P(=O)H to P-OH Tautomerism: A Theoretical and Experimental Study

Article abstract of DOI:10.1021/acs.joc.5b01618

Phosphinylidene compounds R¹R²P(O)H are important functionalities in organophosphorus chemistry and display prototropic tautomerism. Quantifying the tautomerization rate is paramount to understanding these compounds' tautomerization behavior, which may impact their reactivities in various reactions. We report a combined theoretical and experimental study of the initial tautomerization rate of a range of phosphinylidene compounds. Initial tautomerization rates are found to decrease in the order H₃PO₂ > Ph₂P(O)H > (PhO)₂P(O)H > PhP(O) (OAlk)H > AlkP(O)(OAlk)H ≈ (AlkO)₂P(O)H, where "Alk" denotes an alkyl substituent. The combination of computational investigations with experimental validation establishes a quantitative measure for the reactivity of various phosphorus compounds, as well as an accurate predictive tool.

Full text of DOI:10.1021/acs.joc.5b01618

Article
pubs.acs.org/joc
P(O)H to P−OH Tautomerism: A Theoretical and Experimental
Study
Benjamin G. Janesko,* Henry C. Fisher, Mark J. Bridle, and Jean-Luc Montchamp*
Department of Chemistry, Texas Christian University, Box 298860, Fort Worth, Texas 76129, United States
S
* Supporting Information
ABSTRACT: Phosphinylidene compounds R1R2P(O)H are important func-
tionalities in organophosphorus chemistry and display prototropic tautomerism.
Quantifying the tautomerization rate is paramount to understanding these
compounds’ tautomerization behavior, which may impact their reactivities in
various reactions. We report a combined theoretical and experimental study of
the initial tautomerization rate of a range of phosphinylidene compounds. Initial
tautomerization rates are found to decrease in the order H3PO2 > Ph2P(O)H >
(PhO)2P(O)H > PhP(O) (OAlk)H > AlkP(O)(OAlk)H ≈ (AlkO)2P(O)H,
where “Alk” denotes an alkyl substituent. The combination of computational investigations with experimental validation
establishes a quantitative measure for the reactivity of various phosphorus compounds, as well as an accurate predictive tool.
stable,4 that the issue of the best representation between the
INTRODUCTION
■
classic P(V) form 1a and the phosphonium form 1b has been
Organophosphorus compounds are critically important in the
synthesis of pharmaceuticals, herbicides, pesticides, and
previously studied,5 and that the resonance form 1b must be
more heavily represented on the basis of electronegativities.) In
contrast, the less stable P(III) form 1c is the reactive species in
most reactions involving phosphinylidenes. For example,
phosphine ligands. Methods for formation of phosphorus−
carbon bonds continue to receive a great deal of attention.1
Phosphinylidene2 (hydrophosphoryl) compounds 1 are an
dimethyl H-phosphonate’s reaction with chloroacetone is
proposed to proceed through base-catalyzed tautomerization.6
Catalyzed imine hydrophosphonylation is proposed to involve
the catalyst’s stabilization of the P(III) phosphite.7 One of us
proposed that base-promoted alkylation of alkyl phosphinates
and H-phosphinates involved base-catalyzed tautomerization or
stabilization of deprotonated P(III).8,9 Several reactions involve
trapping the P(III) form by coordination to transition-metal
complexes10 or Lewis acids,11 or through silylation. Reference
12 reviews additional evidence for the reactivity of 1c.
important family of organophosphorus compounds, which
includes phosphinates (hypophosphites) 2, H-phosphonates 3,
H-phosphinates 4, and secondary phosphine oxides 5 (Figure
1).
Figure 1. Important types of phosphinylidene compounds.
Substituent effects on tautomerism are particularly critical for
phosphinylidene reactivity. While the equilibrium in eq 1
generally lies far toward the P(V) form, small electronic
differences due to substituents R1 and R2 dramatically affect the
overall rates of reactions involving phosphinylidenes. For
example, the “special” reactivity of aryl H-phosphinates 4 (R1
= aryl, R2 = OAlk) in addition to alkenes has been attributed to
stabilization of the P(III) lone pair in 1c through the aryl’s
electron-withdrawing effect.13−15 However, it is at present
unclear whether these substituent effects arise from thermody-
namic stabilization of the reactive species 1c, from kinetic
acceleration of the rate of 1c formation, or from other effects.
Thus, a fundamental understanding of substituent effects on eq
1 is critical to the practical development of new synthetic
organophosphorus chemistry.
Tautomerization is an important class of chemical reactions
involving the interconversion of constitutional isomers (tau-
tomers). A common subclass is prototropy, in which a hydrogen
atom moves from one atom to another. Perhaps the best-known
example is the keto−enol tautomerism taught in sophomore
organic classes. The phosphinylidene moiety also displays
prototropic tautomerism (eq 1).
