Rhodium-catalysed aryloxycarbonylation of iodo-aromatics by 4-substituted phenols with carbon monoxide or paraformaldehyde

Article abstract of DOI:10.1016/j.mcat.2018.07.021

Rhodium-catalysed phenoxycarbonylation of aryl iodides were carried out under carbon-monoxide atmosphere and in the absence of CO, using paraformaldehyde as an alternative surrogate for carbonylation reactions. Both strategies proved to be efficient for the synthesis of the corresponding phenyl esters. High pressure reactions provided the ester products with good selectivity, however lower activity was achieved compared to palladium containing systems. Using paraformaldehyde as carbon-monoxide source special reaction conditions are required, thus dramatic changes observed during optimisation reactions. Using in situ generated Rh-diphosphine catalyst systems, remarkable influence of ligand structure and solvent composition was observed on the activity and chemoselectivity. The substrate scope and the substituent effect were also investigated.

Full text of DOI:10.1016/j.mcat.2018.07.021

MolecularCatalysis457(2018)67–73
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Rhodium-catalysed aryloxycarbonylation of iodo-aromatics by 4-substituted
phenols with carbon monoxide or paraformaldehyde
⁎
Anas Abu Senia, László Kollára,b, László T. Mikac, Péter Pongrácza,b,
a
Department of Inorganic Chemistry, University of Pécs and János Szentágothai Science Centre, Hungary
MTA-PTE Research Group for Selective Chemical Syntheses, H-7624, Pécs, Hungary
b
c
Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3., Budapest, H-1111, Hungary
A R T I C L E I N F O
A B S T R A C T
Keywords:
Rhodium-catalysed phenoxycarbonylation of aryl iodides were carried out under carbon-monoxide atmosphere
and in the absence of CO, using paraformaldehyde as an alternative surrogate for carbonylation reactions. Both
strategies proved to be efficient for the synthesis of the corresponding phenyl esters. High pressure reactions
provided the ester products with good selectivity, however lower activity was achieved compared to palladium
containing systems. Using paraformaldehyde as carbon-monoxide source special reaction conditions are re-
quired, thus dramatic changes observed during optimisation reactions. Using in situ generated Rh-diphosphine
catalyst systems, remarkable influence of ligand structure and solvent composition was observed on the activity
and chemoselectivity. The substrate scope and the substituent effect were also investigated.
Carbonylation
Carbon monoxide
Paraformaldehyde
Rhodium
Phenyl ester
1. Introduction
solution phase. This cheap surrogate is easy to handle, atom economic
and proved to be applicable in various homogeneous carbonylation
Since the discovery of the ‘Roelen reaction’ (‘oxo-synthesis’) [1] and
Reppe carbonylation [2] in the middle of the past century transition
metal catalysed carbonylation reactions have become the most devel-
oping field of homogeneous catalysis. 25 years later, Heck et al. re-
ported the carbonylation reaction of aryl iodides and nucleophiles (al-
cohols or amines) [3], which process has been developed as a valuable
new method for carbon-carbon bond formation. These reactions utilize
carbon monoxide (CO) as C1 building block and can be efficiently
catalysed by transition metals such as Rh, Ru or Pd, from which the
homogeneous Pd-based catalyst systems have been reported as most
outstanding ones for CeC couplings [4–7] so far. In spite of the ex-
cellent activity of rhodium-containing systems in the hydroformylation
reactions and other non-carbonylative transformations [8–10] their
feasibility in the area of other carbonylations such as amino- and al-
koxycarbonylations, carbonylative coupling reactions have been less
investigated.
The intrinsic properties of widely used carbon monoxide such as
toxicity, flammability, as well as the required special equipment for
handling justify several efforts of seeking non-gaseous CO sources.
Their introduction into these reactions could lead to the realization of
environmentally friendly alternatives. Among numerous CO sources
[11,12] and precursors [13], formaldehyde as well as solid paraf-
ormaldehyde could be considered as most promising alternatives in
reactions [14]. The use of formaldehyde in Rh-catalyzed hydro-
formylation reactions as CO surrogate is known for many years [15],
however the application of this alternative CO source in homogeneous
catalytic carbonylation reactions have become wide-spread only in the
last decade.
Beside the thoroughly studied transfer hydroformylation of alkenes
in the presence of paraformaldehyde [16,17] hydroalkoxycarbonyla-
tion of alkenes were also achieved. Ruthenium and palladium catalysts
proved to be able to transform internal and terminal alkenes to methyl
esters with moderate or good yields [18,19]. Notably, much more re-
sults can be found in the literature on the transition metal catalysed
carbonylation of aryl halides with aldehydes. Kakiuchi and co-workers
reported the efficient cyclisation of aryl bromides in the presence of
various aldehydes [20]. Cinnamaldehyde and pentafluorobenzaldehyde
give better results than paraformaldehyde, but the latter is still the
prosperous choice in term of atom-economy (by-product is only hy-
drogen). Numerous other carbocyclic or heterocyclic systems were
synthesized using similar strategy, i.e. indenones [21], benzoxazinones
[22] or arylbenzofurans [23] catalysed by palladium phosphine sys-
tems. Additionally, paraformaldehyde was also effective in palladium
catalysed alkoxycarbonylation of aryl bromides to form esters in the
presence of alcohols [24]. Recently, para-chlorobenzaldehyde-sup-
ported alkoxycarbonylation of aryl iodides was also published using
⁎
Corresponding author at: MTA-PTE Research Group for Selective Chemical Syntheses, H-7624, Pécs, Hungary.
