Organoiron- And Fluoride-Catalyzed Phosphinidene Transfer to Styrenic Olefins in a Stereoselective Synthesis of Unprotected Phosphiranes

Article abstract of DOI:10.1021/jacs.9b07069

Catalytic phosphiranation has been achieved, allowing preparation of trans-1-R-2-phenylphosphiranes (R = t-Bu: 1-t-Bu; i-Pr: 1-i-Pr) from the corresponding dibenzo-7-(R)-7-phospha-norbornadiene (RPA, A = C₁₄H₁₀, anthracene) and styre

Full text of DOI:10.1021/jacs.9b07069

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Communication
Organoiron- and Fluoride-Catalyzed Phosphinidene Transfer to Styrenic
Olefins in a Stereoselective Synthesis of Unprotected Phosphiranes
Michael B. Geeson, Wesley J. Transue, and Christopher C. Cummins
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07069 • Publication Date (Web): 13 Aug 2019
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Journal of the American Chemical Society
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Organoiron- and Fluoride-Catalyzed Phosphinidene
Transfer to Styrenic Olefins in a Stereoselective
Synthesis of Unprotected Phosphiranes
9
Michael B. Geeson, Wesley J. Transue, and Christopher C. Cummins∗
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received August 13, 2019; E-mail: ccummins@mit.edu
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R
R
P
10 styrene
P
0.15 [Me4N]F
Abstract: Catalytic phosphiranation has been achieved, al-
0.1 [Fp(THF)][BF4]
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lowing preparation of trans-1-R-2-phenylphosphiranes (R = t-
Bu: 1-t-Bu, i-Pr: 1-i-Pr) from the corresponding dibenzo-7-
(R)-7-phospha-norbornadiene (RPA, A = C14H10, anthracene)
and styrene in 73% and 57% isolated yields, respectively.
The co-catalyst system requires tetramethylammonium fluoride
(TMAF) and [Fp(THF)][BF4] (Fp = Fe(η5-C5H5)(CO)2). In
the case of the t-Bu derivative, the reaction mechanism was
probed using stoichiometric reaction studies, a Hammett anal-
ysis, and a deuterium labeling experiment. Together, these
suggest the intermediacy of iron-phosphido FpP(F)(t-Bu) (2),
generated independently from the stoichiometric reaction of
[Fp(t-BuPA)][BF4] with TMAF. Two other plausible reaction
intermediates, [Fp(t-BuPA)][BF4] and [Fp(1-t-Bu)][BF4], were
prepared independently and structurally characterized.
THF, 85 ℃, 6‒12 h
−A
Ph
RPA
R = t-Bu, i-Pr
1-t-Bu (73%, >99:1 dr, 0.53 g)
1-i-Pr, (57%, 92:8 dr, 0.40 g)
Scheme 1. Preparation of phosphiranes 1-t-Bu and 1-i-Pr from the
corresponding RPA compounds.
7-phosphanorbornene.16 In contrast, when R is an alkyl sub-
stituent (for example, t-BuPA), the corresponding triplet
phosphinidene is not transferred to unsaturated substrates,
instead leading to recovery of starting material and forma-
tion of some (t-BuP)3.16 Therefore, we sought to develop
a process in which t-BuPA could be used as a reagent
for catalytic tert-butyl phosphinidene transfer to alkenes,
producing phosphirane products.
Cyclopropanation, aziridination, and epoxidation reactions
are widely used to construct strained three-membered rings
desirable for further synthetic elaboration.1 Transition-
metal catalysts have been widely used to facilitate these
transformations under mild reaction conditions with good
stereoselective and enantioselective control.1 In contrast, the
phosphorus analog (“phosphiranation”) remains in its in-
fancy — this despite the documented utility of phosphiranes
as catalyst ligands,2 polymer precursors,3 and synthetic in-
termediates.4 Only a handful of transition metal-promoted
phosphirane syntheses have been reported,5–8 and catalytic
phosphiranation to give unprotected λ3-phosphiranes re-
mains unknown despite decades of interest.6,7,9–11
Perhaps one reason for the underdevelopment of catalytic
phosphinidene transfer reactions stems from the lack of avail-
ability of appropriate precursors, a limitation recently artic-
ulated by de Bruin and Schneider.12 Hallmarks of good sub-
strates for group-transfer chemistry feature stable, neutral
leaving groups, such as N2 or iodobenzene.1 In the case of
phosphorus, only a limited number of catalytic group trans-
fer reactions are known, generally involving activation of
P–H bonds of primary phosphines in reactions disclosed by
the groups of Waterman13 and Layfield.14
Table 1. Control experiments.
