A phosphetane catalyzes deoxygenative condensation of α-keto esters and carboxylic acids via PIII/PV=O redox cycling

Article abstract of DOI:10.1021/ja511889y

A small-ring phosphacycle is found to catalyze the deoxygenative condensation of α-keto esters and carboxylic acids. The reaction provides a chemoselective catalytic synthesis of α-acyloxy ester products with good functional group compatibility. Based on both stoichiometric and catalytic mechanistic experiments, the reaction is proposed to proceed via catalytic P^III/P^V=O cycling. The importance of ring strain in the phosphacyclic catalyst is substantiated by an observed temperature-dependent product selectivity effect. The results point to an inherent distinction in design criteria for organophosphorus-based catalysts operating via P^III/P^V=O redox cycling as opposed to Lewis base (nucleophilic) catalysis.

Full text of DOI:10.1021/ja511889y

Communication
pubs.acs.org/JACS
A Phosphetane Catalyzes Deoxygenative Condensation of α‑Keto
Esters and Carboxylic Acids via PIII/PVO Redox Cycling
Wei Zhao, Patrick K. Yan, and Alexander T. Radosevich*
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
S
* Supporting Information
ABSTRACT: A small-ring phosphacycle is found to
catalyze the deoxygenative condensation of α-keto esters
and carboxylic acids. The reaction provides a chemo-
selective catalytic synthesis of α-acyloxy ester products
with good functional group compatibility. Based on both
stoichiometric and catalytic mechanistic experiments, the
reaction is proposed to proceed via catalytic PIII/PVO
cycling. The importance of ring strain in the phosphacyclic
catalyst is substantiated by an observed temperature-
dependent product selectivity effect. The results point to
an inherent distinction in design criteria for organo-
phosphorus-based catalysts operating via PIII/PVO redox
cycling as opposed to Lewis base (nucleophilic) catalysis.
Figure 1. Phosphetane-catalyzed deoxygenative condensation of α-keto
esters and protic pronucleophiles via PIII/PVO O-atom-transfer.
among them being silane-based reductants (R3Si−H) pioneered
by Horner10 and Mislow.11 The issue of phosphine oxide
reduction, especially in the context of PIII/PVO catalysis, is
thus better framed as one of kinetics, with improved rates holding
the key to catalytic applications. Several recent reports noted the
superiority of cyclic phospholanes in silane-mediated phosphine
oxide-catalyzed transformations. In 2009, O’Brien was the first to
show PIII/PVO redox catalysis employing a phospholane P-
oxide in the context of a catalytic Wittig reaction.12,13 Additional
results from O’Brien14 and Rutjes15 further established the
importance of phosphacycles (e.g., 1 and 2)16 in PIII/PVO
redox catalysis.17 However, attempts to employ these catalysts
(20 mol% loading) in the deoxygenative condensation of methyl
benzoylformate (6) and 4-fluorobenzoic acid (7) in the presence
of phenylsilane as reductant resulted in predominately reduction
of 6 to methyl mandelate (9) (entries 1 and 2, Table 1).18 By
contrast, we found that use of the highly strained four-
membered-ring phosphetane P-oxide 3 leads to formation of
the deoxygenative condensation product 8 (entry 3), albeit in
low yield and as the minor component of the product mixture 8/
9. Optimization of catalyst and conditions (entries 4−7)
converged on aminophosphetane P-oxide 5 as the preferred
precatalyst, giving deoxygenative condensation product 8 in 89%
isolated yield. This transformation marks a direct catalytic
synthesis of α-acyloxy esters from carbonyl and carboxylic
compounds that would most commonly be accomplished by
two-step carbonyl reduction/acylation strategies. Control experi-
ments (entries 8 and 9) confirm that formation of 8 depends on
the presence of a P-based catalyst.
atalyzed oxygen-atom-transfer reactions, in both natural1
2
Cand synthetic systems, are a focus of persistent interest.
While many of the most significant synthetic O-atom-transfer
reactions are oxygenative in nature,3 opportunity exists to
develop synthetically attractive deoxygenative methods with
catalysts operating within a suitably reducing redox couple.
