Selective CH Functionalization of Methane, Ethane, and Propane by a Perfluoroarene Iodine(III) Complex

Article abstract of DOI:10.1002/anie.201406185

Direct partial oxidation of methane, ethane, and propane to their respective trifluoroacetate esters is achieved by a homogeneous hypervalent iodine(III) complex in non-superacidic (trifluoroacetic acid) solvent. The reaction is highly selective for ester formation (>99 %). In the case of ethane, greater than 0.5 M EtTFA can be achieved. Preliminary kinetic analysis and density functional calculations support a nonradical electrophilic CH activation and iodine alkyl functionalization mechanism. Gas up: Direct partial oxidation of methane, ethane, and propane to their respective trifluoroacetate (TFA) esters is achieved by a homogeneous hypervalent iodine(III) complex in non-superacidic solvent (HTFA). The reaction is highly selective, and for ethane, greater than 0.5 M Et=TFA can be achieved. Preliminary kinetic analysis and density functional calculations support a nonradical electrophilic CH activation and iodine alkyl functionalization mechanism.

Full text of DOI:10.1002/anie.201406185

Angewandte
Chemie
DOI: 10.1002/anie.201406185
Hypervalent Compounds
Selective CH Functionalization of Methane, Ethane, and Propane by
a Perfluoroarene Iodine(III) Complex**
Michael M. Konnick,* Brian G. Hashiguchi, Deepa Devarajan, Nicholas C. Boaz,
T. Brent Gunnoe, John T. Groves, Niles Gunsalus, Daniel H. Ess,* and Roy A. Periana*
Abstract: Direct partial oxidation of methane, ethane, and
propane to their respective trifluoroacetate esters is achieved by
a homogeneous hypervalent iodine(III) complex in non-
superacidic (trifluoroacetic acid) solvent. The reaction is
highly selective for ester formation (> 99%). In the case of
ethane, greater than 0.5m EtTFA can be achieved. Preliminary
kinetic analysis and density functional calculations support
a nonradical electrophilic CH activation and iodine alkyl
functionalization mechanism.
Scheme 1. Generalized catalytic cycle and net reaction for CH function-
alization.
T
he direct conversion of C1–C3 alkanes in natural gas into
upgraded commodities at low temperature and high selectiv-
ity remains a significant challenge.[1] Catalytic systems based
upon nonradical homogeneous CH activation and metal–
methane because of the instability of higher-order oxygenated
products.[2]
À
alkyl (M R) functionalization, together designated herein as
We have recently reported that main-group electrophilic
salts with filled d orbitals (d10) such as TlIII(TFA)3 and
PbIV(TFA)4 (TFA = trifluoroacetate) promote efficient and
selective stoichiometric CH activation and functionalization
of methane (MeH), ethane (EtH), and propane (PrH) in
more practical weaker acid solvents (HOAc, HTFA).[6] The
high rates of these main-group electrophiles were attributed
to substantially faster alkane coordination which could be
conceptually ascribed to the lack of ligand-field stabilization
energies for d10 systems.[6] Importantly, this work also sug-
gested that other main-group electrophiles may promote CH
functionalization.[6]
While the simplicity of main-group electrophilic salts is
attractive, we are also interested in designing main-group
electrophilic systems with stable ligands which can be
elaborated to tune electronics and sterics, to control selectiv-
ity and reactivity. We were particularly interested in hyper-
valent iodine electrophiles, given their wide range of oxida-
tion states, tunability by ligation, and extensive development
for a wide variety of metal-free organic transformations.[7]
Importantly, several stable organo-ligated species of this class
[e.g. R-I(X)2, R-I(X)4] are well known,[7] thus opening the
possibility for rational ligand design. Herein, we describe
a homogeneous stoichiometric reaction for the oxy-function-
alization of C1–C3 alkanes by a well-defined hypervalent
iodine compound C6F5IIII(TFA)2 (1). In the HTFA solvent,
1 cleanly and selectively oxidizes C1–C3 alkanes (RH) to
generate the respective trifluoroacetate esters (RTFA) and
C6F5II (2) in high yields for EtH and PrH. Experiments and
density functional calculations disfavor a 1eÀ pathway and
CH functionalization, remains a viable strategy to achieve this