E-mail address: pongracz@gamma.ttk.pte.hu (P. Pongrácz).
https://doi.org/10.1016/j.mcat.2018.07.021
Received 29 May 2018; Received in revised form 13 July 2018; Accepted 19 July 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

A.A. Seni et al.
MolecularCatalysis457(2018)67–73
octanol and phenols as O-nucleophiles [25].
141 (1), 105 (100), 93 (1), 77 (52), 65 (7), 51 (17).
We
recently
reported
the
palladium-catalysed
hydro-
Phenyl 4-fluorobenzoate (3ba): δH (500 MHz, CDCl3) 8.263 (2 H,
dd, 9.0 Hz, 5.5 Hz, Ph), 7.469 (2 H, t, 8.0 Hz, Ph), 7.314 (1 H, t, 7.5 Hz,
Ph), 7.203–7.252 (4 H, m, Ph). δC (125.7 MHz, CDCl3) 166.2 (d,
255.2 Hz), 164.2, 150.9, 132.8 (d, 8.8 Hz), 129.5, 126.0, 125.8, 121.7,
115.8 (d, 22.6 Hz). IR (KBr (cm–1)): 3075, 3064, 1734, 1597, 1506,
1273, 1193, 1166, 1078, 854, 759, 687. MS m/z (rel int.): 216 (8, M+),
123 (100), 95 (40), 75 (15), 65 (5), 51 (4).
Phenyl 4-chlorobenzoate (3ca): δH (500 MHz, CDCl3) 8.179 (2 H, d,
8.5 Hz, Ph), 7.527 (2 H, d, 8.5 Hz, Ph), 7.472 (2 H, t, 7.5 Hz, Ph), 7.320
(1 H, t, 7.5 Hz, Ph), 7.246 (2 H, dd, 8.0 Hz, 1 Hz, Ph). δC (125.7 MHz,
CDCl3) 164.4, 150.8, 140.2, 131.6, 129.6, 129.0, 128.1, 126.1, 121.6.
IR (KBr (cm–1)): 3092, 3056, 1732, 1612, 1495, 1280, 1076, 853, 756,
723. MS m/z (rel int.): 232, 234 (7, 2, M+), 139, 141 (100, 33), 111,
113 (33, 11), 93 (2), 75 (20), 65 (7), 50 (9).
aryloxycarbonylation of a set of styrenes under carbon monoxide at-
mosphere towards the corresponding arylpropanoic acid aryl esters
[26]. The substituent effect on the regio- and enantioselectivity was
also investigated regarding both the substrate (2- and 4-substituted
styrenes) and the O-nucleophile (4-susbstituted phenols) with Pd-DIOP
system. We decided to extend our studies on the phenoxycarbonylation
reaction of aryl iodides using rhodium catalysts focusing on aromatic O-
nucleophiles. The reactions were carried out both under carbon mon-
oxide atmosphere and in the presence of paraformaldehyde as an al-
ternative source of CO.
2. Experimental
2.1. General
Phenyl 4-bromobenzoate (3 da): δH (500 MHz, CDCl3) 8.101 (2 H, d,
8.5 Hz, Ph), 7.694 (2 H, d, 8.5 Hz, Ph), 7.457–7.488 (2 H, m, Ph), 7.320
(1 H, t, 7.5 Hz, Ph), 7.235–7.251 (2 H, m, Ph). δC (125.7 MHz, CDCl3)
164.5, 150.8, 132.0, 131.7, 129.6, 128.8, 128.5, 126.0, 121.6. IR (KBr
(cm–1)): 3091, 3053, 1731, 1563, 1492, 1287, 1162, 1083, 850, 753,
668. MS m/z (rel int.): 276, 278 (6, 6, M+), 183, 185 (100, 100), 155,
157 (29, 29), 104 (6), 76 (24), 65 (13), 51 (6).
Phenyl 4-methylbenzoate (3ea): δH (500 MHz, CDCl3) 8.131 (2 H, d,
8.0 Hz, Ph), 7.461 (2 H, t, 7.5 Hz, Ph), 7.345 (2 H, d, 8.0 Hz, Ph), 7.301
(1 H, t, 7.5 Hz, Ph), 7.246 (2 H, d, 8.0 Hz, Ph), 2.488 (3 H, s, CH3). δC
(125.7 MHz, CDCl3) 165.3, 151.1, 144.4, 130.2, 129.5, 129.3, 126.9,
125.8, 121.8, 21.8. IR (KBr (cm–1)): 3088, 3039, 2954, 2923, 2856,
1725, 1610, 1477, 1271, 1193, 1080, 751, 688. MS m/z (rel int.): 212
(5, M+), 119 (100), 91 (43), 65 (23), 51 (3).
The [Rh(nbd)Cl)]2 precursor was synthesized from rhodium
trichloride according to the standard procedure [27]. The [Rh(acac)
(CO)2] was also synthesized by published method [28]. Ligands (TPP,
DPPP, Xantphos, DPPB, DPPF) phenols, iodoarenes and dry toluene
were purchased from Sigma-Aldrich and used without further pur-
ification. All reactions were carried out under argon atmosphere using
standard Schlenk-techniques. The 1H- and 13C NMR spectra were re-
corded on a Bruker Avance-III 500 spectrometer. Chemical shifts are
reported in ppm relative to TMS (downfield) for 1H- and 13C NMR
spectroscopy. Conversions and selectivities were determined using GC
and GC–MS. The esters were purified by column chromatography (Si-
lica gel, 0.063 mm; CHCl3) and isolated as pure solids.
