First ever observation of the intermediate of phosphonium salt and ylide hydrolysis: P-hydroxytetraorganophosphorane

Article abstract of DOI:10.1039/c4cc08644a

P-Hydroxytetraorganophosphorane, the long-postulated intermediate in phosphonium salt and ylide hydrolysis, has been observed and characterised by low temperature NMR, finally definitively establishing its involvement in these reactions. The results require modification of the previously accepted mechanism for ylide hydrolysis: P-hydroxytetraorganophosphorane is generated directly by 4-centre reaction of ylide with water. This journal is

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ARTICLE TYPE
First Ever Observation of the Intermediate of Phosphonium Salt &
Ylide Hydrolysis: P-Hydroxytetraorganophosphorane
Peter A. Byrne,*, a,b Yannick Ortin,a Declan G. Gilheanya
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
methyltris(pentafluorophenyl)phosphonium triflate,23 and of
P-hydroxytetraorganophosphorane,
the
long-postulated
50 phospholium iodides,20 respectively, are exceptional in that
they exhibit 1st order dependence on hydroxide concentration.
intermediate in phosphonium salt & ylide hydrolysis, has been
observed and characterised by low temperature NMR, finally
definitively establishing its involvement in these reactions.
10 The results require modification of the previously accepted
R3
I
Ph
P
Ph
P
I
R1
R2
R1
Ph
P
P
P
mechanism
for
ylide
hydrolysis:
P-
Ph
Ph
Ph
Ph
6 R1 = Ph
7 R1 = Me
OH
hydroxytetraorganophosphorane is generated directly by 4-
centre reaction of ylide with water.
1 R1 = R2 = Ph, R3 = NO2
2 R1 = t-Bu, R2 = Me, R3 = H
3 R1 = Et, R2 = Me, R3 = H
4
5
8
Chart 1. Phosphonium salts, ylides, & Pꢀ
hydroxytetraorganohydroxyphosphorane 8.
The alkaline hydrolysis of phosphonium salts has been
15 afforded extensive attention in the chemical literature, and has
been put to good use in the synthesis of tertiary phosphine
oxides.1,2 Phosphonium ylide hydrolysis too has been
employed in diverse synthetic applications.3 Recently, several
examples of tandem or domino reactions have been
20 reported4,5,6 in which the reactivity of an ylide is utilised (e.g.
for the synthesis of dihydrobenzofurans,4 or peptidyl ketones5)
followed by hydrolytic extrusion of phosphine oxide from the
resulting adduct (which may be a phosphonium ylide, betaine,
or salt). Furthermore, as a result of recent developments in
25 phosphine oxide reduction methodology (and indeed the
reduction of the P=O bond in related species),7 phosphonium
salt hydrolysis followed by phosphine oxide reduction has now
become an attractive route to phosphines (and derivatives).
This is of particular importance for chiral phosphonium salts,
30 which are hydrolysed stereospecifically8 and thus give the
possibility of obtaining enantiopure or enantioꢀenriched chiral
phosphines.
55
The mechanism that has been proposed for alkaline
hydrolysis of phosphonium salts is shown in Scheme
1.11,12,13,14,16,17,24 Nucleophilic attack of hydroxide at
phosphorus gives
a
hydroxyphosphorane (with apical
oxygen8,25,26). This is deprotonated by hydroxide to give an
60 oxyanionic phosphorane, which expels a carbanion (probably
protonated in the process of its expulsion) to give phosphine
oxide and alkane or arene.27 Largely on the basis that ylide
hydrolysis gives the same products as phosphonium salt
hydrolysis,15 it has generally been concluded that ylide
65 hydrolysis proceeds by the same mechanism with one extra
(initial) step ꢀ protonation of the ylide by water to give
phosphonium hydroxide.10,28,29
H
R1
P
O
P
R4
O
O
P
-R4H
R1
R3
R1
R3
OH
OH
R2
P
R4
R2
R4
R2
OH
R1
R2
H
O
H2O
R3
R3
H
Scheme 1. The mechanism of alkaline hydrolysis of a phosphonium salt.