Deviation from standard conditionsa Yield (%)b
None
90
5
No [Fp(THF)][BF4]
No TMAF
5
No [Fp(THF)][BF4] or TMAF
t-BuPH2 instead of t-BuPA
(t-BuP)3 instead of t-BuPA
0
0
0
a 0.06 M t-BuPA in THF with reagent ratios shown in
Scheme 1, 85 ◦C, 24 h
b Yield of 1-t-Bu determined by1integration of the product
relative to a standard by 31P{ H} NMR spectroscopy
Following unproductive screening of a variety of cata-
lysts (S1.2) selected for their ability to effect cyclopropa-
nation or aziridination, we scored a hit by using sources
of the Fp+ cation in conjunction with fluoride. In analogy
to known reactivity of [Fp(alkene)][BF4] compounds with
phosphines,18–20 we sought to promote P–C bond formation
by treatment with t-BuPA. Treatment of a slurry of [Fp-
(styrene)][BF4]21 in dichloromethane with a stoichiometric
amount of t-BuPA led to the rapid dissolution of all mate-
rial. Analysis of this reaction mixture by electrospray ion-
ization mass spectrometry (ESI-MS) and NMR spectroscopy
We have developed dibenzo-7-phosphanorbornadiene
compounds (RPA, A = anthracene, C14H10, Scheme 1),
readily available from RPCl2 and MgA·3THF,15 as use-
ful synthetic equivalents for phosphinidenes.16,17 When
R is a π-donating substituent, such as a dimethylamino
group, the Me2NPA species can undergo a thermal uni-
molecular fragmentation to give anthracene and a free
singlet (amino)phosphinidene (Me2NP) that can add to
unsaturated substrates such as 1,3-cyclohexadiene to give a
(
31P NMR: +141.8 ppm) was consistent with the addition of
t-BuPA to the iron-coordinated styrene complex to produce
an addition product (3) containing a phosphonium and iron-
alkyl functionality within the same molecule (Scheme 2).
Unfortunately, 3 could not be isolated in pure form. This
was in part attributed to its relatively short lifetime in solu-
tion; after 24 h at 23 ◦C it had undergone complete conver-
sion to [Fp(t-BuPA)][BF4] and free styrene, suggesting that
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[BF4]−
Ph
Fp
84% yield after precipitation by addition of pentane. Using
DCM
H
t-BuPA +
1
2
3
4
5
6
7
8
the same synthetic procedure, [Fp(t-BuPA)][BF4] could be
prepared from [Fp(THF)][BF4] and t-BuPA in 98% yield.
Both [Fp(1-t-Bu)][BF4] and [Fp(t-BuPA)][BF4] were char-
acterized by their 31P NMR shifts (+220.7 and −76.2 ppm,
respectively), in addition to structural characterization by
X-ray crystallography (Fig. 1). The crystallographic study
of [Fp(1-t-Bu)][BF4] confirms the spectroscopically assigned
trans arrangement of the phenyl and tert-butyl substituents
of the phosphirane ring of 1-t-Bu. The bond angles com-
prising this ring were found to be 49.26(9), 64.2(1) and
66.6(1) at P1, C3, and C2, respectively. With both Fp+-
coordinated phosphines characterized, the reaction was
monitored at 85 ◦C by 31P NMR spectroscopy, confirming
the presence of [Fp(t-BuPA)][BF4] in the reaction mixture
under conditions relevant to catalysis. [Fp(1-t-Bu)][BF4]
was not observed under the same conditions, suggesting it
either does not form or that 1-t-Bu is rapidly displaced by
a different ligand.
