Perfluoroalkylation of Alkenes by Frustrated Lewis Pairs

Article abstract of DOI:10.1002/chem.201604414

The activation of perfluoroalkyl iodides by the frustrated Lewis pair tris(pentafluorophenyl)borane and tri-tert-butylphosphine is described. By abstraction of both a fluorine and an iodine atom, an iodophosphonium fluoroborate salt is formed. In the presence of alkenes the corresponding iodoperfluoroalkylation products are generated regioselectively. First mechanistic investigations support a radical mechanism.

Full text of DOI:10.1002/chem.201604414

DOI: 10.1002/chem.201604414
Communication
&
Fluorine Chemistry
Perfluoroalkylation of Alkenes by Frustrated Lewis Pairs
Ilona Behrends,[b] Susanne Bꢀhr,[b, c] and Constantin Czekelius*[a]
have a number of unique properties, such as high (group) elec-
tronegativity, low polarizability, and bond dissociation energies.
They differ significantly from the heavier halogens and require
special functionalization protocols.[14] Based on the reported
activation of the non-polarized HÀH bond by FLPs we became
interested in a potential related activation of perfluoroiodoal-
kanes RfÀI and subsequent functionalization of unsaturated hy-
drocarbons such as alkenes or alkynes.[15]
Abstract: The activation of perfluoroalkyl iodides by the
frustrated Lewis pair tris(pentafluorophenyl)borane and
tri-tert-butylphosphine is described. By abstraction of both
a fluorine and an iodine atom, an iodophosphonium fluo-
roborate salt is formed. In the presence of alkenes the cor-
responding iodoperfluoroalkylation products are generat-
ed regioselectively. First mechanistic investigations sup-
port a radical mechanism.
We started our investigation with the well-established FLP
system B(C6F5)3/PtBu3 (1/2).[2b] Upon addition of different per-
fluoroalkyl iodides to a solution of the FLP in dichloromethane
the salt [tBu3PI][(C6F5)3BF] (3) was generated and isolated in
54–85% yield (Table 1). This result raised the question for the
Pairs of a Lewis acid and a Lewis base which are sterically too
demanding to form adducts are known as frustrated Lewis
pairs (FLPs).[1] Their chemistry has evolved dramatically from
curiosities to a vivid research area since the first reports that
small molecules such as dihydrogen can be activated.[2] Since
then, hydrogenation protocols for unsaturated hydrocarbons
and reductions of C=N bonds have been well-documented.[3]
Apart from dihydrogen also other small molecules, such as si-
lanes,[4] carbon monoxide,[5] carbon dioxide,[6] sulfur dioxide,[7]
nitrogen oxides[8] as well as alkyl fluorides and bromides,[9]
were successfully activated. It was also shown that the ele-
ment–halogen bond (EÀX) in X2, HX, BX3, SOCl2, and CX4 can
be broken by Lewis pairs.[9e,10] The first FLPs were based on an
electron-deficient borane and an electron-rich phosphine
either as separate molecules or sharing a bridging scaffold.
Today, this concept has been extended also to transition-metal
complexes and chiral catalysts.[1,11]
Table 1. Formation of [tBu3PI][(C6F5)3BF] (3) from perfluoroiodoalkanes.[a]
Entry
Perfluoroiodoalkane
Time
Yield
(RfÀI)
(3) [%]
1
2
3
4
5
6
F3C-I (4a)
20 min
24 h
24 h
20 min
60 min
30 min
65
62
54
85
75
70
F3C-CF2-CF2-CF2-I (4b)
F3C-(CF2)6-CF2-I (4c)
F3C-CFI-CF3 (4d)
C6F11-I (4e)
C6F5CF2-I (4 f)
[a] Reactions were performed in CH2Cl2 at RT for the time indicated.
Recently, we have developed a stereoselective hydroper-
fluoro-alkylation protocol of electron-deficient C=C bonds for
the synthesis of amino acids and butanolides.[12] Due to the
great importance of fluorinated compounds in pharmacy, agro-
chemistry, and materials science, past years have witnessed
a substantial expansion of available synthetic tools for their ef-
ficient preparation.[13] Fluorine and organofluorine compounds
process leading to this fluoroborate salt.[10c] In the case of per-
fluorocyclohexyl iodide (4e) the formation of perfluorocyclo-
hexene (5)[16] was evident by 19F NMR spectroscopy indicating
a b-fluoride elimination process (Scheme 1). Such a mechanism
is not possible for perfluoroiodoalkanes such as CF3I (4a) or
perfluorobenzyl iodide (4 f). For these substrates an a-fluoride
elimination with concomitant perfluorocarbene formation
needs to be considered.
[a] Prof. Dr. C. Czekelius
Institute for Organic Chemistry and Macromolecular Chemistry
Heinrich-Heine-Universitꢀt Dꢁsseldorf
Universitꢀtsstraße 1, 40225 Dꢁsseldorf (Germany)
E-mail: constantin.czekelius@hhu.de
We became interested whether such a carbene intermediate
may be trapped in the presence of an alkene. Surprisingly,
[b] Dr. I. Behrends, S. Bꢀhr
Institute for Organic Chemistry, Freie Universitꢀt Berlin
Takustraße 3, 14195 Berlin (Germany)
[c] S. Bꢀhr
Current address: Technische Universitꢀt Berlin
Straße des 17. Juni 115, 10623 Berlin (Germany)
Supporting information and ORCID from the author for this article are
available on the WWW under http://dx.doi.org/10.1002/chem.201604414.
