Siladifluoromethylation and Deoxo-trifluoromethylation of PV-H Compounds with TMSCF3: Route to PV-CF2- Transfer Reagents and P-CF3 Compounds

Article abstract of DOI:10.1021/acs.orglett.9b00381

A method for siladifluoromethylation of dialkyl phosphonates and secondary phosphine oxides with TMSCF₃ to produce nucleophilic P^V-CF₂- transfer reagents is disclosed, with multigram scale reactions included. Condition-dependent divergent reactivity under the established conditions is demonstrated by the formation of trifluoromethylphosphines. Both one-pot transformations are operationally simple and employ inexpensive materials. Mechanistic investigations suggest the divergent reactivity originates from a common intermediate, with Li⁺ concentration directing the chemoselectivity.

Full text of DOI:10.1021/acs.orglett.9b00381

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Siladifluoromethylation and Deoxo-trifluoromethylation of PV−H
Compounds with TMSCF3: Route to PV−CF2 Transfer Reagents and
−
P−CF3 Compounds
Vinayak Krishnamurti,‡ Colby Barrett,‡ and G. K. Surya Prakash*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California
90089-1661, United States
S
* Supporting Information
ABSTRACT: A method for siladifluoromethylation of dialkyl phospho-
nates and secondary phosphine oxides with TMSCF3 to produce
nucleophilic PV−CF2− transfer reagents is disclosed, with multigram
scale reactions included. Condition-dependent divergent reactivity under
the established conditions is demonstrated by the formation of
trifluoromethylphosphines. Both one-pot transformations are operation-
ally simple and employ inexpensive materials. Mechanistic investigations
suggest the divergent reactivity originates from a common intermediate,
with Li+ concentration directing the chemoselectivity.
ine-tuning the chemical and physical properties of
Fmolecules through fluorofunctionalization is common-
place in materials1 and medicinal2 chemistry. The versatile
−CF2− unit, which is a bioisostere of etherial oxygens,
carbonyls, secondary amides and methylenes, that also
demonstrates lipophilic hydrogen bond donor properties,3
has inspired a plethora of gem-difluorination4 and difluor-
omethylation5 protocols. A prime example of −CF2−
bioisosterism is the use of −CF2PV groups in creating
metabolically stable, bioactive molecules,6 especially nucleoside
mono/di/triphosphate analogues.7 The −CF3 group has also
shown remarkable applicability in the creation of new materials
and bioactive compounds, spurring on a race for better
trifluoromethylation protocols.8
PVCF2TMS reagents, by virtue of their mild activation
conditions, have been extensively used in reactions with
electrophiles9 and in cross-coupling10 (Figure 1), with
Figure 1. Extensive use of PVCF2TMS reagents, mostly limited to
numerous scaffolds, having been functionalized. Variability of
diethyl phosphonate derivatives.
PV substituents can potentially influence chemical/physical
properties, but examples of such compounds are scarce.
PV−CF3 compounds have been synthesized using CF3Br in
Known reagent preparations have not included such variability,
an Arbuzov-type reaction.15 P−H trifluoromethylation has
with scope limited to −OEt and −Oi-Pr derivatives. Despite
the associated hazards, limited availability, and requirement of
been shown using TMSCF3 with (a) dialkyl phosphonates and
copper(I),16 (b) PIII−X compounds (X = F, NEt2, OPh) and
CsF.17 Electrophilic-type trifluoromethylation with a Yagupol-
skii−Umemoto-type/Togni’s reagent is also reported.18 There
are no reports of single-step deoxygenation−trifluoromethyla-
tion of PV−H compounds to afford PIII−CF3 compounds.
TMSCF3 is a source of nucleophilic −CF3,19 −CF2H,20 and
−C2F5,21 and electrophilic singlet CF2 carbene.22 It can be
an additional silylation step,11 they are accessed from the
12
controlled substance CF2Br2 (Halon 1202): a class I ozone
depleting substance and the most hazardous fluorobromocar-
bon known. Another approach involves C(sp3)−H chlorina-
tion of diisopropyl ((methylthio)methyl)phosphonate, chlor-
ine-fluorine exchange (with 3HF-Et3N), activation with n-
BuLi, and silylation13 (Figure 2). PV−CF2H compounds have
been prepared using TMSCF2Br.14 This work aims to provide
access to such PVCF2TMS and PVCF2H compounds using
inexpensive, safe reagents.
