Straightforward formation of carbocations from tertiary carboxylic acids: Via CO release at room temperature

Article abstract of DOI:10.1039/c8dt04416c

We report an unprecedented mode of reactivity of carboxylic acids. A series of tertiary carboxylic acids, containing at least one phenyl α-substituent, undergo loss of carbon monoxide at room temperature (295 K), by a one pot reaction with 0.5-1 molar equivalents of WCl₆ in dichloromethane. A plausible mechanism for the Ph₃CCO₂H/WCl₆ reaction, leading to [CPh₃][WOCl₅] and Ph₃CCl, is proposed on the basis of DFT calculations. The analogous reactions involving CEt(Ph)₂CO₂H, CMe(Ph)₂CO₂H and CMe₂(Ph)CO₂H selectively afforded stable hydrocarbons (alkene or indene, depending on the case), apparently resulting from the rearrangement of elusive tertiary carbocations.

Full text of DOI:10.1039/c8dt04416c

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Straightforward formation of carbocations from
tertiary carboxylic acids via CO release at room
temperature†
Cite this: DOI: 10.1039/c8dt04416c
Received 6th November 2018,
Accepted 23rd November 2018
Niccolò Bartalucci,a Marco Bortoluzzi,b Stefano Zacchini, c Guido Pampaloni
a
a
DOI: 10.1039/c8dt04416c
and Fabio Marchetti
*
rsc.li/dalton
We report an unprecedented mode of reactivity of carboxylic susceptible to metal mediated CO release under thermal treat-
acids. A series of tertiary carboxylic acids, containing at least one ment,8 proceeding with an oxidative addition mechanism.3,9
phenyl α-substituent, undergo loss of carbon monoxide at room
Herein, we present the unique chemistry recognized for a
temperature (295 K), by a one pot reaction with 0.5–1 molar series of tertiary carboxylic acids, containing a variable number
equivalents of WCl6 in dichloromethane. A plausible mechanism of phenyl α-substituents, when allowed to interact with a high
for the Ph3CCO2H/WCl6 reaction, leading to [CPh3][WOCl5] and valent metal chloride at room temperature. Thus, the 1 : 1 molar
Ph3CCl, is proposed on the basis of DFT calculations. The analo- reaction of WCl6 with triphenylacetic acid in dichloromethane
gous reactions involving CEt(Ph)2CO2H, CMe(Ph)2CO2H and afforded [CPh3][WOCl5], 1, and Ph3CCl, 2, in the ca. 1 : 2 molar
CMe2(Ph)CO2H selectively afforded stable hydrocarbons (alkene or ratio (NMR), see Scheme 1. Abundant formation of carbon mon-
indene, depending on the case), apparently resulting from the oxide was detected by gas chromatography.
rearrangement of elusive tertiary carbocations.
The salt 1 was isolated in 32% yield after work-up, and then
characterized by X-ray diffraction, analytical and spectroscopic
methods (see the ESI† for details). The crystal structure of 1
(Fig. 1) consists of an ionic packing of [CPh3]+ cations10 and
[WOCl5]− anions,11 both displaying the expected bonding
parameters.