Phenyl 4-tert-butylbenzoate (3fa): δH (500 MHz, CDCl3) 8.166 (2 H,
d, 8.0 Hz, Ph), 7.559 (2 H, d, 8.5 Hz, Ph), 7.456 (2 H, t, 7.5 Hz, Ph),
7.282–7.349 (1 H, m, Ph), 7.235 (2 H, d, 8.0 Hz, Ph), 1.402 (9 H, s, tert-
butyl). δC (125.7 MHz, CDCl3) 165.2, 157.4, 151.1, 130.1, 129.5, 126.8,
125.8, 125.6, 121.8, 35.2, 31.1. IR (KBr (cm–1)): 3057, 2963, 2932,
2901, 2870, 1735, 1606, 1495, 1269, 1184, 1072, 765, 702. MS m/z
(rel int.): 254 (1, M+), 239 (4), 161 (100), 146 (11), 118 (14), 91 (10),
77 (6), 65 (7), 50 (3).
Phenyl 4-acetylbenzoate (3ga): δH (500 MHz, CDCl3) 8.322 (2 H, d,
8.0 Hz, Ph), 8.106 (2 H, d, 8.5 Hz, Ph), 7.476 (2 H, t, 7.5 Hz, Ph), 7.325
(1 H, t, 7.5 Hz, Ph), 7.260 (2 H, d, 8 Hz, Ph), 2.704 (3 H, s, COCH3). δC
(125.7 MHz, CDCl3) 197.4, 164.3, 150.8, 140.8, 133.4, 130.4, 129.6,
128.4, 126.2, 121.6, 26.9. IR (KBr (cm–1)): 3057, 2963, 2834, 1732,
1678, 1489, 1405, 1317, 1271, 1215, 1162, 1091, 859, 762, 691. MS
m/z (rel int.): 240 (6, M+), 147 (100), 119 (15), 104 (11), 91 (17), 76
(14), 65 (11), 51 (4).
Phenyl 4-phenylbenzoate (3 ha): δH (500 MHz, CDCl3) 8.312 (2 H,
d, 8.5 Hz, Ph), 7.774 (2 H, d, 8.5 Hz, Ph), 7.691–7.709 (2 H, m, Ph),
7.529 (2 H, t, 7.5 Hz, Ph), 7.442–7.498 (3 H, m, Ph), 7.321 (1 H, t,
7.5 Hz, Ph), 7.277 (2 H, dd, 8.0 Hz, 1 Hz, Ph). δC (125.7 MHz, CDCl3)
165.1, 151.0, 146.4, 140.0, 130.7, 129.5, 129.0, 128.3, 127.4, 127.3,
125.9, 121.8. IR (KBr (cm–1)): 3039, 1730, 1608, 1494, 1404, 1266,
1190, 1083, 825, 742. MS m/z (rel int.): 274 (3, M+), 181 (100), 152
(42), 127 (3), 102 (1), 76(2), 65 (7), 51 (3).
4-Fluorophenyl benzoate (3ab): δH (500 MHz, CDCl3) 8.233 (2 H,
dd, 8 Hz, 1 Hz, Ph), 7.668–7.698 (1 H, m, Ph), 7.555 (2 H, t, 7.5 Hz, Ph),
7.200–7.241 (2 H, m, Ph), 7.125–7.177 (2 H, m, Ph). δC (125.7 MHz,
CDCl3) 165.2, 160.3 (d, 243.9 Hz), 146.8, 133.7, 130.2, 129.3, 128.6,
123.1(d, 7.5 Hz), 116.2 (d, 22.6 Hz). IR (KBr (cm–1)): 3113, 3065, 1731,
1504, 1294, 1188, 1087, 1064, 808, 706. MS m/z (rel int.): 216 (5,
M+), 105 (100), 83 (8), 77 (63), 58 (8), 51 (23).
2.2. Aryloxycarbonylation of iodoarenes under carbon monoxide
atmosphere
In
a typical experiment, catalyst precursor [Rh(acac)(CO)2]
(2.68 mg; 0.01 mmol) and Xantphos (11.57 mg; 0.02 mmol) in toluene
(10 mL) containing 1.0 mmol substrate, 2.0 mmol nucleophile and
1.2 mmol Et3N were transferred under argon atmosphere into a 100 ml
stainless steel autoclave followed by its pressurization with CO up to
total 90 bar and placed in a pre-heated oil bath at 120 °C. The mixture
was then stirred with a magnetic stirrer for 48 h. The pressure was
monitored throughout the reaction. After cooling and venting of the
autoclave at given reaction time, the solution was removed and im-
mediately analyzed by GC and GC–MS.
