Organometallics 1995,14, 3196-3202
3196
C-S Bond Scission of Substituted Thiophenes at Rhodium. Factors Influencing the Regioselectivity of the Insertion and the Stability of the Resulting Metallathiacycles Claudio Bianchini," M. Victoria Jimenez, Andrea Meli, and Francesco Vizza Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, ISSECC-CNR, Via J . Nardi 39, 50132 Firenze, Italy Received January 23, 1995@ The fragment [(triphos)RhHl, generated i n situ by thermolysis of the trihydride (triphos)RhH3 in refluxing tetrahydrofuran, reacts with a variety of substituted thiophenes to give C-S insertion products of the formula (triphos)Rh(y3-SCR=CRCH=CH2) ( R = H, R = Me, Et, COMe, C02Et; R = H, R = Me, COMe, OMe; triphos = MeC(CHzPPh&. Irrespective of the position and electronic character of the substituent in the thiophene, insertion is seen exclusively into the C-S bond away from the substituent, consistent with a determinant steric control. The electronic properties of the substituent in the thiophene play an important role in determining the stability of the C-S insertion products in chlorinated solvents. Among the thiophenes investigated, only 2,5-MezT, for steric reasons, and 2-OMeT, for electronic reasons, do not form butadienethiolate complexes by reaction with [(triphos)RhH]. The competitive reactivity of various substituted thiophenes toward the fragment [(triphoslRhHl has been studied by NMR spectroscopy. The reactivity decreases in the order: 2-COzEtT > 2-COMeT > 3-COMeT >> 3-OMeT > T = 2-MeT > 3-MeT, consistent with a predominant electronic effect.
Introduction In recent years, an intense effort has been devoted t o the fundamental understanding of the mechanisms which are operative in the hydrodesulfurization (HDS) reaction of fossil fue1s.l A mechanistic approach which continues to attract considerable attention is the study of the coordination and reactivity of simple model substrates such as thiophene (T)with soluble metal complexes.2-8 However, since alkyl-substituted thiophenes in fossil fuels are much more prevalent than T itself, it is expected that information of effective practical relevance can best @Abstractpublished in Aduance ACS Abstracts, May 15, 1995. (1) ( a ) Mitchell, P. C. H. The Chemistry of Some Hydrodesulphurisation Catalysts Containing Molybdenum; Climax Molybdenum Co. Ltd.: London, 1967. (b) Schuman, S. C.; Shalit, H. Catal. Reu. 1970, 4, 245. (c) Weisser, 0.;Landa, 0. Sulfide Catalysts: Their Properties and Applications; Pergamon: Oxford, U.K., 1973. (d) Gates, B. C.; Katzer, J . R.; Schuit, G. C. A. Chemistry of Catalytic Properties; McGraw-Hill: New York, 1979. (e) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980. (f, Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series 429; American Chemical Society: Washington, DC, 1990. (g)McCulloch, D. C. In Applied Zndustrial Catalysis; Leach, B. E., Ed.; Academic: New York, 1983; Vol. 1, p 69. (h) Lyapina, N. K. Russ. Chem. Reu. (Engl. Transl.) 1982, 51, 189. (i) Challenger, F. Aspects of the Organic Chemistry of Sulfur; Butterworths: London, 1959. (2)(a) Sanchez-Delgado, R. A. J . Mol. Catal. 1994, 86, 287. (b) Rauchfuss, T. B. Prog. Inorg. Chem. 1991,39, 259. ( c )Angelici, R. J. Coord. Chem. Reu. 1990, 105, 61. ( d ) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387. (3) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Frediani, P.; Herrera, V.; Sanchez-Delgado, R. A. J . Am. Chem. SOC.1993,115,2731. (4) Jones, W. D.; Chin, R. M. J . A m . Chem. SOC.1994, 116, 198.