It is a common feature of coordination chemistry that the
addition of carboxylic acids (including triphenylacetic acid12)
to metal chlorides proceeds with HCl release and affords the
corresponding stable metal-carboxylates.13 However, the gene-
Carboxylic acids are key functional groups in organic chem-
istry, being involved in a wide variety of synthetic routes.1 In a
number of useful transformations, the carboxylic acid unit
may act as a leaving one, with the elimination of either carbon
dioxide2,3 or, less frequently, carbon monoxide.4,5 Concerning
decarbonylation reactions, leading to valuable alkenes4 or C–C
coupling products,5 these are usually promoted by low valent
transition metal compounds and require harsh conditions
(typically, temperatures in the range 150–300 °C). In the
absence of sacrificial anhydride as an additive, the process is
believed to proceed with the intermediate formation of carbox-
ylato ligands.6 Remarkably, the decarbonylation of a palla-
dium-acetate complex was recently detected upon thermal
decomposition in the gas phase by means of a mass spec-
trometry experiment.7 Also carboxylic acid chlorides may be
aUniversity of Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 13,
I-56124 Pisa, Italy. E-mail: fabio.marchetti1974@unipi.it; https://people.unipi.it/
fabio_marchetti1974/; Tel: +39 050 2219245
bUniversity Ca’ Foscari Venezia, Dipartimento di Scienze Molecolari e Nanosistemi,
Via Torino 155, I-30170 Mestre, VE, Italy
cUniversity of Bologna, Dipartimento di Chimica Industriale “Toso Montanari”, Viale
Risorgimento 4, I-40136 Bologna, Italy
†Electronic supplementary information (ESI) available: Experimental details,
characterization of the products, X-ray crystallography, DFT structures, NMR
spectra. CCDC 1869885 (1) and 1869886 (4a). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c8dt04416c
Scheme 1 Reactions of WCl6 with tertiary α-phenyl carboxylic acids at
room temperature.
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To the best of our knowledge and to a general extent, the
one pot formation of carbocations from tertiary carboxylic
acids (through CO elimination) is unprecedented.
Based on the experimental facts, a plausible pathway for
the WCl6/Ph3CCO2H reaction was traced by DFT calculations
(Scheme 2).
The first interaction between the reactants is likely to occur
via coordination of the carbonyl oxygen to the metal centre,
reinforced by H⋯Cl hydrogen bonds (intermediate a in
Scheme 2). Subsequent chloride migration should lead to
[WCl5{OCCl(OH)CPh3}] (b). The formal elimination of one HCl
molecule from b affords a mixture of [WOCl4] and Ph3CC(O)Cl
as the stationary point (c), with a strong lowering of the Gibbs
free energy.
In accordance with this DFT outcome, monitoring the
WCl6/Ph3CCO2H reaction by IR and NMR spectroscopy
revealed the intermediate formation of Ph3CC(O)Cl.
Accordingly, the reaction of WOCl4 with Ph3CC(O)Cl, freshly
prepared from Ph3CCO2H and PCl5, finally yielded a mixture
of 1 and 2. It has to be remarked that CO extrusion from
Ph3CC(O)Cl cannot involve here the classical oxidative
addition pathway, on considering that WCl6 and WOCl4
contain the metal atom in their highest oxidation state.
The coordination of acyl chloride to [WOCl4] (d) is only
slightly favourable from a thermodynamic point of view. More
importantly, all the computational attempts to find a tran-
sition state for the decarbonylation of Ph3CC(O)Cl indicated
Fig. 1 Molecular structure of [CPh3][WOCl5], 1, with key atoms labelled.
Thermal ellipsoids are at the 50% probability level. Selected bond dis-
tances (Å) and angles (°): W(1)–O(1) 1.650(4), W(1)–Cl(1) 2.4898(16),
W(1)–Cl(2) 2.3171(16), W(1)–Cl(3) 2.3401(15), W(1)–Cl(4) 2.3154(18),
W(1)–Cl(5) 2.3280(15), C(1)–C(2) 1.445(9), C(1)–C(8) 1.449(9), C(1)–C(14)
1.443(9), O(1)–W(1)–Cl(1) 173.91(17), Cl(3)–W(1)-Cl(5) 172.86(6), Cl(2)–W
(1)-Cl(4) 170.21(6), C(2)–C(1)–C(14) 121.1(6), C(8)–C(1)–C(14) 118.8(6),
C(2)–C(1)–C(8) 120.0(6).
ration of acid chlorides by Cl/O interchange may be viable
when oxophilic metal species are involved.14 With reference to
tungsten hexachloride, it was previously found that the reac-
tions with a number of aliphatic and aromatic carboxylic acids
are non-selective, leading to mixtures of metal carboxylates
and acid chlorides.15
Scheme 2 DFT-calculated reaction pathway for the conversion of WCl6 and Ph3CCO2H to [CPh3][WOCl5] (1), Ph3CCl, 2, and [WOCl4]. Relative
Gibbs free energies in kcal mol−1. Transition states in red. C-PCM/ωB97X calculations, dichloromethane as a continuous medium.
