Opening and hydrogenation of dinaphtho[2,1-b:1′,2′-d]thiophene (DNT) by soluble rhodium and iridium complexes. Homogeneous hydrogenolysis of DNT to 1,1′-binaphthalene-2-thiol by rhodium catalysis

Article abstract of DOI:10.1021/om9604785

The fragment [(triphos)IrH], generated in situ by thermolysis of (triphos)Ir(H)₂(C₂H₅) in THF, reacts with dinaphtho[2,1-b:1′,2′-d]thiophene (DNT) at temperatures higher than 100 °C to give a temperature-invariant 3:2 mixt

Full text of DOI:10.1021/om9604785

4604
Organometallics 1996, 15, 4604-4611
Op en in g a n d Hyd r ogen a tion of
Din a p h th o[2,1-b:1′,2′-d ]th iop h en e (DNT) by Solu ble
Rh od iu m a n d Ir id iu m Com p lexes. Hom ogen eou s
Hyd r ogen olysis of DNT to 1,1′-Bin a p h th a len e-2-th iol by
Rh od iu m Ca ta lysis
Claudio Bianchini,*,1a Davide Fabbri,1b Serafino Gladiali,1c Andrea Meli,1a
Wolfgang Pohl,1a and Francesco Vizza1a
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione,
CNR, Via J . Nardi 39, I-50132 Firenze, Italy, and IATCAPA, CNR, and Dipartimento di
Chimica, Universita` di Sassari, Via Vienna 2, I-07100 Sassari, Italy
Received J une 13, 1996X
The fragment [(triphos)IrH], generated in situ by thermolysis of (triphos)Ir(H)2(C2H5) in
THF, reacts with dinaphtho[2,1-b:1′,2′-d]thiophene (DNT) at temperatures higher than 100
°C to give a temperature-invariant 3:2 mixture of the two diastereomeric C-S insertion
products (triphos)IrH(η2(C,S)-C20H12S) (3a ,b; triphos ) MeC(CH2PPh2)3). In the temperature
range from 70 to 100 °C, the reaction gives kinetic mixtures of the C-S insertion products
and of the complex (triphos)IrH(η2-C20H12S) (2), which is suggested to contain an intact DNT
molecule bound to iridium via a double bond from a naphthyl ring. Complex 2 in THF
transforms into 3a ,b even at 70 °C. Hydrogenation of 3a ,b (30 atm of H2, 60 °C) in THF
gives the dihydride thiolate complex (triphos)Ir(H)2(SC20H13), which is protonated by strong
acids, converting to the dimer [(triphos)IrH(µ-SC20H13)2HIr(triphos)](BPh4)2. The latter
compound is straightforwardly obtained by reaction of 3a ,b with protic acids. The complex
(triphos)RhH3 (6) reacts with DNT (THF, 70 °C) to give exclusively the dihydride thiolate
product (triphos)Rh(H)2(SC20H13). The latter complex reacts in THF with KOBut in the
presence of H2 (5 atm) at room temperature to give the trihydride 6 and potassium 1,1′-
binaphthalene-2-thiolate. In the presence of a strong base (KOBut), the π-alkyne complex
[(triphos)Rh(η2-MeO2CCtCCO2Me)]PF6 in THF behaves as a catalyst precursor for the
homogeneous hydrogenolysis of DNT to 1,1′-binaphthalene-2-thiol (30 atm of H2, 160 °C).
In the proposed mechanism, the [(triphos)RhH] fragment acts as the catalyst while the added
base plays a dual role: it serves to generate the catalyst from the precursor and accelerates
the reaction rate by influencing the rate-determining step positively.
In tr od u ction
benzo[b]thiophene (BT), and dibenzo[b,d]thiophene (DBT)
to give stable C-S insertion products (Scheme 1).7
Irrespective of the final structure of the C-S insertion
product, all the reactions illustrated in Scheme 1
proceed through the intermediacy of isolable kinetic
(hydrido)metallathiacycles having the structure of the
iridadibenzothiabenzene complex (Scheme 1). From
In very heavy oils, polyalkylated thiophenes and
fused-ring thiophenes constitute the majority of sulfur-
containing species (Figure 1) and the substrates which
are most difficult to degrade under hydrodesulfurization
(HDS) conditions.2,3
In contrast to studies under actual reactor conditions
with industrial catalysts,3a,b modeling studies of the
reactions between soluble metal complexes and higher
ring systems than dibenzo[b,d]thiophene are absent in
the relevant literature.4
(4) (a) Angelici, R. J . Acc. Chem. Res. 1988, 21, 387. (b) Angelici, R.
J . Coord. Chem. Rev. 1990, 105, 61. (c) Rauchfuss, T. B. Prog. Inorg.
Chem. 1991, 39, 259. (d) Sa´nchez-Delgado, R. A. J . Mol. Catal. 1994,
86, 287. (e) Angelici, R. J . In Encyclopedia of Inorganic Chemistry;
King, R. B., Ed.; Wiley: New York, 1994; Vol. 3, p 1433. (f) Angelici,
R. J . Bull. Soc. Chim. Belg. 1995, 104, 265. (g) Bianchini, C.; Meli, A.
J . Chem. Soc., Dalton Trans. 1996, 801.
(5) (a) Bianchini, C.; Meli, A.; Laschi, F.; Ramirez, J . A.; Zanello,
P.; Vacca, A. Inorg. Chem. 1988, 27, 4429. (b) Ott, J .; Venanzi, L. M.;
Ghilardi, C. A.; Midollini, S.; Orlandini, A. J . Organomet. Chem. 1985,
291, 89.
In earlier work, we have shown that the 16-electron
systems [(triphos)MH] (M ) Rh,5 Ir;6 triphos ) MeC(CH2-
PPh2)3), generated in situ by thermolysis of appropriate
18-electron precursors in THF, react with thiophene (T),
X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) (a) ISSECC-CNR, Firenze. (b) IATCAPA-CNR, Sassari. (c)
Universita` di Sassari.
(2) (a) Willey, C.; Iwao, M.; Castle, R. N.; Lee, M. L. Anal. Chem.
1981, 53, 400. (b) Kabe, T.; Ishihara, A.; Tajima, H. Ind. Eng. Chem.
Res. 1992, 31, 1577.
(3) (a) Girgis, M. J .; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30,
2021. (b) Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C. J .
Catal. 1979, 57, 509. (c) Geneste, P.; Amblard, P.; Bonnet, M.; Graffin,
P. J . Catal. 1980, 61, 115. (d) Singhal, G. H.; Espino, R. L.; Sobel, J .
E. J . Catal. 1981, 67, 446. (e) Gates B. C. Catalytic Chemistry; Wiley:
New York, 1992; Chapter 5, p 390.
