DEDIAZONIATION OF ARENEDIAZONIUM SALT WITH TRIVALENT PHOSPHORUS COMPOUNDS BY ONE-ELECTRON TRANSFER MECHANISM

Article abstract of DOI:10.1016/0040-4039(91)80095-N

Trivalent phosphorus compounds, triphenylphosphine and triethyl phosphite, gave rise to dediazoniation of an arenediazonium salt in alcoholic solvent at ambient temperature in the dark.The reaction was well interpreted in terms of radical-chain mechanism initiated by an electron transfer from the trivalent phosphorus to a diazonium salt, which postulates a cation radical from the trivalent phosphorus as an intermediate.

Full text of DOI:10.1016/0040-4039(91)80095-N

Tetr~hedron Letters, Vo132, No.40, pp 5601-5604, 1991
Printed m Great Britain
0040-4039/91 $3.00 + 00
Pergamon Press pie
DEDIAZONIATION OF ARENEDIAZONIUM SALT WITH TRIVALENT PHOSPHORUS
COMPOUNDS BY ONE-ELECTRON TRANSFER MECHANISM
Shinro Yasui,* Masayuki Fujii,t Chihiro Kawano,t Yukihiro Nishimura,t and Atsuyoshi Ohnott
Tezukayama College, Gakuen-Minami, Nara 631 J a p a n
tDepartment of Chemistry, Faculty of Engineering, Kinki University, Iizuka, Fukuoka 820 Japan
t~'lnstitute for Chemical Research, Kyoto University, Uji, Kyoto 611 Japan
Abstract:
Trivalent phosphorus compounds, triphenylphosphine and triethyl phosphite, gave rise to
dediazoniation of an arenediazonium salt in alcoholic solvent at ambient temperature in the dark. The reaction
was well interpreted in terms of radical-chain mechanism initiated by an electron transfer from the trivalent
phosphorus to a diazonium salt, which postulates a cation radical from the trivalent phosphorus as an
intermediate.
It has been widely accepted that exothermic conversion of a trivalent phosphorus compound to the
corresponding pentavalent one takes place usually through nucleophilic attack of a trivalent phosphorus
compound to a substmte.1) On the other hand, several examples have shown that a trivalent phosphorus acts
as an electron donor to a good electron acceptor,2) which leads us to an idea that apparent nucleophilic attack ofa
trivalent phosphorus to a substrate may be composed of an electron transfer and a subsequent radical-pair
coupling processes. In this matter, it is of special importance to evaluate intrinsic ability for a trivalent
phosphorus compound to release an electron.
We have now found that a trivalent phosphorus compound gives rise to dediazoniation of an
arenediazonium salt in an alcoholic solvent at ambient temperature in the dark. This briefreport will present,
for this reaction, a radical-chain mechanism initiated by an electron transfer from a trivalent phosphorus to the
diazonium salt, with referring possible reactions of the postulated cation radical from the trivalent phosphorus
compound.
4
-Nitrobenzenediazonium fluoroborate (1) were allowed to react with an appropriate amount of
triphenylphosphine (2a) or triethyl phosphite (2b) in an alcoholic solvent at 200C in the dark for 30 min. The
analyses of the reaction mixture by a gas chromatograph (GC) and a gas chromatograph-mass spectrometer
(
GCMS) confirmed the formation of nitrobenzene (3) and the corresponding oxidized products from 2a-b,
triphenylphosphine oxide (4a) or triethyl phosphate (4b), respectively, along with a small amount of 4,4'-
dinitrodiphenyl (5) (eq.1). 3)
nitrophenylphosphonate (6b).
The reaction with 2b also gave a significant amount of diethyl 4-
5
601

5
602
in R'OH
N O 2 ~ _ XN f B F 4-
+
R3 P
• No2©+ (No2-
4
1
2a; R=Ph
3
(major)
5 (trace)
2
b; R=OEt
(
eq. 1)
100
The yields of the products
determined by GC for the reactions in
methanol were plotted against the ratio of
an initial amount of 2 to that of 1 (2/1)
A
3
\
(
Figures 1A and B). Importantly, more
than theoretical amount of 3 was
formed when the ratio 2/I is less than ca.
.5, which indicates participation of a
a
5o
~ 4
0
chain process in the reaction. That the
formation of 3 was suppressed under
oxygen atmosphere suggests that this
process is a radical-chain.
As listed in
0
.5
1[0
Table 1, the results of the experiments in
deuterated solvents clearly show that a
2/1
hydrogen at the a-carbon rather than a
1
00
hydroxyl hydrogen is introduced into 3,
which is expected for the radical-chain
mechanism where the aryl radical A r-
generated from 1 abstracts a hydrogen
6
b
from the solvent alcohol forming an ct-
5
0
hydroxyalkyl radical as a chain-carrier. .~
The formation of the dimeric produc! 5 is
also m consistence with this mechanism.
According to this mechanism, the solvent
alcohol would eventually be oxidized to
the corresponding carbonyl compound.
The results in Table 2 show that the
reactions in b e n z y l alcohol and
cyclohexanol gave benzaldehyde and
cyclohexanone, respectively, again
verifyrag the radical-chain mechanism.
_
A
~
......
0
5
10
2
/!
F i g u r e 1. Yields of the products varying the initial amount
of 2 based on 1. A and B represent the reactions with 2a
and 2b, respectively. Initial amount of 1, 0.10 mmole in
1
.0 ml of methanol. O, [ ] , / x , and ~ denote the yields of
Decrease in the yield of 3 after ~t reaches
3
, 4, 5, and 6b under N2 atmosphere, respectively.
•
the maximum as the ratio 2/1 increases is
denotes the yield of 3 under 02 atmosphere