[Fp(styrene)][BF4]
H
t-Bu
23 ℃, <5 min
P
H
DCM
[Fp(t-BuPA)][BF4]
23 ℃, 18 h
+ styrene
3
Scheme 2. Reaction of t-BuPA with [Fp(styrene)][BF4].
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the formation of 3 is reversible (S1.8).
Having observed C–P bond-formation, [Fp(styrene)][BF4]
and other sources of Fp+ were screened as catalysts for the
phosphiranation reaction, leading to observation of the de-
sired product by 31P NMR spectroscopy (−165.0 ppm). A
complication was soon encountered as different sources of
Fp+ gave wide ranges of yield. The best performing re-
actions employed [BF4]− as the counter anion, which was
frequently observed to decompose at the reaction temper-
ature (85 ◦C) as assayed by 19F NMR spectroscopy. This
prompted an investigation of the possible catalytic role of
fluoride, generated upon [BF4]− decomposition. Addition of
the fluoride source tetramethylammonium fluoride (TMAF)
in catalytic quantities (10 mol%) led to clean and repro-
ducible formation of the desired phosphirane. The optimized
reaction conditions comprise heating t-BuPA and styrene
(10 equiv) with [Fp(THF)][BF4] (10 mol%) and TMAF
(15 mol%) in THF at 85 ◦C for 12 h (Scheme 1). Con-
trol experiments confirm the requirement of both catalysts
for the formation of 1-t-Bu (Table 1). Using other poten-
tial sources of tert-butyl phosphinidene in place of t-BuPA,
t-BuPH2 and (t-BuP)3, did not lead to the formation of
1-t-Bu.
Fe1
Fe1
C3
P1
P1
C2
C1
[Fp(1-t-Bu)]+
[Fp(t-BuPA)]+
Figure 1. Molecular structures of [Fp(1-t-Bu)][BF4] and [Fp(t-
BuPA)][BF4] with thermal ellipsoids set at the 50% probability
level. Selected hydrogen atoms and the tetrafluoroborate anions
have been omitted for clarity. Selected bond distances and angles
(A, ◦) [Fp(1-t-Bu)][BF4]: P1–Fe1: 2.2283(6); P1–C2: 1.811(2);
˚
The product was assigned exclusively as trans-1-t-Bu-2-
phenylphosphirane (1-t-Bu) by comparison with previous
literature reports22,23 as well as characterization by multinu-
clear NMR spectroscopy, high-resolution mass spectrometry
(HRMS) and elemental analysis. Evidence for the relative
stereochemistry of the t-Bu and phenyl substituents on the
phosphirane ring is provided by 1H NMR spectroscopy: the
proton occupying the same face of the ring as the phospho-
P1–C3: 1.846(2); C2–C3: 1.524(3). [Fp(t-BuPA)][BF4]: Fe1–P1:
2.258(2).
In order to shed light on a plausible mechanism by which
phosphirane 1-t-Bu forms under the reaction conditions the
stoichiometric reaction of [Fp(t-BuPA)][BF4] with TMAF
was studied. Treatment of [Fp(t-BuPA)][BF4] with equimo-
lar TMAF (CH2Cl2, 23 ◦C) resulted in a rapid color change
from bright yellow to bright orange. Analysis by NMR
spectroscopy (1H, 31P, and 19F) indicates formation of iron-
phosphido FpP(F)(t-Bu) (2) and anthracene, resulting from
attack of fluoride at the phosphonium-like phosphorus cen-
ter.17 Iron-phosphido 2 was characterized by the chemi-
cal shift of the 31P and 19F nuclei, found at +370.2 ppm
(DFT calc. = +391.2 ppm) and −202.6 ppm (DFT calc.