(6) Bianchini, C.; Barbaro, P. L.; Meli, A.; Peruzzini, M.; Vacca, A.;
Vizza, F. Organometallics 1993, 12, 2505.
(7) (a) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.;
Herrera, V.; Sa´nchez-Delgado, R. A. J . Am. Chem. Soc. 1993, 115, 2731.
(b) Bianchini, C.; J ime´nez, M. V.; Meli, A.; Moneti, S.; Vizza, F. J .
Organomet. Chem. 1995, 504, 27. (c) Bianchini, C.; Meli, A.; Peruzzini,
M.; Vizza, F.; Moneti, S.; Herrera, V.; Sa´nchez-Delgado, R. A. J . Am.
Chem. Soc. 1994, 116, 4370. (d) Bianchini, C.; Frediani, P.; Herrera,
V.; J ime´nez, M. V.; Meli, A.; Rinco´n, L.; Sa´nchez-Delgado, R. A.; Vizza,
F. J . Am. Chem. Soc. 1995, 117, 4333. (e) Bianchini, C.; J ime´nez, M.
V.; Meli, A.; Moneti, S.; Vizza, F.; Herrera, V.; Sa´nchez-Delgado, R. A.
Organometallics 1995, 14, 2342.
S0276-7333(96)00478-5 CCC: $12.00 © 1996 American Chemical Society

Opening and Hydrogenation of a Thiophene
Organometallics, Vol. 15, No. 21, 1996 4605
over molecular sieves, and purged with nitrogen prior to use.
Potassium tert-butoxide (KOBut, 95%) was purchased from
Aldrich and used without further purification. All other
chemicals were commercial products and were used as received
without further purification. Dinaphtho[2,1-b:1′,2′-d]thiophene
(DNT) was prepared by a literature method.8 Starting materi-
als (triphos)Ir(H)2(C2H5),9 (triphos)Rh(H)3,5b and [(triphos)Rh-
10
(η2-MeO2CCtCCO2Me)]PF6 were prepared as previously
described. Deuterated solvents for NMR measurements were
dried over molecular sieves. 1H, 13C{1H}, and 31P{1H} NMR
spectra were obtained on either a Bruker ACP 200 (200.13,
50.32, and 81.01 MHz, respectively) or a Varian VXR 300
(299.94, 75.43, and 121.42 MHz, respectively) spectrometer.
All chemical shifts are reported in ppm (δ) relative to tetra-
methylsilane, referenced to the chemical shifts of residual
solvent resonances (1H, 13C) or 85% H3PO4 (31P). Broad-band
and selective 1H{31P} NMR experiments were carried out on
the Bruker ACP 200 instrument equipped with a 5 mm inverse
probe and a BFX-5 amplifier device. The computer simulation
of NMR spectra was carried out with a locally developed
package containing the programs LAOCN311 and Davins12
running on a Compaq Deskpro 386/25 personal computer. The
initial choices of shifts and coupling constants were refined
by iterative least-squares calculations using experimental
digitized spectra. The final parameters gave a satisfactory fit
between experimental and calculated spectra, the agreement
factor R being less than 1% in all cases. The 10 mm sapphire
NMR tube was purchased from Saphikon, Milford, NH, while
the titanium high-pressure charging head was constructed at
ISSECC-CNR (Firenze, Italy). Conductivities were measured
with an Orion Model 990101 conductance cell connected to a
Model 101 conductivity meter. The conductivity data were
obtained at sample concentrations of ca. 10-3 M in nitroethane
solutions at room temperature. Infrared spectra were recorded
on a Perkin-Elmer 1600 Series FTIR spectrophotometer using
samples mulled in Nujol between KBr plates. GC analyses
were performed on a Shimadzu GC-14 A gas chromatograph
equipped with a flame ionization detector and a 30 m (0.25
mm i.d., 0.25 µm film thickness) SPB-1 Supelco fused silica
capillary column. GC/MS analyses were performed on a
Shimadzu QP 5000 apparatus equipped with a column identi-
cal with that used for GC analyses.
1
F igu r e 1. (a) H (CDCl3) and (b) 13C{1H} NMR (CD2Cl2;
the asterisk denotes the solvent resonance) spectra of
“Cerro Negro” crude oil. Source: INTEVEP SA, Los Teques,
Venezuela.
Sch em e 1
Rea ction of (tr ip h os)Ir (H)2(C2H5) (1) w ith DNT. A.
NMR Exp er im en t. A sample of 1 (30 mg, 0.035 mmol)
together with a 4-fold excess of DNT (40 mg, 0.14 mmol) were
dissolved in THF-d8 (0.8 mL), and this solution was then
transferred into a 5 mm NMR tube under nitrogen. After two
freeze/pump/thaw cycles at -196 °C, the tube was flame-sealed
and then placed into an oil bath preheated to 70 °C. After 2
h, the tube was cooled to room temperature and 31P{1H} and
1H NMR spectra were recorded at room temperature. In
addition to the starting complex 1 (70%), three other products
(2, 3a , and 3b) were observed in a ratio of 50:31:19 on the
basis of 31P integration. Compounds 3a and 3b were unam-
biguously identified as diastereomeric C-S insertion products
of the formula (triphos)IrH(η2(C,S)-C20H12S) by comparison
with authentic samples (see below). On the basis of the
31P{1H} NMR AMQ pattern and of hydride and olefinic
this study, it was concluded that the energy barrier to
C-S insertion increases in the order T e BT < DBT.7
In this paper, we describe the reactions between the
[(triphos)MH] fragments and dinaphtho[2,1-b:1′,2′-d]-
thiophene (DNT), which is an actual contaminant in
oil,2a and also report the first example of catalytic
hydrogenolysis of DNT to 1,1′-binaphthalene-2-thiol.
1
resonances in the H NMR spectrum (see below), we assigned
2 as (triphos)IrH(η2-C20H12S), in which an intact DNT molecule
binds the metal in an η2 mode via a C-C bond from one of the
two naphthyl rings (most likely the C6-C6a carbon atoms; vide
Exp er im en ta l Section
(8) Fabbri, D.; Delogu, G.; De Lucchi, O. J . Org. Chem. 1993, 58,
1748.
(9) Barbaro, P. L.; Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.;
Vizza, F. Organometallics 1991, 10, 2227.
(10) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.;
Ramirez, J . A. Organometallics 1990, 9, 226.
(11) (a) Bothner-By, A. A.; Castellano, S. QCPE 1967, No. 11, 111.
(b) Castellano, S.; Bothner-By, A. A. J . Chem. Phys. 1964, 41, 3863.
(12) Stephenson, D. S.; Binsch, G. J . Magn. Reson. 1980, 37, 409.