goal (Scheme 1).[2] Some of the most efficient systems we
have developed utilize catalysts based on electrophilic
transition metals with unfilled d orbitals (d<10), such as
platinum,[3] palladium,[4] and gold.[5] One difficulty limiting
the reactivity of these electrophilic systems is the slow rate of
alkane coordination, which is necessary for efficient CH
activation.[2,6] To address this challenge, superacid solvents,
such as H2SO4, have been used to facilitate anion ligand
dissociation and alkane coordination.[2] However, the use of
a superacid solvent is impractical because of the high costs for
product separation and corrosivity, and is limited primarily to
[*] Dr. M. M. Konnick, Dr. B. G. Hashiguchi, N. Gunsalus,
Prof. R. A. Periana
The Scripps Energy & Materials Center, Department of Chemistry
The Scripps Research Institute, Jupiter, FL 33458 (USA)
E-mail: mkonnick@scripps.edu
rperiana@scripps.edu
Dr. D. Devarajan, Prof. D. H. Ess
Department of Chemistry and Biochemistry
Brigham Young University, Provo, UT 84602 (USA)
E-mail: dhe@byu.edu
N. C. Boaz, Prof. J. T. Groves
Department of Chemistry and Princeton Institute for the Science
and Technology of Materials, Princeton, NJ 08544 (USA)
Prof. T. B. Gunnoe
Department of Chemistry, University of Virginia
Charlottesville, VA 22904 (USA)
[**] This work was supported as part of the Center for Catalytic
Hydrocarbon Functionalization, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences, under Award No. DE-SC0001298.
À
provide evidence supporting a CH activation and M R
functionalization mechanism, through the formation of an
alkyl(perfluoroaryl)-l3-iodane intermediate.
The selective oxidation of unactivated alkanes by hyper-
valent iodine is rare,[7,8] with only a few reports describing
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406185.
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selective alkane oxygenation.[9–12] To our knowledge only one
electrophilic, non-oxo system (which was limited to MeH and
operated in superacid media) has been reported for high-
yielding, selective light-alkane oxygenation with hypervalent
iodine,[11] with no reports concerning EtH and PrH. In a prior
report,[9] I(TFA)3 was found to functionalize tertiary alkanes
to esters within minutes at room temperature, however
secondary CH functionalization was sluggish (requiring days
or weeks), primary CH functionalization could not be
observed (neopentane), and C1–C3 alkanes were not exam-
ined. To begin, we evaluated the reactivity of I(TFA)3 in
HTFA as the solvent towards MeH, EtH, and PrH under
more forcing reaction conditions (3 h, 1508C) than previously
described.[9] We also examined the commonly utilized hyper-
valent iodine reagent C6H5I(OAc)2 (DIB). While I(TFA)3 and
DIB both indicated the formation of alkane-derived products
reaction proceeds with no intermediate iodine species
observed in greater than 90% mass balance (Figure S1).
Control experiments showed that 1 was stable under the
reaction conditions (under 500 psi Ar) in the absence of the
hydrocarbons and that the hydrocarbons did not react in the
absence of 1. Consistent with the expected relative reactivity
of an electrophilic system, the order of reactivity was
determined to be PrH (ca. 89%) > EtH (80%) @ MeH
(5%; Table 1, entries 1–3). The reaction was highly efficient
with EtH and PrH, and at the saturation concentration of
1 (ca. 0.8m), 0.56m EtTFA could be observed (see Figure S2
in the Supporting Information). Notably, the selectivity for
the monofunctionalized and 1,2-difunctionalized products
depended upon substrate. The monoester is heavily favored
(24:1) for EtH while about a 1:1 ratio is observed for propane.
Control experiments suggest that this difference likely arises
because of the relative reactivity of the monoester products:
while EtTFA is completely unreactive towards 1, 2-PrTFA is
readily converted into 1,2-Pr(TFA)2 under our standard
reaction conditions (see Table S1 and Figures S3 and S4 in
the Supporting Information) in the presence of 1.
1
by H NMR spectroscopy, quantitative analysis was compli-
cated by either the formation of I2[9] or a side reaction where
DIB reacts with the aryl backbone of a second DIB
equivalent.