2
rus lone pair is associated with a much larger JP–H cou-
pling constant (18.8 Hz) than are the two on the opposing
face (2.6 and 2.2 Hz, respectively).24 No evidence for the cis
isomer was observed by NMR spectroscopy. Use of i-PrPA
in place of t-BuPA led to the new compound 1-i-Pr with
only a small drop in diastereomeric ratio (Scheme 1).
Though previously observed by 31P NMR spectroscopy
as one component of a mixture of several phosphorus-
containing species, phosphirane 1-t-Bu evidently has not
previously been isolated as a pure substance.22,23 We found
that 1-t-Bu could be purified by simple distillation at re-
duced pressure as a colorless liquid (73%, 0.53 g) that froze
at −35 ◦C, and could be stored for months at this tempera-
ture with no signs of decomposition. These observations are
consistent with the properties reported for related phosphi-
ranes.25,26
1
= −226.0 ppm), respectively, along with a JP–F value of
823.3 Hz. 1H NMR data and HRMS were also consistent
with the formulation of 2. In terms of its relevance to catal-
ysis, iron-phosphido 2, which may be regarded as a phos-
phinidenoid,27 was observed by NMR spectroscopy under
the standard reaction conditions at 85 ◦C in THF-d8, along
with [Fp(t-BuPA)][BF4], t-BuPA, and 1-t-Bu. So far, at-
tempts to isolate 2 have been unsuccessful, in part due to
its high solubility in organic solvents. Interestingly, a closely
related literature compound (Fp*P(Cl)(t-Bu), Fp* = Fe(η5-
C5Me5)(CO)2) is reported as being nucleophilic at phospho-
rus, reacting with the strong alkylating agent methyl iodide
to give the phosphonium iodide [Fp*P(Cl)(t-Bu)(Me)][I].28
A Hammett study was carried out to illuminate the
nature of the rate determining step (RDS). Competition
experiments were performed under the standard reaction
The Fp+-coordinated phosphirane complex [Fp(1-t-
Bu)][BF4] was prepared by independent synthesis in or-
der to determine its spectroscopic properties and possi-
ble role as an observable reaction intermediate. Treat-
ment of [Fp(THF)][BF4] with 1.1 equivalents of 1-t-Bu in
dichloromethane gave rise to [Fp(1-t-Bu)][BF4], isolated in
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Journal of the American Chemical Society
0.40
0.30
0.20
0.10
0.00
-0.10
-0.20
and phenyl substituents occupy cis and trans positions on
y = 0.59x + 0.03
R2 = 0.998
1
2
3
4
5
6
7
8
9
CF3
the phosphirane ring, respectively (Fig. 3B). This observa-
tion indicates a stepwise pathway (ionic or radical) in which
a reaction intermediate has a sufficient lifetime for C–C bond
rotation to occur. Nucleophilic attack on styrene by phos-
phido 2 to give intermediate [4] (Fig. 3C), containing a C–C
single bond, would fulfill this requirement. Under the re-
action conditions, the bulk cis-β-deuterostyrene in solution
was found to undergo scrambling to give a mixture of cis-
and trans-β-deuterostyrene (3:1 ratio). However, this ra-
tio is significantly less than that observed for the cis- and
trans isomers (with respect to the Ph and D substituents)
of the product phosphiranes (1:1.5 ratio), suggesting that
the observation of both the cis and trans isomers (with re-
spect to Ph and D) in the product phosphiranes is not an
artifact of bulk styrene isomerization (S1.13). Additionally,
isomerization of the bulk styrene did not occur in a control
experiment (standard conditions without t-BuPA), and is
thus tentatively accounted for by reversible addition of iron-
phosphido 2 to styrene. In this context, it is noteworthy
that the related Fp-phosphido species Fp–P(Ph)2 catalyzes
the isomerization of excess dimethyl maleate to dimethyl fu-
marate.31
Cl
Me
OMe
-0.30
-0.10
0.10
σ
0.30
0.50
para
Figure 2. Hammett plot determined by competition experiments
with p-substituted styrenes. Error bars correspond to the 95%
confidence intervals.