2.3. Aryloxycarbonylation of iodoarenes using paraformaldehyde as CO
surrogate
In a typical experiment, complex precursor [Rh(nbd)Cl)]2 (4.80 mg;
0.01 mmol) and DPPP (20.62 mg; 0.05 mmol) in 10 ml of solvent mix-
ture consists of toluene:ethyl acetate (4:6) containing 0.5 mmol sub-
strate, 3 mmol of nucleophile, 16 mmol paraformaldehyde, 1.5 mmol
Na2CO3, 1.25 mmol MgSO4, and 1.0 mmol CuCl were transferred under
argon atmosphere into three-necked round bottom flask and placed in a
pre-heated oil bath. The mixture was then refluxed at 100 °C at atmo-
spheric pressure using a balloon and stirred with a magnetic stirrer for
24 h. After cooling of the flask, the solution was removed and im-
mediately analyzed by GC and GC–MS.
2.4. Characterisation of the products
Phenyl benzoate (3aa): δH (500 MHz, CDCl3) 8.23–8.24 (2 H, m,
Ph), 7.65–7.68 (1 H, m, Ph), 7.54 (2 H, t, 7.5 Hz, Ph), 7.46 (2 H, t,
7.5 Hz, Ph), 7.31 (1 H, d, 7.5 Hz, Ph), 7.24–7.26 (2 H, m, Ph). δC
(125.7 MHz, CDCl3) 165.2, 151.1, 133.7, 130.3, 129.7, 129.6, 128.7,
126.0, 121.8. IR (KBr (cm–1)): 3070, 3050, 1742, 1590, 1495, 1450,
1260, 1200, 1170, 1060, 830, 700. MS m/z (rel int.): 198 (11, M+),
4-Chlorophenyl benzoate (3ac): δH (500 MHz, CDCl3) 8.230 (2 H,
dd, 8 Hz, 1 Hz, Ph), 7.671–7.701 (1 H, m, Ph), 7.556 (2 H, t, 7.5 Hz, Ph),
7.417–7.448 (2 H, m, Ph), 7.193–7.223 (2 H, m, Ph). δC (125.7 MHz,
CDCl3) 165.0, 149.4, 133.8, 131.3, 130.2, 129.6, 129.2, 128.7, 123.1.
IR (KBr (cm–1)): 3083, 3056, 1734, 1489, 1283, 1219, 1082, 1061, 808,
706. MS m/z (rel int.): 232, 234 (8, 3, M+), 105 (100), 98 (5), 77 (63),
68

A.A. Seni et al.
MolecularCatalysis457(2018)67–73
63 (5), 51 (23).
surrogate. Both strategies proved to be efficient in ester syntheses. First,
the optimisation of the reaction was performed by the modification of
the catalyst system as well as the reaction conditions using iodobenzene
(1a) and phenol (2a), respectively. Furthermore, para-substituted io-
doarenes as well as para-substituted phenols were tested in the carbo-
nylation reactions.
4-Bromophenyl benzoate (3ad): δH (500 MHz, CDCl3) 8.216–8.232
(2 H, m, Ph), 7.670–7.700 (1 H, m, Ph), 7.539–7.597 (4 H, m, Ph),
7.138–7.168 (2 H, m, Ph). δC (125.7 MHz, CDCl3) 164.9, 150.0, 133.8,
132.6, 130.2, 129.2, 128.7, 123.6, 119.0. IR (KBr (cm–1)): 3083, 3061,
1733, 1486, 1281, 1217, 1060, 806, 707. MS m/z (rel int.): 276, 278 (5,
5, M+), 171, 173 (1, 1), 143, 145 (3, 3), 105 (100), 77 (55), 63 (8), 51
(20).
3.1. Reactions under carbon monoxide atmosphere
4-Trifluoromethylphenyl benzoate (3ai): δH (500 MHz, CDCl3)
8.316–8.338 (1 H, m, Ph), 8.230–8.269 (1 H, m, Ph), 7.696–7.762 (2 H,
m, Ph), 7.500–7.596 (2 H, m, Ph), 7.427–7.456 (1 H, m, Ph),
7.384–7.409 (1 H, m, Ph), 7.205–7.270 (1 H, m, Ph). δC (125.7 MHz,
CDCl3) 164.7, 153.5, 134.0, 130.3, 129.0, 128.7, 128.2 (quartet,
32.8 Hz), 126.9 (quartet, 3.8 Hz), 123.9 (quartet, 271.6 Hz), 122.3. IR
(KBr (cm–1)): 3066, 2923, 1734, 1612, 1600, 1517, 1453, 1340, 1272,
1134, 1071, 817, 705. MS m/z (rel int.): 266 (1, M+), 133 (4), 113 (2),
105 (100), 83 (2), 77 (56), 63 (2), 51 (19).
4-Isopropylphenyl benzoate (3aj): δH (500 MHz, CDCl3)
8.233–8.252 (2 H, m, Ph), 7.651–7.683 (1 H, m, Ph), 7.531–7.562 (2 H,
m, Ph), 7.303–7.331 (2 H, m, Ph), 7.155–7.183 (2 H, m, Ph), 2.980 (1 H,
septet, 7.0 Hz, (CH3)2CH), 1.308 (6 H, d, 7.0 Hz, (CH3)2CH). δC
(125.7 MHz, CDCl3) 165.4, 148.9, 146.4, 133.5, 130.2, 129.8, 128.6,
127.4, 121.4, 33.7, 24. IR (KBr (cm–1)):): 3052, 3030, 2972, 2960,
2932, 2892, 1732, 1508, 1450, 1268, 1195, 1081, 1064, 809, 712. MS
m/z (rel int.): 240 (13, M+), 105 (100), 91 (8), 77 (48), 65 (3), 51 (11).