Gen er a l In for m a tion . All reactions and manipulations
were routinely performed under a nitrogen atmosphere by
using standard Schlenk techniques. High-pressure, high-
temperature reactions under controlled pressure of hydrogen
were performed with a stainless steel Parr 4565 reactor
equipped with a Parr 4842 temperature and pressure control-
ler. Tetrahydrofuran (THF) was distilled from LiAlH4, stored

4606 Organometallics, Vol. 15, No. 21, 1996
Bianchini et al.
infra).13 The tube was then kept at 70 °C in an oil bath for
several 2 h periods, and NMR spectra were recorded after
every period; during this time, 1 was gradually consumed,
while 2 converted to 3a and 3b. After ca. 24 h, 60% conversion
of 1 was achieved with the following product distribution: 2
(37%), 3a (39%), and 3b (24%). The tube was then placed into
an oil bath preheated to 100 °C. After 4 h at this temperature,
PA), -32.6 (t, J (PMPQ) ) 16.7 Hz, PM), -38.0 (dd, PQ). 1H NMR
(THF-d8, 20 °C, 200.13 MHz): δ -9.11 (ddd, J (HPtrans) ) 160.8
Hz, J (HPcis) ) 14.0, 10.2 Hz, Ir-H), 5.78 (dd, J (HP) ) 8.3 Hz,
J (HH) ) 7.4 Hz, C-H), 6.4 (masked by aromatic proton
resonances, the correlation between this signal and that at
5.78 ppm was determined by a 1H,1H 2D-COSY experiment,
C-H). Broad-band 1H{31P} NMR (THF-d8, 20 °C, 200.13
MHz): δ -9.11 (s, Ir-H), 5.78 (d, J (HH) ) 7.4 Hz, C-H), 6.4
(masked, C-H).
This reaction between 1 and DNT was repeated in a
sapphire high-pressure NMR (HPNMR) tube. After 24 h at
70 °C, the tube was cooled down to room temperature and then
analyzed by 31P{1H} NMR spectroscopy, which showed product
distribution identical with that in the preparative-scale reac-
tion. The tube was pressurized with 3 atm of CO and then
was introduced into a NMR probe at 20 °C. Even the first
31P{1H} NMR spectrum showed the complete disappearance
of 2. Formed in its place was the known (triphos)IrH(CO)
complex.14
1
31P{1H} and H NMR spectra, recorded at room temperature,
showed an 80% conversion of 1 and a ratio between 3a and
3b of ca. 3:2. Compound 2 was detected only in traces. On
the basis of this product distribution, one may easily infer that,
at 100 °C, the conversion of 2 to both 3a and 3b proceeds more
rapidly than the reaction of 1 with DNT. At 100 °C, complete
conversion of 1 was achieved after a further 10 h with identical
diastereomeric preference for 3a (ratio of 3:2).
B. Syn t h esis of (t r ip h os)Ir H (η2(C,S)-C20H 12S) (Dia -
ster eom er ic 3:2 Mixtu r e of 3a a n d 3b). A Parr reactor was
charged with a solid sample of 1 (0.20 g, 0.24 mmol) and a
solution of DNT (0.27 g, 0.96 mmol) in THF (50 mL) under
nitrogen at room temperature and then heated to 120 °C for
4 h. The reactor was cooled to room temperature, and the
contents were transferred into a Schlenk-type flask. The
volatiles were removed under vacuum, and the residue was
characterized as a ca. 3:2 mixture of 3a and 3b. Recrystalli-
zation from CH2Cl2 and ethanol gave sandy white micro-
crystals which were collected by filtration and washed with
n-pentane; yield 85%. The two diastereomers, in a 3:2 ratio,
could not be separated by either recrystallization or liquid
chromatography. No isomerization reaction occurred by heat-
ing solutions of the mixture of 3a and 3b in THF up to 160 °C
for 2 h. Anal. Calcd (found) for C61H52IrP3S: C, 66.47 (66.33);
Rea ction of th e Dia ster eom er ic Mixtu r e of 3a a n d 3b
w ith H2. Syn th esis of (tr ip h os)Ir (H)2(SC20H13) (4).
A
solution of a 3:2 mixture of 3a and 3b (0.20 g, 0.18 mmol) in
THF (30 mL) was pressurized with hydrogen to 30 atm at room
temperature in a Parr reactor and then heated to 100 °C for
2 h. The reactor was then cooled to room temperature; after
it was depressurized and vented under a nitrogen stream, the
contents were transferred into a Schlenk-type flask. Addition
of n-heptane (40 mL) led to the precipitation of 4 as pale yellow
crystals in almost quantitative yield. Anal. Calcd (found) for
C
61H54IrP3S: C, 66.35 (66.01); H, 4.93 (4.87); S, 2.90 (2.79).
IR: ν(Ir-H) 2071 (s), 2038 (sh) cm-1 31P{1H} NMR (CD2Cl2,
.
H, 4.75 (4.64); S, 2.91 (2.81). IR: ν(Ir-H) 2088 (m) cm-1
.
20 °C, 81.01 MHz): AMQ spin system, δ -2.4 (t, J (PAPM) )
J (PAPQ) ) 13.8 Hz, PA), -26.8 (t, J (PMPQ) ) 13.8 Hz, PM), -28.8
(t, PQ). 31P NMR (CD2Cl2, 20 °C, 81.01 MHz): δ -2.4 (br s,
PA), -26.8 (br d, J (PMH) ) ca. 150 Hz, PM), -28.8 (br d, J (PQH)
) ca. 150 Hz, PQ). 1H NMR (CD2Cl2, 20 °C, 200.13 MHz): δ
-9.2 (doublet of multiplets, J (HPtrans) ) ca. 150 Hz, 2H, Ir-
H). Broad-band 1H{31P} NMR (CD2Cl2, 20 °C, 200.13 MHz):
δ -9.23 (J (HAHB) ) 5.49 Hz, Ir-HA), -9.26 (Ir-HB); these
parameters have been used for the computer simulation of the
hydride resonances (AB spin system) in the experimental
spectrum. The reaction between the diastereomeric mixture
of 3a and 3b and hydrogen was also carried out in an HPNMR
tube under 30 atm of hydrogen pressure and was followed by
1H and 31P NMR spectroscopy. The reaction occurred even at
ca. 60 °C. At 80 °C, all 3a and 3b were consumed in ca. 2 h
to give 4 with no detection of intermediate species; during the
course of the reaction the ratio between the two isomers 3a
and 3b remained practically constant.