Scheme 1. FLP-mediated dehalogenation of perfluorocyclohexyl iodide.
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of both borane 1 and phosphine 2 terminal as well as internal
cis-alkenes 6 react with perfluorobutyl iodide (4b) to the corre-
sponding addition products 7 (Table 2). In the case of trans-
olefins as well as styrenes no apparent reaction was observed
and the starting material was reisolated (Table 2, entries 4 and
12). In polar solvents, such as THF, the reaction proceeds very
slowly. This may be explained by an unfavorable Lewis adduct
formation with the highly electron-deficient borane. For a simi-
lar reason, substrates incorporating polar heteroatoms are not
transformed efficiently. When 4-octyne was submitted to the
Lewis pair mediated perfluoroalkylation reaction the corre-
sponding tetrasubstituted alkene was obtained (Table 2,
entry 13).
Table 2. Iodoperfluoroalkylation of unsaturated hydrocarbons by FLPs.[a]
Entry
Alkene
Product
Time
[h]
Yield
[%]
1
2
12
12
73
95
In order to get insight into the mechanism of the observed
iodoperfluoroalkylation reaction we first investigated potential
interactions between perfluoroalkyl iodide and the individual
members of the Lewis pair. Mixtures of the electron-deficient
borane B(C6F5)3 (1) and either perfluorobutyl iodide (4b) or 1-
3
22
24
33
–
4[b]
1
hexene (6a) showed no significant shift in the H and 19F NMR
5
6
7
12
20
12
55
54
37
spectra compared to the starting material. In contrast, when
equal amounts of the perfluoroalkyl iodide 4b and the elec-
tron-rich phosphine 2 were dissolved in CD2Cl2 the 31P NMR
spectrum showed a significant upfield shift by 6 ppm.[17] Like-
wise, in the 19F NMR spectrum a upfield shift of the RCF2I
signal by 12 ppm was observed (Figure 1). This result is in
agreement with the assumption of an iodine-phosphorous in-
teraction.
8
12
17
24
81
56
84
A mechanistic rationale for the observed activation of per-
fluoroalkyl iodides and subsequent addition to alkenes may
follow an ionic pathway (a) via intermediate I or radical path-
ways (b, c, and d) via intermediate II (Figure 2). The electron-
rich phosphine 2 forms a halogen bond to the perfluoroalkyl
iodide 4. Potentially supported by a coordination of the
borane to a fluorine substituent either heterolytic (upper path)
or homolytic (lower path) RfÀI bond cleavage occurs. Following
the upper pathway an iodophosphonium species is formed.
The cyclic iodonium intermediate I obtained upon halogen
transfer to the alkene (pathway a) is then opened on the ste-
rically more accessible site by the perfluoroalkyl anion. Alterna-
tively, the interaction of B(C6F5)3 with the alkene 6 may result
in the attack of the perfluoroalkyl anion (pathway b). It is
known that electron-deficient boranes can activate alkenes, as
it has been shown specifically for the addition of FLP 1/2 to
different alkenes such as 1-hexene forming a betaine prod-
uct.[18] However, such alkyltris(pentafluorophenyl)borates are
known to be stable and are therefore not likely to form radical
intermediate II.[19] Through pathway c the electron-rich phos-
phine 2 may reductively form a perfluoroalkyl radical, which
adds to the alkene followed by radical chain iodine transfer
from RfÀI to intermediate II.[20] Furthermore, a perfluoroalkyl
borate could be formed by means of pathway d. Homolytic
bond cleavage and formation of a radical pair may then be the
initiation step of a radical iodoperfluoroalkylation from II.
In a test reaction 1,6-heptadiene (6n) was added to perfluo-
robutyl iodide (4b) in the presence of 10 mol% of the FLP 1/2.
A product mixture of the double-addition product 7n and the
corresponding cyclopentane derivative 8 (as mixture of cis/
9
10
11
40
12
56
–
12[b]
13
24
18
[a] Reactions were performed in CH2Cl2 at RT for the time indicated.
[b] No reaction was observed.
when perfluorobutyl iodide (4b) was added in excess to
B(C6F5)3/PtBu3 (1/2) in the presence of 1-hexene (6a) the corre-
sponding iodoperfluoroalkylation product 7a was formed and
isolated in 73% yield (Table 2, entry 1). The reaction proceeded
completely regioselective with the iodo substituent located in
the former 2-position.
Control experiments revealed that both borane 1 and phos-
phine 2 need to be present for this transformation; no product
was obtained in the absence of any of them. In addition, no re-
action was observed when 2,2,2-trifluoro-1-iodoethane was
submitted to the reaction conditions indicating that sufficient
polarization of the CÀI bond is essential.