Perfluorous aryl and alkyl iodine(III) reagents are useful
and potentially recyclable electrophilic oxidizing agents,
In addition to alkanes, 1 is also effective for the selective
monofunctionalization of benzene (PhH) to PhTFA at lower
temperatures (1258C; Table 1, entry 4). Reducing the temper-
ature of this reaction to 1008C resulted in the complete
consumption of 1 with the generation of a new species in
III
À
which contain a stable RF
I
bond and no oxidizable CH
functionality.[7l,13] These traits made the commercially avail-
able C6F5IIII(TFA)2 (1), an ideal candidate for alkane
oxidation. Upon heating a 100 mm solution of 1 at 1508C
for 3 hours in 100 mm TFAA/HTFA under 500 psi of MeH or
EtH (125 psi for PrH), 1H NMR analysis of the crude reaction
mixtures indicated the generation of the respective trifluor-
oacetate monoesters and 1,2-TFA-diesters (for EtH and PrH)
[Eq. (1), Table 1].[14,15] The reactions are highly selective and
1
solution. By using H and 19F NMR spectroscopy, we tenta-
tively identified the new species as the diaryl-l3-iodane
[C6F5IIII(C6H5)][TFA] (3). Only trace levels of PhTFA and 2
were observed (entry 5; see Figure S5 and S6 in the Support-
ing Information). Further heating of this solution (at 1258C)
resulted in the complete conversion of 3 into PhTFA and 2.
This transformation is consistent with 3 as an intermediate in
the conversion of PhH into PhTFA [Eq. (2)]. Although both
no hydrocarbon-derived products are observed other than the
respective monoesters and 1,2-diesters (see Figure S1 in the
Supporting Information). 19F NMR analysis of the solutions
post-reaction showed that 1 is cleanly converted into 2 as the
the generation of diaryl-l3-iodanes from a l3-iodane and
arene in acidic media and the functionalization of nucleo-
philes by diaryl-l3-iodanes are well established,[7,16] only a few
examples of direct one-pot aromatic CH oxygenation by l3-
iodanes have been reported in the literature.[7a,b,e,17]
Table 1: Oxidation of C1–C3 alkanes and benzene by 1.[a]
Entry RH
T [8C] Conv.[b] Product (yield [%])[b]
Analogous to the observed chemistry of benzene with
1 [Eq. (2)], one mechanistic possibility for alkane function-
alization is a reaction that proceeds by the formation of an
alkyl(perfluoroaryl)-l3-iodane intermediate ([C6F5I(alkyl)]-
[TFA]), which is then rapidly functionalized to generate
RTFA. The reductive functionalization of alkyl-l3-iodanes to
oxygenated and olefinic products has long been known.[18]
However, alkyl-l3-iodanes are known to be highly susceptible
to reductive functionalization by weak nucleophiles at low
temperatures (< 08C),[7a,b,e] and it is unlikely that this
intermediate could be detected during our reactions de-
scribed above. Therefore, in an effort to probe the reactivity
of the proposed [C6F5I(alkyl)][TFA] intermediate directly, we
examined the reaction of SnEt4 with 1 in HTFA (see the
1
2
MeH 150
5%
MeTFA (5%)
EtH
PrH
150
150
80%
89%
99%
99%
EtTFA (73%); 1,2-Et(TFA)2 (3%)[c]
2-PrTFA (30%); 1,2-Pr(TFA)2 (27%)[c]
C6H5TFA (91%)
3[d]
4[e]
5[e]
PhH 125
PhH 100
[C6F5IIII(C6H5)][TFA] (99%)[f]
[a] General reaction conditions: 100 mm 1, 500 psi RH (2.78 mmol,
27.8 equiv based on ideal gas law), 1 mL 100 mm TFAA/HTFA, 3 h.
[b] Conversions and product yields are reported relative to [1]0; and were
determined by 19F and 1H NMR spectroscopy, respectively, by compar-
ison to an internal standard (C6F6 and CH2Cl2) which was added post
reaction. [c] 2 equiv of 1 are consumed per 1 equiv 1,2-product.