Phosphinylidenes’ prototropic tautomerism appears to be
critical to their reactivity.3 The so-called P(V) form 1a/1b is
almost invariably the most stable species.3a,b (Note that strong
electron acceptors such as R1 = R2 = CF3 can make 1c more
Received: July 13, 2015
© XXXX American Chemical Society
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Scheme 1. Deuteration of Phosphinylidene Compounds Using an Excess of D2O
Figure 2. Proton-coupled 31P NMR spectra at 161.97 MHz obtained during the deuteration of diethyl H-phosphonate (1 M in CH3CN) using D2O
3
3
(11 equiv) at room temperature: (EtO)2P(O)H, δ 10.05 (dq, JP−H = 704, JP−H = 9 Hz); (EtO)2P(O)D, δ 9.73 (tq, JP−D = 107, JP−H = 9 Hz).
Figure 3. Relative intensity of the P−H starting material’s 31P NMR signal (1 M in CH3CN) during deuteration with D2O. The figure depicts
compounds with half-lives greater than 3 h.
There have been several previous systematic computational
studies of how groups R1 and R2 affect the thermodynamics of
phosphinylidene tautomerization. These largely bear out the
trends discussed above, with electron donor groups destabilizing
the P(III) form and electron acceptor groups stabilizing the
P(III) form.4,5 Computational studies have also demonstrated
that the tautomerization in eq 1 requires catalysis. Uncatalyzed
tautomerization of phosphine oxide (R1 = R2 = H) proceeds
through a strained three-membered ring, and has a prohibitively
high predicted reaction barrier of ∼60 kcal mol−1.5,16,17
Substantially lower tautomerization barriers are predicted
when P(V) dimers exchange a pair of protons,18−20 or in
phosphine oxide complexes with transition metals or bicyclic
guanidines.21
R1 and R2. Early experimental studies mainly considered the
kinetics of oxidation. With the popularization of computational
methods, combined experimental and theoretical works have
started to appear.16,18,19,22 However, these previous works each
consider only a single specific case of phosphinylidene 1, or do
not combine theory and experiment, making it difficult to draw
general conclusions about reactivity.
The present work seeks to remedy this deficiency in the
literature. We report a combined experimental and computa-
tional study of the initial rate of tautomerization of 17 different
phosphinylidenes R1R2P(O)H bearing a range of substituents
R1 and R2. Our results provide a fundamental understanding of
substituent effects on phosphinylidene tautomerization kinetics,
a frame of reference to rationalize previous experimental results,
and a step toward development of new phosphinylidene
chemistry.
There have also been a few previous studies of the kinetics of
phosphinylidene tautomerization, for special choices of groups
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Figure 4. Relative intensity of the P−H starting material’s 31P NMR signal (1 M in CH3CN) during deuteration with D2O. The figure depicts
compounds with half-lives of less than 3 h.
Of the compounds studied, both NaH2PO2 (entry 1) and
Bu2P(O)H (entry 2) do not undergo observable deuteration
under these conditions, even after several days. This is broadly
consistent with the expected effects of these compounds’
electron-donating substituents, which should destabilize tran-
sition states leading to the reactive P(III) form and stabilize the
P(V) form. Other possible effects on Bu2P(O)H are discussed
below. Phosphorous acid (H3PO3; entry 3) has the longest
observed half-life at around 48 h. Hypophosphorous acid
(H3PO2; entry 16) reacts 3 orders of magnitude faster than
H3PO3 with a half-life of only 180 s. This difference in reactivity
between H3PO2 and H3PO3 is consistent with the higher
reactivity of H3PO2 seen in radical and Pd-catalyzed hydro-
phosphinylation reactions, and with the lower first pKa of
H3PO3 (1.8 versus 2.0, respectively, although the difference is
small), meaning that the phosphorus atom is slightly more
positively charged in phosphorous acid. In some cases,
additional effects might contribute to the dramatic difference
between H3PO2 and H3PO3, including solubility and the pH of
the reaction. For example, phosphorous acid is insoluble in
organic solvents. and at pH 7 it exists as the dianionic and
monoanionic forms (second pKa = 6.7). In general and for
identical “R” groups, phosphinates ROP(O)H2 (2) are always
more reactive than the corresponding H-phosphonates (RO)2P-
(O)H (3) in all reactions investigated.23 (Note that it was not
possible to measure the rate of deuteration with organic esters of
hypophosphorous acid since these compounds are rapidly
hydrolyzed to H3PO2.)
The observed deuteration rates of phosphinic acid and
phosphine oxide derivatives in Table 1 are consistent with the
electron donor/acceptor character of substitutents R1 and R2.
Replacing a −OH group with an n-octyl group increases the
deuteration rate by a factor of 9 (compare entry 8 to entry 3).
Phenyl groups further increase the deuteration rate (entry 11).
This confirms the special reactivity of aryl-substituted
phosphinylidenes over alkyl-substituted ones, due to their
more rapid tautomerization rates. This special reactivity can also
be seen in the relative deuteration rates of diphenylphosphine
oxide (entry 15), octylphenylphosphine oxide (entry 13), and
dibutylphosphine oxide (entry 2).