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conditions, using 5 equiv each of styrene and a para-
substituted styrene as the substrates.29 The analysis showed
that electron-poor styrenes react more rapidly than electron-
rich styrenes, resulting in a Hammett parameter (ρ) of +0.59
(Fig. 2). This small positive value indicates build-up of neg-
ative charge in the RDS, consistent with attack of 2 to-
ward styrene as the corresponding elementary step. A pre-
vious Hammett analysis of the addition of para-substituted
styrenes to transient (CO)5W-coordinated phosphinidenes
gave a negative value for ρ of −0.60,30 highlighting the differ-
ence in mechanism between known reactions of electrophilic
phosphinidene complexes with olefins versus our proposed
pathway involving a nucleophilic iron-phosphido species.
In light of the forgoing mechanistic experiments, we put
forward the working hypothesis shown in Fig. 3C. Initial
ligand substitution of t-BuPA with [Fp(THF)]+ results in
[Fp(t-BuPA)]+. The addition of a fluoride anion to [Fp(t-
BuPA)][BF4], resulting in compound 2, is conceptually re-
lated to the ability of chloride to promote anthracene loss
from a phosphonium derived from an RPA compound that
we reported recently17 and was the subject of a more de-
tailed computational study by Grimme and coworkers.32
Next, addition of styrene to 2 furnishes intermediate [4] ca-
pable of rotation about the newly-formed C–C single bond,
and which could plausibly go on to form [Fp(1-t-Bu)]+ with
the ejection of fluoride. Closure of the phosphirane ring in
this sequence presumably dictates the trans stereochemistry
found in the product.
This work introduces a novel catalytic styrene phosphira-
nation reaction. Phosphiranes are excellent target molecules
for transition-metal catalyzed syntheses that do not suffer
from product inhibition, as the phosphirane three-membered
ring confers high s character on the included phosphorus
lone pair with consequent diminished ligating ability and
ease of dissociation relative to typical tertiary phosphines.33
Importantly, the reaction allows for facile preparation of
two phosphiranes in good yield, enabling their develop-
ment as a ligands for transition metals and as potential
phosphorus-containing polymer precursors.3,34,35 The phos-
phiranes products are chiral and the potential future use of
a chiral catalyst raises the possibility of preparing P-chiral
phosphiranes from RPA compounds using readily accessible
organoiron and fluoride catalysts.
t-Bu
Fp
+
A
P
[Me4N]F
DCM of THF, 23 ℃
[BF4]−
P
Fp
t-Bu
F
− [Me4N][BF4]
− A
2
t-Bu
P
B
C
0.1 [Me4N]F
0.1 [Fp(THF)][BF4]
t-BuPA + 10
D
Ph
THF, 85 ℃, 12 h
Ph
D
3:2
t-Bu
Fp
+
trans:cis (Ph/D)
P
F−
1-t-Bu
[Fp(t-BuPA)]+
A
t-BuPA
t-Bu
Fp
+
P
P
Fp
2
t-Bu
[Fp(1-t-Bu)]+
F
Ph
Ph
styrene
−
F−
+
P
Fp
t-Bu
F
[4]
Acknowledgments
Figure 3. A: Stoichiometric reaction of [Fp(t-BuPA)][BF4] with
TMAF. B: Deuterium labeling study. C: Proposed catalytic cycle
leading to the formation of 1-t-Bu.
We thank the National Science Foundation (NSF-CHE-
1664799) for financial support of this research and the MGH-
PCC for computational resources.
Finally, cis-β-deuterostyrene was tested as a substrate un-
der the standard reaction conditions, in order to differenti-
ate between stepwise and concerted reaction mechanisms.
2H NMR spectra confirmed the formation of two isomers
of deuterated phosphirane product, in which the deuterium
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(17) Szkop, K. M.; Geeson, M. B.; Stephan, D. W.; Cummins, C.
Supplementary Information
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Synthesis of acyl(chloro)phosphines enabled by phosphinidene
transfer. Chem. Sci. 2019, 10, 3627–3631.
