1042
M. Alajar´ın et al. / Tetrahedron Letters 42 (2001) 1041–1043
From the equation in Scheme 1 it is obvious that the
fragmentation of 1a into 2a and 3 is in fact a hydrolytic
process, and that the trace amounts of water contained
in the CDCl3 should be the actual responsible. How-
ever, we did not observe a similar fragmentation in
other solvents such as wet DMSO-d6, benzene or tolu-
ene. We then suspected that traces of HCl in the
commercial CDCl3 were also essential for promoting
the hydrolytic fragmentation of 1a. This was confirmed
by adding a catalytic amount of 1N aqueous HCl to a
refluxing solution of 1a in wet CHCl3, which ended in
the virtually quantitative formation of 2a and 3 after
short reaction times (2–3 h).
equivalent of 10N aqueous HCl was added at once, at
room temperature, to a chloroform solution of 1a
(Ar=4-H3C-C6H4), the salt 6 cleanly formed, which
could be isolated and characterised. Obviously we next
checked the evolution of 6 under the conditions in
which the fragmentation of 1 occur. Unexpectedly,
aminophosphonium chloride 6 remained unchanged for
months in CDCl3 solution or in refluxing wet CHCl3
(Scheme 2). This result revealed that the rapid and
complete conversion of 1a into 6 blocked the fragmen-
tation process.
However, the addition of one equivalent of Et3N to the
CDCl3 solution of 6 cleanly converted it into 2a and the
corresponding 3 in less than 24 h. Most probably, the
base is required for assisting the water addition to the P
atom of the phosphonium function in 6, the first step of
the classical alkaline hydrolysis of phosphonium salts.3
If the fragmentation of 6 follows the well-established
mechanistic scheme of those hydrolytic reactions, the
next step should be the detachment of one substituent
(the best leaving group) from the pentacoordinated
phosphorus of the resulting species, with the assistance
of a base, thus yielding the new PꢁO bond.
In our hands, the sulphur analogous 1b (Ar=4-
H3CC6H4, 4-H3COC6H4)7 behaved similarly, giving rise
to diphenylmethylphosphane sulphide 2b and the corre-
sponding phosphinamide 3.
It has been reported8 that di-l5-phosphazenes 1c de-
rived from dppm experimented acid-catalysed hydroly-
sis to give phosphanilides 3, phosphane oxide 2a and
the corresponding aniline. In light of our present
results, we presume these last two compounds come
from the hydrolysis of the PꢁN function of the phos-
phazenes 2c.
On this basis, we believe that a reasonable sequence for
explaining the acid-catalysed hydrolytic fragmentation
of compounds 1 could be that represented in Scheme 3,
a sort of reversible catalytic cycle in which 1 acts first as
a base fixing the proton at its N atom, followed by
nucleophilic attack of water to the most electrophilic P
atom, assisted by 1 as base. The resulting pentacoordi-
nated phosphorus species 7 is then cleaved, in an irre-
versible step, into 2 and 3, a process facilitated by the
action of the X atom of the P=X function as internal
base and by the release of 2 as leaving group.
In summary, compounds of general structure 1 have
been shown to fragment hydrolytically giving the spe-
cies 2 and 3 by a rare PꢀC bond fission, which requires
the catalytic action of HCl. It should be noted that
other dppm derivatives, such as 4 and 5 depicted below,
did not undergo this kind of fragmentation.
Ph2P
X
PPh2
Y
Ph2P
PPh2
X
X = O, S, Se, NR
X, Y = O, S, Se
This tentative mechanistic scheme accounts for the
most significant facts of the conversion of 1 into 2+3,
especially that only a catalytic amount of HCl is
required, whereas the complete protonation of 1 by one
equivalent of acid stops the process at the aminophos-
phonium salt 6, providing no external base is added.
While l5-phosphazenes are known to be easily cleaved
to amines and phosphane oxides under acidic or basic
hydrolytic conditions,9 the fragmentation described
here is relevant in following a notably different course.
The previously reported fragmentation of tetraalkyl
methylenediphosphonates and resembling species10 may
be mechanistically related to the one disclosed here.
5
4
The two phosphorus atoms of the dppm fragment being
at P(V) oxidation state and at least one of them in
phosphazene form seem to be mandatory for the occur-
rence of the acid-catalysed fragmentation.
Concerning the sequence of chemical events leading to
the PꢀC bond fission of compounds 1, we reasoned that
this sequence, promoted by the catalytic proton, should
involve the initial protonation of the more basic centre
in 1, the nitrogen atom. This would lead to the
aminophosphonium chlorides type 6. In fact, when one
H2O / CDCl3
Ph2P
NH
PPh2
O
+
2a
3
H2O / Et3N / CDCl3
Ar
Cl
Ar = 4-H3CC6H4
6
Scheme 2.
Alajarín, Mateo