4-Methoxyphenyl benzoate (3ak): δH (500 MHz, CDCl3) 8.238 (2 H,
d, 7 Hz, Ph), 7.668 (1 H, t, 7.5 Hz, Ph), 7.545 (2 H, t, 7.5 Hz, Ph),
7.156–7.189 (2 H, m, Ph), 6.964–6.997 (2 H, m, Ph), 3.863 (3 H, s,
OCH3). δC (125.7 MHz, CDCl3) 165.6, 157.4, 144.5, 133.5, 130.2,
129.7, 128.6, 122.5, 114.6, 55.6. IR (KBr (cm–1)): 3113, 3070, 2999,
2959, 2932, 2834, 1730, 1505, 1271, 1195, 1087, 1064, 808, 709. MS
m/z (rel int.): 228 (19, M+), 123 (3), 105 (100), 95 (3), 77 (48), 63 (1),
51 (13).
4-Phenoxyphenyl benzoate (3 al): δH (500 MHz, CDCl3) 8.247 (2 H,
dd, 8.0 Hz, 1 Hz, Ph), 7.663–7.698 (1 H, m, Ph), 7.556 (2 H, t, 7.5 Hz,
Ph), 7.369–7.411 (2 H, m, Ph), 7.207–7.239 (2 H, m, Ph), 7.139–7.173
(1 H, m, Ph), 7.074–7.117 (4 H, m, Ph). δC (125.7 MHz, CDCl3) 165.3,
157.3, 154.9, 146.4, 133.7, 130.2, 129.8, 129.5, 128.6, 123.4, 122.9,
119.7, 118.9. IR (KBr (cm–1)):): 3055, 1735, 1600, 1586, 1498,
1488,1450, 1269, 1184, 1093, 1066, 810, 710. MS m/z (rel int.): 290
(25, M+), 185 (3), 157 (1), 129 (4), 105 (100), 77 (56), 63 (3), 51 (17).
4-Biphenyl benzoate (3am): δH (500 MHz, CDCl3) 8.263–8.280
(2 H, m, Ph), 7.671–7.706 (3 H, m, Ph), 7.629–7.646 (2 H, m, Ph), 7.569
(2 H, t, 7.5 Hz, Ph), 7.476–7.506 (2 H, m, Ph), 7.399 (1 H, t, 7.5 Hz, Ph),
7.322–7.351 (2 H, m, Ph). δC (125.7 MHz, CDCl3) 165.3, 150.4, 140.4,
139.1, 133.7, 130.2, 129.6, 128.8, 128.6, 128.3, 127.4, 127.2, 122.0. IR
(KBr (cm–1)): 3057, 3027, 1732, 1486, 1272, 1220, 1087, 1065, 759,
703. MS m/z (rel int.): 274 (20, M+), 169 (3), 141 (6), 115 (9), 105
(100), 77 (40), 51 (9).
In general, the reaction was carried out in the presence of triethy-
lamine as a base in toluene while the catalyst to substrate ratio was kept
at 1/100. Incomplete, but satisfying conversions can be reached after
48 h which cannot be enhanced with longer reaction times. Beside the
targeted esters, side products were formed in all cases in small extent,
because of the reduction of the iodobenzene (benzene formation) or
ester hydrolysis (carboxylic acid formation). The chemoselectivity of
the reaction varied between 80–95%, where lower ester yields were
achieved at elevated temperatures (120 °C).
All the tested ligands, including the parent monodentate triphe-
nylphosphine (TPP), formed catalytically active complexes (Table 1,
entries 1,2,6,11,13) though with DPPB lower activities and selectivities
were obtained (entries 11, 12). DPPP, DPPF and Xantphos gave similar
conversions, but the latter one provided notably higher ester chemos-
electivity (67%, 81% and 95%, respectively). The results with TPP was
promising, but 90% chemoselectivity lags behind the tem. Therefore,
the Rh-xantphos system was selected for further investigations re-
garding the scope of the reaction (See below, Table 2). Significant
differences were observed for different rhodium precursors, that is,
reactions using dinuclear [Rh(nbd)Cl]2 showed lower activity in the
phenoxycarbonylation reaction (compare entries 7 and 10; 11 and 12)
than chlorine-free Rh(CO)2(acac). It can be supposed that the formation
of the catalytically active Rh(CO)n(diphosphine) (n = 1,2) species took
place much easier in the latter case due to two reasons: i) no dehalo-
genation (in case of Rh(I) species) or dehydrohalogenation on the in-
fluence of the base (in case of Rh(III) species formed in the catalytic
cycle) was necessary, ii) no preliminary activation of CO was necessary.
Using Xantphos as ligand no significant difference was detected
between catalyst precursors (entry 3 and 5) keeping metal to ligand
ratio of 1:2. As expected, the carbon-monoxide pressure has a sig-
nificant effect on the carbonylation reaction. Lower pressures resulted
lower conversions (compare entries 2 and 4, or 8 and 9). The best result
was obtained by using Xantphos as ligand under 90 bar of CO at 120 °C,
which conditions were selected for subsequent experiments (entry 3).