[d] 125 psi PrH (0.70 mmol, 7.0 equiv based on ideal gas law).
[e] 690 mm C6H6, 250 mm 1, 500 psi Ar. [f] Tentative product identifica-
tion.
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À
Supporting Information for details). Sn R species (including
reaction indicated that minimal levels of over oxidation
examples where R is a sp3-hybridized carbon center)[19] are
occurs (< 1% 13CO2).[21] An intramoleuclar H/D KIE value of
3.0 (Æ 0.5) was obtained for the reaction by examining
CH3CD3 (see the Supporting Information for details). Similar
to the reported TlIII and PbIV chemistry,[6] the examination of
1 with EtH in DTFA indicated no H/D exchange occurs (see
well known to undergo facile R-transfer reactions with IIII
species to generate IIII R. While the species [C6F5I(Et)-
[7]
À
(TFA)] could not be observed, addition of SnEt4 to a solution
of 1 in HTFA at 508C resulted in clean formation of EtTFA
and smaller amounts of 1,2-Et(TFA)2.[20] This is consistent
with the product selectivities observed for the reaction of EtH
and 1 described above.
Scheme S8), thus suggesting that if a IIII Et intermediate is
À
formed, functionalization occurs rapidly relative to protonol-
ysis.
To further examine the reaction mechanism, we per-
formed a more detailed experimental and theoretical analysis
of the oxidation of EtH by 1. A time-course study, monitored
by 1H and 19F NMR spectroscopy, indicated that the reaction
is well behaved and first-order in 1, thus displaying clean
exponential decay through three half-lives with matching
exponential growth of EtTFA and 1,2-Et(TFA)2 as the
reaction proceeds (Figure 1). Importantly, the ratio of
The high reproducibility, selectivity, well-defined kinetics,
and constant EtTFA to 1,2-Et(TFA)2 product ratios observed
with EtH and 1 suggest that the reaction does not proceed by
a free-radical chain mechanism. A comparison of the reaction
of EtH with 1 to the reaction of EtH with other strong
oxidants known to promote free-radical pathways (K2S2O8,
NaBO3) under otherwise identical reaction conditions sup-
ports this proposal, as these reactions are much less selective
for EtTFA and generate significant amounts of unidentified
aliphatic products (see Scheme S9). To probe for the presence
of a free-radical pathway or other 1eÀ homolytic pathways
(such as hydrogen-atom transfer; HAT) further, we next
examined the reactivity of toluene (which contains homolyti-
cally weak benzylic CH bonds) with 1. Under our standard
reaction conditions (3 h, 1508C; see the Supporting Informa-
tino for details), quantitative generation (based upon starting
[1]) of p-MeC6H4TFA and o-MeC6H4TFA in approximately
a 3:1 ratio was observed with no formation of any benzylic
oxy-functionalized products expected from a radical pathway
[Eq. (3)].
Figure 1. Reaction timecourse of the oxidation of EtH by 1. Reaction
conditions: [1]0 =103 mm, 500 psi EtH, 1508C, 1 mL 100 mm TFAA/
HTFA, 0–4.5 h.
[EtTFA] and [1,2-Et(TFA)2] was determined to remain
constant throughout the reaction (ca. 25:1). Additionally,
control experiments have shown EtTFA is completely
unreactive towards 1 (see Table S1 and Figure S4). These
results support a mechanism where both the EtTFA and 1,2-
Et(TFA)2 products are formed from an unobserved common
intermediate, and not a sequential functionalization of EtH to
EtTFA to 1,2-Et(TFA)2. Variation of EtH pressure indicated
Importantly, the observed selectivity with 1 (e.g. a prefer-
ence for homolytically stronger, aromatic CH bonds) sharply
contrasts with the previously described work by Ochiai et al.,
where the IIII complex 4 (see below) displayed a clear
preference for benzylic (over aromatic or vinylic) CH oxy-
genation.[22] Mechanistic work by Ochiai et al. indicated that
functionalization with 4 proceeds by a HAT mechanism
where after I-OOtBu bond homolysis to generate 5, abstrac-
tion of a hydrogen atom from a CH bond by 5 (or the
benzoyloxy-centered radical analogue of 5) occurs.[22] Sup-
porting this proposal, large H/D KIE effects of 10–14 were
observed for CH/D oxygenation with 4,[22] and is character-
istic of HAT from a CH bond.[23] This result also differs
greatly from the H/D KIE value of 3.0 observed with 1 and
EtH as described above.