The hydrolysis of a phosphonium ylide (by addition of water
to an anhydrous solution of the ylide)2,9,10 and that of the
35 parent phosphonium salt under alkaline conditions afford the
same products – phosphine oxide and the alkane or arene that
results from scission of the PꢀC bond to the group that gives
the most stable carbanion (i.e. the order of ease of PꢀC bond
70
The existence of an intermediate in these reactions is a
necessity in view of the kinetic order of the reaction (& in
particular that it is 2nd order in hydroxide). Based on analogy
with many other instances in which phosphoranes are
involved,25,30,31,32,33,34,35 including the relatively stable P-
75 alkoxytetraorganophosphoranes that have been observed
spectroscopically,35 it is reasonable to conclude that the
intermediate in question is a trigonal bipyramidal (TBP) Pꢀ
hydroxytetraorganophosphorane, as shown in Scheme 1.
Consistent with this interpretation are the results of hydrolysis
80 reactions of phosphonium salts in which the phosphorus atom
is in a cycle of five or fewer members (see the ESI for
additional discussion on this topic).† Certain chiral small ring
phosphonium salts (and also compound 2; see Chart 1)24,36
have been shown to undergo hydrolysis with retention of
clavage is p-nitrobenzyl
>
benzyl
>
phenyl
>
40 alkyl).8,11,12,13,14,15,16,17,18,19,20,21 Hydrolysis of phosphonium salts
has been shown to be 3rd order (1st order in phosphonium salt,
2nd order in hydroxide) for benzyltriphenylphosphonium,11,12,14
benzyltrialkylphosphonium,14
phosphonium, mixed
mixed
alkyl/phenyl
benzyl/alkyl/phenyl
phosphonium,14
45 tetraphenylphosphonium,12,14 and tetrabenzylphosphonium
salts.13 3rd order hydrolysis has also been observed for cyclic
phosphonium salts.22 The hydrolysis reactions of paraꢀ
nitrobenzyltriphenylphosphonium bromide (1, Chart 1),12,23 of
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configuration at phosphorus,37 while other cyclic phosphonium 30 been observed or characterised spectroscopically.44,45,46,47,48
salts undergo ring opening in the course of hydrolysis,20,22,38
including dibenzophospholium salts such as 4 (Chart 1).18,39 In
TBP phosphoranes, the leaving group is obliged to depart from
an apical site.25,40 However, in a cyclic hydroxyphosphorane,
Since low temperature NMR analysis of oxaphosphetanes
(OPAs)32,33,34 and of alkoxyphosphoranes35 was known to be
possible, and had proven to be so useful in the analysis of the
reactions in which these species are involved, we undertook to
5
the apical sites are expected to be occupied by hydroxide and 35 determine
if
spectroscopic
observation
of
P-
one ring PꢀC bond.25,26,30,35,41 Thus, for decomposition of the
intermediate to occur, the reaction must proceed either with
ring cleavage or with stereomutation by pseudorotation to
hydroxytetraorganophosphorane(s) could be realised. We also
considered that these experiments might allow us to clarify
what seemed to us to be an inconsistency in the accepted
mechanism of ylide hydrolysis.10,15,28 The supposition that the
10 place the leaving group in an apical site in the transition
state.42 If starting from a chiral phosphonium salt, the ligand 40 first step of ylide hydrolysis involves formation of
permutation that occurs during pseudorotation can reveal itself
in the product phosphine oxide. Racemisation or retention of
configuration (the latter is observed in the examples cited
15 above) at phosphorus in the product indicates the intervention
phosphonium hydroxide is inconsistent with the relative pKas
of H2O (31 in DMSO)49 and phosphonium ylides (e.g. pKa = 22
in DMSO for deprotonation of MePh3P+),50 in organic media.
Since stepwise protonation and subsequent hydroxide addition
of stereomutation in the intermediate. That both ring cleavage 45 seems therefore to be an impossible process in organic media,
and phosphorus ligand permutation (resulting in retention of
configuration at phosphorus) are known to occur during
phosphonium salt hydrolysis gives strong support to the
20 existence of a TBP intermediate. The hydrolysis of 3, (in
we hoped to establish if there exists any evidence for the
formation (or otherwise) of phosphonium hydroxide in these
reactions.