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that the coordination adduct d is not on the route. Instead,
In summary, we have described for the first time the one
the salt [Ph3CCO][WOCl5] (e) is more probably implicated in pot CO elimination reaction from the carboxylic acid function
the formation of 2, being originated from chloride interchange occurring at room temperature, as favoured by phenyl
between Ph3CC(O)Cl and [WOCl4]. Compound e is about α-substituent(s) and promoted by stoichiometric amounts of
16.7 kcal mol−1 less stable than c, and the related transition W(VI) hexachloride. Thus, tertiary carboxylic acids undergo
state (ts1) is energetically close to e. It has to be mentioned conversion into the corresponding acid chlorides, presumably
that the possible existence of “oxocarbonium” species, analo- followed by the formation of oxocarbonium cations and then
gous to the cation in e, and their tendency to lose CO were pre- carbocations (also Ph3CCl was obtained from triphenylacetic
viously demonstrated by Olah.16 The driving force of the entire acid). The latter are relatively stabilized by the α-phenyl substi-
process appears to be the decomposition of [Ph3CCO]+ into tuents, and evolve to stable hydrocarbons according to their
[CPh3]+ and CO (f), and the related activation energy is only characteristic nature and chemistry. The viability of this
about 13.7 kcal mol−1 (ts2). The resulting salt, 1, can undergo unusual reactivity seems to be related to the multitasking role
chloride migration to give 2 and [WOCl4] (g). Although the played by WCl6,28 being a strong Lewis acid, effective chlorinat-
−
Gibbs energy difference between f and g does not consider the ing agent and precursor of the stabilizing WOCl5 countera-
reticular energy of [CPh3][WOCl5] and the aggregation of nion.11 It is noteworthy that, in contrast, the typical decarbony-
[WOCl4] units,17 the DFT data agree with the experimental lation of carboxylic acid chlorides requires high temperatures
observation of a mixture of 1 and 2 as the final products.
and intermediate oxidative addition to redox active metal
The present investigation was extended to a selection of centres.
phenyl-acetic acids, whose reactions with WCl6 were checked
by GC and NMR analyses. Significant amounts of CO were
found to be released from CMe2(Ph)CO2H, CPh2(Me)CO2H,
Conflicts of interest
CPh(C5H10)CO2H (1-phenyl-1-cyclohexanecarboxylic acid).18 There are no conflicts to declare.
CPh2(Et)CO2H, CPh2(Pr)CO2H, CPh2(CH2CH2Br)CO2H, and
Organic products could be cleanly isolated in a limited
number of cases, and in particular the alkene Ph2CvCH(Me),
3, was obtained from WCl6/CPh2EtCO2H. The reactions of
WCl6 with CMe2(Ph)CO2H and CPh2(Me)CO2H were con-
Acknowledgements
veniently performed using the 1 : 2 stoichiometry, and afforded We are grateful to Mr Francesco Del Cima and Dr Debora Preti
19
the indenes 4a–b over a precipitate of gray WO2Cl2
elemental analysis), as the by-product (Scheme 1).
(IR, for executing GC analyses, to Ms Gioia Marrazzini for assist-
ance with the experimental work and to the University of Pisa
Compounds 3 and 4a–b were efficiently isolated in 62–88% for financial support (Fondi di Ateneo 2017).
yields after the hydrolysis of the respective reaction mixtures
and subsequent purification of the organic phase by chrom-
atography on alumina, and then characterized by elemental
analysis, IR and NMR spectroscopy. Furthermore, the mole-
Notes and references
cular structure of 4a was elucidated by a single crystal X-ray
diffraction study (a view of the structure is supplied in ESI
Fig. S34†).
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