Along these lines, we recently investigated a deoxygenative
condensation of α-keto esters with various protic pronucleo-
philes mediated by the conversion R3PIII→R3PVO (Figure 1).4
These transformations are initiated by Kukhtin−Ramirez
addition5 of α-keto ester substrates to a reducing trivalent
phosphorus reagent; subsequent proton transfer from a protic
pronucleophile and Arbuzov-like displacement of phosphine
oxide lead to the observed α-functionalized ester products.6 We
report the finding that a strained small-ring phosphacycle is
capable of catalyzing the deoxygenative condensation of α-keto
esters and carboxylic acids in a manner that involves formal PIII/
PV oxidation state cycling via in situ reduction of a PVO linkage
by a hydrosilane (Figure 1). These results, which expand the
emerging area of phosphine oxide-catalyzed reactions,7 illustrate
the extent to which the design criteria for organophosphorus
catalysts operating by PIII/PV redox cycling diverge from those of
the more well-established organophosphorus-based nucleophilic
catalysts. More broadly, the results further suggest opportunities
for geometrically constrained main-group compounds as redox
catalysts in organic transformations.8
Phosphetanes have not featured widely in organophosphorus
catalysis,19 presumably as a function of their perceived
instability20 and the weak nucleophilicity that attends the severe
geometric deformation enforced by the small ring (vide infra).21
The longstanding challenges associated with recycling and
turnover of the phosphine oxide byproducts of stoichiometric
phosphine-mediated transformations are typically rationalized
on enthalpic grounds.9 Notwithstanding this thermodynamic
argument, there exist numerous reagents that promote the
conversion PVO→PIII with good chemoselectivity, chief
Received: November 19, 2014
© XXXX American Chemical Society
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DOI: 10.1021/ja511889y
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Journal of the American Chemical Society
Communication
Table 1. Phosphacycles as Catalysts for Deoxygenative
Condensation
Table 2. Examples of Deoxygenative Condensation with
Varying Carboxylic Acid Components
a
a
yield (%)
entry
R3PO
solvent
T (°C)
8
9
b
1
1
2
3
4
5
5
5
PhMe
100
100
100
100
100
80
0
0
73
83
47
42
7
2
3
PhMe
PhMe
27
41
84
80
89
0
b
4
PhMe
5
PhMe
c
6
c
PhMe
7
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
80
8
9
none
80
0
0
pyrrolidine
80
0
c
d
e
10
10
80
73
a
Yields of isolated products. Reactions conducted on 0.56 mmol scale.
b
c
Yields based on recovered starting material. Yield of 9 not
d
e
determined. See Scheme 1 for structure. See ref 25.
a
Yields are for isolated products.
Notwithstanding this fact, phosphetanes of diverse substitution
are readily prepared on multigram scale (i.e., via McBride
synthesis22), and these compounds exhibit the requisite
stoichiometric reactivity to underpin the proposed catalytic O-
atom-transfer sequence (Scheme 1). Specifically, aminophos-
Table 3. Examples of Deoxygenative Condensation with
Varying α-Keto Ester Components
a
Scheme 1. Preparation and Stoichiometric Reactivity of
a
Aminophosphetanes
a
Reagents and conditions: (a) i. 2,4,4-trimethyl-2-pentene, AlCl3; ii.
pyrrolidine, 81% for two steps. (b) PhSiH3, PhMe, rt, 67%. (c)
PhC(O)CO2Me (6), p-FC6CH4CO2H (7), PhMe, 50 °C, 92%.
phetane P-oxide 5 is prepared in 81% yield by reaction of
phosphorus trichloride and 2,4,4-trimethyl-2-pentene in the
presence of aluminum trichloride, followed by amination of the
intermediate phosphinoyl chloride with pyrrolidine.23 Moreover,
the aminophosphetane P-oxide 5 is readily reduced by PhSiH3
under mild conditions to the tricoordinate aminophosphetane
10.24 Furthermore, tricoordinate 10 promotes the deoxygenative
condensation of 6 and 7, yielding the corresponding α-acyloxy
ester 8 with regeneration of aminophosphetane P-oxide 5. Based
on these stoichiometric reactivities, we postulate that the
deoxygenative condensation observed in Table 1 proceeds by
catalytic O-atom-transfer via 5/10 cycling. In concert with this
proposal, we find that the tricoordinate aminophosphetane 10
also serves as a functional catalyst for the deoxygenative
condensation (Table 1, entry 10).25
a
b
Yields are for isolated products. Ar = p-tBuC6H4−.
The results of our studies into the scope of this catalytic
transformation are collected in Tables 2 and 3. With respect to
the carboxylic acid component, a suite of benzoic acid derivatives
bearing electron-rich (11) and electron-deficient substituents
(14) gave excellent yields of the corresponding α-acyloxy esters.