Received: January 29, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00381
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
substituting LiI with NaCl, 2a was not observed; the hitherto
unknown 3a was formed selectively (trial 5), highlighting the
need for Li+.28 The combination of LiH and LiCl afforded 2a
in near-quantitative yields (trial 9). The method was extended
to a series of phosphonates and phosphine oxides (see Scheme
1).
Scheme 1. Siladifluoromethylation and Deoxo-
a
trifluoromethylation of P−H Compounds
Figure 2. Reported syntheses of siladifluoromethyl and trifluor-
omethyl phosphorus compounds.
prepared from fluoroform23 and is safer and less expensive than
many other fluoroalkylation reagents. Using TMSCF3 to add
−CF2TMS to nucleophiles has garnered appreciable interest,
allowing direct synthesis of less-accessible difluoromethylsi-
lanes from a single −CF2− and TMS− source. The seminal
work of Mikami and co-workers on the siladifluoromethylation
of boron24 and sp3, sp2, and sp carbon25 centered nucleophiles
is a prime example. An HMPA-mediated siladifluoromethyla-
tion of di/triarylmethanes is reported.26 Our group recently
detailed a synthesis of SII−CF2H compounds via SII−CF2TMS
intermediates.27
Optimization trials (Table 1) were performed using 1a.
Among the bases screened, hydrides proved most effective,
Table 1. Select Optimization Studies on Diethyl
a
Phosphonate (1a)
b
NMR Yield (%)
a
e
e
trial
salt
base
TMSCF3 (equiv)
2a
3a
4a
1
2
3
LiI
LiI
LiI
LiI
NaCl
NaI
LiCl
LiCl
LiCl
LiOtBu
LiH
NaH
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
4.0
22
66
66
0
0
5
73
84
99
0
0
0
11
0
0
0
3
5
2
0
0
a
Reactions performed with 0.5 mmol of 1. Yields in parentheses
c
determined by 19F NMR of reaction mix with PhF (0.5 mmol) as
internal standard. Single asterisk symbol (*) indicates that 20 mmol
of 1 was used; double asterisk symbol (**) indicates that the
4
−
0
5
6
7
8
NaH
NaH
NaH
LiH
89
24
4
0
0
b
component has been oxidized for easy isolation. 2.5 equiv TMSCF3.
c2.0 equiv LiOtBu, 2.0 equiv LiCl. 4.0 equiv LiCl. 1.2 equiv LiCl
d
e
d
added.
9
LiH
a
Unless specified, trials were performed using 1a (0.5 mmol), in DMF
b
(0.2 M). Based on 19F NMR using fluorobenzene (0.5 mmol) as an
internal standard. LiBF4 (1.2 equiv) was added. Reaction was
performed in DMF (0.4 M). 1.2 equiv.
Phosphonate 2a, used extensively in phosphadifluoromethy-
lation reactions,9,10 was obtained in near-quantitative yield
(99%) (Scheme 1). To investigate steric effects at the P atom,
2b was prepared from 1b in excellent yield (89%). Even the
egregiously sterically encumbered 1c underwent facile trans-
formation to 2c (94%). At 20 mmol scale, 2a (83%, 4.3 g) and
2b (63%, 3.6 g) were obtained in high yields, demonstrating
scalability and applicability in bulk reagent synthesis. Although
reagents 2a−2c enable steric variability at the P center, the
cleavable alkoxy groups may limit applicability in some
scenarios. Cyclic phosphonate 2d was prepared in excellent
yield (73%) as a potentially more-stable reagent. Diphenyl
phosphonate and dibenzyl phosphonate, however, did not
produce the corresponding products 2, possibly because of the
c
d
e
affording 2a in 66% yield (trials 2 and 3). Amide and alkoxide
bases proved ineffective for this substrate, with appreciable
decomposition of 1a. Under these conditions, no reaction was
observed in THF, Et2O, hexanes, MeCN, or DMPU; with the
latter undergoing significant decomposition. Binary systems of
DMF with THF, hexanes, MeCN, or Et2O did not produce
favorable results. Our working hypothesis was that the reaction
proceeded through a difluorocarbene intermediate; therefore,
utilizing lithium or sodium salts could accelerate decom-
position of [CF3]− to difluorocarbene. Interestingly, upon
B
DOI: 10.1021/acs.orglett.9b00381
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
enhanced leaving group ability of the −OR groups. Nonethe-
less, the method seems well-suited for dialkyl phosphonates.
