Reversible decomposition of mono(α-hydroxy)-phosphines and their reaction with α,β-unsaturated aldehydes

Article abstract of DOI:10.1139/V09-021

The mono(α-hydroxy)phosphines R₂PCH(OH)R' (R = Ph, R' = H, Et, CH₂Ph, Ph, p-X-C_6H4 R = cyclohexyl, R' = Ph) are prepared under solvent-free conditions by a 1:1 reaction of Ph₂PH with the appropriate aldehyde, and their stabilities (with respect to reversible dissociation into reactants), studied in DMSO, Et₂O, and MeOH, increase with decreased basicity of the hydroxyphosphine; for example, for the Ph₂PCH(OH)C₆H₄-p-X phosphines, stability decreases in the order: X = CN > Cl > F > H > Me > OMe. A 1:1 room-temperature reaction of the (α-hydroxy)phosphines (except for R' = H) with cinnamaldehyde in DMSO slowly yields the known mono- and di-phosphines Ph₂PCH(Ph)CH₂CHO (4a) and Ph₂PCH(Ph)CH ₂CH(PPh₂)OH (10a), and the corresponding R'CHO aldehyde. In MeOH, the sequentially formed intermediates, PhCH=CHCH(OH)PPh₂, PhCH(OH)C H=CHPPh₂, and Ph₂PCH(Ph)CH=CHOH, were detected en route to 4a and 10a. Reaction of cinnamaldehyde with Ph₂PCH ₂OH gives 4a and the hemiacetal Ph₂PCH₂OCH ₂OH formed from the reactant hydroxyphosphine with the co-product formaldehyde. Reactions carried out in MeOH are faster because of the formation of hemiacetals from the phosphine-containing aldehyde products; thus, 4a is seen as Ph₂PCH(Ph)CH₂CH(OMe)(OH), which on dissolution in Et₂O, reverts to the aldehyde. The reaction rates and equilibrium concentrations of the various species depend on the R' group of the reactant phosphine; the rates of consumption of the hydroxyphosphines in the reactions with cinnamaldehyde decrease in the order: Ph₂PCH(OH)Ph > Ph ₂PCH(OH)Et > Ph₂PCH(OH)CH₂Ph ? Ph ₂PCH₂OH. The reactivity pattern of Ph₂PCH(OH)Ph with sinapaldehyde [3,5-(OMe)₂-4-OH-cinnamaldehyde] in DMSO follows that seen for cinnamaldehyde. Reaction of Ph₂PH with cinnamaldehyde in DMSO affords 4a and 10a via the same intermediates seen with the Ph ₂PCH(OH)R' reagents, but these latter reactions are thought to occur via direct attack on cinnamaldehyde by the hydroxyphosphine rather than via Ph₂PH.

Full text of DOI:10.1139/V09-021

582
Reversible decomposition of mono(a-hydroxy)-
phosphines and their reaction with a,b-
unsaturated aldehydes
Dmitry V. Moiseev, Paolo Marcazzan, and Brian R. James
Abstract: The mono(a-hydroxy)phosphines R2PCH(OH)R’ (R = Ph, R’ = H, Et, CH2Ph, Ph, p-X-C6H4; R = cyclohexyl,
R’ = Ph) are prepared under solvent-free conditions by a 1:1 reaction of Ph2PH with the appropriate aldehyde, and their
stabilities (with respect to reversible dissociation into reactants), studied in DMSO, Et2O, and MeOH, increase with de-
creased basicity of the hydroxyphosphine; for example, for the Ph2PCH(OH)C6H4-p-X phosphines, stability decreases in
the order: X = CN > Cl > F > H > Me > OMe. A 1:1 room-temperature reaction of the (a-hydroxy)phosphines (except
for R’ = H) with cinnamaldehyde in DMSO slowly yields the known mono- and di-phosphines Ph2PCH(Ph)CH2CHO (4a)
and Ph2PCH(Ph)CH2CH(PPh2)OH (10a), and the corresponding R’CHO aldehyde. In MeOH, the sequentially formed inter-
mediates, PhCH=CHCH(OH)PPh2, PhCH(OH)CH=CHPPh2, and Ph2PCH(Ph)CH=CHOH, were detected en route to 4a
and 10a. Reaction of cinnamaldehyde with Ph2PCH2OH gives 4a and the hemiacetal Ph2PCH2OCH2OH formed from the
reactant hydroxyphosphine with the co-product formaldehyde. Reactions carried out in MeOH are faster because of the
formation of hemiacetals from the phosphine-containing aldehyde products; thus, 4a is seen as Ph2PCH(Ph)CH2CH(O-
Me)(OH), which on dissolution in Et2O, reverts to the aldehyde. The reaction rates and equilibrium concentrations of the
various species depend on the R’ group of the reactant phosphine; the rates of consumption of the hydroxyphosphines in
the reactions with cinnamaldehyde decrease in the order: Ph2PCH(OH)Ph > Ph2PCH(OH)Et > Ph2PCH(OH)CH2Ph >>
Ph2PCH2OH. The reactivity pattern of Ph2PCH(OH)Ph with sinapaldehyde [3,5-(OMe)2-4-OH-cinnamaldehyde] in DMSO
follows that seen for cinnamaldehyde. Reaction of Ph2PH with cinnamaldehyde in DMSO affords 4a and 10a via the
same intermediates seen with the Ph2PCH(OH)R’ reagents, but these latter reactions are thought to occur via direct attack
on cinnamaldehyde by the hydroxyphosphine rather than via Ph2PH.
Key words: (a-hydroxy)phosphines, a,b-unsaturated aldehydes, diphenylphosphine, hydrophosphination.