(5) Jones, W. D.; Chin, R. M.; Crane, T. W.: Baruch. D. M. Organometallics 1994, 13, 4448. (6)Dong, L.; Duckett, S. B.; Ohman, K. F.; Jones, W. D. J . Am. Chem. SOC.1992, 114, 151. (7) Chen, J.; Daniels, L. M.; Angelici, R. J . J . A m . Chem. SOC.1990, 112, 199. (8) Selnau, H. E.; Merola, J . S. Organometallics 1993, 12, 1583.
be obtained from studies of the reactions between substituted thiophenes and metal fragments. Indeed, through an approach of this type, some research groups have recently answered important questions about the HDS mechanism. Angelici has shown, inter alia, that methyl-substituted thiophenes are $(S)- and/or q5-adsorbed to the catalyst surface through a comparison of their adsorption coefficients on a sulfided Co-Mo/AlzOs HDS catalyst, with the equilibrium constants for thiophene binding in model complexe~.~ Jones has proved that an S-bound species is an intermediate precursor to thiophene C-S bond cleavage just by looking a t the different selectivities of the reactions between a unique metal fragment and various alkyl-substituted thiophenes.6J0 Homogeneous reactions of substituted thiophenes with metal complexes have been investigated by a few other re~earchers.'J~-'~ Scheme 1 summarizes all the known reactions leading t o C-S bond scission. An inspection of Scheme 1 clearly shows that both steric and electronic factors can interfere with the observed regioselectivities of the metal insertion into C-S bonds of different thiophenes. However many fundamental questions about C-S cleavage of substi( 9 ) ( a )Benson, J. W.; Angelici, R. J. Organometallics 191)3,12, 680. (b) Benson, J . W.; Angelici, R. J. Organometallics 1992, 11, 922. (10)Jones, W. D.; Dong, L. J . A m . Chem. SOC.1991, 113, 559. (11)Jones, W. D.; Chin, R. M. J . A m . Chem. SOC.1992,114, 9851. (12) Buys, I. E.; Field, L. D.; Hambley, T. W.; McQueen, A. E. D. J . Chem. Soc., Chem. Commun. 1994,557. (13) Luo, S.; Skaugset,A. E.; Rauchfuss, T. B.; Wilson, S. R. J . A m . Chem. SOC.1992, 114, 1732. (14) Hachgenei, J . W.; Angelici, R. J . J . Organomet. Chem. 1988, 355, 359. (15) Ogilvy, A. E.; Drapanjac, M.; Rauchfuss, T. B.; Wilson, S. R. Organometallics 1988, 7 , 1171.
0276-733319512314-3196$09.0010 0 1995 American Chemical Society
C-S Bond Scission of Substituted Thiophenes at Rh
Organometallics, Vol. 14, No. 7, 1995 3197 Scheme 1
ref 6,lO
*
kJ \
's
L
\s
100 "C
Rh
ref 6.10
Ca.l:l
1' ref 14 ref 6,lO
ref 11
ref 12
L' ref 12
tuted thiophenes are still unanswered; for example, it is still not understood what the competitive selectivities of different thiophenes are and what the relative stabilities of the C-S insertion products as a function of the electronic and steric nature of the substituent(s) in the thiophene are. This paper describes our effort to address some of these questions through a comparative study of the reactions of various substituted thiophenes with the 16electron metal fragment [(triphos)RhHl (triphos = MeC(CH2PPh2)3).16
Experimental Section General Information. All reactions and manipulations were routinely performed under a nitrogen atmosphere by (16)( a )Bianchini, C.; Frediani, P.; Herrera, V.; Jimenez, M. V.; Meli, A.; Rincdn, L.; Sbnchez-Delgado, R. A.; Vizza, F. J.A m . Chem. SOC., in press. (b)Bianchini, C.; Meli, A,; Laschi, F.; Ramirez, J . A,; Zanello, P.; Vacca, A. Inorg. Chem. 1988,27,4429. (c)Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Albinati, A. Organometallics 1990, 9, 2283.
using standard Schlenk techniques. Tetrahydrofuran (THF) was distilled from LiAlH4 and n-heptane from sodium. The solvents were stored over molecular sieves and purged with nitrogen prior to use. 2-MeT, 2-EtT, 3-MeT, 2,5-MezT, 2-OMeT, 3-OMeT, 2-COMeT, 3-COMeT, and 2-COzEtT were purchased from Aldrich and used without further purification. All other chemicals were commercial products and were used as received without further purification. The literature method was used for the preparation of (triphos)RhHs (l1.l' All metal complexes were collected on sintered-glass frits and washed with appropriate solvents before being dried under a stream of nitrogen. Infrared spectra were recorded on a Perkin-Elmer 1600 Series FT-IR spectrophotometer using samples mulled in Nujol between KBr plates. Deuterated solvents for NMR measurements were dried over molecular sieves. 'H NMR spectra were obtained on a Bruker ACP 200 (200.13 MHz) spectrometer. 'H NMR shifts are recorded relative to residual 'H resonance in the deuterated solvent. l3C{lH}NMR spectra were recorded on the Bruker ACP 200 instrument operating (17) Ott, J.;Venanzi, L. M.; Ghilardi, C. A.; Midollini, S.; Orlandini, A. J . Organomet. Chem. 1986, 291, 89.