5
603
attributable to the consumption of Ar. by the attack to 2 when an excess amount of 2 exists in the reaction
mixture, resulting in many unexpected reactions.4) Similar yield-substrate ratio profile has been observed in the
dediazoniation of I with 1-benzyl-1,4-dihydronicotinamide, where the reaction proceeds through a radical-chain
mechanism.5)
T a b l e 1. Dediazoniation of
with in deuterated
T a b l e 2.
Solvent effect on product
1
2
distribution in the reaction with 2a.a;
methanols, a)
R'
Yield, %b)
tdvalent-
solvent
phosphorus
No in R'OH
3
4
5
C=Oc)
%D
in 3b)
1
2
3
Me
Benzyl
cyclo-Hex
34 43
32 18 < 1
44 25 < 1
6
d)
7
2
a
CH3OD
CD3OD
CH3OD
CD3OD
< 0.5
99.5
< 0.5
99.5
38
2
b
a)0.10 mmol of 1 and 2a were reacted in
.0 ml of R'OH. At 20°C under N2 atmos-
1
phere in the dark. b)Based on 1 used.
C)Benzaldehyde and cyclohexanone for No.
a)0.05 mmole of 1 and 2 were
reacted in 0.5 ml of methanol.
2
and 3, respectively,
formaldehyde was not determined.
d)Yield of
At 20 °C under N2 atmosphere
in the dark. b)Determined on
GCMS.
Now, the well established radical-chain mechanism for the present reaction in hand, we can
unambiguously manifest that an electron transfer from 2 to 1 is the process that generates the radical Ar. as well
as 2+. . It should be mentioned that no additive is required in this reaction. Thus, this chain process can be
summarized as in eqs.2-4.
e-transfer
1
+ 2
~
At.
+
2+.
(eq.2)
-
N2, -BF4"
Ar.
+
R " R ' " C H O H
~.- ArH
3
+
R ' R " C - O H
W'R"'C=O
(eq.3)
(eq.4)
R " R ' C - O H
+
1
~
Ar-
+
+
H÷
-
N2, -BF 4
The cation radical 2+. can interact with a solvent molecule to give phosphoranyl radical 7, which
collapses into the final product 4 (eqs.5 and 6). 6) Importantly, 2b is converted into 4b with transesterification
to diethyl methyl 4-nitrophenylphosphonate (4b') in methanol, supporting the existence of 7 as an intermediacy
along this pathway.
The product ratio 4 b : 4 b ' = 23:77 was observed on GC, which roughly reapresents
statistical distribution predicted by eq.6.

5604
2
a +'
+
R'OH
~
W
Phsl;OR'
~ PhsP=O
(eq.5)
(eq.6)
-
7 a
-
R'.
4 a
2
b+" + R'OH
~'- (EtO)31~OR'
•-- --(EtO)3P=O + (EtO)2(R'O)P=O
-
H +
7 b
-
R'. or -Et.
4 b
4 b '
The observation that the reaction with the phosphite 2b affords the phosphonate 6b as well (see Figure
B) is also explained taking into account the radicals Ar. and 2+'. Thus, this product is formed in the Arbuzov
1
fashion if both radicals couple to give the phosphonium ion 8b (eq.7). An ionic reaction between 1 and 2b, if
any, would afford an azo-compound such as Ar-N=N-P÷R3 7)
O
+
~
H
At.
+
2b+"
It-- NO2~--~-P(OEt)3
~
NO2~x-'P(OEt)2
(eq.7)
_Et÷
8
b
6b
In conclusion, we have verified the formation of the cation radical 2+" during the dediazoniation of an
arenediazonium salt. More direct evidence will be available by ESR. The study is now in progress.
R e f e r e n c e s a n d N o t e s
I) For example see; D. J. H. Smith, "Comprehensive Organic Chemistry, The Synthesis and the Reactions of
Organic Compounds, Vol. 2" ed. by I. O. Sutherland, Pergamon Press, Oxford (1979), pp 1146 - 1156; R.
S. Edmundson, ibid., pp. 1203 - 1223.
2
) S. Yasui andA. Ohno, TetrahedronLett., 32, 1047 (1991); R. L Powell and C. D. Hall, Z Am. Chem.
Soc., 91, 5403 (1969).
3
) Arenediazonium salt with an electron-releasing substituent, 4-methoxylbenzene&azonium fluoroborate, was
also dediazoniated by 2 to give 3 m comparable yield.
4
5
) In fact, the chromatogmm of HPLC for the reaction mixture showed some unidentified peaks as well.
) S. Yasui, K. Nakamum, and A. Ohno, J. Org. Chem., 49,878 (1984).
6
) W. G. Bentrude and R. A. Wielesek, Z Am. Chem. Soc., 91,2406 (1969); W. G. Bentrude and T. B. Min,
ibid., 94, 1025 (1972).
7) D -L. F. DeTar and M. N. Turetzky, J. Am. Chem. Soc., 77, 1745 (1955); T. J. Broxton and M. J
McLeish, J. Org. Chem, 48,191 (1983).
(
Recewed in Japan 21 June 1991)