Compound 3a : 31P{1H} NMR (CD2Cl2, 20 °C, 121.42 MHz)
AMQ spin system, δ -8.59 (t, J (PAPM) ) 14.4 Hz, J (PAPQ) )
13.3 Hz, PA), -24.05 (dd, J (PMPQ) ) 16.8 Hz, PM), -49.89 (dd,
PQ); 1H NMR (CD2Cl2, 20 °C, 299.94 MHz) δ -7.88 (dt, J (HPQ)
) 154.4 Hz, J (HPA) ) J (HPM) ) 11.8 Hz, Ir-H); broad-band
1H{31P} NMR (CD2Cl2, 20 °C) δ -7.88 (s, Ir-H); 13C{1H} NMR
(THF-d8, 20 °C, 50.32 MHz) δ 168.3 (d, J (CP) ) 4.4 Hz, Ir-
C). Compound 3b: 31P{1H} NMR (CD2Cl2, 20 °C, 121.42 MHz)
AMQ spin system, δ -8.59 (t, J (PAPM) ) 14.2 Hz, J (PAPQ) )
13.3 Hz, PA), -24.26 (dd, J (PMPQ) ) 16.7 Hz, PM), -49.20 (dd,
PQ); 1H NMR (CD2Cl2, 20 °C, 299.94 MHz) δ -7.83 (dt, J (HPQ)
) 154.0 Hz, J (HPA) ) J (HPM) ) 11.5 Hz, Ir-H); broad-band
1H{31P} NMR (CD2Cl2, 20 °C) δ -7.83 (s, Ir-H); 13C{1H} NMR
(THF-d8, 20 °C, 50.32 MHz) δ 168.8 (d, J (CP) ) 4.2 Hz, Ir-
C).
C. Syn th esis of (tr ip h os)Ir H(η2-C20H12S) (2). A Parr
reactor was charged with a solid sample of 1 (0.20 g, 0.24
mmol) and a solution of DNT (0.27 g, 0.96 mmol) in THF (50
mL) under nitrogen at room temperature and then heated to
70 °C. After 24 h, the reactor was cooled to room temperature
and the contents were transferred into a Schlenk-type flask.
The volatiles were removed under vacuum, and the 31P{1H}
NMR spectrum of the residue gave the following mixture
composition: 1 (34%), 2 (27%), 3a (24%), and 3b (15%).
Notwithstanding several attempts, we did not succeed into
isolating a pure sample of 2 from the reaction mixture by either
recrystallization or liquid chromatography. The 31P{1H} and
1H NMR chemical shifts and coupling constants were thus
obtained from samples of 2 invariably contaminated by 1, 3a ,
and 3b. 31P{1H} NMR (THF-d8, 20 °C, 81.01 MHz): AMQ
pattern, δ -10.8 (dd, J (PAPM) ) 14.3 Hz, J (PAPQ) ) 9.3 Hz,
Rea ction of th e Dia ster eom er ic Mixtu r e of 3a a n d 3b
w it h H BF 4‚OE t 2. Syn t h esis of [(t r ip h os)Ir H (µ-SC20
-
H
13)2HIr (tr ip h os)](BP h 4)2 (5). A slight excess of HBF4‚OEt2
(40 mL, 0.20 mmol) was added to a solution of a 3:2 isomeric
mixture of 3a and 3b (0.20 g, 0.18 mmol) in CH2Cl2 (20 mL)
at room temperature. There was an immediate color change
from pale yellow to red-orange. After ca. 20 min, NaBPh4 (0.85
g, 0.25 mmol) in ethanol (30 mL) was added to the reaction
mixture. On partial evaporation of the solvents under a brisk
stream of nitrogen, red microcrystals of 5 precipitated. They
were filtered off and washed with ethanol and n-pentane; yield
80%. Anal. Calcd (found) for C170H146B2Ir2P6S2: C, 71.77
(70.99); H, 5.17 (5.18); S, 2.25 (2.11). ΛM: 102 Ω-1 cm2 mol-1
IR: ν(Ir-H) 2088 (s) cm-1 31P{1H} NMR (THF-d8, -30 °C,
.
.
81.01 MHz): ABM spin system, δ 10.5 (dd, J (PAPB) ) 42.3 Hz,
J (PAPM) ) 8.6 Hz, PA), 9.1 (dd, J (PBPM) ) 10.2 Hz, PB), 1.6 (t,
PM). 1H NMR (THF-d8, -30 °C, 200.13 MHz): δ -1.29 (dt,
J (HPtrans) ) 131.6, J (HPcis) ) 10.1 Hz, Ir-H). The reaction
between 3a ,b and HBF4‚OEt2 was repeated in a NMR tube in
THF-d8: again 5 was the only product detectable on the NMR
(13) Numbering scheme for DNT:
(14) J anser, P.; Venanzi, L. M.; Bachechi, F. J . Organomet. Chem.
1985, 296, 229.

Opening and Hydrogenation of a Thiophene
Organometallics, Vol. 15, No. 21, 1996 4607
At t em p t ed Ca t a lyt ic H yd r ogen a t ion of DNT b y
(tr iph os)Rh (H)2(SC20H13) (7) in a Sapph ir e HP NMR Tu be.
A 10 mm sapphire HPNMR tube was charged with a THF-d8
(2 mL) solution of 6 (40 mg, 0.06 mmol) and a 20-fold excess
of DNT (320 mg, 1.2 mmol) under nitrogen and then placed
into an NMR probe, preheated to 70 °C, for 3 h. After the
31P{1H} and 1H NMR spectra were recorded to test the
quantitative formation of 7, the tube was cooled to room
temperature and pressurized with hydrogen to 30 atm. The
tube was then placed into the NMR probe preheated to 120
1
°C. The 31P{1H} and H NMR spectra of this sample, recorded
every 30 min for 3 h, showed the presence of only 7 during
the entire experiment. After the tube was cooled to room
temperature and depressurized, a 1 µL sample of the solution
was withdrawn by a microsyringe and analyzed by GC and
GC/MS. No hydrogenation product of DNT was detected.
Rea ction of (tr ip h os)Rh (H)2(SC20H13) (7) w ith KOBu t
u n d er a Hyd r ogen Atm osp h er e in a Sa p p h ir e HP NMR
Tu be. An HPNMR tube containing a THF-d8 solution (2 mL)
of 7, prepared as described above from the trihydride 6 (40
mg, 0.06 mmol) and a 3-fold excess of DNT (50 mg, 0.18 mmol),
was opened under a nitrogen atmosphere. A 6-fold excess of
solid KOBut was introduced into the tube, which was then
closed and pressurized with hydrogen to 5 atm. The 31P{1H}
and 1H NMR spectra of this sample, recorded at room tem-
perature after ca. 12 h, showed the presence of only the
trihydride 6. After the tube was depressurized, the contents
were transferred into a Schlenk-type flask and acidified with
aqueous HCl to ca. pH 5. A sample of the solution, analyzed
by GC and GC/MS, showed the presence of DNT and 1,1′-
binaphthalene-2-thiol (GC/MS (EIMS, 70 eV; m/e (%)): 286
(100) M+, 285 (60) M - H+, 253 (47) M - SH+, 252 (54) M -
SH2+, 126 (58) C10H6+) in a ca. 2:1 ratio.