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Resume : On a prepare des mono(a-hydroxy)phosphines R2CH(OH)R’ [R = Ph, R’ H, Et, CH2Ph, Ph, p-X-C6H4; R = cy-
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clohexyle; R’ = Ph] dans des conditions sans solvant, en faisant reagir des quantites equimoleculaires (1:1) de Ph2PH et
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de l’aldehyde approprie. On a de plus etudie leurs stabilites par rapport a leur dissociation en reactifs dans le DMSO,
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Et2O et MeOH qui augmente avec une diminution de la basicite de l’hydroxyphosphine; par exemple, pour les phosphi-
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nes Ph2PCH(OH)C6H4-p-X, la stabilite diminue dans l’ordre X = CN > Cl > F > H > Me > OMe. Une reaction de quan-
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tites equimoleculaires des (a-hydroxy)phosphines (a l’exception de celle dans laquelle R’ = H) et de cinnamaldehyde,
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dans le DMSO, conduit lentement a la formation des mono- et diphosphines connues, Ph2PCH(Ph)CH2CHO (4a) et
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Ph2PCH(Ph)CH2CH(PPh2)OH (10a), ainsi qu’a l’aldehyde correspondant, R’CHO. Dans le methanol, les produits
PhCH=CH(OH)PPh2, PhCH(OH)=CHPPh2 et Ph2PCH(Ph)CH=CHOH, qui se forment successivement comme intermediai-
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res entre 4a et 10a. La reaction du cinnamaldehyde avec le Ph2PCH2OH conduit a la formation du produit 4a et de l’he-
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miacetal, Ph2PCH2OCH2OH, qui resulte de la reaction du reactif hydroxyphosphine avec le formaldehyde obtenu comme
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coproduit. Les reactions effectuees dans le MeOH sont plus rapides en raison de la formation d’hemiacetals a partir des
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produits aldehydiques contenant une phosphine; ainsi, le produit 4a observe sous la forme de Ph2PCH(Ph)CH2CH(O-
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Me)(OH) qui, par dissolution dans Et2O se retransforme en aldehyde. Les vitesses de reaction et les concentrations a
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l’equilibre des diverses especes dependent de la nature du groupe R’ present sur la phosphine; les vitesses de consomma-
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tion des hydroxyphosphines dans les reactions avec le cinnamaldehyde diminuent dans l’ordre Ph2PCH(OH)Ph >
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Ph2PCH(OH)Et > Ph2PCH(OH)Ph > Ph2PCH(OH)CH2Ph > Ph2PCH2OH. Le patron de reactivite du Ph2PCH(OH)Ph avec
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le sinapaldehde [3,5-(Ome)2-4-OH-cinnamaldehyde], dans le DMSO, est similaire a celui du cinnamaldehyde. La reaction
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du Ph2PH avec le cinnamaldehyde, dans le DMSO, conduit a la formation des produits 4a et 10a par le biais des memes
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intermediaires que ceux observes avec les reactifs Ph2PCH(OH)R’; on croit toutefois que ces dernieres reactions se pro-
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duisent par le biais d’une attaque directe de l’hydroxyphosphine sur le cinnamaldehyde plu˛tt que par le biais d’une at-
taque via Ph2PH.
Received 27 October 2008. Accepted 13 January 2009. Published on the NRC Research Press Web site at canjchem.nrc.ca on 30 March
2009.
D.V. Moiseev, P. Marcazzan, and B.R. James.1 Department of Chemistry, The University of British Columbia, Vancouver, BC V6T
1Z1, Canada.
1Corresponding author (e-mail: brj@chem.ubc.ca).
Can. J. Chem. 87: 582–590 (2009)
doi:10.1139/V09-021
Published by NRC Research Press

Moiseev et al.
583
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Mots-cles : (a-hydroxy)phosphines, aldehydes a,b-insatures, diphenylphosphine, hydrophosphination.
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[Traduit par la Redaction]
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Introduction
Scheme 1. 1:1 and 2:1 reactions of Ph2PH with cinnamaldehyde; *
indicates a chiral centre (ref. 4).
Collaborative research involving our group has revealed
that (hydroxymethyl)phosphines, particularly P(CH2OH)3
(abbreviated THP), are excellent bleaching and brightness
stabilization agents for pulps, and interaction of such phos-
phines with conjugated carbonyl components of lignin is im-
portant in the bleaching process.1 A current, widely used
pulp-bleaching agent is sodium dithionite (Na2S2O4), indus-
trially called ‘‘hydrosulfite’’, and the collaborative studies
revealed a remarkable synergistic effect in the use of a com-
bination of THP and Na2S2O4 for the bleaching;2 this led us
to study the complicated interaction between these two
chemicals in aqueous solution, where some bis(hydroxyme-
thyl)phosphine PH(CH2OH)2 was observed.3 This encour-
aged us to study the reaction of secondary phosphines with
lignin-model compounds, exemplified by cinnamaldehyde,
where use of neat reagents gave isolable products from hy-
drophosphination of first the C=C and subsequently the C=O
bonds (Scheme 1).4 The isolated (a-hydroxy)phosphine II
was found in solution to decompose reversibly to I and
Ph2PH with rates that were highly solvent-dependent.4 The
analogous reversible decomposition of Ph2PCH(OH)Et to
propionaldehyde and Ph2PH was also demonstrated,4 and
this confirmed the generality of such a process, which was
first noted for the benzaldehyde/Ph2PH system;5 a more re-
cent study with phenolic aldehydes has also demonstrated
such equilibria.6 This chemistry was clearly relevant for our
studies involving THP, a tris(a-hydroxy)phosphine, and we
decided to investigate reactivity between cinnamaldehyde
and mono(a-hydroxy)phosphines, which could well be
present in the bleaching systems formed via addition of PH
of the observed PH(CH2OH)2 across carbonyl and (or) ole-
finic functionalities. This paper presents our findings on
these systems, studied in organic solvents. The studies com-
plement those of our recent papers, which describe the inter-
actions in aqueous solution between cinnamaldehyde and
P[(CH2)2OH]3,7 and between substituted cinnamaldehydes
and non-functionalized tertiary phosphines.8
There is literature describing reactivity of (a-hydroxy)phos-
phines (mostly THP) with a,b-unsaturated carbonyl-containing
compounds in organic solvents, where the phosphorus atom
attacks the activated C=C bond followed by loss of an alde-
hyde; e.g., THP with acrylonitrile gives P(CH2CH2CN)3 with
loss of formaldehyde, the net reaction being addition of P–H
across the C=C bond (eq. [1]).9,10 Similarly, reaction of the
bis(a-hydroxy)phosphines RP(CH2OH)2 (R = Me, Et) with
acrylonitrile can lead to formation of RP(CH2CH2CN)2.11
Reaction of THP with methyl acrylate in organic solvents
generates the phosphine oxide O=P(CH2CH2CO2Me)3
formed because of the presence of trace water,12 while THP
(formed from the tetrakis(hydroxy)phosphonium salt
[P(CH2OH)4]Cl and base) reacts with acrylamide to give a
mixture of [(HOCH2)P(CH2CH2NHCH2OH)3]OH and
O=P(CH2CH2NHCH2OH)3, and with acrylic acid to yield a
phosphobetaine (eq. [2]).10 In this relatively early literature
work, studied during the early days of 31P NMR, the products,
although likely correct, were not always well-characterized.