RESULTS AND DISCUSSION
■
Measured Initial Rates of Deuteration. The kinetics of
phosphinylidene tautomerization are directly correlated with the
rate of phosphinylidine deuteration by excess D2O. P(III)
phosphinylidines formed by any method will readily exchange
the P−OH proton for deuterium, as depicted in Scheme 1.
Deuteration rates may be directly observed and quantified over
time using 31P NMR spectroscopy as depicted in Figure 2.
Excess D2O ensures that tautomerization is rate-limiting.
Figures 3 and 4 plot the decay of starting material quantified
from the 31P NMR spectra. First-order kinetics give excellent fits
to the data, with the logarithm of the substrate concentration
being nearly linear (R2 > 0.99) with time. Table 1 shows the
resulting initial deuteration rates.
Table 1. Pseudo-First-Order Kinetic Data for the Initial Rate
of Decay of the Starting Phosphinylidene after Addition of
D2O
entry
R1
R2
rate constant (s−1
)
half-life (s)
>3 days
1
2
3
4
5
6
H
NaO
Bu
Bu
>3 days
OH
OH
3.91 × 10−6
8.18 × 10−6
9.00 × 10−6
2.35 × 10−5
2.57 × 10−5
3.55 × 10−5
4.73 × 10−5
1.33 × 10−4
2.09 × 10−4
7.40 × 10−4
1.16 × 10−3
2.20 × 10−3
4.61 × 10−3
3.87 × 10−3
2.68 × 10−3
177366 (49 h)
84695 (24 h)
77025 (21 h)
29446 (8.2 h)
26992 (7.5 h)
19525 (5.4 h)
14648 (4.1 h)
5200 (1.4 h)
3324 (55 min)
937 (15.6 min)
596 (9.9 min)
315 (5.2 min)
150
Me2C(CH2O)2
EtO
EtO
EtO
Oct
a
7
(MeO)2P(S)H
8
9
OH
BnO
EtO
OH
Oct
BnO
Ph
10
11
12
13
14
15
16
17
Ph
(Me2CO)2
Ph
Oct
PhO
Ph
PhO
Ph
H
OH
b
179
DOPO
259
a
b
Dimethyl H-thiophosphonate. DOPO = 6H-Dibenzo[c,e][1,2λ5]-
oxaphosphinine 6-oxide.
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The observed deuteration rates of H-phosphonate and H-
phosphinate esters also showed comparable results with regard
to the substituent effect on the rate of deuteration. However, the
rates are overall slightly slower than those of the corresponding
phosphinic acid and phosphine oxide derivatives. H-Phospho-
nate deuteration rates increased along the series neopentylene
glycol H-phosphonate < diethyl H-phosphonate < dibenzyl H-
phosphonate < diphenyl H-phosphonate (entry 4 < entry 5 <
entry 9 < entry 13). This is again consistent with the substitutent
effects, and is confirmed experimentally.9 Replacement of ethoxy
with a phenyl group accelerates the reaction (compare entries 5
and 10). Notably, dimethyl thio-H-phosphonate (entry 7)
shows an increase in the deuteration rate consistent with less
stabilization of the positive charge at phosphorus by the sulfur
atom.
Table 3. B3LYP/6-311++G(3df,3pd) Tautomerization
Energy (ΔE, kcal mol−1) and P(V) Form P−H and PO
Bond Lengths (Å) for the Phosphinylidenes in Table 1 and
a
Some Test Cases
entry
R1
R2
ΔE
R(P−H)
R(PO)
1
2
H
NaO
Bu
24.7
7.8
1.439
1.417
1.393
1.393
1.396
1.413
1.399
1.409
1.395
1.404
1.404
1.397
1.416
1.395
1.415
1.404
1.403
1.412
1.409
1.499
1.481
1.465
1.463
1.466
1.472
1.923
1.471
1.468
1.472
1.471
1.459
1.481
1.462
1.482
1.468
1.467
1.477
1.465
Bu
3
OH
OH
10.0
6.4
4
Me2C(CH2O)2
5
EtO
EtO
EtO
Oct
10.2
10.2
9.8
6
7
(MeO)2P(S)H
8
OH
Oct
BnO
Ph
9.8
9
BnO
EtO
OH
10.8
9.9
10
11
12
13
14
15
16
17
Ph
9.2
COMPUTATIONAL RESULTS
■
(Me2CO)2
7.5
Choice of Basis Set. The remainder of this work uses
electronic structure calculations to help rationalize the initial
deuteration rates in Table 1. We would prefer to use relatively
inexpensive Kohn−Sham density functional theory (DFT)
calculations in modest atom-centered Gaussian basis sets, an
approximation that is enormously successful in many areas.24,25
However, ref 5 suggests that accurate calculations of the gas-
phase tautomerization energy (ΔE) of phosphine oxide (R1 = R2
= H) require very large basis sets. Before continuing, we thus
test whether the widely used25 B3LYP/6-311++G(3df,3pd)
approximation suffices to model phosphine oxide tautomeriza-
tion.