Ca ta lytic Hyd r ogen a tion of DNT by [(tr ip h os)Rh (η2-
MeO2CCtCCO2Me)]P F 6 (8) in th e P r esen ce of KOBu t. In
a typical experiment, a solution of the π-alkyne complex 8 (14
mg, 0.014 mmol) in THF (30 mL) and 50-fold excesses of both
DNT (198 mg, 0.7 mmol) and KOBut (78 mg, 0.7 mmol) were
placed into the Parr reactor under a nitrogen atmosphere.
After the reactor was pressurized with hydrogen to 30 atm at
room temperature, the mixture was heated to 160 °C and then
immediately stirred (650 rpm). After 16 h, the reactor was
cooled to room temperature and slowly depressurized. The
contents of the reactor were transferred into a Schlenk-type
flask and acidified with aqueous HCl to ca. pH 5. A sample
of the solution, analyzed by GC and GC/MS, showed the
following compound distribution: DNT (62(1)%), 1,1′-binaph-
thalene-2-thiol (38(1)%), 1,1′-binaphthalene (traces, <0.2%).
Catalytic reactions carried out in the presence of excess
elemental Hg (1000:1) to test the homogeneous character of
the reactions gave practically identical results in terms of
activity and chemoselectivity.18
1
F igu r e 2. 1H NMR (a, 70 °C; b, 20 °C) and H{31P} NMR
(c, 20 °C) resonances of the hydride ligands in 7 (THF-d8,
200.13 MHz).
time scale. Compound 5 is formed quantitatively also by
reaction of 4 with HBF4‚OEt2.15 The evolution of H2 during
the reaction was seen by 1H NMR spectroscopy (in situ
experiment).
Rea ction of (tr ip h os)Rh (H)3 (6) w ith DNT in a Sea led
NMR Tu be. A. NMR Exp er im en t. A 5 mm NMR tube was
charged with a THF-d8 (0.8 mL) solution of 6 (40 mg, 0.06
mmol) and a 3-fold excess of DNT (50 mg, 0.18 mmol) under
nitrogen, flame-sealed, and placed in a NMR probe preheated
to 70 °C. The 31P{1H} and 1H NMR spectra of this sample,
recorded every 30 min, showed the gradual conversion of 6 to
a new product, which we assigned as (triphos)Rh(H)2(SC20H13
)
(7) on the basis of its 31P{1H} NMR AMNX pattern, of a broad
doublet of multiplets at ca. -7.5 ppm in the hydride region of
1
the H NMR spectrum (Figure 2a), and of related precedents
in the literature.16,17 Complete conversion of 6 to 7 occurred
in ca. 3 h; no intermediate species was observed. The probe
was then cooled to room temperature and 31P{1H}, 1H (Figure
2b), and broad-band 1H{31P} NMR spectra were acquired
(Figure 2c): 31P{1H} NMR AMNX pattern, δ 37.95 (J (PAPM)
) 24.71 Hz, J (PAPN) ) 25.06 Hz, J (PARh) ) 114.44 Hz, PA),
-1.35 (J (PMPN) ) 22.91 Hz, J (PMRh) ) 80.05 Hz, PM), -1.54
(J (PNRh) ) 80.35 Hz, PN), these parameters have been used
for the computer simulation of the experimental spectrum; 1H
NMR δ -7.5 (doublet of multiplets, J (HPtrans) ) ca. 170 Hz,
2H, Rh-H); broad-band H{31P} NMR δ -7.43 (J (HAHB) ) 4.09
1
Hz, J (HARh) ) 13.03 Hz, Rh-HA), -7.56 (J (HBRh) ) 13.42
Hz, Rh-HB), these parameters have been used for the com-
puter simulation of the hydride resonances (ABX spin system)
in the experimental spectrum. The tube was then opened
under a nitrogen atmosphere, and 31P{1H} and 1H NMR
spectra were recorded at room temperature every 2 h. Fol-
lowing the elimination of hydrogen, 7 decomposed, although
slowly, to unidentified products. Almost complete decomposi-
tion was achieved after ca. 24 h.
Resu lts
B. Syn th esis of (tr ip h os)Rh (H)2(SC20H13) (7). A 10 mm
NMR tube was charged with a THF-d8 (2 mL) solution of 6
(80 mg, 0.12 mmol) and a 3-fold excess of DNT (100 mg, 0.36
mmol) under nitrogen, flame-sealed, and placed into an oil
Rea ction of (tr ip h os)Ir (H)2(C2H5) (1) w ith DNT.
The reaction of 1 in THF with DNT at temperatures
higher than 100 °C gives a diastereomeric mixture of
(triphos)IrH(η2(C,S)-C20H12S) (3a and 3b) in a 3:2 ratio,
which is invariant with temperature up to 160 °C.
Following the insertion into a C-S bond of DNT, iridium
becomes a chiral center19 and thus originates two
diastereomers, as DNT is atropisomeric and exhibits C2
symmetry.20 Unfortunately, we were unable to either
separate the two diastereomers or grow crystals suitable
for an X-ray analysis. Accordingly, the structural
1
bath at 70 °C for 3 h. After the 31P{1H} and H NMR spectra
of this sample were recorded at room temperature to test the
quantitative formation of 7, the tube was opened under a
nitrogen atmosphere. The solution was cannulated into a
Schlenk-type flask and concentrated to dryness under vacuum.
The 31P{1H} and 1H NMR spectra of the residue showed the
presence of 7 and various decomposition products (ca. 30%).
(15) Bianchini, C.; Casares, J . A.; J ime´nez, M. V.; Meli, A.; Moneti,
S.; Vizza, F.; Herrera, V.; Sa´nchez-Delgado, R. A. Organometallics
1995, 14, 4850.
(16) Bianchini, C.; Herrera, V.; J ime´nez, M. V.; Meli, A.; Sa´nchez-
Delgado, R. A.; Vizza, F. J . Am. Chem. Soc. 1995, 117, 8567.
(17) Bianchini, C.; Casares, J . A.; Meli, A.; Vizza, F. Polyhedron, in
press.
(18) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891.
(19) Hommeltoft, S. I.; Cameron, A. D.; Shackleton, T. A.; Fraser,
M. E.; Fortier, S.; Baird, M. C. Organometallics 1986, 5, 1380.
(20) Fabbri, D.; Dore, A.; Gladiali, S.; De Lucchi, O.; Valle, G. Gazz.
Chim. Ital. 1996, 126, 11.