½1ꢀ
PðCH2OHÞ3 þ 3CH2¼CHCN ! PðCH2CH2CNÞ3
þ 3CH2O
½2ꢀ
½PðCH2OHÞ4ꢀCl þ NaOH þ CH2¼CHCO2H !
ðHOCH2Þ3PþCH2CH2CO2ꢁ
þ CH2O þ H2O þ NaCl
Our studies reported here describe mainly reactions of
cinnamaldehyde with the hydroxyphosphines R2PCH(OH)R’
(R = Ph, R’ = H, Et, CH2Ph, Ph, or p-substituted-Ph) in
DMSO and in MeOH, as well as reversible decomposition
of the hydroxyphosphines.
Experimental section
Cinnamaldehyde, sinapaldehyde [3,5-(OMe)2-4-OH-cinna-
maldehyde], PhCHO, and 4-X-C6H4CHO (X = CN, Cl, F,
Me, and MeO), all Aldrich products, were distilled under re-
duced pressure or recrystallized, and stored under Ar before
use. Ph2PH and Cy2PH (Cy = cyclohexyl) from Strem Chem-
icals were used as received. Ph2PCH2OH was prepared as re-
ported from Ph2PH and paraformaldehyde,13 but purified by
trituration using hexane/CHCl3 (6:1), while Ph2PCH(OH)Et,
Ph2PCH(OH)CH2Ph, Ph2PCH(OH)Ph, and Cy2PCH(OH)Ph
were prepared from the appropriate secondary phosphine and
aldehyde by our recently described procedure under Ar using
Schlenk glassware or a glovebox.4 1H, 1H{31P}, and 13C{1H}
NMR data for a DMSO-d6 solution, and elemental analyses,
of the new (a-hydroxy)phosphines are given in Table S1 (see
Supplementary data). The 31P{1H} singlets for these phos-
phines in different solvents are listed in Table S2 (see Sup-
plementary data). Organic solvents were dried over the
appropriate reagents, and then distilled under Ar, while
CD3OD and DMSO-d6 (Cambridge Isotope Laboratory)
were used as received. 31P{1H} NMR spectra were recorded
on a Bruker AV300 spectrometer (121 MHz) at 300 K unless
1
otherwise stated; H and 13C NMR spectra were similarly re-
1
corded on an AV400 instrument (400 MHz for H, 100 MHz
for 13C{1H}). A residual deuterated solvent proton (relative
to external SiMe4) and external 85% aq. H3PO4 were used as
references: br = broad, s = singlet, d = doublet, t = triplet, p =
pentet, and m = multiplet. J values are given in Hertz. When
necessary, atom assignments were made by means of H–1H,
1
1H–13C{1H} (HMQC and HMBC) and 1H–31P{1H} NMR
correlation spectroscopies.
Published by NRC Research Press

584
Can. J. Chem. Vol. 87, 2009
Stability (with respect to reversible dissociation) of
mono(a-hydroxy)phosphines in organic solvents
In a glovebox, the phosphine (0.05 mmol) was dissolved
in 1 mL of a selected solvent, and a sample (~0.7 mL) of
the solution was placed under Ar in a J-Young NMR tube;
31P{1H} NMR spectral changes were then monitored as a
function of time at 300 K.
Scheme 2. Preparation and reversible decomposition of (a-hydro-
xy)phosphines.
Reactions of Ph2PH with aromatic aldehydes in MeOH
In a glovebox, Ph2PH (18.6 mg, 0.1 mmol) was added to
a MeOH solution of the aldehyde (0.1 mmol in 1 mL), and
31P{1H} NMR spectral changes of a sample were then moni-
tored as described above.
vent studied where decomposition of 2a was observed
(~15% equilibrium, with t1/2 ꢂ 3 h). Establishing overall
equilibria requires that measureable equilibria exist also be-
tween the aldehyde and its methanol hemiacetal, which is
consistent with literature data, for example, in the case of
2d, for benzaldehyde15 (see below). The decompositions of
2a–2d occur faster and to a greater extent in MeOH than in
Et2O, and hemiacetal formation must be one factor, although
the presence of trace water in the MeOH possibly plays a
role by favouring formation of a phosphonium intermediate.