Ph
Oct
PhO
Ph
7.2
PhO
Ph
9.2
6.8
H
OH
7.0
DOPO
5.4
H
H
−0.3
−5.7
CF3
CF3
a
Positive values indicate that the P(V) form is more stable.
are sorted from lowest to highest experimental deuteration rates.
The results are generally consistent with ref 22. Test calculations
on R1 = R2 = H (phosphine oxide) and R1 = R2 = CF3 (ref 7) are
included for comparison. Consistent with previous work,3 all
experimentally tested phosphinylidenes have ΔE > 5 kcal mol−1,
suggesting that the P(V) form dominates the equilibrium. R1 =
R2 = CF3 has the P(III) form more stable, consistent with ref 7.
The initial rate of phosphinylidene deuteration will thus be
dominated by the rate of the forward reaction P(V) → P(III) in
eq 1.
Table 3 shows that tautomerization energies match some, but
not all, of the trends in the deuteration initial rates in Table 1.
For example, phosphine oxides R1, R2 = (OH, OH) < (EtO,
EtO) < (BnO, BnO) have increasing deuteration rates but
decreasing stability of the P(III) form. Replacing a OH group
with an alkyl group gives a substantial increase in deuteration
rate but a negligible change in predicted equilibrium (compare
entry 8 to entry 3). This is consistent with our hypothesis that
deuteration initial rates are dominated by the kinetics, not the
thermodynamics, of eq 1. Table 3 also shows that computed
P(V)O bond lengths tend to increase with the deuteration
rate, suggesting that more “product-like” P(V) structures with
longer PO bonds give faster tautomerization. H-Phospho-
nates R1, R2 = (PhO, PhO), (H, OH), (Me2CO)2, and DOPO
are exceptions.
Choice of Model Solvent. As discussed above, the high
barrier computed for gas-phase phosphinylidene tautomeriza-
tion suggests that this tautomerization must be catalyzed.16−21
Under our experimental conditions, the catalyst is likely
phospinylidene dimerization or the 11 equiv of D2O.
Simulations of deuteration initial rates (i.e., the rate of the
forward reaction in eq 1) must account for catalysis by water.
Simulations of such solvent effects may either treat the solvent
as a continuum or treat explicit solvation by ∼1−10 solvent
molecules.26 Here we explore how implicit and explicit solvent
models, and Gibbs free energy corrections, affect the predicted
Table 2 reports the basis set dependence of B3LYP DFT
calculations on phosphine oxide’s P(V) → P(III) tautomeriza-
Table 2. Number of Basis Functions and Computed P(V) →
P(III) Tautomerization Energy (ΔE, kcal mol−1) from
B3LYP Calculations on Phosphine Oxide Using Various
Basis Sets
basis set
6-31G(d)
no. of basis functions
ΔE
40
96
−3.29
−2.59
−0.35
0.31
6-311++G(2d,2p)
6-311++G(3df,3pd)
cc-pV(Q+d)Z
ref 5
155
255
1.22
tion energy (ΔE). Positive values mean that P(V) is more stable,
a sign convention opposite that of ref 5. P(III) is taken to be in
the trans-phosphinous acid form (the phosphorus lone pair and
the P−OH hydrogen are on the same side of the molecule).
B3LYP calculations in the large cc-pV(Q+d)Z basis set give a
tautomerization energy within 0.9 kcal mol−1 of the reference.
The smaller 6-311++G(3df,3pd) basis gives results within 1.6
kcal mol−1 of the reference. Both values are well within the ∼4−
5 kcal mol−1 weighted mean absolute deviation seen for typical
DFT calculations on general main group thermochemistry,
kinetics, and noncovalent interactions.24 Smaller basis sets
degrade the results, consistent with the importance of
phosphorus hypervalency. The B3LYP/6-311++G(3df,3pd)
approximation thus appears appropriate for the present work.
Tautomerization Equilibria. We next consider the
predicted tautomerization equilibria of the tested phosphinyli-
dene derivatives. Table 3 reports the B3LYP/6-311++G-
(3df,3pd) tautomerization energy (ΔE) and the P(V) form’s
computed P−H and PO bond lengths. As in Table 1, results
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rate of phosphine oxide tautomerization. Table 4 shows the total
and Gibbs free energy barriers computed for uncatalyzed
Table 4. B3LYP/6-311++G(3df,3pd) Total Energy Barrier
(ΔE⧧) and Gibbs Free Energy Barrier (ΔG⧧) for Phosphine
a
Oxide P(V) → P(III) Tautomerization
catalyst
uncatalyzed
ΔE⧧
ΔG⧧
ΔEr
ΔGr
62.3
65.3
28.2
9.3
59.1
62.0
36.8
28.7
continuum water
one explicit H2O molecule
two explicit H2O molecules
−6.2
−16.2
0.5
5.2
a
Results are reported for uncatalyzed tautomerization and three
models of water-catalyzed tautomerization: implicit water, one explicit
H2O molecule, and two explicit H2O molecules. Energies are relative
to isolated P(V) H3PO and isolated water. Relative energies ΔEr and
ΔGr of P(V) H3PO complexed with explicit solvent are included for
comparison. All values are given in kilocalories per mole.