4608 Organometallics, Vol. 15, No. 21, 1996
Bianchini et al.
η2(C6,C6a) coordination for chemical reasons (i.e. the lack
of C-H activation products during the reaction which
transforms 1 into 3a ,b; vide infra).
Sch em e 2
When a mixture of 2, 1, 3a , and 3b in THF-d8 is
pressurized with 3 atm of CO (HPNMR experiment),
only 2 reacts with CO at room temperature to give the
known hydride carbonyl (triphos)IrH(CO).14
The combined chemical and NMR data are thus
consistent with a structure for 2 in which iridium is
octahedrally coordinated by a fac triphos ligand, by a
terminal hydride, and by an intact DNT molecule. This
uses a naphthyl double bond for coordination. The
observation of a single set of NMR resonances for the
phosphorus atoms of 2 is consistent with the fact that
the iridium atom is not a stereocenter.
Hyd r ogen a tion a n d P r oton a tion of th e Ir id iu m
C-S In ser tion P r od u cts. The two diasteromers 3a ,b
react with H2 (30 atm) in THF at a temperature higher
than 60 °C to give the dihydride thiolate complex
(triphos)Ir(H)2(SC20H13) (4) as a racemic mixture (Scheme
4). The result of this reaction parallels that of the
analogous DBT derivative (triphos)Ir(H)2(SC12H9).7e,15
The reaction between the diastereomeric mixture of
3a and 3b and H2 (30 atm) was also studied in an
HPNMR tube. No intermediate species was detected
during the transformation into 4. The latter complex
is stable in both the solid state and solution.
The elimination of H2 from 4 is achieved by reaction
with a protic acid such as HBF4‚OEt2 in THF. As was
previously observed for the 2-(phenylthio)phenolate
dihydride (triphos)Ir(H)2(SC12H9),15 the elimination of
dihydrogen (probably via direct attack of the added
proton at one of the terminal hydrides)22 is followed by
dimerization of the unsaturated fragment [(triphos)IrH-
(SC20H13)] to the stable binuclear complex [(triphos)IrH-
(µ-SC20H13)2HIr(triphos)](BPh4)2 (5). This dimer is
straightforwardly obtained by protonation of 3a ,b, a
reaction that has been demonstrated to proceed via
regioselective attack of the proton at the metalated
carbon atom.15
The lack of mirror symmetry, a consequence of the
atropisomeric nature of the 1,1-binaphthalene-2-thiolate
ligand, makes all the phosphorus donor atoms (AMQ
pattern) and the hydride ligands in the racemic complex
4 chemically and magnetically inequivalent. In con-
trast, due to the presence of two stereogenic 1,1′-
binaphthalene-2-thiolate bridging ligands, two dia-
stereomers (namely the racemate R,R and S,S and the
meso form R,S) would be expected for 5. The experi-
mental observation of a single ABM pattern in the
31P{1H} NMR spectrum down to -75 °C suggests that
the protonation reactions of 3a ,b and 4 proceed with
complete diastereoselectivity and afford the more sym-
metric meso form (in the racemate all six phosphorus
atoms would be inequivalent). All the other chemico-
physical characteristics (NMR, IR, ΛM, etc.) of 4 and 5
are quite comparable to those of the related BT- and
DBT-derived analogs (triphos)Ir(H)2[o-S(C6H4)C2H5],
[(triphos)IrH[µ-o-S(C6H4)C2H5]2HIr(triphos)](BPh4)2,
assignments in Scheme 2 are provisional, in the sense
that the labeled stereochemistries for 3a and 3b could
be the opposite.
The coordination geometry of the stable C-S insertion
products 3a and 3b can unequivocally be established
by multinuclear NMR spectroscopy, as well as by a
comparison with the NMR characteristics of the analo-
gous DBT derivative (triphos)IrH(η2(C,S)-C12H8S).7e In
particular, the two diastereomers exhibit temperature-
1
invariant 31P NMR AMQ patterns and distinct H and
13C NMR signals for the terminal hydride (trans to the
most upfield phosphorus resonance) and the metalated
carbon atom of the cleaved DNT (J (CP) = 4 Hz).
In situ variable-temperature NMR experiments in
THF-d8 show that the formation of the mixture of 3a
and 3b is accompanied, even at the early stages of the
reaction, by that of another species of the formula
(triphos)IrH(η2-C20H12S) (2), which persists in solution
up to 70 °C and then transforms into the C-S insertion
products. Although the exclusive formation of 2 has not
been observed at any stage of the reaction between 1
and DNT, the time dependence of the 2:3a ,b ratio is
consistent with a kinetic mixture where the C-S
insertion products are thermodynamically more stable
than the η2(C,C) isomer (Scheme 3).
Compound 2 is obtained in ca. 27% yield by reaction
of 1 in THF with DNT at 70 °C for 24 h. Although
largely contaminated by the starting complex 1 and by
3a ,b, complex 2 can satisfactorily be characterized by
1
31P{1H} and H NMR spectroscopy.
Complex 2 is stereochemically rigid in solution on the
NMR time scale. The 31P{1H} NMR spectrum consists
of a canonical AMQ pattern, while the 1H NMR contains
a resonance at -9.11 ppm (ddd) characteristic of a
terminal hydride ligand (J (HPtrans) ) 160.8 Hz, J (HPcis
)
) 14.0, 10.2 Hz). Most importantly, the 1H NMR
spectrum also contains a resonance at 5.78 ppm (1H,
dd, J (HP) ) 8.3 Hz, J (HH) ) 7.4 Hz) and a correlated
signal at 6.4 ppm (1H,1H 2D-COSY; selective decoupling
experiments), which resemble those reported by J ones
for the isoelectronic η2-naphthalene complex (C5Me5)-
Rh(PMe3)(η2-C10H8).21 The observed multiplicity in the
1
1H (dd) and broad-band H{31P} (d) NMR spectra of the
olefin-type hydrogen at 5.78 ppm and the lack of
coupling connections between the hydrogen at 6.4 ppm
with any other hydrogen nuclei in the molecule suggest
that an intact DNT ligand in 2 binds the metal center
via either the C6-C6a or C5-C6 carbon atoms from a
naphthyl ring.21 Although we cannot exclude either of
these bonding modes, we are inclined to favor the
(22) (a) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1987, 109,
5865. (b) Chinn, M. S.; Heinekey, D. M. J . Am. Chem. Soc. 1990, 112,
5166. (c) Collman, J . P.; Wagenknecht, P. S.; Hembre, R. T.; Lewis, N.
S. J . Am. Chem. Soc. 1990, 112, 1294. (d) Bianchini, C.; Peruzzini,
M.; Zanobini, F. J . Organomet. Chem. 1990, 390, C16. (e) Morris, R.
