Improved Synthetic Route to Heteroleptic Alkylphosphine Oxides

Article abstract of DOI:10.1021/acs.organomet.7b00304

A new method for the synthesis of heteroleptic alkylphosphine oxides (R₂R¹P=O, where R ≥ R¹) from secondary phosphine oxides (or SPOs, R₂HP=O) is presented. These reactions were fast at room temperature, sterically selective, high yielding, and >95% pure after an aqueous wash. Deprotonation of an SPO generates a phosphinite anion ([R₂P-O]^-) that was found to be highly selective for nucleophilic P-C bond formation (as opposed to O-C bond formation) with alkyl halides. Surprisingly, most strong organometallic bases failed to deprotonate SPOs to their respective phosphinite anions (pK_as for most SPOs are <27). Only sodium bis(trimethylsilyl)amide (NaHMDS) cleanly formed the phosphinite anion, which was stable in solution (0.1 M, 23 °C in THF) for over 24 h. The need for a very specific base to deprotonate suggests that both ion pairing and the conjugate acid play a role in stabilizing the phosphinite anion. Phosphinite anion reactivity followed the expected trend for an S_N2 mechanism on reaction with alkyl halides; elimination products were never observed. A wide variety of heteroleptic alkylphosphine oxides were isolated in near-quantitative yield with only an aqueous wash as purification. This methodology was then used to make new bis(phosphine oxide)alkanes and unsymmetrical α,ω-bis(phosphine oxide)alkanes (R₂P(O)(CH₂)₃P(O)R¹₂) on the benchtop with unprecedented ease.

Full text of DOI:10.1021/acs.organomet.7b00304

Article
pubs.acs.org/Organometallics
Improved Synthetic Route to Heteroleptic Alkylphosphine Oxides
Alexander J. Kendall, Daniel T. Seidenkranz, and David R. Tyler*
University of Oregon, Department of Chemistry and Biochemistry, 1253 University of Oregon, Eugene, Oregon 97403, United States
S
* Supporting Information
ABSTRACT: A new method for the synthesis of heteroleptic
alkylphosphine oxides (R2R1PO, where R ≠ R1) from
secondary phosphine oxides (or SPOs, R2HPO) is presented.
These reactions were fast at room temperature, sterically
selective, high yielding, and >95% pure after an aqueous wash.
Deprotonation of an SPO generates a phosphinite anion ([R2P−
O]−) that was found to be highly selective for nucleophilic P−C
bond formation (as opposed to O−C bond formation) with alkyl
halides. Surprisingly, most strong organometallic bases failed to
deprotonate SPOs to their respective phosphinite anions (pKas for most SPOs are <27). Only sodium bis(trimethylsilyl)amide
(NaHMDS) cleanly formed the phosphinite anion, which was stable in solution (0.1 M, 23 °C in THF) for over 24 h. The need
for a very specific base to deprotonate suggests that both ion pairing and the conjugate acid play a role in stabilizing the
phosphinite anion. Phosphinite anion reactivity followed the expected trend for an SN2 mechanism on reaction with alkyl halides;
elimination products were never observed. A wide variety of heteroleptic alkylphosphine oxides were isolated in near-quantitative
yield with only an aqueous wash as purification. This methodology was then used to make new bis(phosphine oxide)alkanes and
unsymmetrical α,ω-bis(phosphine oxide)alkanes (R2P(O)(CH2)3P(O)R1 ) on the benchtop with unprecedented ease.