3198 Organometallics, Vol. 14, No. 7, 1995
Bianchini et al.
Table 1. 3lP(lH} NMR Spectral Data for the New Complexes" complex
pattern
2e
AMQX AMQXd AMQX AMQX AMQXd AMQX AMQXd AMQX AMQXd AMQX AMQX AMQXd AMQX
3 4 5 6
7 8 9
chem shift, ppmb d(A) d(M) cHQ) 31.4 0.2 -4.0 31.4 -0.3 -3.5 30.8 -1.7 -4.3 31.2 -1.4 -5.5 31.5 -1.6 -5.6 30.3 3.4 -4.3 3.7 -4.2 30.7 30.9 2.3 -4.3 0.5 -4.2 31.4 30.8 0.2 -5.4 31.2 2.1 -4.0 2.2 -3.9 31.8 30.3 0.6 -5.7
coupling const, Hz J(AM)
J(AQ)
J(MQ)
J(ARh)
J(MRh)
J(QRh)
34.6 33.3
29.1 30.0
39.4 44.3
108.3 108.4
119.4 118.3
106.4 107.8
34.2
29.3
42.7
108.6
118.4
107.4
33.9
30.9
27.9
106.2
123.6
104.0
33.6 36.4
31.1 31.4
28.1 44.5
106.6 111.1
122.5 119.8
104.6 107.5
35.4 35.4
32.0 28.5
34.9 41.5
108.7 107.4
120.3 120.0
104.6 105.6
All spectra were recorded at 20 "C in CDzClz solutions unless otherwise stated. The chemical shifts (d's) are relative to 85%H3P04; downfield values are assumed as positive. See ref 16a. At -10 "C.
at 50.32 MHz. The 13C{lH}NMR shifts are given relative to the solvent resonance. 31P{1H)NMR spectra were recorded on a Bruker ACP 200 spectrometer operating a t 81.01 MHz. Chemical shifts are relative to external 85% H3P04 with downfield values reported as positive. Broad band and selective lH131P)NMR experiments were carried out on the Bruker ACP 200 instrument equipped with a 5-mm inverse probe and a BFX-5 amplifier device. 13C DEFT, lH-13C 2D HETCOR, and IH-lH 2D COSY NMR experiments were conducted on the Bruker ACP 200 spectrometer. The computer simulation of NMR spectra was carried out with a locally developed package containing the programs L4OCN3l8 and Davinsl9 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.
General Procedure for the Preparation of (triphoslRh(q3-SCR=CRCH=CH2) ( R = H, R = Me (3),Et (41, COMe (5),COzEt (6);R = H, R = Me (7), COMe (8), OMe (9)).To a stirred suspension of (triphos)RhHs (1;0.50 g, 0.68 mmol) in THF (40 mL) was added a 10-fold excess of the appropriate thiophene, and then the mixture was heated at reflux temperature. Within a few minutes the solid dissolved. After ca. 5 h, the resulting orange-brown solution was concentrated to ca. 10 mL. Whereas 4 and 9 precipitated during the concentration process, the precipitation of the other compounds was accomplished by portionwise addition of n-heptane (20 mL). All the compounds, obtained as orange microcrystals, were collected by filtration and washed with n-pentane; yield 7 0 4 5 % . In independent isothermal reactions, carried out in THF-ds in the temperature range from 60 t o 120 "C, only the isomer formed by 1-5 insertion was observed by 31P{'H} NMR spectroscopy. Characterizationof 3-9. Selected NMR spectral data for the metal complexes are collected in Table 1 PP{lH} NMR) and Table 2 PH, l3C{lH) NMR). 13C DEFT, 13C-lH 2D HETCOR, and 'H-IH 2D COSY spectra allowed the total and unequivocal assignment of all hydrogen and carbon resonances for all metal complexes as labeled in Tables 1 and 2. (triphos)Rh(q3-SC(Me)=CHCH=CH2) (3). Anal. Calcd (found) for C.dbP3RhS: c, 66.83 (66.65); H, 5.61 (5.58);Rh, 12.45 (12.22);S, 3.88 (3.59). IR: v(C=C) 1568 (m) cm-l. (triphos)Rh(q3-SC(Et)=CHCH=CH2) (4). Anal. Calcd (found) for C47H48PaRhS: C, 67.14 (67.00); H, 5.75 (5.78);Rh, 12.24 (12.00); S, 3.81 (3.61). IR: v(C-C) 1565 (m) cm-l. (triphos)Rh(qS-SC(COMe)=CHCH==CHz) (5). Anal. Calcd (found)for C47H460P3RhS: C, 66.04 (65.83);H, 5.42 (5.31); Rh, 12.04 (12.00); S, 3.75 (3.57). IR: v(C-0) 1650 (s), v(C=C) 1536 (m) cm-l. ~~~~~
~
(18)(a)Bothner-By, A. A.; Castellano, S. QCPE 1967, 11, 111. (b) Castenallo, S.; Bothner-By, A. A. J. Chem. Phys. 1964, 41, 3863. (19)Stephenson, D. S.; Binsch, G. J . Mugn. Reson. 1980, 37, 409.