Other carboxylic acids, both saturated (17) and unsaturated
(18), are likewise reductively coupled in excellent yield. The
inherently low Lewis acidity of the P-based catalyst results in
good compatibility with heteroatom functionality including
heteroarenes (16, 20, 22), sulfides (19), and free anilines (21).
By virtue of the Kukhtin−Ramirez-initiated mechanism, the
B
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Journal of the American Chemical Society
Communication
reaction is selective for reductive coupling at the α-keto ester
carbonyl; carbonyl functionality elsewhere in the reactants
including both ketone (24) and even aldehyde (23) groups are
carried through the transformation without complication.
Perhaps most significantly, the phosphetane-catalyzed method
shows good chemoselectivity for reductive coupling even in the
presence of sensitive electrophilic functionalities that are known
substrates for reaction with tricoordinate P compounds. For
instance, both electron-deficient nitroarenes (14) and benzylic
halides (25) are unaffected during the event.
The reaction is similarly amenable to diverse α-ketoester
substrates (Table 3). Functionalized benzoylformate derivatives
were reactive, giving good to excellent yields. Other hetero-
aromatic α-keto esters (32, 33) were also reactive. Alkyl α-keto
esters were found to be suitable substrates for both inter- (34,
35) and intramolecular (36, 37) implementations.
Figure 2. Correlation between product selectivity and temperature over
the range 24 °C < T < 100 °C (see Table 4). Linear fit: ln[(8/9)/T] =
−5.26T−1 + 10.59, R2 = 0.995.
The importance of ring strain in facilitating PIII/PVO cycling
is illustrated by an observed dependence of product selectivity on
temperature. As described above, reaction of 6 and 7 with 20 mol
% 5 and 1.2 equiv of phenylsilane at 100 °C in PhMe results
predominantly in catalytic deoxygenative condensation product
8 (Table 4, entry 1). Under otherwise identical reaction
From these results, we surmise that the consumption of the
phosphine oxide by silane represents the selectivity-determining
branch point and that the strain imposed at P by the four-
membered ring is essential for obtaining the deoxygenative
condensation transformation. Previous experimental27 and
theoretical28 studies implicate an intramolecular hydride shift
from Si to P (38→39, Figure 3) as the rate-controlling step in
Table 4. Dependence of Product Selectivity on Temperature
a
yield (%)
entry
T (°C)
8
9
1
2
3
4
100
75
84
73
48
18
7
22
43
74
50
24
a
Yields are for isolated products.
conditions, however, decreasing reaction temperature (entries
2−4) results in an increase in the formation of 9 at the expense of
8. In each case, the combined yields of 8 and 9 exceed >90%. We
independently verified that the reduction product 9 is not an
intermediate en route to 8: (1) the product ratio 8/9 for a room
temperature reaction (i.e., Table 4, entry 4) remains unchanged
upon subsequent heating to 100 °C for 20 h, and (2) resubjection
of 9 to the high-temperature conditions depicted in Table 4 (1.0
equiv 9, 1.2 equiv 7, 1.2 equiv PhSiH3, 20 mol% 5, 0.2 M in
PhMe, 100 °C, 20 h) does not lead to 8.26 Thus, we infer that the
observed product ratios are formed under kinetic control. A
linear relationship is obtained in the plot of ln((8/9)/T) vs 1/T
(Figure 2), from which an isokinetic temperature (Tiso = 48 °C)
and differential activation parameters (ΔΔH⧧ = +10.5(9) kcal/
mol, ΔΔS⧧ = −26(6) cal/mol·K) can be extracted. The
interpretation of these values suggests that the process leading
to 8 is disfavored relative to 9 on enthalpic terms, but favored at
higher temperatures on entropic terms. Moreover, the
magnitude of ΔΔS⧧ is suggestive of differing molecularity in
the rate-controlling steps for the formation of 8 and 9,
respectively. In situ spectroscopy at both the high and low
temperature extremes show phosphetane P-oxide 5 (31P δ 58.8
ppm) to be the only observable P species in solution throughout
the course of the catalytic transformation.