2
useful as biologically active molecules,22 as novel materials,23,24
INTRODUCTION
■
and as ligands in coordination chemistry.25,26
Despite the importance of phosphines in modern inorganic
We began by studying the reaction of secondary phosphine
chemistry, catalysis, materials, medicinal chemistry, and organic
oxides (SPOs) with alkyl halide electrophiles to form a P−C
synthesis, the challenging nature of phosphine synthesis
bond (yielding tertiary phosphine oxides), which is currently a
continues to thwart the development of new phosphines.1−3
poorly developed area of chemistry. Most known trans-
There is a movement in synthetic chemistry to make these
formations of SPOs with electrophiles use arylphosphine
traditionally onerous chemical preparations benchtop
oxide nucleophiles and carbonyl electrophiles.27−31 Reports of
friendly.4−6 Our laboratory has played a part in this movement
dialkyl SPOs reacting with alkyl electrophiles to form P−C
by developing a method to synthesize arylphosphines on the
bonds are both exceedingly rare and narrow in scope.32−36
benchtop.7 We sought to extend the notion of benchtop
A promising study by Hays showed that H-phosphonates
chemistry to alkylphosphines. For example, dimethylphos-
could be directly converted to heteroleptic alkylphosphine
phines are a severely underrepresented class of compounds that
oxides using Grignard reagents, with the key intermediate being
have outstanding ligand properties,8 but their usefulness is
a nucleophilic phosphinite anion (Figure 1).37 Although the
overshadowed by their synthetic inaccessibility due to the air,
Hays method has been successfully applied to a narrow scope
water, and silica instability of the precursors and intermediates
of phosphine syntheses, as reported by Doyle,38 it has been
used in traditional syntheses.9−11 Easy synthetic access to
otherwise neglected. The reaction is plagued by poor yields,
dimethyl(alkyl)phosphines would be exceedingly useful to
egregious workup conditions, and very limited substrates. Of
chemists, because current synthetic approaches require either
most concern is the wasteful excess of alkyl bromide and the
HPMe2 or Me2PCl, both of which are exceedingly difficult and
dangerous to synthesize, handle, and purify.12−15 The syntheses
of known dimethyl(alkyl)phosphines often entail a considerable
resource investment of researcher time.
Our previous work reported a synthetic method that used
electrophilic phosphonate reagents to prepare heteroleptic
phosphines and that focused on air-, water-, and silica-stable
precursors.7 In contrast, the work reported here focused on
developing a general synthesis for heteroleptic alkylphosphines
using nucleophilic, air-stable phosphorus precursors to yield air-
Figure 1. Hays method of tertiary phosphine oxide synthesis (R1 = Et;
R2 = Me, Et; R3 = 1° alkyl).
stable tertiary phosphine oxides. Air-free handling is thus
avoided until the final step, reduction16−21 to a phosphine. It is
Received: April 20, 2017
worth noting that phosphine oxides are in and of themselves
© XXXX American Chemical Society
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vulnerability to considerable side reactions. These factors make
Hays’ synthetic route to heteroleptic phosphine oxides
unfeasible, and consequently it has been seldom used in
modern syntheses.
We hypothesized that the formation of a nucleophilic
phosphinite anion directly from the deprotonation of a
secondary phosphine oxide would be an easy way to avoid
the unfavorable product selectivity, reaction conditions, and
difficult isolations associated with the Hays reaction. Herein we
report a new method for the deprotonation of SPOs to form
phosphinite anions and their subsequent selective, clean, and
fast transformations into a wide variety of tertiary phosphine
oxides.
RESULTS AND DISCUSSION
■
Figure 2. 31P{1H} NMR overlay showing crude reaction mixtures of
diethylphosphine oxide with various bases followed by reaction with
iodoethane. The diethylphosphine oxide resonance appears at 53.1
ppm, whereas the triethylphosphine oxide resonance appears at 41.2
ppm.
Deprotonation screening studies. Our initial approach
was deprotonation of an SPO with a Grignard reagent to
generate the same phosphinite anion as that made by Hays
(Scheme 1). Unexpectedly, Grignard reagents did not
Scheme 1. General Scheme Showing the Proposed Stepwise
Reaction Starting with Deprotonation of an SPO To Form a
Nucleophilic Phosphinite Anion Followed by a Reaction
with an Electrophile To Yield a Heteroleptic Tertiary
Phosphine Oxide (R, R1 = Alkyl)
temperature (under N2), is not prone to decomposition (for at
least 24 h), reacts chemoselectively at the P atom (over the
oxygen), and quantitatively produces clean products. (Mech-
anistic details are discussed below.)