(triphos)Rh(q3-SC(COSt)=CHCH=CH~) (6). Anal. Calcd (found) for C48H4802P3RhS: C, 65.16 (64.88); H, 5.47 (5.39); Rh, 11.63 (11.39);S, 3.62 (3.52). IR: v(C=O) 1690 (s), v(C==C) 1561 (m) cm-'. (triphos)Rh(q3-SCH=C(Me)CH=CHz) (7). Anal. Calcd (found)for C46H46P3RhS: c, 66.83 (66.55); H, 5.61 (5.54);Rh, 12.45 (12.26);S, 3.88 (3.64). IR: v(C-C) 1570 (m) cm-l. (triphos)Rh(q3-SCH=C(COMe)CH=CH2) (8). Anal. Calcd (found)for C47H460P3RhS: C, 66.04 (65.74); H, 5.42 (5.32); Rh, 12.04 (11.90);S, 3.75 (3.61). IR: v(C=O) 1611 (s), v(C=C) 1510 (m) cm-'. (triphos)Rh(q3-SCH4(OMe)CH=CH2) (9). Anal. Calcd (found) for C46H460P3RhS: c, 65.56 (65.53); H, 5.50 (5.43);Rh, 12.21 (12.08); S, 3.80 (3.67). IR: v(C=C) 1550 (m), v(C-0) 1136 (s) cm-'. Reaction of 1 with 2,5-Me~T.Following a procedure analogous to that reported above, the reaction between 1 and 2,5-Me~Tled to the formation of the known dimeric complex (triphos)RhH(p-H)2HRh(triphos). 16b Reaction of 1 with 2-OMeT. Under the reaction conditions reported above, the reaction between 1 and 2-OMeT led to a mixture of several unidentified compounds.
General Procedure for the Intermolecular Competition Reaction of 1 with T w o Different Thiophenes. A 5-mm NMR tube was charged under nitrogen with a solution of 1 (20 mg, 0.027 mmol) and a 10-fold excess of a 1:l mixture of two different thiophenes in THF-ds (0.7 mL), flame-sealed, and kept at 70 "C (oil bath). After 5 h, the tube was cooled to room temperature and the product composition was determined by 31P{1H}NMR spectroscopy. The results obtained are reported in Table 3. The product ratios reported in the table are invariant with time up t o 100 "C, which was the highest temperature investigated.