that the reaction is also first-order in EtH (Figure 2). The
13
À
reaction of 1,2-[ C]-EtH with 1 showed that both Et TFA
and 1,2-Et(TFA)2 are derived solely from EtH (see Fig-
ure S7), and GC/MS analysis of the headspace gases post-
M06/6-31 + G(d,p)[LANL2DZ(d,p)] density functional
calculations were carried out to predict the alkane oxidation
mechanism and understand the high selectivity for the
monoester product over the diester product for reaction of
1 with EtH. Calculations were carried out with an explicit
HTFA solvent embedded in an implicit HTFA solvent model
(see the Supporting Information for details). The ground state
of the C6F5IIII(TFA)2 complex involves trans-k1-TFA ligands
interacting with C6F5IIII. Calculations indicate prohibitively
Figure 2. Initial rate dependence of EtH pressure on the generation of
EtTFA from EtH and 1. Reaction conditions: [1]0 =101 mm, 50–500 psi
EtH, 1408C, 1 mL TFAA/HTFA, 0.5 h.
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high energies for one-electron oxidation between 1 and EtH
at DHꢀ > 80 kcalmolÀ1 (see Scheme S10). We also explored
hydrogen-atom abstraction, which has
a
reasonable
DHꢀ value of 38.8 kcalmolÀ1, but is higher in energy than
a CH activation pathway.
À
Calculations suggest that CH activation and M R func-
À
Figure 4. DFT-calculated I O bond dissociation enthalpies for 4 and
1 in HTFA solvent. (M06/6-31+G(d,p)[LANL2DZ(d,p)]).
tionalization provides a viable mechanism for EtH oxidation
(Figure 3). In this pathway, EtH coordination occurs with
TFA ligand displacement (with the TFA anion remaining
À
between 1 and 4 is clearly revealed by the calculated I O
À
bond homolysis enthalpies (Figure 4). The I O bond disso-
ciation enthalpy for 4 is compatible with a radical mechanism
at 30.6 kcalmolÀ1. Whereas with 1, the bond dissociation
enthalpy is much higher (60.1 kcalmolÀ1), thus suggesting that
homolysis to form 7 is unlikely.
In summary, we report that efficient and selective oxy-
functionalization of light alkanes to oxy-esters in a non-
superacid solvent (HTFA) can be mediated by hypervalent
iodine. Experiment and theory disfavors one-electron mech-
anisms and provides evidence supporting alkane CH activa-
À
tion and M R functionalization by the hypervalent iodine
complex 1. Considering the recent developments on the use of
hypervalent iodine catalytically,[7f,i,j,25] it is feasible that further
studies may allow development of catalytic systems designed
to selectively transform unactivated alkanes and arenes into
alcohols and phenols, respectively. These investigations are
currently underway.
Received: June 12, 2014
Published online: && &&, &&&&
À
Figure 3. DFT-calculated CH activation and M R functionalization
pathway. Enthalpies are in kcalmolÀ1. (M06/6-31+G(d,p)[LANL2DZ-
À
Keywords: alkanes · C H activation · hypervalent compounds ·
iodine · main-group elements
(d,p)]).
.
within the solvation sphere of the IIII center), thus requiring
DH = 9.4 kcalmolÀ1. Subsequent deprotonation of the CH
bond by the TFA (TS1) requires DHꢀ = 31.6 kcalmolÀ1, and
exothermically generates the alkyl(perfluoroaryl)-l3-iodane
intermediate [C6F5I(Et)][TFA] (6). Importantly, the
DHꢀ value for CH activation of the methyl group of EtTFA
by 1 was calculated to be 36.3 kcalmolÀ1. This DH value is
5 kcalmolÀ1 larger for EtTFA (relative to TS1) and supports
our mechanism where the 1,2-diester is not formed from
functionalization of EtTFA by 1. Our CH activation model is
further validated by the approximate calculated KIE of 3.4
for CH3CD3 based on TS1, and is within the error range of the
experimental value described above.