We have suggested previously4 that decomposition of (a-
hydroxy)phosphines could occur via an acid–base interac-
tion between the OH proton and phosphine lone pair to give
a phosphonium intermediate that re-arranges to the aldehyde
and secondary phosphine (Scheme 5 in ref. 4); this implies
also that a more electron-rich P atom gives a less stable (a-
hydroxy)phosphine, and the above data for Ph2PCH(OH)Ph
(2d) versus those for Cy2PCH(OH)Ph (2e) support such a
premise. Of note, the tertiary phosphines P[(CH2)nOH]3 (n =
1, 3) react quite differently with benzaldehyde in aqueous
solution at ~90 8C via a redox reaction to give benzyl alco-
hol and the corresponding phosphine oxide.16
We then investigated, by NMR-scale reactions in MeOH
(see Experimental section), the effect of a p-substituent (X)
in a series of benzaldehydes on the stability of the (a-hy-
droxy)phosphines derived from Ph2PH (eq. [3]). The data
are summarized in Fig. S4, which shows that the rates and
degrees of formation of the (a-hydroxy)phosphines decrease
in the order: CN > Cl > F > H > Me > OMe; an excellently
linear Hammett relationship results from plotting the relative
equilibrium concentration vs. s (Fig. S5). Thus, similar to
the effect of the nature of the secondary phosphine, elec-
tron-withdrawing groups of the aldehyde favour the forma-
tion of the (a-hydroxy)phosphines and, consistent with this,
the pentafluorophenyl derivative Ph2PCH(OH)C6F5 is known
to be stable.5 It should be pointed out that the linearity of
the Hammett plot may be fortuitous in that the presence of
the aldehyde Ð hemiacetal equilibria is ignored, while the
degree of formation of the hemiacetal is reported to be fav-
oured with increasing electron-withdrawing nature of X.15
Indeed, experimental NMR and enthalpy data15 imply rela-
tively little hemiacetal formation for the benzaldehydes that
we studied, although there are significant discrepancies be-
tween some of the measured and calculated values;15 one
set of data is X = OMe (1.5% formation of hemiacetal), Me
(2.1%), H (3.7%), and Cl (13%). The data of Figs. S4 and
S5 must reflect the effect of X on (a-hydroxy)phosphine
stability because the effect of X on the hemiacetal equilibria
would itself give an apparent decrease in formation of the
hydroxyphosphines by removal of the aldehyde.
Reactions of Ph2PH or a mono(a-hydroxy)phosphine
with an a,b-unsaturated aldehyde
As above, the phosphine (0.1 mmol) was added under Ar
to a solution of the aldehyde (0.1 mmol) in air-free DMSO/
DMSO-d6 or MeOH/CD3OD (1 mL), and a sample was
monitored by NMR spectroscopy at 300 K.
Results and discussion
Synthesis and decomposition of (a-hydroxy)phosphines
The (a-hydroxy)phosphines 2a–2e were prepared quantita-
tively as white solids by reaction of Ph2PH (1a) or Cy2PH
(1b) with the corresponding aldehydes under solvent-free
conditions at room temperature (Scheme 2), following the
procedure reported earlier for 2b;4 the 1H, 1H{31P}, and
13C{1H} NMR data in DMSO-d6 (Table S1 of the Supple-
mentary data) are assigned as done previously for 2b.4 We
have shown4 that 2b in solution at ambient conditions rever-
sibly decomposes into Ph2PH and propionaldehyde, greater
stability being seen generally in higher donor number (DN)
solvents such as DMSO, DMF, and pyridine,14 and for this
reason, the NMR data obtained in DMSO-d6 are listed. The
31P{1H} data in four solvents (Table S2 of the Supplemen-
tary data) reveal a qualitative decomposition trend, and semi-
quantitative studies on 2a–2e were undertaken in DMSO,
Et2O, and MeOH by monitoring the 31P{1H} singlet; decom-
position in CHCl3 and CH2Cl2 was significantly faster.
In DMSO (DN = 29.8), 2a–2d are stable, with only trace
amounts of 1a being detected even after several hours, while
2e essentially decomposes completely over 12 h (Fig. S1).
In Et2O (DN = 19.2), 2e fully decomposes within 5 min,
while Fig. S2 shows decomposition curves for 2b–2d in this
solvent, where the rates and degree of decomposition of 2d
are the highest, an equilibrium with ~90% conversion being
set up with t1/2 ꢂ 5 h; for 2c, an equilibrium is established
with ~45% conversion (t1/2 ꢂ 12 h), while for 2b, there is
only ~60% conversion after 10 days, and no decomposition
is evident for 2a.
In MeOH, although a good donor (DN = 30.0), all the (hy-
droxy)phosphines show some instability, because they now
decompose to the secondary phosphine and some hemiacetal
formed from the aldehyde and MeOH; data in Fig. S3 show
that the rates and the degree of decomposition decrease in
the order: 2e >> 2b > 2d > 2c > 2a (with 2e, the 31P{1H}
spectrum after 5 min showed only the resonance of 1b).
Rapid removal of the aldehyde as hemiacetal impedes regen-
eration of the (hydroxy)phosphines. Methanol is the only sol-
½3ꢀ
Published by NRC Research Press

Moiseev et al.
585
Fig. 1. The in situ 31P{1H} spectrum during reaction of cinnamal-
dehyde (3a) with Ph2PCH2OH (2a) (1:1, Ar, 300 K, DMSO-d6):
(A) after 2 days and (B) 22 days.
Fig. 2. The in situ 31P{1H} spectrum of the reaction of cinnamalde-
hyde (3a) with Ph2PCH2OH (2a) (1:1, Ar, 300 K, MeOH): (A) after
25 min, (B) 50 min, (C) 2 h, (D) 4 h, (E) 8 h, (F) 22 h, and (G)
48 h.