With the optimum reaction conditions, the effect of the para-sub-
stituents on the substrate’s reactivity was studied by varying both
substrate iodoarene and the nucleophile phenol, as well (Table 2). 4-
Substituted aryl iodides were found to be more reactive substrates than
iodo benzene, regardless of whether electron withdrawing or donating
substituents are located in para position. The corresponding phenyl 4-
substituted benzoates (3ba–3 ha) were formed in 72–97%. However,
aryl iodide bearing tert-butyl group afforded particularly lower con-
version and chemoselectivity (entry 5). The highest yield (97%) was
provided by the acetyl substituted substrate (1 g). Similarly, using para-
substituted phenols the corresponding aryl benzoates (3ab-3am) were
obtained in high yields independently on the electronic properties of
the substituents (91–98%).
3. Results and discussions
The transformation of the substrate iodobenzene (1a) and O-nu-
cleophile phenol (2a) in the presence of rhodium catalysts to afford
phenyl benzoate (3aa) was selected as a model reaction (Scheme 1).
Two methods of carbonylation were conducted considering the source
of the carbonyl group. Carbon monoxide was used in a conventional
aryloxycarbonylation process and paraformaldehyde as its economical
Noteworthy, similar results, i.e. increased reactivity of the substrate
(4-iodotoluene) were observed in Pd-catalysed aryloxycarbonylation
both with electron-withdrawing or electron donating substituents in the
para-position of the phenol nucleophile [29]. Surprisingly, not only the
O-nucleophile but the iodoarene substrates behaved similarly as follow:
enhanced reactivity of the 4-substituted iodoarenes was observed pos-
sessing substituents either with positive or negative Hammett-para-
substituent constant values (vide supra). Since not the expected elec-
tronic effect due to para-substituents of the substrate was observed, it
can be stated that the rate-determining step of the aryloxycarbonylation
Scheme 1. Aryloxycarbonylation reactions with carbon monoxide or paraf-
ormaldehyde.
69

A.A. Seni et al.
MolecularCatalysis457(2018)67–73
Table 1
Optimization of the phenoxycarbonylation reaction under CO atmosphere.a.
c
Entry
Precursor
Ligand (eq.)
Temperature [oC]
p(CO)
[bar]
Conv.b
[%]
Yield (3aa)
Chemosel.
[%]
1
2
3
4
5
6
7
8
Rh(CO)2(acac)
Rh(CO)2(acac)
Rh(CO)2(acac)
Rh(CO)2(acac)
[Rh(nbd)Cl]2
Rh(CO)2(acac)
Rh(CO)2(acac)
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
Rh(CO)2(acac)
[Rh(nbd)Cl]2
Rh(CO)2(acac)
Rh(CO)2(acac)
TPP (4)
90
90
120
90
120
90
120
90
90
120
90
90
90
90
90
60
90
90
90
90
60
90
90
90
90
90
> 99
62
80
58
79
76
92
27
8
60
20
7
64
82
90
51
76
52
76
72
62
21
6
53
19
4
57
67
90
82
95
90
96
95
67
78
75
88
95
57
89
82
Xantphos (2)
Xantphos (2)
Xantphos (2)
Xantphos (4)
DPPP (2)
DPPP (2)
DPPP (4)
DPPP (4)
DPPP (4)
9
10
11
12
13
14
DPPB (2)
DPPB (4)
DPPF (2)
90
120
DPPF (2)
a
Reaction conditions: precursor: 0.01 mmol, substrate 1 mmol, phenol: 2 mmol, Et3N: 1.2 mmol, toluene: 10 mL, reaction time: 48 h.
Determined by GC.
Chemoselectivity toward ester (3aa) product (moles of 3aa / moles of converted 1a x 100).
b
c
should be after the formation of the palladium(II)-acyl complex, i.e., the
of the substituents on the aryl and phenol moieties may be supposed.
Consequently, the reductive elimination step (or alternatively, the
phenolic cleavage of the acyl-complex (E) could be considered as a rate-
determining one.
rate and selectivity of the reaction are determined followed the oxi-
dative addition step (see proposed catalytic cycle below; Fig. 1).
When the widely accepted mechanism of palladium-catalysed al-
koxycarbonylation [30,31] as well as the observation of Miura et al is
considered [29], the following catalytic cycle for the rhodium-catalysed
reaction can be supposed. The oxidative addition of the iodoarene onto
rhodium(I) species resulted in rhodium(III)-aryl complex (B). It is fol-
lowed by CO insertion forming the corresponding rhodium(III)-acyl
intermediate (C). The nucleophilic attack of the phenol (D) is accom-
panied by HI elimination resulting in the rhodium(III)-acyl-phenoxy
complex (E). The reductive elimination of the phenyl benzoate product
gave the coordinatively unsaturated rhodium(I) species (F) which
yields the 'starting' complex via CO coordination.
3.2. Reactions in the presence of paraformaldehyde as CO source
The rhodium-catalysed phenoxycarbonylation reaction was carried
out using the above substrates and phenols as well as rhodium-pre-
cursors in the absence of carbon monoxide enabling direct comparison
between two methodologies. Based on preliminary results, the reaction
seemed to be feasible using paraformaldehyde as CO source.
First, the reaction conditions were optimized using the model re-
action involving substrate (1a) and phenol nucleophile (2a) (Table 3).
Similarly to the experiments carried out under carbon-monoxide at-
mosphere, in situ generated catalysts were used with substrate to cat-
alyst ratio of 50/1. Disappointingly, the experiments performed in
If the formation of C (or D) was the rate-determining step, it should
be influenced by the acidity of the phenol, i.e., a Hammett-plot should
be obtained. Based on the reproducible catalytic results, the 'interplay'
Table 2
Aryloxycarbonylation reaction with substituted iodoarenes and phenols.a.