Figure 6. Computed reaction paths for the H3PO tautomerizations in
Table 3, plotted as functions of the P−H and O−H bond lengths.
Points denote the positions of the transition states.
tautomerization and for three solvent models: continuum
solvent, one explicit H2O molecule, and two explicit H2O
molecules. Figure 5 shows the computed transition-state
geometries. Energies of the initial P(V)−explicit water
complexes are included for completeness.
tautomerizations begin with contraction of the complex. The
O−H bond shrinks by ∼0.4 Å, and the dissociating P−H bond
undergoes a small contraction in the water-catalyzed tautome-
rization. The complex then undergoes simultaneous O−H bond
shortening and P−H bond lengthening, passing through the
transition states shown in Figure 5. Barrier heights are inversely
correlated with the transition state’s P−H bond length. Once
the O−H bond approaches its equilibrium value, the complex
re-expands, lengthening the P−H bond to form the P(III)
product. As in Table 3, these results suggest that product-like
species with a soft and readily dissociated P−H bond will tend to
have lower reaction barriers.
Predicted Deuteration Initial Rates. We next compare
the experimental deuteration initial rates in Table 1 to
simulations. As discussed in the Computational Methods,
deuteration half-lives are predicted from rate constants given
by the Eyring equation, using the B3LYP/6-311++G(3df,3pd)
total energy barrier (ΔE⧧) to P(V) tautomerization catalyzed by
a single H2O molecule. Table 5 reports the computed total
energy barriers. Figure 7 presents the correlation between
computed and experimental deuteration half-lives.
Figure 6 shows that the computed H2O-catalyzed half-lives
recover most experimental trends. Computed H-phosphinate
half-lives follow experimental trends: (EtO, Oct) > (OH, Oct) >
(EtO, Ph) > (OH, Ph) > (OH, H) > DOPO (entry 6 > entry 8
> entry 10 > entry 11 > entry 16 > entry 17). Conputed H-
phosphonate half-lives also follow experimental trends: (EtO,
EtO) ≈ (CH3)2C(CH2O)2 > (MeO)2P(S)H > (PhO, PhO)
(entries 5 and 4 > entry 7 > entry 14). Computed half-lives
capture the special reactivity of aryl phosphinylidenes: (EtO,
Ph), (OH, Ph), and (Ph, Oct) have lower tautomerization
barriers than (EtO, Oct), (OH, Oct), and (Bu, Bu), respectively
(entries 10 and 6, 11 and 8, 13 and 2). Computed half-lives also
capture trends among different classes of phosphinylidenes.
Secondary phosphine oxides (SPOs) have short half-lives, while
H-phosphonates tend to have rather long half-lives. The
calculations are not a perfect representation of experiment:
computed tautomerization barriers are ∼10 kcal/mol too high,
giving half-lives that are ∼8 orders of magnitude too large.
However, the good agreement of trends supports our previous
arguments that this model suffices to capture this system’s
chemistry.
Figure 5. Calculated transition-state geometries for the H3PO
tautomerizations in Table 3. From left to right, the figure shows
uncatalyzed tautomerization, tautomerization in continuum water, and
tautomerization catalyzed by one and two explicit H2O molecules.
Table 4 and Figure 5 show that uncatalyzed tautomerization
has a strained three-membered ring transition state and a
prohibitively high barrier, consistent with previous work.5
Continuum solvent does not capture water’s catalytic effect,
giving tautomerization barriers that are even higher than those
of the uncatalyzed reaction. Test calculations (Supporting
Information) confirm that tested phosphinylidene derivatives
have high uncatalyzed and continuum solvent tautomerization
barriers. In contrast, a single water molecule dramatically
reduces ΔE⧧ and ΔG⧧ by simultaneously accepting a proton
from P and donating a proton to O. Adding a second explicit
H2O gives a less strained transition state and a lower ΔE⧧.
However, the entropic cost of ordering the second water
molecule yields only a modest effect on ΔG⧧. These results
suggest that one explicit H2O provides a qualitatively correct
treatment of water-catalyzed tautomerization, making it a
reasonable model system for exploring substituent effects on
deuteration initial rates.