Reactions of (a-hydroxy)phosphines with a,b-unsaturated
compounds
(seen in CD3OD as CD3OCH2OD, dH 4.65). Compound 6 is
formed as a mixture of diastereomers (eq. [5]), observed in
the 31P{1H} spectrum as two singlets dP 0.9 and 0.1 (Fig. 2),
the diastereomeric ratio (d.r.) of a presumed mixture of S,S/
R,R- and S,R/R,S-enantiomers changing from 0.37 (after
25 min) to 1.25 (after 2 days). The phosphine 4a is almost
insoluble in MeOH, but slowly dissolves as the hemiacetal
6, when the dP 0.9 diastereomer is again initially preferred,
the d.r. changing from 0.65 (90 min) to 1.25 (5 days)
(Fig. S8). The same d.r. values result from integration of
On a 1:1 reaction of Ph2PCH2OH (2a) with cinnamalde-
hyde (3a) in DMSO-d6, the 31P{1H} resonance of 2a (dP –
14.7) was very slowly displaced by two almost equal inten-
1
sity singlets at dP 0.4 and –20.0 (Fig. 1A); H–1H COSY and
1H–31P{1H} HMQC data (Figs. S6 and S7) show that the
singlets are associated, respectively, with the known phos-
phine 4a (I in Scheme 1)4 and the phosphinated hemiacetal
co-product 5 (eq. [4]). The net reaction would seem to be
the formation of 4a by nucleophilic attack of 2a at the C=C
bond of 3a (but see below), with concomitant liberation of
formaldehyde, which is not detected because it interacts via
1
the H signals, where the CH(OD)-protons appear as ddd
½5ꢀ
the OH group of 2a to afford 5. The P–CH2 protons of 5 ap-
1
pear in the H spectrum as a doublet at dH 4.27 (2JPH
=
patterns at dH 4.17 and 4.09 with similar coupling constants
5.0 Hz), which collapses to a singlet in the H{31P} spec-
trum. The corresponding protons of 2a appear at dH 4.25 as
a doublet of doublets due to extra coupling to the OH proton
1
3
4
(3JHH = 8.9, JHH = 2.7, and JPH = 0.8 Hz), and the PhCH-
protons are seen as overlapping ddd patterns at dH 3.76 and
3
2
3.73 (3JHH = 12.0, JHH = 3.4, and JPH = 6.7 Hz). 13C{1H}
3
(2JPH = 6.8, JHH = 5.5 Hz), and this reduces to a doublet in
NMR data for 6 are as follows: d 142–127 (s and d, Ph
the H{31P} spectrum. The CH2OH protons of 5 appear as a
1
1
groups), d 97.9 (s, CHOMe), 42.2 (d, JPC = 11.0, PhCH),
doublet at dH 4.67 both in the H and H{31P} spectra (3JHH
= 7.9 Hz), and the OH proton is seen as a triplet at dH 6.45.
After 22 days, integration of the 31P{1H} signals (Fig. 1B)
shows that 4a and 5 are each formed in ~20% yield (based
on 2a). Attempts to isolate pure 5 from reaction of CH2O
with 2a, which itself is prepared from the reversible reaction
of CH2O with Ph2PH,13 have thus far been unsuccessful.
Phosphines of this type have been synthesized previously
from CH2O and the (a-hydroxy)phosphine P(CH2OH)3.17
There is no evidence for hemiacetal formation from 4a and
2a, consistent with higher reactivity (electrophilicity) of
CH2O versus that of 4a.
1
1
2
and 41.7 (d, JPC = 20.6, CH2) for one diastereomer;
1
d 97.8 (s, CHOMe), 42.1 (d, JPC = 11.3, PhCH), and
2
41.0 (d, JPC = 20.6, CH2) for the second diastereomer; the
assignments for the PhCH and CH2 carbons, although unu-
2
1
sual in that JPC > JPC, are based on definitive correlation
NMR data for the corresponding C atoms of 4a, which
have such d and J values.4
Of note, we have previously synthesized 4a by the reac-
tion of 3a with Ph2PH using neat reagents4 (Scheme 1 in
the Introduction), and it is plausible that the formation of
4a illustrated in eq. [4] might involve the same chemistry,
Ph2PH being formed by decomposition of 2a with formalde-
hyde as co-product (Scheme 2). This will be discussed later
when data from reactions of cinnamaldehydes with the other
(hydroxy)phosphines 2b–2d have been considered (here, a
different set of products and intermediates were observed).
For example, a 1:1 reaction of Ph2PCH(OH)Et (2b) with
cinnamaldehyde (3a) in DMSO-d6 generates after 30 min
two new singlets at dP –3.5 and 2.6 in the 31P{1H} spectrum
½4ꢀ
The reaction of 2a with 3a in MeOH is much faster than
that in DMSO, the products now being the hemiacetal 6
(formed from 4a and MeOH) and formaldehyde–hemiacetal
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586
Can. J. Chem. Vol. 87, 2009
Fig. 3. The in situ 31P{1H} spectrum of the reaction of cinnamalde-
hyde (3a) with Ph2PCH(OH)Et (2b) (1:1, Ar, 300 K, DMSO): (A)
after 30 min, (B) 6 h, (C) 22 h, (D) 46 h, and (E) 12 days; 10a is a
mixture of diastereomers (a-I and b-I) and (a-II and b-II) (Ref. 4).
Fig. 4. The consumption of the starting phosphine in the reactions:
(A) 2c + 3a, (B) 2b + 3a, (C) 2d + 3b, (D) 2d + 3a, and (E) 1a +
3a. Reactions were carried out under standard conditions (1:1, Ar,
300 K, DMSO).
Scheme 3. Formation of intermediates in reactions of (a-hydroxy)-
phosphines with a,b-unsaturated compounds.
Fig. 5. The relative concentration of the intermediate 7 in the reac-
tions: (A) 2c + 3a, (B) 2b + 3a, (C) 2d + 3b, (D) 2d + 3a, and (E)
1a + 3a. Reactions were carried out under standard conditions (1:1,
Ar, 300 K, DMSO).