Entry
R1 (iodoarene)
R2 (phenol)
Conv.b
[%]
Yieldc (ester) [%]
1
2
3
4
5
6
7
8
F (1b)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
F (2b)
Cl (2c)
Br (2d)
CF3 (2i)
iPr (2 j)
OMe (2k)
OPh (2l)
Ph (2 m)
80
95
97
97
27
> 99
96
> 99
> 99
> 99
98
72 (3ba)
88 (3ca)
87 (3 da)
88 (3ea)
12 (3fa)
97 (3ga)
86 (3 ha)
94 (3ab)
98 (3ac)
95 (3ad)
93 (3ai)
95 (3aj)
91 (3ak)
95 (3 al)
98 (3am)
Cl (1c)
Br (1d)
Me (1e)
tBu (1f)
Ac (1 g)
Ph (1 h)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
9
10
11
12
13
14
15
98
98
99
> 99
a
Reaction conditions: precursor: 0.01 mmol, ligand: 0.02 mmol, substrate 1 mmol, nucleophile: 2 mmol, Et3N: 1.2 mmol, toluene: 10 mL, p(CO) = 120 bar,
T = 120 °C, reaction time: 48 h.
b
Determined by GC.
Isolated yields.
c
70

A.A. Seni et al.
MolecularCatalysis457(2018)67–73
reactions can be supressed (entry 7). In order to increase the dissolution
of the in situ forming formaldehyde, ethyl acetate was added to the
reaction mixture as a co-solvent (entries 8–13) resulting in significantly
better by using increased ligand ratio. Interestingly, only DPPP ligand
showed acceptable performance under the selected reaction conditions
(entry 8). In the presence of DPPF or DPPB still low ester yields can be
obtained with slightly increased selectivity. Further increase of ethyl
acetate ratio caused very low catalyst activity (entry 13). It is worth
noting, that the Rh(CO)2(acac) precursor gave poorer results contrary
to reactions under CO pressure (entry 3, 12).
The optimized procedure was applied to the phenoxycarbonylation
of para-substituted iodoarenes (1a–h) and phenols (2a–m) (Table 4).
Unfortunately, any substitution on the iodobenzene ring dramatically
decreases the ester yield under the selected reaction conditions. Con-
version values also decreased in numerous cases, but side reaction,
namely hydrodeiodination obviously dominated against the carbony-
lation reaction (entries 1–7). This reductive pathway was described by
other authors earlier [25], and efficient palladium-catalysed hydro-
dehalogenation with paraformaldehyde was also published by Lee and
co-workers [32]. Beside the reduction of aryl halide, oxidation of the
alcohol nucleophile was suggested providing aldehyde or formate as
products.
Fig. 1. Proposed mechanism of aryloxycarbonylation with carbon monoxide.
toluene or DMF gave low conversion and negligible yields of the cor-
responding ester (entries 1, 2). That is, in most cases not the ester
formation but the side reactions, such as hydrodeiodination of the
substrate and acid formation were also favoured. Numerous optimiza-
tion reactions were conducted with different solvents and bases.
Na2CO3 provided much better results than that of obtained in the
presence of triethyl amine, and the introduction of drying agent
(MgSO4) also enhanced the chemoselectivity supressing the hydrolysis
of the target esters. It should be also noted, that the addition of copper
(I)-chloride considerably increased the reaction rate. The same phe-
nomenon was described with palladium systems, where copper(I) plays
important role on forming active catalytic species by removing phos-
phine ligands from palladium complexes [29]. In case of increased li-
gand/precursor ratio (see optimised conditions later) the potential
formation of bis-diphosphine rhodium complexes can reduce the
number of catalytically active species. It is conceivable that copper(I)
can create vacant sites by receiving these additional ligands from rho-
dium enhancing the activity.
Contrary to iodoarene substitution, no such negative effect was
observed using various phenol derivatives (entries 8–14). The trans-
formed amount of the substrate varied in wide range from 49% to total
conversion, without any correspondence with the character of the
substituents. Most of the nucleophiles gave lower conversions than the
parent phenol, except fluoro and bromo substituted derivatives showed
exceeded results (entries 8 and 10). The chemoselectivity of the car-
bonylation reaction was over 88% in all cases, in addition exclusive
ester formation occurred with chloro, phenoxy and phenyl substituted
phenols (entries 9, 13, 14 respectively).
On the basis of our experiments and reported results [18,30–32], a
plausible reaction mechanism can be proposed for the arylox-
ycarbonylation of iodoarenes, and for the reduction path via rhodium
hydride intermediate (Fig. 2). First, the depolymerisation of paraf-
ormaldehyde occurs at elevated temperature to produce formaldehyde,
which can undergo an oxidative addition with rhodium(I) species (F).
The forming (hydrido)(acyl)rhodium(III) complex (G) can react further
in two directions. Reductive elimination of H2 gives the carbonyl-rho-
dium complex (A), which is the key intermediate of the phenox-
ycarbonylation pathway (cycle II). Upon this way, the formed carbonyl-
rhodium complex can follow the route described earlier (Fig. 1), via
aryl iodide addition (B), CO insertion (C) and by the attack of the nu-
cleophile (D).