To investigate the tolerance of P−N bonds, 1e was subjected
to the reaction conditions, and phosphonamidate 2e (20%)
was isolated. Diarylphosphine oxides 4f and 4g were prepared
in good to excellent yields (55% and 73%) as a mixture of the
corresponding PV−CF2TMS and PV−CF2H compounds; thus,
they were isolated as the corresponding PV−CF2H products.
Interestingly, the most electron-rich phosphine oxide 2h was
obtained selectively as the PV−CF2TMS compound (40%).
The greater electron density at the P atom, leading to a less-
stable anion upon desilylation, may be responsible for this
heightened stability. A similar reaction with bis(4-
(trifluoromethyl)phenyl)phosphine oxide did not produce
any significant amount of product 2. It appears that electron-
deficient aryl units are not compatible with this procedure.
Difluoromethyl dialkyl phosphine oxide 4i was obtained in
29% yield.
either the oxonium trifluoromethide IV or trifluoromethyl-
phosphorane V. Similar to the mechanism of deoxygenation
(reduction) of phosphine oxides using hexachlorodisilane, a
reductive elimination of TMS2O would produce compound 5.
As a demonstration of their synthetic utility, 2a, 2b, 2d, and
2h were reacted with 6sm to produce PVCF2− compounds 6
(see Scheme 3). Interestingly, the stability of the O−TMS
Scheme 3. Demonstration of 2 as Reagents and the
a
Significance of Variability at P
Surprisingly, in the absence of LiCl, 1i showed an
unprecedented deoxo-trifluoromethylation to afford the
corresponding trifluoromethyl phosphine 5i in 62% yield
(Scheme 1), which underwent slow oxidation in air. Similarly,
5j was obtained in good yield (47%, 62% by NMR). These
results demonstrate a divergence in reactivity under similar
conditions, dictated by the amount of LiCl present. Finally, 5g
was obtained (50%) as an air-stable solid. This is the first
report of a tandem deoxygenation−trifluoromethylation of
secondary phosphine oxides using TMSCF3 to produce −CF3
phosphines, wherein the oxidation state of P changes from +5
to +3.
a
Unoptimized, isolated yields. See the Supporting Information for full
experimental details. The single asterisk symbol (*) indicates that
TMS may be fully cleaved on prolonged stirring.
To elucidate potential reaction pathways (Scheme 2) and
probe intermediates involved in the divergent reactivity,
bond depends on the identity of the R groups at P. 6a and 6b
were readily formed. 6′d underwent slower hydrolysis under
the same conditions, producing a mixture of O−H and O−
TMS compounds. 6h was unreactive to the hydrolytic
conditions. The difference in stability to the basic, aqueous
conditions reinforces the importance of variability at the P
atom in tuning the chemical properties of molecules.
Scheme 2. Plausible Reaction Pathways
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00381.
Procedures, characterization, and images of spectra
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gprakash@usc.edu.
ORCID
control experiments were performed (see the Supporting
Information for full discussion of experiments). We envision
that the divergent reactivity results from differences in the
mode of reaction of intermediate II with TMSCF3. In path 1,
we propose that intermediate II is formed by silylation of I
G. K. Surya Prakash: 0000-0002-6350-8325
Author Contributions
‡These authors contributed equally.
Author Contributions
−
with TMSCF3. This releases CF3 that will decompose in the
presence of excess Li+ to produce electrophilic difluorocar-
bene. Trapping of difluorocarbene by II may result in the ylide-
intermediate III, which could undergo an intramolecular or
intermolecular silylation at the CF2 carbon to afford product 2.
In contrast, path 2 may be a consequence of an additional
silylation of II by another equivalent of TMSCF3, producing
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.9b00381
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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ACKNOWLEDGMENTS
■
The authors thank the Loker Hydrocarbon Research Institute
for funding. V.K. thanks the Morris−Smith Foundation for
funding.
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