Results Thermolysis of the Trihydride (triphos)R,h& in the Presence of Substituted Thiophenes. The complex (triphos)RhHs (1) has previously been shown to behave as a thermal precursor for the generation of the 16-electron fragment [(triphos)RhH1,l6 which is active toward the oxidative addition of C-S bonds from T or benzo[blthiophene (BT).16" Thermolysis of 1 in THF occurs already a t 60 "C and produces H2. In the absence of substrates able to trap the unsaturated Rh(1)fragment, the latter dimerizes to the known complex (triphos)RhH(p-H)zHRh(triphos).16b This reaction path is observed for the thermal reaction of 1 with 2,5-Me2T (Scheme 2). With the exception of 2-OMeT, all the other substituted thiophenes shown in Chart 1react with [(triphos)RhHl t o give orange C-S insertion products of the
C-S Bond Scission of Substituted Thiophenes at Rh Table 2. 'H and W{'H}
Organometallics, VoE. 14, No. 7, 1995 3199
NMR Spectral Data for the New Complexes"
'H NMR complex
assignt HI
13C{lH} NMR
d (multiplicity, J)bc 5.98 (m. WHdRh) = 1.0. WHdHd = 4.5) 5.87 (m; V(H;Rh) = 2.0; 3J(H;H;) = 6.1) 3.26 (m, 2J(H3Rh)= 1.1, 3J(H3Hz)= 7.5) 2.86 (m, 2J(Hz.Rh)= 2.0, V(HZH3) = 9.3) 1.63 (m, 2J(H2Rh)= 1.7, 2J(Hz.H2)= 0.5) 5.86 (m, WHdRh) = 1.2, 3J(H4H3)= 5.3) 3.11 (m, 2J(Hz.Rh)= 2.1, W H Z H ~=) 9.1) 3.00 (m, 2J(H3Rh)= 1.2, 3J(H3H2)= 6.8) 2.00 (br s, 4J(H6H4)= 1.1) 1.75 (m, 2J(H2Rh)= 1.3, 2J(H2,H2)= 0.5)
assignt
5.87 (m, V(H4Rh) = 1.0, V(H4H3) = 4.2) 3.10 (m, 2J(Hz.Rh)= 2.1, WHyH3) = 9.2) 3.00 (m, 2J(H3Rh)= 1.5, WH3H2) = 7.5) 2.3h 1.61 (m, WHzRh) = 1.0) 1.16 (t) 7.4' 3.13 (m, 2J(H3Rh)= 1.4, WH3H4) = 5.2) 3.05 (m, 2J(HrRh)= 1.9, %J(H2H3) = 9.2) 2.38 (s) 1.74 (m, 2J(H2Rh)= 1.4), V(H3Hz) = 7.6)
c3 c2
c3
cz c4 c5
c3 c2
CS c4
c5
63.1 (br d, 2J(CP)= 32.8) 41.2 (br d, 2J(CP)= 36.2) 24.0 (s) e e
CS CI c4
e
c5
e
c6
198.2 (s) 59.5 (m) 40.0 (m) 27.8 (s)
c2
c7 c4
c5
CS c7
c3 c2
CS c4
c5
5.51 (m, 3J(H5Rh)= 0.43, 4J(H5H3)= 1.13) 3.01 (m, 2J(H2,Rh)= 2.04, 3J(H2,H3)= 9.11) 2.96 (m, 2J(H3Rh)= 0.90, 3J(H3H2)= 8.31) 1.94 (d, 4J(H6H5)= 1.24) 1.68 (m, 2J(H2Rh)= -1.28, 2J(HzHz.)= -1.81) 7.7' (4J(H5H3)= 1.3) 4.11 (m, 2J(H3Rh)= 1.3, 3J(H3Hz)= 7.7) 2.62 (m, 2J(Hz.Rh)= 1.9, %T(HrH3)= 9.6) 2.28 (s) 1.58 (m, 2J(HzRh)= 1.6, 2J(H2H2)= 0.5)
e e
64.9 (m) 42.4 (dd, 2J(CP)= 24.4, 10.1) 31.5 (9) 16.6 (s)
c3
7.7' PJ(H4H3) = 4.6) 4.18 (q, V(H7Hs) = 7.2) 3.12 (m, 2J(H3Rh)= 1.5, V(H3H2) = 7.1) 3.02 (m, 2J(HrRh)= 1.8, 3J(Hz.H3)= 9.4) 1.70 (m, 2J(H2Rh)= 1.1, 2J(H~Hz) = 0.5) 1.31 (t)
b (multiplicity, J)'J 65.7 (dt. 2J(CP)= 33.8. 9.2. WCRh) = 9.2) 40.8 (br'd, 2J(CP)= 32:O)
c3
c2
CS c4
c5
e e
168.5 (s) 60.7 (s) 59.6 (dd, 2J(CP)= 20.7,9.4) 40.0 (dd, 2J(CP)= 18.5, 11.8) 15.1(s) e e