(Fig. 3A), which are assigned to the racemates of the phos-
phines species 7a and 8a, respectively (Scheme 3). A kinetic
intermediate, (a-hydroxy)phosphine 7a, is formed by nucle-
ophilic attack of the P atom at the aldehyde C atom. In the
1H spectrum (Fig. S9), the g- and b-vinyl protons (dH 6.43
and 6.19, respectively) give dd and ddd patterns, which col-
1
lapse to a doublet and a doublet of doublets in the H{31P}
spectrum, respectively (Fig. S10) [3JHH (trans H atoms) =
4
3
16.0, JPH = 3.3, JPH = 4.6 Hz]; the a-H appears as a broad
doublet centered at dH 5.21 both in the 1H and in the
1H{31P} spectrum, and the broad singlet of the OH proton
is seen at dH 5.73. Intramolecular re-arrangement within 7a,
involving OH migration from the a- to the g-C, affords 8a.
The downfield shift of the 31P resonance (compared to that
for 7a) is consistent with the relative positions of the C=C
Fig. 6. The relative concentration of the intermediate 8 in the reac-
tions: (A) 2c + 3a, (B) 2b + 3a, (C) 2d + 3b, (D) 2d + 3a, and (E)
1a + 3a. Reactions were carried out under standard conditions (1:1,
Ar, 300 K, DMSO).
1
moiety. The b- and a-vinyl H proton signals are upfield-
shifted (dd and ddd at dH 4.78 and 4.48, respectively) versus
3
those for 7a, the JHH value of 10.8 Hz implying a cis con-
3
formation (2JPH = 6.5, JPH = 6.6 Hz), while the g-H signal
appears at dH 6.23 and overlaps with the b-H resonance of
1
7a; the H{31P} spectra support the assignments (Figs. S9
and S10). The cis conformation might entail hydrogen-bond-
1
ing between the OH proton and the P atom. The H–31P{1H}
correlation spectra of 7a and 8a are shown in Fig. S11. The
eventual higher concentration of 8a (vs. that of 7a) implies a
higher thermodynamic stability of the former (Figs. 3A–3D).
The hydroxyphosphines (2c–2d) also react with cinnamal-
dehyde (3a) according to Scheme 3. Figure 4 shows the rate
of consumption of 2b–2d in the reaction with 3a under iden-
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Moiseev et al.
587
Fig. 7. The relative concentration of the diphosphine 10 in the re-
actions: (A) 2c + 3a, (B) 2b + 3a, (C) 2d + 3b, (D) 2d + 3a, and
(E) 1a + 3a. All reactions were carried out under standard condi-
tions (1:1, Ar, 300 K, DMSO).
Fig. 8. The relative concentration of the phosphine 4a or 4b in the
reactions: (A) 2c + 3a, (B) 2b + 3a, (C) 2d + 3b, (D) 2d + 3a, and
(E) 1a + 3a. All reactions were carried out under standard condi-
tions (1:1, Ar, 300 K, DMSO).
tical conditions (1:1, DMSO, 300 K, Ar), while Figs. 5 and
6 show the relative concentrations of the corresponding in-
termediates 7 and 8, respectively. The degree of the con-
sumption of 2b–2d in their reaction with 3a to form 7a/8a
(Scheme 3) decreases in the order: 2d > 2b > 2c (Fig. 4),
and also the extents of generation of 7a (Fig. 5) and of 8a
(Fig. 6) decrease in the same order. This means that within
the upper reaction of Scheme 3, benzaldehyde is a better
‘‘leaving group’’ than propionaldehyde, which is better than
phenylacetaldehyde; with a more electrophilic aldehyde
component of the reactant hydroxyphosphine, this equili-
brium will be shifted to the left, and recall that in the reac-
tion of 2a (involving CH2O, the most electrophilic of
aldehydes) with 3a, intermediates of the type 7 or 8 were
not detected, and 4a and 5 were the products (see above).
With the 2b–2d systems, the concentrations of 7a and 8a do
decrease slowly with formation of 4a (Figs. 3, 5, and 6). How-
ever, 4a can also then react with the starting phosphine 2b–2d
to generate the known diphosphine 10a as a mixture of diaster-
eomers, which is seen in the 31P{1H} spectrum as two sets of
two equal intensity singlets (dP 1.5, –5.6 and –0.7, –7.4)
(Figs. 3C-3E); eqs. [6] and [7] summarize the later stages
of the chemistry (9a was detected in MeOH, see below). In
recent work, we have characterized 10a, where it was iso-
lated from reaction of cinnamaldehyde with Ph2PH via inter-
mediate formation of 4a4 (see Scheme 1 in the Introduction)
where 4a and 10a are labelled I and II, respectively.
Of note, the concentration of 10a generated is independ-
ent of the reactant phosphine (2b–2d) used (Fig. 7), while
the rate of formation of 4a depends on the starting phos-
phine and decreases in the order: 2c > 2b > 2d (Fig. 8).
These findings can be rationalized qualitatively by consider-
ing the reverse reactions of the aldehydes with the diphos-
phine 10a (eq. [7], Figs. 4 and 8), where again a more
reactive aldehyde will shift the equilibrium to the left. The
concentrations of 2b and 8a decrease simultaneously after
the relatively fast stage (10 h), while the concentration of
7a remains constant (Figs. 4–6), and this requires the upper
reaction of Scheme 3 to be an equilibrium. This was con-
firmed by addition of EtCHO (the precursor to 2b) to a sol-
ution containing 7a and 8a such as the one used to measure
spectrum A in Fig. 3: the 7a:2b ratio decreased rapidly from
0.15 to 0.02 on addition of two equiv. of EtCHO to the sol-
ution, while the 8a:2b ratio increased from 0.15 to 0.30. The
final product in these systems is 4a (evident after 2 months!),
and this requires also the reaction of eq. [7] to be reversible,
and this was confirmed by addition of EtCHO to a DMSO
solution of 10a, when production of 2b and 4a was seen by
using 31P{1H} NMR spectroscopy.