Figure 3. Mechanistic proposal for deoxygenative condensation.
silane-mediated phosphine oxide reduction. The energetic
barrier to this transformation is associated with significant
bond angle contraction that attends conversion of the
pseudotetrahedral phosphine oxide to a trigonal bipyramidal
phosphorane. The strain accrued to the phosphetane minimizes
the necessary geometric reorganization and thereby increases the
rate of PVO→PIII reduction relative to intermolecular
hydrosilylation. This line of reasoning connects with the
rationale put forth by Westheimer29 and Hudson30 on the effect
of ring strain on phosphate and phosphine reactivity, and related
arguments underpin the strain-induced Lewis acidity in the main
group (P,31 Si,32 Ge,33 Al34). Subsequent deoxygenative
condensation promoted by tricoordinate 10 via Kukhtin−
Ramirez intermediate 40 and 41 then follows to regenerate 5
and close the catalytic cycle.
There is an electronic corollary to the finding that highly
strained four-membered phosphacycles enable deoxygenative
condensation of α-keto esters and carboxylic acids. The
imposition of acute angles at P by the small ring (as in 10)
leads to accrual of s-character in the valence lone pair,35 with a
concomitant decrease in nucleophilicity. This phenomenon runs
C
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Journal of the American Chemical Society
Communication
(13) For non-redox phosphine oxide catalysis, see: (a) Marsden, S. P.;
McGonagle, A. E.; McKeever-Abbas, B. Org. Lett. 2008, 10, 2589.
(b) Denton, R. M.; An, J.; Adeniran, B.; Blake, A. J.; Lewis, W.; Poulton,
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Acc. Chem. Res. 2000, 33, 432.
(14) (a) O’Brien, C. J.; Nixon, Z. S.; Holohan, A. J.; Kunkel, S. R.;
Tellez, J. L.; Doonan, B. J.; Coyle, E. E.; Lavinge, F.; Kang, L. J.;
Przeworski, K. C. Chem.Eur. J. 2013, 19, 15281. (b) O’Brien, C. J.;
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2013, 19, 5854.
counter to the design of typical organophosphorus nucleophilic
catalysts,36 which favor structures with broad interior angles that
result in higher energy, p-rich frontier lone pairs. Theoretical
efforts to substantiate and metrically assess the underlying
enthalpic and electronic components of the strained phospha-
cyclic design are ongoing in our laboratories. Experimental work
also continues with the goal of expanding the scope and
performance of phosphetane-based PIII/PV redox catalysis.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) (a) Van Kalkeren, H. A.; Bruins, J. J.; Rutjes, F. P. J. T.; van Delft,
F. L. Adv. Synth. Catal. 2012, 354, 1417. (b) Van Kalkeren, H. A.;
Leenders, S. H. A. M.; Hommersom, C. R. A.; Rutjes, F. P. J. T.; van
Delft, F. L. Chem.Eur. J. 2011, 17, 11290. (c) Van Kalkeren, H. A.; te
Grotenhuis, C.; Haasjes, F. S.; Hommersom, C.; Rutjes, F. P. J. T.; van
Delft, F. L. Eur. J. Org. Chem. 2013, 2013, 7059. (d) Van Kalkeren, H. A.;
van Delft, F. L.; Rutjes, F. P. J. T. Pure Appl. Chem. 2013, 85, 817.
(16) In this and related chemistry, the phosphine oxide precatalyst is
presumably reduced to the corresponding reactive trivalent phosphine
in situ.
■
S
AUTHOR INFORMATION
Corresponding Author
*radosevich@psu.edu
■
Notes
(17) There are also limited examples of acyclic phosphines in PIII ⇌ PV
catalysis: (a) Harris, J. R.; Haynes, M. T., II; Thomas, A. M.; Woerpel, K.
A. J. Org. Chem. 2010, 75, 5083. (b) Kosal, A. D.; Wilson, E. E.; Ashfeld,
B. L. Angew. Chem., Int. Ed. 2012, 124, 12202. (c) Lenstra, D. C.; Rutjes,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the Pennsylvania State
University. A.T.R. gratefully acknowledges early career support
from the Alfred P. Sloan Foundation.
F. P. J. T.; Mecinovic, J. Chem. Commun. 2014, 50, 5763.
́
(18) For Lewis base-catalyzed hydrosilylation of carbonyl compounds,
see: Kobayashi, S.; Yasuda, M.; Hachiya, I. Chem. Lett. 1996, 25, 407.
(19) For applications as ligands, see: Li, W.; Zhang, X. In
Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis;
Kamer, P. C. J., van Leeuwen, P. W. N. M., Eds.; John Wiley & Sons,
Ltd.: New York, 2012; pp 56−57.
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D
DOI: 10.1021/ja511889y
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