68.6 (dd, 2J(CP)= 36.0, 15.8) 41.8 (br d, 2J(CP)= 34.8) 24.1 (s) e e
c4
190.1 (9) 60.8 (dd, 2J(CP)= 19.8, 10.7) 41.1 (br d, 2J(CP)= 18.5) 26.8 (5) e
c5
e
CS c3 c2
c7
4.86 (br d, %KHbRh)= 1.0, 4J(H5H3)= 1.2) 3.62 ( 8 ) 3.04 (m, 2J(H2,Rh)= 2.1, V(HrH3) = 8.9) 2.87 (m, 2J(H3Rh)= 1.4, V(H3Hz) = 7.3) 1.63 (m, 2J(H2Rh)= 1.2, 2J(H2H2)= 0.5) a All spectra were recorded at room temperature in CD2C12 solutions at 200.13 ('H NMR) and 50.32 MHz (13C{'H}NMR) unless otherwise stated. Chemical shifts are given in ppm and are relative to either residual lH resonance in the deuterated solvent ('H NMR) or the deuterated solvent resonance (l3C(IH}NMR). Key: s, singlet; d, doublet; t, triplet; q, quartet; qt, quintet; m, multiplet; br, broad. Coupling constants (J)are in hertz. The J(HH) values were determined on the basis of lH131P}NMR experiments. See ref 16a. e Masked by the phenyl carbons of the triphos ligands. f The 13C{IH}NMR spectrum was recorded in DMFd7. The 13C{IH}NMR spectrum was recorded in benzene-d6. * The two He protons are masked by the aliphatic chain protons of the triphos ligand. In benzene-&, however, they clearly appear as a second-order multiplet that was computed as the AB part of an AB& spin system with the following magnetic parameters: 2J(HgH6) = 14.4 Hz, V(HgH7) = 3J(H6H7) = 7.4 Hz. Masked by the aromatic protons of the triphos ligand. The chemical shift was determined from a 'H-IH 2D COSY experiment. The 1H{31P}NMR resonances of Hz and H3 were computed as the AB part of an ABCDEaF spin system with F = Rh. The 13C{lH} NMR spectrum was recorded in THF-ds. Due to the instability of 9 in CD2C12 and its low solubility in all the other organic solvents, no 13C{IH}NMR spectrum could be recorded. J
general formula (triphos)Rh(q3-SCR=CRCH=CHd( R = H, R = Me (31,Et (41, COMe (51, COzEt (6); R = H, R = Me (71, COMe (81, OMe (9))(Scheme 3). Irrespective of the position and electronic character of the substituent in the thiophene, insertion is seen exclusively into the C-S bond away from the substituent. In fact, though the butadienethiolate substituents perturb the local magnetic field of the neighboring nuclei
differently (vide infra),the spectroscopic data of the q3(S,C,C)-butadienethiolate complexes 3-9 are in excellent correlation with those of (triphos)Rh(q3-SCH=CHC H = C H Z )(2) ~ ~and ~ (tri~hos)1r(q~-SCH=CHCH=CHz)~ (10)obtained by reaction of [(triphos)RhH] and [(triphos)IrHl with T,respectively. Thus, like 2 and 10, complexes 3-9 are assigned octahedral structures in which the rhodium center is coordinated by the three
3200 Organometallics, Vol. 14, No. 7, 1995
Bianchini et al.