Evidence that 4a likely derives from 7a via the enol inter-
mediate 9a (formed from 7a by migration of the Ph2P group
from the a- to g- position) (eq. [6] was obtained by carrying
out 1:1 reactions of 2b–2d with cinnamaldehyde in MeOH/
CD3OD, which are much faster than the reaction in DMSO.
Figure 9 shows a 31P{1H} NMR profile of the reaction for
2d in MeOH, which is complete in 1 h. The final products
of the reaction are the diastereomeric mixture of the hemia-
cetal 6 formed from 4a (eq. [5], Fig. 9) and traces of the di-
astereomers of diphosphine 10a. The 1:1 stoichiometry
determines that 6 is essentially the sole product, while the
observed intermediacy of 10a again reveals the reversibility
of the eq. [7] reaction. The 10a diastereomers are detected at
dP 2.6, –4.5, and 0.3, –6.6 (Fig. 9C). Figure 10 shows the
1H–31P{1H} HMQC correlation spectrum of this system in
CD3OD recorded after ~10 min, when the intermediates 7a,
8a, and 9a are readily identified. The 31P signals of 7a (dP –
2.3) and 8a (dP 3.9) are downfield-shifted from those seen in
DMSO, while the associated a-, b-, and g-proton signals are
at chemical shifts similar to those observed in DMSO-d6
(see above) but are better resolved; e.g., the a-H of 7a ap-
4
pears at dH 5.17 as a pseudo-doublet of triplets (2JPH = JHH
3
1
= 1.5, JHH = 6.5 Hz). Three H resonances correlating with
the 31P{1H} resonance at dP –0.7 in the H–31P{1H} HMQC
1
spectrum (Fig. 10) are assigned to the enol species 9a: the
vinyl a- and b-protons appear at dH 4.10 and 5.02, respec-
3
3
tively, as dd and ddd patterns (4JPH = 6.8, JPH = 7.1, JHH
3
= 9.1 Hz), the JHH value again implying a cis conformation
at the olefinic bond, while the g-H appears as a dd pattern
centered at dH 6.15 and overlaps with the g-H of 7a; the
3JHH coupling constant to the b-H is 12.3 Hz (2JPH is diffi-
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588
Can. J. Chem. Vol. 87, 2009
Fig. 9. The in situ 31P{1H} spectrum of the reaction of cinnamaldehyde (3a) with Ph2PCH(OH)Ph (2d) (1:1, Ar, 300 K, MeOH): (A) after
8 min, (B) 11 min, (C) 20 min, (D) 30 min, (E), and (F) 50 min; 10a is a mixture of diastereomers (a-I and b-I) and (a-II and b-II) (Ref. 4).
1
Fig. 10. The in situ H–31P{1H} HMQC spectrum of the reaction of cinnamaldehyde (3a) with 2d (1:1, Ar, 300 K, CD3OD) after ~10 min.
Published by NRC Research Press

Moiseev et al.
589
Fig. 11. The in situ 31P{1H} spectrum of the reaction of sinapaldehyde (3b) with Ph2PCH(OH)Ph (2d) (1:1, Ar, 300 K, DMSO): (A) after
40 min, (B) 3 h, (C) 7 h, (D) 24 h, (E) 5 days, and (F) 12 days; 10a is a mixture of diastereomers (a-I and b-I) and (a-II and b-II) (Ref. 4).
cult to determine). In CD3OD, the product 6 is monodeuter-
ated in the b-position, consistent with an enol-OD intermedi-
ate being involved in the formation of 4a.
complicated because of the various equilibria involved, the
reaction pathways are considered substantiated.
Because Ph2PH (1a) is a decomposition product of the (a-
hydroxy)phosphines (2a–2d) in organic media (Scheme 2),
1
1a was always detected by 31P{1H} and H NMR in small
½6ꢀ
amounts (<6%) in all the reactions discussed above (dP –39.7
in DMSO, dP –39.8 in MeOH; dH 5.22, d, 1JPH = 222 Hz; see
Figs. S9 and S12), and further, since 1a is known to react
with 3a to give 4a,4 we thus studied the reaction of 3a with
1a in DMSO to see the plausibility of whether 4a was being
formed by this ‘‘decomposition’’ route. The expected inter-
mediates and product discussed above (7a, 8a, and 4a) were
detected; the consumption rate of Ph2PH was slower than in
the case of the hydroxyphosphines 2c and 2d (Fig. 4), pre-
sumably due to the lower nucleophilicity of the secondary
phosphine compared with that of the tertiary phosphines and,
as a result, the generation of 7a and 8a is also slower (Figs. 5
and 6). The formations of monophosphine 4a (Fig. 8) and di-
phosphine (10a) (Fig. 7) are faster because no aldehyde is
present in the reaction mixture (see the reversible reactions
involving R’CHO shown in Scheme 3 and eq. [7]). The slower
reactions seen with Ph2PH (vs. 2c and 2d) are consistent with
reaction pathways for the hydroxyphosphines via their direct
attack on the unsaturated aldehydes rather than via Ph2PH,
although this latter route might contribute also; direct attack
is expected to form (via nucleophilic attack of the phosphine
at the C=O moiety) an (a-hydroxy)phosphonium intermediate
such as PhCH=CHCH(OH)–R2P+CH(O–)R’,8,17b,19 which sub-
sequently forms 7a with release of R’CHO (Scheme 3). When
Ph2PH was reacted with 3a (1:1) in MeOH, intermediates
7a–10a were seen, while the only final product was hemiace-
tal 6 (see eq. [5]) formed from 4a (see Scheme 1, where I :
4a); removal of the MeOH yielded 6 as a pink, viscous
liquid, which could be dissolved in Et2O and then removal
of the Et2O regenerated 4a (see also the discussion on
eq. [5]). From an experimental point of view, the solid
mono(a-hydroxy)phosphines are useful storage agents for
Ph2PH, a foul smelling, easily oxidizable liquid.