Precursor or ligand change did not resulted in any improvement in
both activity and selectivity, even the best performance ligand under
high pressure conditions, XANTPHOS showed moderate efficiency on
paraformaldehyde-mediated carbonylation (entry 6). However, in-
creasing the precursor-ligand ratio from 1 to 5 using DPPP, the side
Table 3
Optimization of the phenoxycarbonylation reaction in the presence of paraformaldehyde.a.
Entry
Precursor
Ligand (eq.)
Solvent
Conversionb
[%]
Ester yieldb [%]
1
2
3
4
5
6
7
8
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
Rh(CO)2(acac)
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
Rh(CO)2(acac)
[Rh(nbd)Cl]2
DPPP (2)
DPPP (2)
DPPB (1)
DPPB (2)
DPPF (2)
XANTPHOS (2)
DPPP (5)
DPPP (2)
DPPP (5)
DPPB (5)
DPPF (5)
DPPB (2.5)
DPPP (5)
Toluene
DMF
Toluene
Toluene
Toluene
Toluene
Toluene
EtOAc:Toluene (6:4)
EtOAc:Toluene (6:4)
EtOAc:Toluene (6:4)
EtOAc:Toluene (6:4)
EtOAc:Toluene (6:4)
EtOAc:Toluene (8:4)
45
29
32
47
44
24
26
40
76
38
43
22
5
19
2
15
15
14
11
26
25
76
25
18
11
5
9c
10c
11c
12c
13c
a
Reaction conditions: precursor: 0.01 mmol, substrate 0.5 mmol, phenol: 3 mmol, solvent: 10 mL, paraformaldehyde: 16 mmol, Na2CO3: 1.5 mmol, MgSO4:
1.25 mmol, temperature: 100 °C, reaction time: 24 h. Mixed solvent (entries 7–12) are given in v/v%.
b
Determined by GC.
1.0 mmol CuCl was used.
c
71

A.A. Seni et al.
MolecularCatalysis457(2018)67–73
Table 4
Phenoxycarbonylation reaction with substituted iodoarenes and phenols.a.
Entry
R1 (iodoarene)
R2 (phenol)
Conv.b
[%]
Yieldb (ester) [%]
1
2
3
4
5
6
7
8
F (1b)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
F (2b)
Cl (2c)
Br (2d)
CF3 (2i)
iPr (2 j)
OPh (2l)
Ph (2 m)
28
67
> 99
43
40
> 99
89
> 99
51
87
49
19 (3ba)
3 (3ca)
6 (3 da)
35 (3ea)
13 (3fa)
2 (3ga)
9 (3 ha)
86 (3ab)
51 (3ac)
83 (3ad)
43 (3ai)
61 (3aj)
61 (3 al)
58 (3am)
Cl (1c)
Br (1d)
Me (1e)
tBu (1f)
Ac (1 g)
Ph (1 h)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
9
10
11
12
13
14
65
61
58
a
Reaction conditions: precursor: 0.01 mmol, ligand: 0.05 mmol, substrate 0.5 mmol, nucleophile: 3 mmol, toluene: 4 mL, EtOAc: 6 mL, Na2CO3: 1.5 mmol, MgSO4:
1.25 mmol, CuCl: 1.0 mmol.
b
Determined by GC.
As another option, the release of CO provides the dihydrido-rho-
dium species (HA), which can be responsible for the reduction of the
iodoarenes (cycle I, path A). Further possible way can be proposed with
the formation of (formyl)(phenoxy)-rhodium complex (HB), which as-
sumes the reductive elimination of phenyl formate as side product (path
B). Both mechanisms lead to the (hydrido)(aryl) species (I), to get the
hydrodehalogenated product and the initial unsaturated complex (F).
4. Summary
Rhodium-catalysed phenoxycarbonylation of aryl iodides were
conducted under carbon monoxide atmosphere and with paraf-
ormaldehyde as CO surrogate. Under optimized reaction conditions
both strategies proved to be appropriate for the synthesis of various
phenyl esters. Increased reactivity can be observed by the substitution
Fig. 2. Proposed reaction mechanism of aryloxycarbonylation and hydrodeiodination with formaldehyde. (B, C and D catalytic intermediates are identical with those
depicted in Fig. 1).
72

A.A. Seni et al.
MolecularCatalysis457(2018)67–73
of both the substrate and the nucleophile in the presence of carbon
monoxide. Paraformaldehyde mediated carbonylations showed dif-
ferent dependence. Considering the electronic properties of the sub-
stituents of phenol no definite effect observed on carbonylation reac-
tion. Surprisingly, iodoarene substitution switches the reaction to
hydrodehalogenation pathway providing arenes instead of the target
ester compounds.
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Acknowledgements
This work was supported by the ÚNKP-17-4-II New National
Excellence Program of the Ministry of Human „Capacities” as well as by
the European Union, co-financed by the European Social Fund Grant
no.: EFOP-3.6.1.-16-2016- 00004 entitled by Comprehensive
Development for Implementing Smart Specialization Strategies at the
University of Pécs. L. T. L. T. Mika is grateful for the support of a József
Varga Research Scholarship from the Faculty of Chemical and
Biochemical Engineering, Budapest University of Technology and
Economics. This work was supported by the GINOP-2.3.2-15-2016-
00049 grant and OTKA K113177.
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