Perfluoroalkyl Grignard Reagents: NMR Study of 1-Heptafluoropropylmagnesium Chloride in Solution

Article abstract of DOI:10.1021/acs.joc.6b00807

We report on the generation of a perfluoroalkyl Grignard reagent (^FRMgX) by exchange reaction between a perfluoroalkyl iodide (^FR-I) and a Grignard reagent (RMgX). ¹⁹F NMR was applied to monitor the generation of n-C₃F₇MgCl. Additional NMR techniques, including ¹⁹F COSY, NOESY, and pulsed gradient spin-echo (PGSE) diffusion NMR, were invoked to assign peaks observed in ¹⁹F spectrum. Schlenk equilibrium was observed and was significantly influenced by solvent, diethyl ether, or THF.

Full text of DOI:10.1021/acs.joc.6b00807

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Article
Perfluoroalkyl Grignard Reagents: NMR Study of 1-
Heptafluoropropylmagnesium Chloride in Solution
Jie Guang, Russell Hopson, Paul G. Williard, Motohiro Fujiu, Kazuyuki Negishi, and Koichi Mikami
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00807 • Publication Date (Web): 13 Jun 2016
Downloaded from http://pubs.acs.org on June 20, 2016
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Perfluoroalkyl Grignard Reagents: NMR Study of
1-Heptafluoropropylmagnesium Chloride in Solution
7
Jie Guang†, Russell Hopson†, Paul G. Williard*†, Motohiro Fujiu‡, Kazuyuki Negishi‡
and Koichi Mikami‡
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†Department of Chemistry, Brown University, Providence, RI 02912 USA
‡
Department of Applied Chemistry, Tokyo Institute of Technology
Tokyo, 152-8552 Japan
e-mail: pgw@brown.edu
TOC
Abstract
We report on the generation of a perfluoroalkyl Grignard reagent (FRMgX) by
exchange reaction between a perfluoroalkyl iodide (FR-I) and a Grignard reagent
(RMgX). 19F-NMR was applied to monitor the generation of n-C3F7MgCl. Additional
NMR techniques, including 19F-COSY, NOESY and pulsed gradient spin-echo
(PGSE) diffusion NMR, were invoked to assign peaks observed in 19F spectrum.
Schlenk equilibrium was observed and was significantly influenced by solvent,
diethyl ether or THF.
Introduction
The first synthesis of a perfluoroalkyl Grignard reagent (FRMgX) was developed
by Haszeldine by reaction of a perfluoroalkyl iodide with magnesium metal.
FRMgX
1
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reacts with CO2, ketones and aldehydes similar to unfluorinated Grignard reagents.
FRMgX has much poorer thermal stability than the corresponding RMgX. Hence it is
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F
much more convenient to produce and use RMgX at low temperature, preferably at -
9
78 °C. However the reaction between perfluoroalkyl iodide and magnesium requires
warming to at least -10 ~ -20 °C to initiate and this causes decomposition of the
FRMgX reagent. Moreover the yields are sensitive to the purity and size of
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magnesium metal.
2
To overcome these drawbacks, McBee and co-workers developed
F
a fast and convenient method to generate RMgX at low temperature.2,3 By mixing
perfluoroalkyl iodide (FRI) and RMgX in ethereal solvents a halogen-magnesium
exchange happens quickly leading to FRMgX and alkyl iodide (RI).
reaction is fast and quantitative even at -78 °C.
4
This exchange
5
In analogy with normal Grignard reagents,
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FRMgX react with electrophiles
leading to organofluorine compounds.
7
However, in practice the applications have
F
been quite limited due to poor stability and reactivity.
8
Decomposition of RMgX
yields a protonated product (FRH), perfluoroalkene and traces of coupling product
(FR-FR) as well as perfluoroalkene polymer.1b Generation of perfluoroalkene
involves either an α or β magnesium-fluoride elimination. A single electron transfer
(SET) pathway was suggested which indicated the existence of a radical intermediate
(FR•) to explain the formation of the other decomposition byproducts.