½7ꢀ
Sinapaldehyde (3b), a lignin-type aldehyde,18 was also re-
acted with Ph2PCH(OH)Ph (2d) in DMSO to compare its re-
activity with that of cinnamaldehyde (3a). The reason for
this study was that the presence of an OH group para to the
unsaturated aliphatic chain of 3a has been shown to change
significantly in aqueous solution the interaction of such al-
dehydes with PR3 phosphines, where R does not contain an
a-OH group.8 However, in DMSO, the reactivity of 2d with
3b is found to be similar to that with 3a, with corresponding
intermediates and products (4b, 7b, 8b, and 10b) being de-
tected by NMR (Fig. 11). Data in Figs. 4–6 (C curves) show
that the consumption of 2d is slower in the reaction with 3b,
leading to lower concentrations of 7b and 8b at a selected
reaction time; the resulting higher concentration of 2d leads
to a higher concentration of 10b (Fig. 7, eqs. [6] and [7]),
although the rate of the generation of phosphine product 4b
is the same as for 4a (Fig. 8).
It cannot be completely ruled out that 4a and 4b derive
via 9a and 9b, respectively, formed at least partially by ini-
tial direct nucleophilic attack of a hydroxyphosphine at the
C=C bond of the a,b-unsaturated aldehyde (eq. [6]). Further,
10a/10b, which are hydroxyphosphines, could react with the
cinnamaldehydes to form 4a/4b giving intermediates 7a/7b
as co-products (cf. Scheme 3, upper equilibrium). However,
the NMR studies support strongly the overall chemistry as
presented in Scheme 3 and eqs. [6] and [7], and, although
Published by NRC Research Press

590
Can. J. Chem. Vol. 87, 2009
(2) Hu, T. Q.; James, B. R.; Williams, T.; Schmidt, J. A.; Moi-
seev, D. V. World Patent. PCT WO 2007 016769 A1. 2007.
(3) Moiseev, D. V.; James, B. R.; Hu, T. Q. Unpublished work.
(4) Moiseev, D. V.; Patrick, B. O.; James, B. R. Inorg. Chem.
2007, 46, 11467. doi:10.1021/ic701597g. PMID:17990874.
(5) Evangelidou-Tsolis, E.; Ramirez, F.; Pilot, J. F.; Smith, C. P.
Phosphorus, Sulfur Silicon Relat. Elem. 1974, 4, 109.
(6) Chikkali, C.; Gudat, D. Eur. J. Inorg. Chem. 2006, 3005.
doi:10.1002/ejic.200600330.
(7) Moiseev, D. V.; James, B. R.; Hu, T. Q. Inorg. Chem. 2007,
46, 4704. doi:10.1021/ic0701559. PMID:17432851.
(8) Moiseev, D. V.; Patrick, B. O.; James, B. R.; Hu, T. Q. In-
org. Chem. 2007, 46, 9389. doi:10.1021/ic7007478. PMID:
17914858.
(9) Valetdinov, R. K.; Kuznetsov, E. V.; Komissarova, S. L. Zh.
Obshch. Khim. 1969, 39, 1744.
(10) Vullo, W. J. Ind. Eng. Chem. Prod. Res. Dev. 1966, 5, 346.
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(12) Valetdinov, R. K.; Zuikova, A. N. Zh. Obshch. Khim. 1978,
48, 1726.
Conclusions
Several
synthesized
mono(a-hydroxy)phosphines,
R2PCH(OH)R’, are confirmed to be generally reversibly
unstable in organic solvents to give R2PH and R’CHO, the
degree of decomposition depending on the following:
(a) the solvent, where stability typically increases with sol-
vent donor number, except in the case of MeOH for which
hemiacetal formation promotes decomposition, and (b) the
nature of substituents R and R’, where more basic phos-
phines promote decomposition. Reactions of the phosphines
with cinnamaldehyde in DMSO and in MeOH yield the
known phosphine Ph2PCH(Ph)CH2CHO (4a) and, depending
on R’, the known diphosphine Ph2PCH(Ph)CH2CH(PPh2)OH
(10a), as well as the liberated R’CHO aldehyde and hemia-
cetal co-products that result from interactions between
aldehyde and alcohol functionalities. A series of sequential
intermediates, more readily detected in MeOH, includes
PhCH=CHCH(OH)PPh2,
PhCH(OH)CH=CHPPh2,
and
Ph2PCH(Ph)CH=CHOH; relative rates of formation of 4a,
10a, and the intermediates as a function of R’ in various
equilibria are described. The reactions are thought not to
proceed via Ph2PH, which is formed by decomposition of
the Ph2PCH(OH)R’ compounds, although the secondary
phosphine reacts with cinnamaldehyde to give the same
products and intermediates.
(13) Ku¨hl, O.; Blaurock, S.; Sieler, J.; Hey-Hawkins, E. Polyhe-
dron, 2001, 20, 2171. doi:10.1016/S0277-5387(01)00811-7.
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1973, 51, 4122. doi:10.1139/v73-616.
(16) Moiseev, D. V.; Patrick, B. O.; James, B. R.; Hu, T. Q. In-
org. Chem. 2006, 45, 10338. doi:10.1021/ic0613560. PMID:
17140243.
Supplementary data
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca) or may be purchased
from the Depository of Unpublished Data, Document Deliv-
ery, CISTI, National Research Council Canada, Ottawa, ON
K1A 0R6, Canada. DUD 3905. For more information on ob-
taining material, refer to cisti-icist.nrc-cnrc.gc.ca/cms/
unpub_e.shtml.
(17) (a) Vullo, W. J. J. Org. Chem. 1968, 33, 3665 doi:10.1021/
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48, 97; (d) Valetdinov, R. K.; Zuikova, A. N.; Yakiminskaya,
N. Sh.; Zyablikova, T. A.; Ofitserov, E. N.; Il’yasov, A.
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Published by NRC Research Press