Ylide hydrolysis reactions are extremely rapid at ambient
which the phosphorus is not constrained in a ring) proceeds 50 temperature (20 °C), and all such reactions investigated by us
stereospecifically with inversion of configuration at P.8,43
resulted only in the observation of phosphine oxide (by 31P
NMR). Indeed, even at ꢀ70 °C, rapid conversion of ylide to
phosphine oxide was observed by 31P NMR for ylides 5 and 6
(Chart 1), with no appearance of any high field signals that
55 could be attributed to an intermediate.
However, the addition of water to a solution of constrained
(cyclic) ylide 7 (Chart 1) at ꢀ80 °C resulted in the formation of
a compound with a chemical shift of ꢀ80.6 ppm in the 31P
NMR (see Fig. 1(a); obtained at ꢀ70 °C).† The high field
60 chemical shift is indicative of
a phosphorane structure
(electronꢀrich at phosphorus).51 This compound was also
characterised by 1H, 13C, gCOSY, gHSQC, 1Hꢀ13C gHMBC
1
1
and Hꢀ31P HMBC. The H NMR (Fig. 1(b)) contains a double
triplet for the methyl group at δH 0.90, two multiplets arising
65 from the diastereotopic CH2 protons (δH 2.32 & 2.68, each of
which couples to methyl group by COSY), and a distinctive
broadened singlet at δH 5.11 for the P-hydroxyl proton. The
1
hydroxyl proton is shown to be coupled to phosphorus by Hꢀ
31P HMBC, and to the methylene carbon by 1Hꢀ13C HMBC,
70 indicating beyond doubt that this species is indeed Pꢀ
hydroxyphosphorane (8, shown in Chart 1). In the 13C NMR of
1
8, a value of JPC of 119 Hz is observed for the methylene
1
carbon.† The magnitude of the value of JPC observed for the
methylene group of 8 is consistent with its occupation of an
75 equatorial position in a pentavalent phosphorane.30,31,33,34,35
This
is
the
first
time
that
a
Pꢀ
hydroxytetraorganophosphorane – the proposed intermediate in
phosphonium salt and ylide hydrolysis – has been observed
spectroscopically. Significantly, a small amount of residual
80 ylide could also be observed in the 31P NMR (δP ꢀ11.6). Since
water (present in excess), ylide and Pꢀhydroxyphosphorane can
1
be seen from the 31P and H NMR to coꢀexist in solution, it is
apparent that an equilibrium involving these three species is in
operation (cf. reactions of ylides and alcohols to give
85 alkoxyphosphoranes35). Furthermore, since the pKa of the
ylide is too low in organic media to effect deprotonation of
water, we conclude that that phosphonium hydroxide (which is
25 Fig. 1. (a) Partial 31P NMR and (b) partial 1H NMR showing signals for Pꢀ
hydroxyphosphorane 8.
Despite
the
strong
evidence
implicating
P-
hydroxytetraorganophosphoranes formation in phosphonium
salt & ylide hydrolysis, this putative intermediate has never
2
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DOI: 10.1039/C4CC08644A
not detected) plays no role in the process of interconversion
Notes and references
between ylide and Pꢀhydroxytetraorganophosphorane. We
propose instead that the phosphorane is formed directly from
ylide + H2O by (reversible) concerted addition of an OꢀH bond
cross the P=C bond. Indeed, it is even possible that the
hydrolysis of pꢀnitrobenzyl phosphonium salt 1 (Chart 1),
which is 1st order in each of phosphonium salt & hydroxide,
may proceed by initial formation of ylide, and hence occurs by
a different mechanism to that shown in Scheme 1. The sample
a
55 Centre for Synthesis & Chemical Biology, University College Dublin,
Belfield, Dublin 4, Ireland. Tel: +353-1-7162308.
b
Current address: Department Chemie, Ludwig–Maximilians–Universität
5
München, Butenandtstr. 5–13 (Haus F), 81377 München, Germany. E-
mail: peter. byrne@cup.lmu.de.
