The Phospha-Bora-Wittig Reaction

Article abstract of DOI:10.1021/jacs.1c06228

We report the phospha-bora-Wittig reaction for the direct preparation of phosphaalkenes from aldehydes, ketones, esters, or amides. The transient phosphaborene Mes*P═B-NR2 reacts with carbonyl compounds to form 1,2,3-phosphaboraoxetanes, analogues of oxap

Full text of DOI:10.1021/jacs.1c06228

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The Phospha-Bora-Wittig Reaction
Andryj M. Borys, Ella F. Rice, Gary S. Nichol, and Michael J. Cowley*
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ABSTRACT: We report the phospha-bora-Wittig reaction for the direct preparation of phosphaalkenes from aldehydes, ketones,
esters, or amides. The transient phosphaborene Mes*PB−NR2 reacts with carbonyl compounds to form 1,2,3-
phosphaboraoxetanes, analogues of oxaphosphetane intermediates in the classical Wittig reaction. 1,2,3-Phosphaboraoxetanes
undergo thermal or Lewis acid-promoted cycloreversion, yielding phosphaalkenes. Experimental and density functional theory
studies reveal far-reaching similarities between classical and phospha-bora-Wittig reactions.
hosphaalkenes are closely related to alkenes.1 The similar
Pelectronegativity of carbon and phosphorus makes CP
π-bonds structurally and chemically similar to alkenes, albeit
with narrower HOMO−LUMO gaps as a result of the weaker
2p-3p π-bond.1 Because the replacement of CC with CP
units alters frontier orbital energies without significantly
polarizing the π-system, phosphaalkenes are attractive
“building blocks” for main-group π-conjugated molecules and
materials.2 Their advance from laboratory curiosity to chemical
workhorse has also seen phosphaalkenes used as ligands for
transition-metal catalyzed transformations,3,4 and incorporated
into inorganic polymers.5,6
The first preparations of phosphaalkenes exploited 1,3-silyl
migration7 or elimination chemistry,8−10 necessitating pre-
formed P−C σ-bonds.11 Synthetically, it is more convenient to
install the “RP” functionality in one step at a late stage. The
Figure 1. (a) Selected phospha-Wittig reagents; (b) the phospha-
bora-Wittig reaction reported here.
direct synthesis of phosphaalkenes from carbonyl compounds,
akin to the Wittig reaction, is therefore particularly attractive
due to the availability and synthetic access to suitable carbonyl
phosphaalkenes directly from a range of carbonyl compounds
remains desirable.
precursors. The first phospha-Wittig reagent, reported by
Mathey in 1988,12,13 enables just such a conversion (I, Figure
1a).
We have recently demonstrated that transient phosphabor-
enes [Mes*PBNR2] (Mes* = 2,4,6-tri-tert-butylphenyl; NR2
= 2,2,6,6-tetramethylpiperidine) can be accessed in solution
and subsequently trapped by unsaturated compounds includ-
ing phenylacetylene to give the corresponding formal [2 + 2]
Several “phospha-Wittig”2,14 reagents are now available.
These compounds can be viewed as phosphinidenes
coordinated by a Lewis base and/or to a Lewis acid.
Organometallic terminal phosphinidene complexes (e.g.,
Cp2ZrPMes*(PMe3), II) can be used to prepare phos-
phaalkenes from aldehydes or ketones.15−17 Phosphoranylide-
nephosphines (ArPPMe3 ↔ ArP−-P+Me3)18,19 (III) perhaps
bear the closest resemblance to classical Wittig reagents
(R2CPPh3 ↔ R2C−−P+Ph3), given the closely related
resonance forms, and the common phosphine-oxide by-
product.
cycloaddition product.22 In 1986, Noth and Glaser reported
̈
that transient methyleneboranes [R2CBNR’2] undergo a
Wittig-type reaction with ketones to give the corresponding
alkene.23 Considering the isoelectronic relationship between
CR2 and PR, and the reported reactivity of phosphinoboranes
R2PBR’2 with CO bonds,24−26 we suspected that that
phosphaborenes might be used to prepare phosphaalkenes.
Despite the similarities between the Wittig and “phospha-
Wittig” reaction, the latter is less well-developed and
understood. Few mechanistic studies have been made.20,21
Furthermore, the reported “phospha-Wittig” reagents can be
unstable or challenging to prepare. Phosphinidene transfer
reactions are generally limited to aldehydes or activated
carbonyl compounds. A widely applicable method of preparing
Received: June 16, 2021
© XXXX The Authors. Published by
American Chemical Society
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Here, we report the development of the “phospha-bora-
Wittig” reaction (Figure 1b). Using the stabilizing Mes*
substituent at P, we demonstrate the synthesis of known and
novel phosphaalkenes directly from a wide range of carbonyl
compounds including ketones, aldehydes, esters and amides.