9
F
Mechanistic studies of RMgX are still insufficient.10 In the solid state, structural
characterization of FRMgX is also lacking. 19F-NMR has been applied several decades
F
ago11 but unambiguous chemical shift values of RMgX are difficult to find in the
literature due to instrumentation used in the earliest studies. However, mechanistic
studies of these reagents are crucial for the improvement of stability and reactivity.
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F
For example, synthetic methodology development involving RMgX reagents shows
that some additives significantly stabilize FRMgX or improve the performance but the
reasons for these improvements are unclear. 12 Therefore, we report directly
observation and characterization of various intermediates in the reactions involving
FRMgX and we supply NMR data in this study.
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Results and Discussion
In order to simplify the spectra, perfluoropropyl magnesium chloride (n-FPrMgCl)
was selected as the model of perfluoroalkyl Grignard reagents. Additionally, this
compound is relatively stable. The preparation of n-FPrMgCl is described in Figure 1,
with chemical shift values (19F) of reactant13 and major byproducts14 listed. Chemical
shift values were measured in diethyl ether at -78 °C and calibrated to the chemical
shift of C6F6 at -164.9 ppm.
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Figure 1. Preparation of n-FPrMgCl, with 19F chemical shift values of reactant and
major byproducts.
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Several reactions are reported that use perfluoroalkyl Grignard reagents at low
temperature followed by warming to -20 °C or room temperature.15, 10e, 10f Our VT
NMR results show that n-FPrMgCl or n-FBuMgCl undergo significant decomposition
at -20 °C and also that they slowly decompose even at -78 °C in both THF and diethyl
ether solutions respectively. At higher concentrations, the decomposition occurs even
more quickly. Hence the successful applications reported at 0 °C are probably due to
a faster reaction of FRMgX with other regents than self-decomposition. Therefore all
NMR experiments mentioned in this manuscript were performed at -78 °C for both
sample preparation and NMR analysis in order to minimize self-decomposition.
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Perfluoropropyl Magnesium Chloride (n-FPrMgCl) in Diethyl Ether Solution
A freshly made ether solution of n-FPrMgCl displays two sets of “FPrMg” peaks in
F
F
19F NMR (Figure 2 bottom). These peaks are labeled as Pr-Mg (1) and Pr-Mg (2).
F
F
After storage at -78 °C for 3.5 h, Pr-Mg (1) can be barely detected and Pr-Mg (2)
slightly increases. However a new “FPrMg” species appears and is labeled as FPr-Mg
(3) (Figure 2 top). Full assignments for the peaks in Figures 2 are presented in Figure
3 and 4. The correlation of two adjacent CF2 peaks from the same “FPrMg” species
3
are supported by the cross peaks in 19F COSY spectra (Figure 5) due to JFF
couplings. There are several minor unassigned peaks in the 19F spectra, which display
numerous cross-peaks in the 19F COSY spectra. These impurities are assigned to
traces of perfluoro-n-hexane and polymer byproducts whose formations involve a
heptafluoropropyl radical intermediate. These long-chain perfluoro byproducts
4
present multiple JFF coupling cross-peaks, which usually have much stronger
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intensities than JFF coupling cross-peaks in 19F COSY spectra. A significant amount
of perfluoropropene (C3F6) was observed after 3.5 h as the decomposition product of
all “FPrMg” complexes in solution.
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3.5h later
CF -C2F I
3
4
CF -CF CF -Mg(2)
3
2
2
CF -CF CF -Mg(3)
3
2
2
CF -C2F Mg(1,2,3)
3
4
C3F
6
C2F -CF -I
5
2
C3F
6
Freshly prepared
CF -CF CF -Mg(1)
3
2
2
CF -C2F I
3
4
CF -C2F Mg(1,2)
3
4
CF -CF CF -Mg(2)
3
2
2
CF -CF -CF I
3
2
2
C2F -CF -I
5
2
C3F
6
Figure 2. 19F NMR, 0.4 M ether solution of n-FPrMgCl at -78 °C.