Experimental and computational details are provided in the
supporting information. This material is available free of
charge via the internet at http://pubs.acs.org. Crystal-
lographic data are available from the CSD under refcodes
1936231 and 1936232.
(18) Lennon, P.; Madhavarao, M.; Rosan, A.; Rosenblum, M.
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Page 5 of 11
Journal of the American Chemical Society
Graphical TOC Entry
1
2
3
4
5
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7
8
A catalytic “phosphiranation” reaction:
R
R
P
P
0.15 [Me4N]F
0.1 Fe catalyst
Ph
+ 10
THF, 85 ℃, 6‒12 h
−C14H10 (A)
Ph
RPA
R = t-Bu, i-Pr
9
The phosphorus analog of cyclopropanation, aziridination, and epoxidation.
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Fe catalyst = [(η5-C5H5)(CO)2Fe(THF)][BF4] ([Fp(THF)][BF4])
ACS Paragon Plus Environment
5

R
R
10 styrene
P
0.15 [Me4N]F
Journal of the American Chemical SoPcaigeety6 of 11
P
0.1 [Fp(THF)][BF4]
ACS Paragon Plus Environment
THF, 85 ℃, 6‒12 h
Ph
−A
RPA
R = t-Bu, i-Pr
1-t-Bu (73%, >99:1 dr, 0.53 g)
1-i-Pr, (57%, 92:8 dr, 0.40 g)
1
2

[BF4]−
Ph
Fp
DCM
H
t-BuPA +
Page 7Joouf r1n1al of the American Chemical Society
[Fp(styrene)][BF4]
H
t-Bu
23 ℃, <5 min
P
H
ACS Paragon Plus Environment
1
DCM
2 [Fp(t-BuPA)][BF ]
4
23 ℃, 18 h
+ styrene
3
4
3

Journal of the American Chemical SoPcaigeety8 of 11
Fe1
Fe1
C3
1
2
3
4
5
6
P1
P1
C2
ACS Paragon Plus Environment
C1
[Fp(1-t-Bu)]+
[Fp(t-BuPA)]+

0.40
y = 0.59x + 0.03
Pa0g.3e09Jouf r1n1al of the American Chemical Society
CF3
R2 = 0.998
0.20
Cl
0.10
1
2
0.00
3 -0.10
4 -0.20
5
6
ACS Paragon Plus Environment
Me
OMe
-0.30
-0.10
0.10
σ
0.30
0.50
para

t-Bu
Fp
+
A
P
[Me4N]F
DCM of THF, 23 ℃
−
Journal of the American Chemical SPoagcieet1y0 of 11
[BF4]
P
Fp
t-Bu
F
− [Me4N][BF4]
− A
2
1
t-Bu
P
2
B
0.1 [Me4N]F
0.1 [Fp(THF)][BF4]
3
4
5
t-BuPA + 10
D
Ph
THF, 85 ℃, 12 h
Ph
D
3:2
6
t-Bu
Fp
+
C
trans:cis (Ph/D)
P
7
8
9
F−
1-t-Bu
10
11
12
13
14
[Fp(t-BuPA)]+
A
t-BuPA
t-Bu
Fp
+
P
P
Fp
2
t-Bu
[Fp(1-t-Bu)]+
15
F
16
17
18
19
20
21
22
Ph
Ph
styrene
−
−
ACS Paragon Plus Environment
F
+
P
Fp
t-Bu
F
[4]

A catalytic “phosphiranation” reaction:
R
R
P
Page 1J1ouorfn1a1l of the American Chemical Society
P
0.15 [Me4N]F
0.1 Fe catalyst
Ph
+ 10
THF, 85 ℃, 6‒12 h
−C14H10 (A)
Ph
1
2
3
4
5
RPA
ACS Paragon Plus Environment
R = t-Bu, i-Pr
The phosphorus analog of cyclopropanation, aziridination, and epoxidation.
Fe catalyst = [(η5-C5H5)(CO)2Fe(THF)][BF4] ([Fp(THF)][BF4])