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Zupan, J. Iskra, J. A. Gladysz, J. Org. Chem. 2009, 74, 3133.
[14] The compound 1 is known to react with H2O to generate the
=
explosive C6F5I O and other reactive iodine species (see
Ref. [7l,15]). Therefore, all reactions were run in the presence
of 100 mm TFA2O to ensure dry conditions. Control experiments
demonstrated that 100 mm TFAA had no effect on reaction
conversions and product selectivity.
[15] a) M. Schmeißer, H. Dahmen, P. Sartori, Chem. Ber. 1967, 100,
1633; b) J. P. Collman, Chem. Eng. News 1985, 63, 2.
[16] T. Dohi, N. Yamaoka, Y. Kita, Tetrahedron 2010, 66, 5775.
[17] H. Liu, X. Wang, Y. Gu, Org. Biomol. Chem. 2011, 9, 1614, and
references therein.
[18] a) R. C. Cambie, B. G. Lindsay, P. S. Rutledge, P. D. Woodgate, J.
Chem. Soc. Chem. Commun. 1978, 919; b) H. J. Reich, S. L.
Peake, J. Am. Chem. Soc. 1978, 100, 4888.
[19] a) M. Ochiai, Y. Nagao, J. Synth. Org. Chem. Jpn. 1986, 44, 660;
b) M. Ochiai, T. Ukita, S. Iwaki, Y. Nagao, E. Fujita, J. Org.
Chem. 1989, 54, 4832, and references therein.
[20] Control experiments demonstrated that SnEt4 did not generate
EtTFA or 1,2-Et(TFA)2 in the absence of 1 under otherwise
identical reaction conditions. No reaction between SnEt4 and
1 was observed at lower temperatures (258C).
[21] 12CO2 and HCF3 are observed in the headspace post-reaction,
and is the result of the known thermal decarboxylation of HTFA.
See: D. M. Jollie, P. G. Harrison, J. Chem. Soc. Perkin Trans. 2
1997, 1415.
[22] M. Ochiai, T. Ito, H. Takahashi, A. Nakanishi, M. Toyonari, T.
Sueda, S. Goto, M. Shiro, J. Am. Chem. Soc. 1996, 118, 7716.
[23] a) J. A. Howard, K. U. Ingold, M. Symonds, Can. J. Chem. 1968,
46, 1017; b) G. A. Russell, J. Am. Chem. Soc. 1957, 79, 3871.
[24] Control experiments with C2H4 and 1 at 1008C for 1 h indicated
the quantitative generation (based upon added 1) of 1,2-
Et(TFA)2 and 2, and there is significant precedent for the
oxidation of alkenes by IIII to generate 1,2-di-functionalized
products. See Ref. [7a,b].
´
Michalkiewicz, M. Jarosinska, I. Łukasiewicz, Chem. Eng. J.
2009, 154, 156.
[12] G. C. Fortman, N. C. Boaz, D. Munz, M. M. Konnick, R. A.
Periana, J. T. Groves, T. B. Gunnoe, J. Am. Chem. Soc. 2014, 136,
8393.
[25] a) F. V. Singh, T. Wirth, Chem. Asian J. 2014, 9, 950; b) T. Dohi,
Y. Kita, Chem. Commun. 2009, 2073.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Hypervalent Compounds
M. M. Konnick,* B. G. Hashiguchi,
D. Devarajan, N. C. Boaz, T. B. Gunnoe,
J. T. Groves, N. Gunsalus, D. H. Ess,*
Gas up: Direct partial oxidation of meth-
ane, ethane, and propane to their
nary kinetic analysis and density func-
tional calculations support a nonradical
electrophilic CH activation and iodine
alkyl functionalization mechanism.
respective trifluoroacetate (TFA) esters is
achieved by a homogeneous hypervalent
iodine(III) complex in non-superacidic
solvent (HTFA). The reaction is highly
selective, and for ethane, greater than
R. A. Periana*
&&&& — &&&&
Selective CH Functionalization of
Methane, Ethane, and Propane by
a Perfluoroarene Iodine(III) Complex
À
0.5m Et TFA can be achieved. Prelimi-
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!