The perfluoroalkylation reaction requires only substoichio-
metric amounts of the FLP. In fact, in the presence of 10 mol%
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tion, the iodoperfluoroalkylation of substrate 6c gave
the product as a mixture of syn/anti-diastereomers,
which would not be expected for the nucleophilic
ring opening of a putative iodonium intermediate or
a potential synchronous process. These results sug-
gest paths c or d as potential mechanistic routes.
However, as no radical intermediates could be de-
tected by EPR so far, no reaction or polymerization
was found with styrene as substrate,[23] and addition
of stoichiometric amounts of the radical scavenger
BHT[24] had no impact on the conversion rate, other
mechanisms should not be excluded at this point
and will be in the focus of future investigations.
In summary, we have shown that frustrated Lewis
pairs can activate perfluoroalkyl iodides. In the pres-
ence of alkenes a regioselective iodoperfluoroalkyl-
ation reaction occurs while in their absence an iodo-
phosphonium fluoroborate salt is formed. Studies
aiming at a better understanding of the mechanism
as well as a potential development of a stereoselec-
tive process are under way and will be reported in
Figure 1. 19F NMR spectrum (376 MHz, 298 K, CD2Cl2) of a mixture of PtBu3 and perfluoro-
butyl iodide compared to free perfluorobutyl iodide.
due course.
Experimental Section
Representative procedure for the iodoperfluoroalkylation of al-
kenes: Tris(pentafluorophenyl)borane (1) (46.4 mg, 90.6 mmol,
10 mol%) and tri-tert-butylphosphine (2) (18.4 mg, 90.9 mmol,
10 mol%) were dissolved in dichloromethane (2.0 mL). Afterwards,
vinylcyclohexene (6b, 120 mL, 0.877 mmol) and perfluorobutyl
iodide (4b, 150 mL, 0.877 mmol) were added by syringe. The reac-
tion mixture was stirred overnight at room temperature. After re-
moval of the solvent the crude product was purified by column
chromatography (silica gel, eluent: cyclohexane) to give the de-
sired product 7b (380 mg, 95%) as a colorless viscous liquid.
1H NMR (500 MHz, 298 K, CDCl3): d=4.35 (td, J=6.7 Hz, J=3.0 Hz,
1H; CHI), 2.84 (mc, 2H; CH2CF2), 1.82–1.78 (m, 2H; CH2Cy), 1.73–1.65
(m, 3H; CH2Cy), 1.43–1.29 (m, 2H; CH2Cy), 1.26–1.08 (m, 3H; CH2Cy),
0.82 ppm (tq, J=11.1 Hz, J=3.1 Hz, 1H; CHCy); 13C{1H} NMR
(126 MHz, 298 K, CDCl3): d=120.9–108.0 (m, CF3, CF2), 44.3 (sbr,
CHCy), 39.1 (t, J=20.8 Hz, CH2CF2), 33.9 (CH2Cy), 30.5 (sbr, CHI) 29.9,
26.2, 25.9, 25.7 ppm (CH2Cy); 19F NMR (376 MHz, 298 K, CDCl3): d=
À80.9 (t, J=10.0 Hz, 3F, CF3), À113.8 (mc, 2F, CF2), À124.4 (mc, 2F,
CF2), À125.8 ppm (mc, 2F, CF2). IR (film): n˜ =2930 (s), 2857 (s), 1451
(m), 1433 (w), 1349 (s), 1134 (s), 1051 (w), 903 (w), 880 (m), 768 (m),
650 (w), 521 cmÀ1 (w); MS (EI) calcd (m/z) for [MÀI]+: 329.0952,
found: 329.0941; elemental analysis calcd (%) for C12H14F9I: C 31.60,
H 3.09; found: C 31.60, H 3.09.
Figure 2. Mechanistic rationale for the activation of perfluoroiodoalkanes.
B=B(C6F5)3 (1); P=PtBu3 (2).
Acknowledgements
Financial support of the German Research Council (DFG) by an
Emmy-Noether grant (CZ 183/1) is most gratefully acknowl-
edged.
Scheme 2. Iodoperfluoroalkylation of 1,6-heptadiene (6n).
trans isomers) was formed (Scheme 2).[21] This regioselective
ring closure of the diene is more consistent with the assump-
tion of a step-wise perfluoroalkyl and halogen atom transfer
via a radical rather than by a cationic intermediate.[22] In addi-
Keywords: alkenes · boranes · fluoroalkylation · frustrated
Lewis pairs · halogenation
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Received: September 19, 2016
Published online on && &&, 0000
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Communication
COMMUNICATION
Frustrated Lewis pairs mediate the re-
gioselective addition of perfluoroalkyl
iodides to non-activated olefins. In the
absence of an alkene, an iodophospho-
nium fluoroborate salt is formed by
a fluoride elimination process. For the
activation of the CÀI bond, dual action
of both the phosphine and the borane
is required (see scheme).
&
Fluorine Chemistry
I. Behrends, S. Bꢀhr, C. Czekelius*
&& – &&
Perfluoroalkylation of Alkenes by
Frustrated Lewis Pairs
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ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!