H. Inorg. Chem. 1992, 31, 1471. (f) Collman, J . P.; Wagenknecht, P.
S.; Lewis, N. S. J . Am. Chem. Soc. 1992, 114, 5665.
(21) (a) Belt, S. T.; Dong, L.; Duckett, S. B.; J ones, W. D.; Partridge,
M. G.; Perutz, R. N. J . Chem. Soc., Chem. Commun. 1991, 226. (b)
Chin, R. M.; Dong, L.; Duckett, S. B.; J ones, W. D. Organometallics
1992, 11, 871.

Opening and Hydrogenation of a Thiophene
Organometallics, Vol. 15, No. 21, 1996 4609
Sch em e 3
Sch em e 4
Sch em e 5
equatorial phosphorus nuclei complicates the reso-
nances of the trans hydride ligands, which are magneti-
cally inequivalent. As is shown in Figure 2b, the
hydride resonances appear at room temperature as a
doublet of multiplets (J (HPtrans) ) 170 Hz) which could
(triphos)Ir(H)2(SC12H9), and [(triphos)IrH(µ-SC12H9)2-
HIr(triphos)](BPh4)2.7c,e,15 Hence, a detailed description
of their spectroscopic properties is not given here.
Rea ction of (tr ip h os)Rh H3 (6) w ith DNT. Unlike
DBT, whose energy barrier to C-S insertion is too high
for the [(triphos)RhH] fragment,7e,17 DNT is readily
cleaved by the 16-electron Rh system even in refluxing
THF (Scheme 5). As a result, the dihydride thiolate
complex (triphos)Rh(H)2(SC20H13) (7) is quantitatively
obtained due to the secondary reaction of the C-S
insertion product with H2 generated upon thermolysis
of the trihydride precursor 6.
The hydrogenation of the C-S insertion intermediate
is more facile than that of the isolable Ir analogs 3a ,b
(Scheme 4), consistent with the chemistry of rhodium,
which forms weaker metal-carbon bonds than iridium.
The substitution of Rh for Ir remarkably influences
the chemistry of the corresponding (dihydride)thiolate
complexes. In fact, under a nitrogen atmosphere, 7 is
much less stable than 4 with respect to the loss of H2,
an experimental observable that has already been
reported for the related 2-(ethylthio)phenolate deriva-
tive (triphos)Rh(H)2[o-S(C6H4)C2H5].16 Unlike the latter
complex, however, the large size of the 1,1-binaphtha-
lene-2-thiolate ligand prevents the dimerization of the
unsaturated fragment [(triphos)Rh(SC20H13)], which
thus decomposes to undefined products.16,17 Nonethe-
less, 7 can be isolated in the solid state (although
contaminated by some decomposition products, ca. 30%)
by rapidly removing the solvent in vacuo. Under 30 atm
of H2 7 is stable up to 160 °C.
1
not be computed. The H{31P} NMR spectrum (Figure
2c) is much less complicated and could successfully be
simulated as an ABX spin system with the following
parameters: δ(HA) -7.43, J (HAHB) ) 4.09 Hz, J (HARh)
) 13.03 Hz; δ(HB) -7.56, J (HBRh) ) 13.42 Hz.
Ca ta lytic Hyd r ogen olysis of DNT. In earlier
work, we have shown that the [(triphos)RhH] fragment
is an active catalyst for the homogeneous hydrogenation
of BT to 2-(ethylthio)phenol.16 Under identical condi-
tions (THF, 30 atm of H2, 160 °C), no hydrogenolysis of
DNT occurs when 6 is employed as catalyst precursor.
A similar result has more recently been observed for
the hydrogenolysis of T catalyzed by [(triphos)RhH] and
has been attributed to the weak basicity of the sulfur
atom of T.17 The poor ligand capability of T substan-
tially contributes to slow down the rate-determining
step (for both BT and T the proposed rate-determining
step is the reductive elimination of the thiol from the
dihydride thiolate species; vide infra).16,17 In order to
increase the reaction rate of T hydrogenolysis, a basic
coreagent (KOBut, KOH, or NaOMe) was successfully
employed.17 It was suggested that the strong base
promotes the elimination of the thiol by formation of a
thiolate salt. We have experimentally proved in this
work that indeed KOBut reacts with the dihydride
thiolate complex 7 in the presence of H2 (5 atm) to give
the trihydride 6 and potassium 1,1′-binaphthalene-2-
thiolate even at room temperature.
The addition of a strong base in a concentration
equivalent to that of the thiophene is a winning strategy
also for the hydrogenolysis of DNT catalyzed by the
[(triphos)RhH] system, as shown in Scheme 6. How-
ever, since 6 independently reacts with strong bases to
give unreactive species, the π-alkyne complex [(triphos)-
Due to the lack of mirror symmetry in the molecule,
the three phosphorus atoms in 7 are magnetically
inequivalent and the 31P{1H} NMR spectrum appears
as an AMNX pattern (X ) Rh) slightly perturbed by
second-order effects. This has successfully been com-
puted with the parameters given in the Experimental
Section. The second-order perturbation affecting the
10
Rh(η2-MeO2CCtCCO2Me)]PF6 (8) was employed as
catalyst precursor. The latter compound, in fact, is

4610 Organometallics, Vol. 15, No. 21, 1996
Bianchini et al.
Sch em e 6
Sch em e 7
capable of generating the [(triphos)RhH] fragment by
treatment with H2 in the presence of strong bases.17
Accordingly, strong bases have a dual role (cocatalyst
and coreagent) in the present hydrogenolysis of DNT,
which occurs in the homogeneous phase (mercury
test).18
The hydrogenolysis rate of DNT is rather low (19 mol
of substrate/mol of catalyst consumed in 16 h), but the
catalytic system is quite robust (80% conversion in 48
h).
tion energy for C-S insertion is lower than that of C-H
insertion, most likely a consequence of the inherent ring
strain in the DNT molecule (the energy barrier to C-S
insertion is actually much higher for DBT than for
DNT).8,19 In other words, C-S insertion would be a
downhill process after slippage of DNT in 2 from η2(C,C)
to η1(S)23,25 (S-bound complexes are, in fact, the im-
mediate precursors to T, BT, and DBT C-S cleavage
by low-valent metal complexes).23 In a sense, the
η2(C6,C6a) structure of 2 as given in Scheme 2 fits better
than the η2(C5,C6) one with a mechanism which involves
a low-energy η2(C,C) to η1(S) slippage.
The enhanced kinetic activity of rhodium vs iridium
does not allow us to detect any intermediate species
during the interaction of the [(triphos)RhH] fragment
with DNT, but there is no reason to exclude the
possibility that the two 16-electron systems behave
similarly. In the case of Rh, the H2 evolved upon
thermolysis of 6 rapidly converts the C-S insertion
product to the dihydride thiolate complex 7.