Tautomerization Mechanism. Figure 5 illustrates that
water-catalyzed phosphinylidene tautomerization involves si-
multaneous dissociation of a P−H bond and formation of an
O−H bond. Figure 6 reports intrinsic reaction coordinate (IRC)
calculations27 of the P(V) → P(III) tautomerization reaction
coordinate. Reaction coordinates are visualized as functions of
two variables, the lengths of the forming O−H and breaking P−
H bonds. Uncatalyzed and catalyzed P(V) → P(III)
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Table 5. B3LYP/6-311++G(3df,3pd) Tautomerization
Energy Barrier (ΔE⧧, kcal mol−1) and Transition-State P−H
and PO Bond Lengths (Å) for the Phosphinylidenes in
CONCLUSIONS
■
The experimental and computational results reported here show
that many of the known trends in phosphinylidene R1R2P(O)H
reactivity, such as metal-catalyzed and free radical addition to
hydrocarbons, are correlated with the rate of prototropic
tautomerism. Measured deuteration initial rates yield a
quantitative measure of relative reactivities. The ease of
tautomerization is found to be H3PO2 > Ph2P(O)H >
(PhO)2P(O)H > PhP(O)(OAlk)H > AlkP(O)(OAlk)H ≈
(AlkO)2P(O)H. Deuteration half-lives computed from H2O-
catalyzed tautomerization barriers capture most experimental
trends, indicating the promise of computation for predictive
studies of phosphinylidene reactivity. Calculations also suggest
that substituents yielding soft, product-like P(V)−H bonds will
tend to increase tautomerization rates. Phosphorus chemists
should pay attention to the large differences in tautomerization
rates and the associated chemical reactivity between the various
phosphinylidene compounds when interpreting and evaluating
the scope of a particular methodology.
a
Table 1 and Some Test Cases
entry
R1
R2
ΔE⧧
R(P−H)
R(PO)
1
2
H
NaO
Bu
Bu
31.3
1.596
1.572
3
OH
OH
4
Me2C(CH2O)2
35.4
36.8
35.1
33.5
34.9
1.725
1.712
1.646
1.842
1.668
1.529
1.537
1.559
2.046
1.554
5
EtO
EtO
EtO
Oct
6
7
(MeO)2P(S)H
8
OH
Oct
BnO
Ph
9
BnO
EtO
OH
10
11
12
13
14
15
16
17
33.5
33.4
33.1
30.8
34.4
30.8
32.7
31.6
28.2
24.2
1.640
1.676
1.717
1.615
1.749
1.625
1.695
1.682
1.668
1.745
1.560
1.553
1.537
1.570
1.538
1.569
1.547
1.548
1.557
1.527
Ph
(Me2CO)2
Ph
Oct
PhO
Ph
PhO
Ph
EXPERIMENTAL SECTION
H
OH
■
DOPO
General Chemistry. All starting materials were purchased from
commercial sources and used as received. The solvents were distilled
under N2 and dried according to standard procedures (toluene and
acetonitrile from CaH2, THF from Na/benzophenone ketyl; N,N-
diisopropylethylamine was distilled from CaH2 and stored over 4 Å
molecular sieves). TLC analyses were performed on sheets precoated
with silica gel 60F254. Compound detection was achieved by exposure
to UV light (254 nm) and by immersion in anisaldehyde stain (by
volume, 93% ethanol, 3.5% H2SO4, 1% AcOH, and 2.5% anisaldehyde)
followed by heating at 150 °C. Flash chromatography experiments were
carried out on silica gel (premium Rf grade) (40−75 μm). Ethyl
acetate/hexane mixtures were used as the eluent for chromatographic
H
H
CF3
CF3
a
The calculations treat catalysis by a single H2O molecule.
1
purifications. The H, 13C, and 31P NMR spectra were recorded on a
400 MHz spectrometer. Chemical shifts for 1H NMR are given in parts
per million relative to internal tetramethylsilane (δ = 0 ppm), using
CDCl3 as the solvent. Chemical shifts for 13C NMR are given in parts
per million relative to CDCl3 (δ = 77.0 ppm). Chemical shifts for 31P
NMR spectra are recorded at 161.97 MHz and given relative to external
85% phosphoric acid (δ = 0 ppm). Abbreviations used for signal
patterns are as follows: s, singlet; d, doublet; t, triplet; q, quadruplet; m,
multiplet.
Figure 7. Calculated vs experimental deuteration half-lives.
Procedure for the Collection of Kinetic Data via 31P NMR for
Structure Influence Studies. NMR kinetics were recorded at room
temperature on a 400 MHz spectrometer, at 161.97 MHz for 31P using
D2O as a solvent lock, with four repetitions, a 2 s relaxation delay, and a
45° pulse angle. Delays between each scan were dependent on the
substrate run (see the Supporting Information for more details). The
NMR spectra were individually processed and integrated using
appropriate software. Each resonance for the P−H compound and
the forming P−D compounds was individually integrated, and the total
sum of integrals was normalized to 100%. The kinetics were calculated
on the basis of the decay of starting P−H compound (i.e., total integrals
for starting material (SM)/100 × concentration of sample, 1 M). The
NMR yields are determined by integration of all the resonances in the
31P spectra.
Table 5 also provides chemical insight into substituent effects
on tautomerization and deuteration. There is a modest
correlation between the measured deuteration rate and the
computed transition-state P−H bond length, with fast
deuteration generally corresponding to long bond lengths.
This is consistent with the above results suggesting that soft,
product-like P−H bonds yield faster tautomerization. In
contrast, transition-state PO bond lengths are not well
correlated with experiment, suggesting that the tautomerization
rate is more affected by the P−H bond.
