Et/Me.
It is noteworthy that the hydrolysis of the second ethoxy group
of phosphonate 1b could also be performed as a neat 2b14 ® 3a
transformation under similar conditions (Figure S3). In this case,
a k2 value of 0.29 h–1 was obtained, which is in excellent agreement
with the value of 0.27 h–1 obtained from the two step experiment
(see Table 1, entry 2). Considering alkyl phosphonates 1a–c, the
sterically hindered isopropyl ester 1c was the most reactive. As
suggested by the ratio of k1/k2 = 2.08/1.33, the reactivities of both
PriO groups are closer than that of the alkoxy groups of ethyl ester
1b or methyl derivative 1a, characterized by the k1/k2 ratios of
0.88/0.27 and 2.67/0.70, respectively. These results originate from
the predominance of the AAl1 mechanism over the AAc2 route for the
PriO derivative. In comparison with the unsubstituted analogue,
the 4-Me group had more significant impact on the hydrolysis rate
of the second EtO group than on that of the first EtO substituent
(k1/k2 = 0.86/0.16 vs. k1/k2 = 0.88/0.27).
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.01.012.
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ꢃ00
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ꢂ0
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0
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ꢄ
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tꢊh
tꢊh
Figure 1 Concentration profiles for the components in the hydrolysis of
phenylphosphonates (a) 1a, (b) 1b, (c) 1c and (d) 1d under optimized
conditions. The R2 values of goodness of fits are 0.995, 0.994, 0.979 and
0.934, respectively.
Received: 17th July 2019; Com. 19/5989
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