CF -C2F -I
3
4
3.5h later
CF -C2F Mg(2)
3
4
CF -C2F Mg(3)
3
4
C3F
6
FPr-I
C3F
6
Freshly prepared
CF -C2F -I
3
4
CF -C2F Mg(2)
3
4
CF -C2F Mg(1)
3
4
FPr-I
C3F
6
C3F
6
Figure 3. 19F NMR, downfield region of Figure 2.
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3.5h later
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CF -CF CF -Mg(2)
3
2
2
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CF -CF -CF I
3
2
2
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CF -CF CF -Mg(3)
3 2 2
C3F
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Freshly prepared
CF -CF CF -Mg(1)
3
2
2
CF -CF CF -Mg(2)
3
2
2
CF -CF -CF I
3
2
2
Figure 4. 19F NMR, upfield region of Figure 2.
Figure 5. {19F, 19F} COSY NMR, 0.4 M ether solution of n-FPrMgCl at -78 °C.
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F
If the sample concentration is lowered to 0.2 M, Pr-Mg (1) is the dominant
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magnesium species in the solution at first, suggesting that Pr-Mg (1) is relatively
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more stable in dilute solution (Figure 6). However, 3.5 h later, Pr-Mg (2) is still the
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dominant intermediate and FPr-Mg (3) begins to appear.
(2)
CF region
3
(1)
FPr-I
3.5h later
(3)
(3)
FPr-Mg(2)
(1)
(2)
FPr-Mg(1)
FPr-I
FPr-I
Freshly prepared
FPr-Mg(3)
FPr-Mg(1)
FPr-Mg(2)
FPr-I
Figure 6. 0.2 M ether solution of n-FPrMgCl at -78 °C.
Perfluoropropyl Magnesium Chloride (n-FPrMgCl) in THF Solution
A much cleaner 19F NMR spectrum was observed when THF was the solvent. As
F
shown in Figure 7, Pr-Mg (3) is the only detectable magnesium species. The 19F
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COSY spectrum (Figure 8) clearly distinguishes two CF2 peaks of Pr-Mg (3), since
the CF3 peak only exhibits a cross peak with CF2 (c) but not with CF2 (b). This
4
indicates there is a four-bond coupling between F(a) and F(c), as JFF is usually
3
stronger than JFF for unbranched perfluoroalkyl groups. 16 It is noteworthy that
identical 19F spectra are obtained when ether is the solvent and when commercial n-
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BuMgCl that we utilized to prepare the n-FPrMgCl reagent was dissolved in THF.
The common feature between these two reactions is the presence of THF when the n-
FPrMgCl is formed. It is noteworthy that the species in solution are not observed in
the same proportions when THF is subsequently added to a solution of the n-FPrMgCl
reagent initially formed in the absence of THF (Figure S4).
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F
It is also worth noting that two CF2 groups of Pr-Mg (3) show significantly
F
different chemical shifts compared with the corresponding CF2 groups in Pr-Mg
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species (1) and (2). Moreover, THF accelerates the formation of Pr-Mg (3). We
assume that FPr-Mg (3) is a dialkyl magnesium “(n-FPr)2Mg” species and both (1) and
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(2) are alkyl magnesium halide “n-FPrMgCl” type species, due to relatively slow R
group exchange (Scheme 1).17
CF -CF CF Mg(3)
3
2
2
CF -CF -CF Mg(3)
3
2
2
CF CF -CF -Mg(3)
3
2
2
FPr-I
CF -CF CF H
CF -CF -CF -H
3
2
2
3
2
2
FPr-I
Figure 7. 0.4 M THF solution of n-FPrMgCl at -78 °C.
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Figure 8. {19F, 19F} COSY NMR, 0.4 M THF solution of n-FPrMgCl at -78 °C.
Schlenk Equilibrium
A
19F pulsed-gradient spin-echo (PGSE) experiment was applied to measure self-
diffusion coefficients of these various magnesium species in ether solution in order to
ascertain information about the aggregation state. As shown in Figure 9, S1 and S2,
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FPr-Mg (2) has similar size to Pr-Mg (3) and is larger than Pr-Mg (1). Moreover,
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exchange cross-peaks are observed between Pr-Mg (1) and (2), and also between
FPr-Mg (2) and (3) as seen in the {19F, 19F} NOESY experiment (Figure 10).