One notable result in Tables 3 and 5 is that the secondary
phosphine oxide R1 = R2 = Bu (entry 1) has a long P(V)O
bond and a low predicted ΔE⧧. These results are consistent with
the computed and experimental results for other secondary
phosphine oxides (entries 13 and 15), but are inconsistent with
the negligible deuteration rate measured for entry 1. We
speculate that other effects, such as aggregation, may play a role
in the slow deuteration of this species. Steric effects may also
play a role since the angle between the two substituents
decreases from P(V) to P(III).
Phosphinylidene compounds were diluted to 1 M using freshly
distilled CH3CN in a 4 dram glass vial (note that solutions should be
stored in the freezer when not in use). By 1 mL syringe, 0.5 mL of
phosphinylidene solution (0.5 mmol) was added to a dry NMR tube,
and the tube was capped until the solution was needed. D2O (99.99
atom %, no internal reference, 0.1 mL, 5.5 mmol, 11 equiv) was added
by autopipet, the tube was recapped, and the tube was inverted one
time to mix the solution. The time from the addition of D2O to the start
of the data collection was noted. Spectra were collected over regular
time intervals at room temperature using four transients as
predetermined by running a test sample, which was also used as a
F
DOI: 10.1021/acs.joc.5b01618
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
reference sample to lock, tune, and shim the spectrometer before each
run. The NMR kinetic experiments were completed in duplicate for
each phosphinylidene sample.
Computational Methods. Calculations use the Gaussian 09
electronic structure program,35 generalized Kohn−Sham density
functional theory36,37 with the B3LYP hybrid exchange-correlation
functional,38,39 and Pople-type40 or cc-pV(Q+d)Z41−43 basis sets taken
from the EMSL Basis Set Exchange.44 cc-pV(Q+d)Z calculations use
the cc-pVQZ basis on hydrogen. Continuum solvent calculations use
the SMD model.45 Alkyl chains are replaced with methyl groups for
computational convenience. No constraints on molecular symmetry are
applied. All geometries are fully optimized. Local minima and transition
states are confirmed to have zero and one negative eigenvalue in their
vibrational Hessian. Test calculations on most transition states confirm
that displacing the molecule along the Hessian eigenvector with a
negative eigenvalue, and reoptimizing the geometry, converges to the
expected reactant or product geometry. Additional test calculations
using the intrinsic reaction coordinate method confirm that the
transition states connect the reactants and products of interest.
Entropic and thermal corrections are evaluated for isolated molecules
using standard rigid rotor harmonic oscillator approximations. (Put
another way, the Gibbs free energy is taken as the “sum of electronic
and thermal free energies” printed in a Gaussian 09 vibrational
frequency calculation.) Pictures of calculated geometries use color
coding: H (white), C (gray), O (red), P (yellow). Bond orders in these
pictures are drawn as a guide to the eye. H2O-catalyzed tautomerization
barriers are evaluated as the difference between the transition-state
energy and the energy of free H2O and free P(V) phosphinylidene. The
computed deuteration half-lives (t1/2 = ln(2)/k) in Figure 6 are
calculated from rate constants k obtained using the Eyring equation k =
kBT/h exp(−ΔG⧧/RT), with T = 298.15 K, and with ΔG⧧
approximated as ΔE⧧ obtained from B3LYP/6-311++G(3df,3pd)
calcuations on the P(V) → P(III) tautomerization barrier, catalyzed
by a single explicit H2O molecule as described above. Computing the
barrier relative to the most stable H2O−P(V) complex gives unrealistic
results where R1 or R2 is a OH group, as forming the transition state
requires breaking hydrogen bonds between P(V) OH groups and the
external water. This issue is extreme for H2O-catalyzed H3PO3 and
NaH2PO2 tautomerization (entries 1 and 3). We were unable to locate
transition states for these reactions, as all geometry optimizations
converged to give H2O hydrogen bound to P−OH groups.
Preparation of Phosphinylidene Compounds. Diethyl phos-
phite, diphenyl phosphite, dibenzyl phosphite, phenylphosphinic acid,
DOPO, and sodium hypophosphite were all used as purchased without
further purification. Hypophosphorous and phosphorus acids were
acquired as 50 wt % solutions in water and preconcentrated in vacuo at
rt for 15 min before use. All other phosphinylidene compounds used for
the kinetic studies were synthesized according to literature procedures.
Dibutylphosphine Oxide (Table 1, Entry 2). This compound
was prepared from diethyl phosphite according to a literature
procedure.28 Yield: 3.44 g (70%). H NMR (400 MHz, CDCl3): δ
1
6.81 (d, JP−H = 446 Hz, 1H), 1.81 (m, 4H), 1.63 (m, 4H), 0.968 (t, J =
7.4 Hz, 6H). 31P NMR (162 MHz, CDCl3): δ 34.9 (dm, JP−H = 455
Hz).
5,5-Dimethyl-1,3,2-dioxaphosphinane 2-Oxide (Table 1,
Entry 5). Aqueous H3PO3 (50 wt %, 8.2 g, 50 mmol) was weighed
into a 250 mL round-bottom flask along with 2,2-dimethyl-1,3-
propanediol (5.21 g, 50 mmol). Toluene (100 mL) was added, and a
Dean−Stark trap prefilled with excess toluene was fitted onto the flask.