We show that the reaction proceeds by a stepwise cyclo-
addition/cycloreversion mechanism, analogous to that consid-
ered operative in the classical Wittig reaction.27
We initially investigated the reaction of diphosphadiboretane
1 with benzophenone. Heating 1 with two equivalents of
benzophenone in C6D6 at 80 °C resulted in consumption of all
starting materials and the emergence of new resonances at δ
15.4 and 38.6 in the 31P and 11B NMR spectra respectively
(reactions with 1 equiv of ketone lead to lower yields of
product due to increased thermal decomposition of 1). X-ray
diffraction experiments on crystalline product confirmed the
identity of the formal [2 + 2] cycloaddition product as 2a
(Figure 2). 2a is analogous to the oxazaboretidines obtained
1,2,3-Phosphaboraoxetanes 2a−e are reminiscent of the
four-membered oxetane intermediates in the classical Wittig
reaction.27 We thus considered that their conversion into
phosphaalkenes may be possible. Elimination of the OBNR2
fragment and its subsequent oligomerization would provide a
thermodynamic incentive through B−O bond formation. The
likelihood of such an elimination appears increased upon
examination of the structures of 2a−e. For example, the
structure of 2a (Figure 2b) reveals a planar, strained, central
PBCO ring. The internal angles at C1 (92.07(8)°) and B1
(94.10(9)°) are particularly narrow. The NR2 substituent at B1
is oriented to allow BN π-bonding, leading to the short B1−
N1 distance (1.410(2) Å).
We did not observe thermal elimination of phosphaalkenes
from the 1,2,3-phosphaboraoxetanes 2a−e, even at elevated
temperatures. However, addition of AlBr3 (1 equiv) immedi-
ately converted 2a−e into their corresponding phosphaalkenes
3a−e. The major initial boron-containing byproduct resonates
at δ 20.0 in the 11B NMR spectrum. Subsequent addition of
pyridine (to sequester AlBr3) led to the replacement of this
signal with one at δ 22.3. After separation from the
phosphaalkene product, the boron-containing byproduct was
identified by NMR and mass spectrometry as [R2NBO]3.23
Al(III) halides promote the intramolecular decomposition of
Mes*-substituted phosphaalkenes.31 We did not observe such
reactivity except with superstoichiometric (to 2a−e) quantities
of AlBr3. A preference for AlBr3 complexation of [R2NBO]3
(consistent with the 11B NMR signal at δ 20.0) over
coordination to phosphaalkenes is thus likely. Conversion of
2a−e to phosphaalkenes could also be achieved more
economically with substoichiometric quantities of AlBr3 (see
the SI).
Phosphaalkenes 3a−e are conveniently prepared in one pot
from 1 and the corresponding ketone or aldehyde. After the
formation of the 1,2,3-phosphaboraoxetanes 2a−e (80 °C, 2
h), AlBr3 addition affords known and novel phosphaalkenes
3a−e in good purity and yield (Figure 2a, 53−95%).
Fluorenylidene phosphaalkenes (e.g., 3c) are promising
components for organic materials based on their optoelec-
tronic and redox properties.32−36
When diphosphadiboretane 1 was reacted with esters in
place of ketones/aldehydes, direct conversion to the 2-alkoxy-
phosphaalkene products occurred (Figure 3). For example, the
reaction of 1 and ethyl acetate led to the new phosphaalkene
Figure 2. (a) Preparation of 1,2,3-phosphaboraoxetanes 2a−e and
their subsequent conversion into phosphaalkenes 3a−e. (Mes* =
2,4,6-tritert-butylphenyl; NR2 = 2,2,6,6-tetramethylpiperidino). (b)
Structure of 2a; thermal ellipsoids at 50% probability, and hydrogen
atoms omitted.51
from the reaction of iminoboranes with ketones and
aldehydes.28 No evidence of the [4 + 2] cycloaddition product
of 1 and benzophenone was observed, in contrast to the
behavior of diazodiboretanes ([RBNR]2).29
Diphosphadiboretane 1 also reacts cleanly with acetone,
forming the dimethyl 1,2,3-phosphaboraoxetane, 2b. In
contrast (to PB), NB bonds react with the enol tautomer
of acetone by 1,2 addition.30 9-Fluorenone, isobutyraldehyde,
or benzaldehyde also react with 1, forming 2c−2e. Aldehyde-
derived 2d/2e have stereogenic P and C centers in their
central PBCO ring. Only one of the expected two pairs of
diastereomers of 2d/2e was observed spectroscopically; either
2d and 2e are formed stereospecifically or inversion at
phosphorus is facile. 2a−e were characterized by NMR
spectroscopy and single-crystal X-ray diffraction (see the
Supporting Information (SI)).