Phosphinite Anion Screening. To determine the breadth
of the deprotonation reaction, a representative series of alkyl
SPOs were deprotonated using NaHMDS and subsequently
reacted with a primary alkyl bromide (Table 1). All of the
Table 1. Phosphinite Nucleophile Screening with
Bromododecane
deprotonate the SPO, which was very unusual given that
SPO pKas range from 21 to 27.39 Attempts to protonate the
species formed by the reaction of the SPO and the Grignard
reagent back to SPO met with failure, only yielding complex
and unidentified reaction mixtures.
This result suggests that the Grignard reagent did not initially
deprotonate the SPO to form a phosphinite anion. Rather, the
SPO appears very susceptible to side reactions that yield
complex mixtures (likely from the ability of SPOs to act as
electrophiles and form hypervalent structures rather than
undergoing deprotonation40).
a
Average of two isolated yields.
Alternative organometallic bases were screened for SPO
deprotonation using the model reaction in Figure 2. These
results showed that generating a stable phosphinite anion
successfully is crucially dependent on both the cation and
conjugate acid of the base. Only potassium bis(trimethylsilyl)-
amide (KHMDS) and sodium bis(trimethylsilyl)amide
(NaHMDS) produced a stable anion from diethylphosphine
oxide that reacts both quantitatively and cleanly with
iodoethane to yield triethylphosphine oxide. It is also notable
that, although the phosphinite anion forms quickly (<15 min at
0 °C), at times up to 24 h (23 °C) after the phosphinite anion
had been generated, addition of iodoethane provided complete
conversions to triethylphosphine oxide. (Control experiments
showed that, under the conditions employed, diethylphosphine
oxide does not react directly with iodoethane in the absence of
a base.) Handling of the starting materials and workup of the
reaction mixtures were performed on the benchtop in air. The
air- and water-sensitive phosphinite anion was generated in situ,
requiring only basic Schlenk techniques during the reaction. In
summary, these results show that the phosphinite anion forms
with NaHMDS or KHMDS very quickly, is persistent at room
reactions screened were very clean, with no observable
elimination products or side reactions even when sterically
encumbered SPOs were used (i.e., Bu2P(O)H). Note that no
t
purification was necessary; the purity of the crude reaction
mixtures was >95% in every case after a water quench and
organic extraction. These results show that the generation of
the phosphinite anion from NaHMDS is a general reaction for
a representative range of alkyl SPOs as well as their subsequent
reactivity with the alkyl bromide to yield heteroleptic
phosphine oxides.
Electrophile Screening. We next explored the reactivity of
the dimethylphosphinite anion with a series of electrophiles
(Table 2) to determine the scope of the reaction. Primary
chlorides, primary bromides, primary iodides, and secondary
bromides were converted to tertiary phosphine oxides in
excellent yields. Benzyl bromides and chlorides were also high
yielding; however, these halides had to be cooled to 0 °C to
mitigate side reactions (likely O−C bond formation on the
basis of 31P NMR spectroscopy revealing peaks around 120
ppm).41 In general, secondary chlorides were resistant to
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Table 2. Reactions of Sodium Dimethylphosphinite with Varying Electrophiles To Yield Tertiary Dimethyl(alkyl)phosphine
Oxides
a
b
c
d
Average of two isolated yields. No observable conversion. Refluxing THF for 16 h, no conversion observed. Reagents purified immediately
e
before use and reactions run under rigorously light free conditions. Reacted at 0 °C for 2 h.
conversion even in refluxing THF (e.g., Table 2, entry 5). The
exceptional chemoselectivity for some electrophiles over others
appears to be the result of two factors: (1) increasing steric
hindrance at the electrophile obstructs nucleophilic attack by
the phosphinite anion (e.g., Table 2, entries 6−9, show
increasing steric hindrance at the electrophile, of which entry 8
(which has 1,3-diaxial steric interactions) appears to retard the
reaction) and (2) lower bond strengths of the leaving groups
(weaker/longer bonds) are more reactive with the phosphinite
anion (e.g., Table 2, entries 8 and 11). These findings are
consistent with an SN2 type mechanism (see below).
Bis-electrophiles were also screened for reactivity. As with the
reactions observed in Table 2, no elimination or other side
reactions were observed when product was formed. A series of
bis-dimethylphosphine oxides were synthesized and isolated in
excellent yields without the need for purification (Table 3).