In consideration of all the observations above and as shown in Scheme 1, we assign
structures to and interpret the Schlenk equilibrium18 of “n-FPrMgCl” in ethereal
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solution as follows. Pr-Mg (1) is n-FPrMgCl monomer, generated immediately upon
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mixing of the two reactants. This monomer is not a stable aggregate and will self-
aggregate to a halide-bridged dimer, which we assign to the species (2), i.e. (FPr-Mg–
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X)2. THF enhances Schlenk equilibrium, i.e. disproportionation of dimer (2) to a
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mixed dimer depicted as (3) consisting of Pr2Mg
•MgX2 with bridging halides. Our
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observations above also clearly suggest that the mixed dimer (3) is more stable when
solvated by THF than by ether. All of these solution-state structures are analogous to
known crystal structures of non-fluorinated Grignard reagents.19
Figure 9. 19F PGSE data and results of 0.4 M ether solution of n-FPrMgCl at -78 °C.
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2
FPr-I
3
1
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Exchange between FPr-Mg(1) and (2)
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1
2
3
FPr-I
Exchange between FPr-Mg(2) and (3)
Figure 10. {19F, 19F} NOESY of 0.4 M ether solution of n-FPrMgCl at -78 °C.
Scheme 1. Schlenk equilibrium observed in ethereal solutions.
Conclusion
Although we have observed Schlenk equilibrium in ether solution of n-C3F7MgCl,
this equilibrium favors the “FRMgX” species in this solvent. n-C3F7MgCl slowly
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decomposes even at -78 °C and generates perfluoropropene (C3F6) as the major
decomposition product. Increasing temperature or increasing concentration
accelerates the decomposition. In THF, Schlenk equilibrium strongly favors the
formation of the mixed dimer consisting of a dialkyl magnesium species, FRMgFR
and MgX2 with bridging halides.
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Experimental Section
Procedures for NMR Experiments. NMR samples were transferred into NMR
tubes via cannula. NMR tubes were evacuated in vacuo, flame-dried and filled with
argon before use. 19F chemical shifts were referenced to C6F6 at -164.9 ppm. All NMR
experiments were acquired on a 600 MHz spectrometer equipped with a z-axis
gradient multinuclear broadband fluorine observe (BBFO) smartprobe. The maximum
spectral width (sw) is no more than 60 ppm for all 2D 19F NMR experiments due to
the requirement of uniform excitation over the entire bandwidth of observed
resonances. {19F, 19F} COSY and NOESY spectra were acquired by standard
programs for 1H acquisition that had been modified for acquisition of 19F spectra.
Mixing time of 19F NOESY experiments was 0.3 ~ 0.8 s. For 19F PGSE experiments,
a 10A z-axis gradient amplifier was employed, with maximum gradient strength of
0.5 T/m. A standard pulse program dstebpgp3s was selected, employing a double
stimulated echo sequence, bipolar gradient pulses for diffusion, and 3 spoil gradients.
Diffusion time was 100 ms, and the rectangular gradient pulse duration was 1000 µs.
Individual rows of the quasi-2-D diffusion databases were phased and baseline
corrected. Actual diffusion coefficients used for D-FW analysis were obtained using
the T1/T2 analysis module in commercially available software.
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General Procedures for preparing FRMgX NMR sample. To a 0.4M RMgX (1.0
mmol) solution in 2.5 ml ethereal solvent at -78 °C under Ar atmosphere was slowly
added slightly excess FR-I (1.1 mmol). The reaction mixture was allowed to stir at -78
°C.
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ASSOCIATED CONTENT
Supporting Information
NMR spectra including PGSE, COSY and 1H. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Email: pgw@brown.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by NSF grant 1058051 to PGW and Japan Science and
Technology Agency (JST) (ACT-C: Advanced Catalytic Transformation for Carbon
Utilization) to KM.
.
References
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Henne, A. L.; Francis, W. C. J. Am. Chem. Soc. 1953, 75, 992-994.
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2 McBee, E. T.; Roberts, C. W.; Meiners, A. F. J. Am. Chem. Soc. 1957, 79, 335-337.
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