Figure 3. Synthesis of phosphaalkenes 4a−c directly from esters and
amides (Mes* = 2,4,6-tritert-butylphenyl; NR2 = 2,2,6,6-tetramethyl-
piperidino).
B
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Figure 4. Computed reaction profiles for the reactions of 1 with acetone and acetamide (M06-2X/def2svp), ΔG298 (ΔH) in kcal mol−1.
4a as a mixture (22:78) of E and Z isomers, identified by
signals in the 31P{1H} NMR spectrum at δ 120.1 and 104.4.
11B NMR spectroscopy revealed the formation of [R2NBO]2,37
indicated by a resonance at δ 28.1, which we confirmed
crystallographically. Monitoring the reaction of 1 and ethyl
acetate by NMR spectroscopy revealed that it proceeds
through a transient 1,2,3-phosphaboraoxetane intermediate:
signals at δ −115.7 (31P{1H}) and δ 39.0 (11B) are consistent
with those for 2a−e (Figure S1). We extended the reaction of
esters with 1 to α-pyrone to afford the exocyclic phosphaalkene
4b as a mixture (58:42) of E and Z isomers. We also prepared
the known 2-aminophosphalkene 4c37,38 from 1 and N,N-
dimethylacetamide. Phosphaalkenes 4a−c were easily isolated
in high purity and yield (70−91%).
We used density functional theory calculations (M06-2X/
def2svp)39 to probe the mechanism of the reaction of 1 with
carbonyl compounds. The first step in the reaction pathway
(Figure 4) is the dissociation of 1 into the monomeric
phosphaborene INT-1.22 Phosphaborenes have orthogonal
PB and BN π systems and can exhibit both nucleophilic
(at P) and electrophilic (at B) reactivity.22 For the initial
interaction with acetone, we thus considered the (i) formation
of a betaine-like intermediate27 by the attack of P at the
carbonyl carbon and (ii) interaction of the carbonyl oxygen
atom with boron. We could not locate a betaine-type structures
as minima. Instead, phosphaborene INT-1 and acetone react
via TS1acetone (+20.07 kcal mol−1) to form acetone adduct
INT-2acetone (+20.10 kcal mol−1).
distance: 1.82 Å vs INT-1 1.75 Å). As a result, intramolecular
attack of the phosphorus center at the carbonyl carbon occurs
via the very early transition state TS-2acetone (+22.32 kcal
mol−1), closing the 4-membered ring. The resulting isolable
phosphaoxaboretane 2b is substantially stable relative to its
precursors (−22.42 kcal mol−1).
Phosphaborene INT-1 and acetamide follow an alternative
pathway. Attempts to optimize acetamide counterparts of
adduct INT-2acetone minimized only to INT-1 and acetamide.
INT-1 and acetamide instead react by cycloaddition through
TS-2acetamide (+12.08 kcal mol−1) to form the phosphaoxabor-
etane P(R)-C(S)-5c or its P(S)-C(R) enantiomer (−7.36 kcal
mol−1). The amino-phosphaboraoxetane 5c can exist as two
pairs of diastereomers due to stereogenic P and C centers in
the 4-membered ring. P(R)-C(S)-5c is less stable than its
diastereomer P(S)-C(S)-5c relative to 0.5 1 + acetamide
(+0.99 vs −7.36 vs kcal mol−1. We could not locate transition
states leading to P(S)-C(S)-5c or its P(R)-C(R) enantiomer
from INT-1 + acetamide; inspection of TS-2acetamide reveals
that inversion of either P or C centers would generate an
unfavorable 1,2 steric interaction between Mes* and NMe2
groups. Experimental insight into the stereochemistry of the
intermediates 5c is limited by the ready interconversion of E
and Z-phosphaalkenes.40,41
Close inspection of the geometry of TS-2acetone and TS-
2acetamide reveals that they adopt markedly different structures
(Table S8). TS-2acetone is highly puckered (P−B−O−C torsion
= 48.8°) with a much more fully formed B−O than P−C bond
(B−O distance 1.561 vs 2a 1.386 Å [the B−O distance
contracts −13%]; P−C 3.015 vs 1.921 Å [−57%]). In contrast,
in TS-2acetamide the developing P−C and B−O bonds form
synchronously (B−O: 2.055 vs 1.393 Å [−48%]; P−C: 2.947
Coordination of acetone to boron in INT-2acetone lengthens
CO and PB bond distances, increasing electrophilicity at
the carbonyl carbon (C−O distance: 1.24 Å vs acetone, 1.20
Å) and nucleophilicity at the phosphorus center (P−B
C
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vs 1.974 Å [−49%]). The developing PBCO ring is much
flatter (P−B−O−C torsion = −24.2°).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c06228.