These results include a protected alcohol (Table 3, entry 4) and
the trivial synthesis of an immediate precursor for 1,3-
bis((dimethylphosphino)methyl)benzene (a bis-dimethylphos-
phine η3-PCP ligand) (Table 3, entry 6), which is otherwise
reported as a laborious effort.42−44
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Table 3. Bidentate Phosphine Oxide Synthesis
Table 4. Unsymmetrical Bidentate Phosphine Oxides
Prepared from α,ω-Dihaloalkanes
a
b
Average of two isolated yields. Quantitative conversion of all
reagents.
bis(phosphine oxide)alkanes (although this is beyond the scope
of the current work).
a
b
Mechanistic Considerations. The intermediate generated
on deprotonation of the SPO appears to be best described as a
phosphinite anion because of the 31P{1H} NMR chemical shift
from 55.8 ppm (diisopropylphosphine oxide) to 121.6 ppm
Average of two isolated yields. Quantitative conversion of all
reagents.
37
i
(Figure 3, R = Pr) in THF. The phosphinite anion prefers
Unfortunately, 1,2-dibromoethane underwent side reactions
with the phosphinite anion. The resulting complex reaction
mixture was not identified. Somewhat oddly, 1,2-(bisalkyl)-
phosphines can be made using the Doyle38 modification to the
Hays preparation shown in Figure 1. (Presumably the different
reactivity is caused by the phosphinite’s attendant cation being
Mg2+ instead of Na+, or perhaps the other chemical species
present in the reaction mixture.)
Synthesis of Unsymmetrical Bidentate Phosphine
Oxides. During our initial studies, it was observed that the
reactions of the phosphinite anion with alkyl iodides were
nearly complete within minutes, those of alkyl bromides were
complete within hours, and alkyl chlorides required overnight
reaction times for complete reactivity. From these observations,
we hypothesized that the observed kinetic selectivity and the
precise control of reaction stoichiometry could be used to
synthesize unsymmetrical bis-phosphine precursors (e.g., R2P-
Figure 3. Intermediate phosphinite anion properties with 31P NMR
shifts noted for R = Pr in THF.
i
(O)(CH2)3P(O)R1 ) by stepwise reaction of unsymmetrical
P−C bond formation over O−C bond formation, with the
exception of oxophilic electrophiles such as chlorosilanes
(which is in good agreement with observations in the
literature).49,50 This observation suggests that the hard oxygen
nucleophile is cation passivated and only attacks strongly
oxophilic substrates. In all cases reported here, bond formation
was observed at the phosphorus exclusively.
The reaction mechanism is consistent with an SN2 attack of
the phosphinite on the electrophile. This mechanism explains
the sensitivity of the reaction to the electrophile’s steric profile
(e.g., sec-butyl bromide and bromocyclopentane react readily,
whereas bromocyclohexane and 2-bromoadamantane are
unreactive (Table 2)) as well as the qualitative reaction rates
observed for the halide leaving groups (I > Br > Cl). In
agreement with this assessment, no reactivity was observed with
2
α,ω-dihaloalkanes with different phosphinite anions (e.g., first
sodium dimethyl phosphinite then sodium di-tert-butyl
phosphinite) (Table 4). The results from this method are
shown in Table 4.
Although unsymmetrical bis-phosphines have been synthe-
sized before,45 to our knowledge, these products are the first
cases of an unsymmetrical bis-phosphine or bis-phosphine
oxide that has di-tert-butylphosphine and dimethylphosphine
moieties. Prior synthetic routes to unsymmetrical bis-
alkylphosphines required arduous reaction conditions and
numerous steps, the efforts of which have only been successful
in a handful of reports.46−48 Because the synthetic routes
described here require no purification, it is likely a true “one-
pot” procedure could easily be optimized for unsymmetrical
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tert-butyl iodide (Table 2, entry 12), indicating that an SN1
mechanism is not operating under the reaction conditions.
Patrick F. Martino, and Chase A. Salazar are acknowledged for
their contributions to this project.
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CONCLUSIONS
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00304.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for D.R.T.: dtyler@uoregon.edu.
ORCID
David R. Tyler: 0000-0002-5032-7533
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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