Rh -Ca ta lyzed Hyd r ogen olysis of DNT. HPNMR
spectroscopy combined with the isolation and charac-
terization of key species related to catalysis has provided
a substantial contribution to the understanding of the
mechanism of hydrogenolysis of BT and T to 2-(ethyl-
thio)phenol and 1-butanethiol, respectively, catalyzed
by [(triphos)RhH] (Scheme 7).16,17
Irrespective of the thiophene, the proposed rate-
determining step is the elimination of the thiol from the
dihydride thiolate intermediates, a process which is
influenced by the steric and electronic nature of the
substrate. For example, the hydrogenolysis of BT is
much faster than that of T, because the latter molecule
is a poorer σ-donor ligand and does not efficiently
promote the elimination of the corresponding thiol.16,17,26
In order to increase the hydrogenolysis rate, external
reagents capable of trapping the thiol have been suc-
cessfully employed.17 As a result, under identical reac-
tion conditions, the rates of hydrogenolysis of both BT
and T catalyzed by [(triphos)RhH] have been found to
increase remarkably when a strong base such as KOBut
was added to the catalytic mixtures.17 The role of the
Discu ssion
Activa tion of DNT a t th e 16-Electr on [(tr ip h os)-
MH] F r a gm en ts. Understanding the modes of adsorp-
tion of thiophenes at the metal centers on the catalyst
surface is of paramount importance for the mechanistic
elucidation of the HDS process. In recent years, valu-
able information in this field has been provided by
studies of the modes of coordination of thiophenes in
organometallic complexes.4 Homogeneous modeling
studies have also strengthened ties between the bonding
mode of the thiophene and its subsequent chemical
reactivity: η1(S) thiophenes may be precursors to C-S
bond scission23 (and hence to hydrogenolysis),4g while
η2(C,C) thiophenes may be intermediates to either C-H
bond cleavage or double-bond hydrogenation.4d,23
The present study of the interaction of DNT with
iridium, which belongs to the class of HDS promoters,24
shows that this fused-ring thiophene can approach the
metal via a naphthyl ring (η2(C,C) bonding mode). We
do not see, however, the formation of kinetic C-H
cleavage products paralleling or preceding the formation
of the C-S insertion products 3a ,b. This result is
surprising, if one recalls that the [(triphos)IrH] fragment
reacts with DBT to give C-H and C-S insertion
products in parallel paths over the temperature range
from 120 to 160 °C.7e Above 160 °C, C-S bond cleavage
prevails over C-H bond cleavage. Competitive C-H
and C-S scissions have been observed also for T and
BT activation at [(triphos)IrH]7b as well as other 16-
electron metal systems (e.g. (C5Me5)Rh(PMe3)).23 In all
cases, it has been concluded that C-H bond cleavage is
kinetically competitive with C-S bond cleavage, but the
latter is thermodynamically preferred. Since there is
no evidence whatsoever for C-H insertion products
during the reaction sequence 1 f 2 f 3a ,b, one may
conclude that, in the peculiar case of DNT, the activa-
(25) (a) Rao, K. M.; Day, C. L.; J acobson, R. A.; Angelici, R. J . Inorg.
Chem. 1991, 30, 5046. (b) Choi, M.-G.; Robertson, M. J .; Angelici, R.
J . J . Am. Chem. Soc. 1991, 113, 4005. (c) Robertson, M. J .; Day, C. L.;
J acobson, R. A.; Angelici, R. J . Organometallics 1994, 13, 179.
(26) (a) Benson, J . W.; Angelici, R. J . Organometallics 1992, 11, 922.
(b) Benson, J . W.; Angelici, R. J . Organometallics 1993, 12, 680.
(23) (a) J ones, W. D.; Dong, L. J . Am. Chem. Soc. 1991, 113, 559.
(b) Dong, L.; Duckett, S. B.; Ohman, K. F.; J ones, W. D. J . Am. Chem.
Soc. 1992, 114, 151.
(24) Startsev, A. N. Catal. Rev.-Sci. Eng. 1995, 37, 353.

Opening and Hydrogenation of a Thiophene
Organometallics, Vol. 15, No. 21, 1996 4611
Sch em e 8
base is essentially sacrificial when the catalyst is
[(triphos)RhH], whereas the base acts also as a cocata-
lyst when the starting (triphos)Rh complex does not
contain hydride ligands (i.e. the π-alkyne complex 8,
which generates the [(triphos)RhH] fragment by het-
erolytic splitting of H2).7c,17 Since the base delivers the
eliminated thiol into the solution as a thiolate salt, the
fastest hydrogenolysis rates are obtained for equivalent
concentrations of substrate and base.
The base-assisted reactions cannot be studied by
HPNMR spectroscopy for technical reasons (the risk of
damaging the sapphire tube is too high). Thus, it
cannot be proved experimentally that the hydrogenoly-
sis of DNT catalyzed by [(triphos)RhH] follows the
general mechanism reported for T and BT. On the other
hand, there is no reason to think of alternative mech-
anisms if one takes into account all the experimental
evidence and chemical arguments discussed here, which
ultimately lead to the catalysis cycle proposed in Scheme
8. This begins with the interaction of [(triphos)RhH]
with DNT to give an η1(S) adduct which is appropriate
to C-S insertion. After C-S bond cleavage, 7 is formed
by sequential steps of C-H reductive elimination and
H2 oxidative addition. The 1,1′-binaphthalene-2-thio-
late dihydride is a stable complex since, unlike BT,16
DNT alone is not capable of promoting the elimination
of the thiol due to steric effects. At this point, the strong
base comes into play: the elimination of the thiol as a
thiolate salt occurs, and the catalyst is regenerated.
The slow rate of transformation of DNT (1.2 mol (mol
of catalyst)-1 h-1) is consistent with the large size of
this substrate. Under similar reaction conditions, in
fact, BT is hydrogenated to 2-(ethylthio)phenol 40 times
faster, despite the less nucleophilic character of its
sulfur atom.7,17,26 On the other hand, the hydrogenoly-
sis of DBT catalyzed by [(triphos)RhH] proceeds with
the same rate as for DNT.17 This result further
confirms that the C-S insertion step is not involved in
the rate-determining step, as the energy barrier to C-S
insertion is lower for DNT than for DBT, while these
two substrates contain sulfur atoms of comparable
basicity and should exhibit also comparable steric
interactions with the metal center.
Ack n ow led gm en t. Thanks are due to the Progetto
Strategico “Tecnologie Chimiche Innovative”, CNR,
Rome, Italy, for financial support. A postdoctoral grant
to W.P. from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
OM9604785