■
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Why is TS-2acetamide substantially lower in energy than TS-
2acetone? We ascribe this to two factors: (i) The synchronous
formation of P−C and B−O bonds in TS-2acetamide proceeds
with a lesser decrease in B−N π-bonding as the PBN
angle is distorted from away from linear in TS-2 (160° vs
129°). (ii) The more planar P−B−C−O ring in TS-2acetamide
enables the formation of C−H···O hydrogen bonds between
the amide oxygen and the methyl groups of the tetramethylpi-
peridine substituent at boron. The C−H···O distances and
angles, and CO···H angles in TS-2acetamide (2.2−2.3 Å, 125−
130°, and 135−145°, Table S10) are ideal for interactions of
this kind, whereas those in the puckered TS-2acetone are not
(Table S9).42 Such interactions can amount to as much as 4
kcal mol−1 with optimum geometry.43
Synthetic procedures, NMR spectra, and computational
details (PDF)
XYZ coordinates (ZIP)
Accession Codes
CCDC 1913519 and 2088200−2088204 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
The second barrier for the cycloreversion of phosphabor-
aoxetanes 2b/5c to phosphaalkenes and transient [R2NBO] is
much higher for 2b than it is for 5c (+38.77 vs +13.02 kcal
mol−1). The high energy of TS-3acetone (+16.35 kcal mol−1) is
consistent with the observed thermal stability of 2b, which
requires the addition of AlBr3 to promote cycloreversion.
Examination of the structures of TS-3 reveal their
asynchronous character: in both cases, compared to precursors
2a/5c, substantial C−O bond elongation (+51%, + 55%) is
observed with only minimal P−B elongation (+14, + 4%, Table
S11). This behavior strongly suggests that the lower energy of
TS-3acetamide vs TS-3acetone can be attributed in part to
stabilization of the developing positive charge at the carbon
center by its NMe2 substituent. Alkoxy substituents can be
expected to fulfill the same π-donor role, which explains the
differing fates in reactions of 1 with amides/esters and
ketones/aldehydes. Similarly, we propose that AlBr3 coordina-
tion to 2a−e lowers the energy of TS-3acetone by polarizing the
C−O (and thus the forming C−P) bonds.
Michael J. Cowley − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom; orcid.org/0000-0003-0664-2891;
Email: michael.cowley@ed.ac.uk
Authors
Andryj M. Borys − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom
Ella F. Rice − EaStCHEM School of Chemistry, University of
Edinburgh, Edinburgh EH9 3FJ, United Kingdom
Gary S. Nichol − EaStCHEM School of Chemistry, University
of Edinburgh, Edinburgh EH9 3FJ, United Kingdom
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c06228
Notes
The authors declare no competing financial interest.
Our studies reveal deep and far-reaching mechanistic
similarities between the reactions of 1 and carbonyl
compounds and the Wittig reaction. We thus propose the
term “phospha-bora-Wittig” to describe phosphaalkene-form-
ing reactions of phosphaborenes with carbonyl compounds. In
the Wittig reaction, the transition state for the cycloaddition of
ylide (Ph3PCHR) and carbonyl compound is subtly
influenced by factors including 1,2 and 1,3 steric interactions
in the cyclic four-membered transition state, dipole/dipole
interactions, and CO···H hydrogen bonds.27,44 1,2 Inter-
actions play a role in the formation of 5c, though the
importance of 1,3 interactions is negated by the two-
coordinate nature of the P/B centers. CH hydrogen bonding
in TS-3 is also observed. We also note the similarity to borata-
Wittig reactions of borata-alkenes, [R2CBR2]−, with carbon-
yl compounds.23,45−49
The classical Wittig reaction is limited in scope for esters/
amides, generally requiring careful substrate modification to
counteract the effect of the OR and NR2 substituents.50 This
limitation is absent in the reactions of 1 with esters or amides.
We ascribe this to the greater electrophilicity of the boron in
RPB=NR2 compared to the phosphorus center in
phosphorus ylides, R3PR2. We are currently working to
extend the scope of this reaction to compounds with other
substituents at phosphorus.
ACKNOWLEDGMENTS
■
M.J.C. wishes to thank Dr. Tobias Krämer, Dr. Rafal Szabla,
and Dr. Stephen Thomas for helpful discussions during this
work. This project has received funding from the European
Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (Grant
Agreement No. ERC-2016-STG-716315).
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contain the supplementary crystallographic data for this paper. These
data are provided free of charge by the joint Cambridge Crystallo-
graphic Data Centre and Fachinformationszentrum Karlsruhe: Access
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F
https://doi.org/10.1021/jacs.1c06228
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