The solution was refluxed overnight (∼16 h) under N2. After cooling,
column chromatography on silica gel (80:20 hexanes/EtOAc to 100%
EtOAc) was completed to obtain a white solid (3.53 g, 47%). 1H NMR
(400 MHz, CDCl3): δ 6.92 (d, JP−H = 676 Hz, 1H), 4.06 (m, 4H), 1.26
(s, 3H), 0.942 (s, 3H). 31P NMR (162 MHz, CDCl3): δ 3.02 (dm, JP−H
= 671 Hz). Spectral properties match previously reported values.29
Ethyl Octylphosphinate (Table 1, Entry 6). This compound was
prepared from octylphosphinic acid according to a literature
procedure.30 After column chromatography on silica gel (80:20
hexanes/EtOAc to 100% EtOAC), a clear oil was obtained (2.76 g,
1
67%). H NMR (400 MHz, CDCl3): δ 7.09 (d, JP−H = 526 Hz, 1H),
4.17 (m, 2H), 1.74 (m, 2H), 1.58 (m, 2H), 1.42−1.28 (m, 13H), 0.88
(t, J = 6.96 Hz, 3H). 31P NMR (162 MHz, CDCl3): δ 39.1 (dm, JP−H
=
526 Hz).
Dimethyl Thiophosphonate (Table 1, Entry 7). This compound
was prepared according to a literature procedure.31 Yield: 1.90 g (30%).
1H NMR (400 MHz, CDCl3): δ 7.36 (d, JP−H = 600 Hz), 3.86 (m, 6H, J
= 15 Hz). 31P NMR (162 MHz, CDCl3): δ 75.2 (dm, JP−H = 590 Hz).
Octylphosphinic Acid (Table 1, Entry 8). This compound was
prepared according to a literature procedure.32 Yield: 15.6 g (70%). 1H
NMR (400 MHz, CDCl3): δ 12.4 (br, 1H), 7.11 (d, JP−H = 541 Hz),
1.77 (m, 2H), 1.61 (m, 2H), 1.41 (m, 2H), 1.29 (m, 8H), 0.92 (t, J =
7.09 Hz). 31P NMR (162 MHz, CDCl3): δ 34.8 (d, JP−H = 536 Hz).
Ethyl Phenylphosphinate (Table 1, Entry 10). This compound
was prepared from phenylphosphinic acid according to a literature
procedure.27 Yield: 2.72 g (80%). 1H NMR (400 MHz, CDCl3): δ 7.61
(d, JP−H = 563 Hz, 1H), 7.80 (m, 2H), 7.62 (m, 1H), 7.55 (m, 2H), 4.18
(m, 2H), 1.40 (t, J = 6.80 Hz, 3H). 31P NMR (162 MHz, CDCl3): δ
24.6 (d, JP−H = 563 Hz).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b01618.
■
S
All computed molecular geometries and energies (TXT)
Kinetic data and graphs and NMR spectra for the
compounds in Table 1 (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: b.janesko@tcu.edu.
*E-mail: j.montchamp@tcu.edu.
Notes
■
4,4,5,5-Tetramethyl-1,3,2-dioxaphospholane 2-Oxide (Table
1 Entry 12). This compound was prepared from diethyl phosphite
according to a literature procedure.29 Yield: 2.93 g (17%). H NMR
1
(400 MHz, CDCl3): δ 7.22 (d, JP−H = 708 Hz, 1H), 1.48 (s, 6H), 1.38
(s, 6H). 31P NMR (162 MHz, CDCl3): δ 16.7 (d, JP−H = 708 Hz).
Octylphenylphosphine Oxide (Table 1, Entry 13). This
compound was prepared according to a literature procedure.33 Yield:
1.43 g (30%). 1H NMR (400 MHz, CDCl3): δ 7.72 (m, 2H), 7.54 (m,
3H), 7.50 (d, JP−H = 463 Hz, 1H), 2.01 (m, 2H), 1.61 (m, 2H), 1.42 (m,
2H), 1.26 (t, J = 7.08 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 132.4,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based in part upon work supported by the
National Science Foundation under Grant CHE-1262254.
Support from the Texas Christian University Invests in
Scholarship (TCU-IS) Program is acknowledged for M.A.B.
and H.C.F.
131.2 (d, JPC = 95.6 Hz), 129.9 (d, JPCC = 11.1 Hz), 128.9 (d, JPCC
=
13.1 Hz), 31.7. 30.7, 30.6 (d, J = 20.1 Hz), 29.5 (d, JPC = 99.6 Hz), 29.0,
22.6, 21.5 (d, JPCCC = 4.02 Hz), 14.1. 31P NMR (162 MHz, CDCl3): δ
21.4 (d, JP−H = 481 Hz).
REFERENCES
Diphenylphosphine Oxide (Table 1, Entry 15). This compound
was prepared from chlorodiphenylphosphine according to a literature
procedure.34 Yield: 4.49 g (89%). 1H NMR (400 MHz, CDCl3): δ 8.09
■
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G
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