60 † Electronic Supplementary Information (ESI) available: Additional
discussion of the mechanism of phosphonium salt hydrolysis, experimental
procedures & characterisation details for new compounds are available.
See DOI: 10.1039/b000000x/
10 of Pꢀhydroxyphosphorane 8 was stored at ꢀ19 °C, and was
1
observed by H & 31P NMR (run at 25 °C) to have survived for
1
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5 days at ꢀ19 °C. Warming of the sample to 25 °C resulted in
the formation of ringꢀopened phosphine oxide 10 (Scheme 2)
almost to the exclusion of the other two possible phosphine
15 oxide products (cf. other cyclic phosphonium salts, especially
4, vide supra). The identity of 10 was confirmed
unambiguously by comparison with the NMR data of an
2
3
authentic sample prepared by independent means
– αꢀ
methylation of phosphine oxide 9 via a phosphinoxy carbanion
20 (see Scheme 2).
4
5
6
O
P
O
P
Ph
Ph
(i) n-BuLi
THF, -78 °C
Ph
Ph
Et
(ii) MeI
7
9
10
Scheme 2. Synthesis of phosphine oxide 10.
In conclusion, we have succeeded in carrying out the first
ever
25 hydroxytetraorganophosphorane,
phosphonium salt and ylide hydrolysis.
spectroscopic
observation
of
a
P-
of
the
intermediate
The 31P NMR
8
9
W. E. McEwen, K. F. Kumli, A. BladeꢀFont, M. Zanger, C. A.
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chemical shift and associated spectral data reported above
establish that the intermediate is a TBP phosphorane, and
validates the operation of the proposed mechanism shown in
30 Scheme 1 for phosphonium salt hydrolysis. The establishment
of the structure of this intermediate provides a basis for a
unifying explanation of the outcomes (ring opening, ring
expansion, retention of stereochemistry at phosphorus) of the
hydrolysis reactions of the various cyclic phosphonium salts.28
35 It seems likely that spectroscopic observation of the Pꢀ
hydroxyphosphoranes arising from hydrolysis of other cyclic
phosphonium salts (e.g. 4) should be possible, at least in some
cases. Given the relative stability of 8 (it survived under inert
atmosphere for several days at ꢀ19 °C), it may be possible to
40 pursue novel synthetic avenues by testing the reactivity of 8
and analogues. Finally, we have provided evidence that the
first step of ylide hydrolysis is not protonation of the ylide, but
rather is concerted addition of an OꢀH bond across the P=C
bond.
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45 Acknowledgements
This work was supported by the Synthesis and Solid State
Pharmaceutical Centre (SSPC) and Science Foundation Ireland
(SFI) under grant number 12/RC/2275. We also thank SFI for
the acquisition at UCD of an NMR spectrometer equipped for
50 low temperature experiments (SFI Infrastructural Award
[12/RI/2341(2)]).
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hydroxyphosphorane were produced by reaction of ylide with
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37 K. L. Marsi J. Chem Soc. Chem. Commun., 1968, 846; W. Hawes, S.
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39 E. M. Richards, J. C. Tebby J. Chem Soc. Chem. Commun. 1967, 957.
40 No exceptions to the apical departure rule for trigonal bipyramidal
phosphoranes have yet been reported.25,26,31,32,33,34,35
41 Note that where there is a choice between two elements, the most
electronegative element occupies the apical site.
42 E. P. A. Couzijn, J. C. Slootweg, A. W. Ehlers, K. Lamertsma J. Am.
Chem. Soc. 2010, 132, 18127; E. L. Muetterties, J. Am. Chem. Soc.
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García López, A. Morán Ramallal, J. González, L. Roces, S. Garcíaꢀ
Granda, M. J. Iglesias, P. OñaꢀBurgos, F. López Ortiz J. Am. Chem.
Soc. 2012, 134, 19504.
43 The benzyl group is apparently cleaved far faster than ligand
permutation
can
occur
at
phosphorus
in
the
Pꢀ
hydroxytetraorganophosphorane.
4
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