Scheme 2
THF, 67OC
H 1
Table 3. Thermolysis of 1 in the Presence of a 1:l Mixture of Different Thiophenes mixture
products (ratio) 3/7(60:40) 9/3(80:20) 5 518 (80:20) 8
2/3(50:50) 8 8 917 (9O:lO) 615 (60:40)
phosphorus atoms of a fuc triphos and by a 2- or 3-substituted butadienethiolate ligand which uses the sulfur atom and the distal olefinic end to bind the metal. As previously found for 2 and 10,3J6athe olefinic moieties of the butadienethiolate ligands form quite robust bonds to the metal center due to a significant dn (metal) n* (ligand) back-bonding. The metallacyclopropane character of the Rh-C2-C3 ring appears t o be more pronounced for the complexes with thio ligands bearing electron-donatingsubstituents (2&c2,3P) = 24-36 Hz)than for the complexes obtained from thiophenes bearing electron-withdrawing substituents (2J(c2,3P) = 18-20 Hz).This effect may be explained
-
by considering that the substituent is closer to the sulfur atom than to the bound double bond. Accordingly, electron-withdrawingsubstituents decrease the electron density at the sulfur atom, thus ultimately decreasing the basicity of the rhodium center. The nature of the substituent also affects the chemical shifts of the neighboring carbon and hydrogen nuclei, particularly of H4 and H5, which, as compared to the analogous resonances in the lH NMR spectrum of the unsubstituted complex 2, move either upfield or downfield depending on the electron-donating/electronwithdrawing character of the substituent (see Table 2). No intermediate Rh species was observed to traverse the conversion of 1 to 3-9 when the progress of the reactions was followed by 31P{1H)NMR spectroscopy at a constant temperature of 60 "C. Compounds 3-9 are stable both in the solid state, even for a short exposure to air, and in solution with non-halogenated solvents. In halogenated solvents (CH2C12, CHC131, the complexes containing electrondonating substituents (Me, Et, OMe) on the butadienethiolate ligand decompose at a rate that increases as the electron-donating character of the substituent in the thio ligand increases. Among the various thiophenes investigated, 2-MeOT is the only one to react with [(triphos)RhHl without yielding a butadienethiolate complex; extensive decomposition occurs with formation of various, undefined rhodium compounds. Thermolysis of the Trihydride 1 in the Presence of Mixtures of Substituted Thiophenes. The competitive reactivity of substituted thiophenes toward the fragment [(triphos)RhHl has been followed in THF-de in sealed NMR tubes charged with 1:l mixtures of the following thiophene couples: T/2-MeT, T/3-COMeT9 2-MeT/3-MeT,2-MeT/3-OMeT,3-MeT/3-OMeT,3-COM-
Chart 1
0 S T
a. d S
-4-L S
S
2-Me1
3-Me1
2-EtT
QOMe 2-OMeT
2 3-Me2T
QCO*Et 3-OMeT
2-COMeT
3-COMeT
2-COzEtT
Scheme 3
I
R = H, 2; Me, 3; Et, 4; COMe, 5; C02Et, 6
R = Me, 7; COMe, 8; OMe, 9
C-S Bond Scission of Substituted Thiophenes at Rh
Organometallics, Vol. 14, No. 7, 1995 3201 Scheme 4 kinetic
thermodynamic
67 O C
eT,V-Vl.AdT,2-MeT/2-COMeT,3-MeT/3-COMeT,2-COMeT/3-COMeT, and 2-COzEtT/2-COMeT. At the complete disappearance of the starting trihydride, the product distribution of each run was determined by 31P{1H} NMR integration. The results obtained, reported in Table 3, give the following trend of competitive reactivity: 2-COzEtT > 2-COMeT > 3-COMeT >> 3-OMeT > T = 2-MeT > 3-MeT. The product ratios reported in Table 3 are invariant with time up to 100 "C, which is the highest temperature investigated. Consistently, the C-S insertion reactions are irreversible in the same temperature range. As an example, the butadienethiolate complex 2 is stable when refluxed in THF in the presence of 2-COzEtT. In conclusion, the observed product ratios represent kinetic distributions, which was confirmed by a preliminary study of the competitive reactions of the [(triphos)RhHlfragment, photochemically generated at 20 "C, with various substituted thiophenes.20 Discussion Opening of Substituted Thiophenes by [(triphos)RhHl. A possible mechanism for the formation of the C-S insertion products 3-9 can be proposed in light of previous studies on the opening of T, BT, or dibenzothiophene (DBT) by the [(triphos)RhHPa and [(triphos)IrHl fragment^.^^^^^^^^ Valuable mechanistic information was obtained with the use of the more kinetically inert iridium system. The fragment [(triphos)IrHl inserts into a C-S bond of BT (20)A 5-mm NMR quartz tube, charged with a solution of 1 (20 mg, 0.027 mmol) and a 10-fold excess of a 1:l mixture of two different thiophenes in THF-ds (0.7 mL), was irradiated with UV light at 20 "C. The photolysis source was a 135-W (principal emission wavelength 366 nm) high-pressure mercury vapor immersion lamp equipped with a water filter to remove excess heat. After 4 h, the product composition was determined by 31P{lH} NMR spectroscopy. The selectivity trend of the competitive reactions was analogous to that observed in the thermal reactions at 70 "C, whereas a different product distribution, still invariant with time and temperature, wa8 observed in some cases. (21)(a) Bianchini, C.; Jimenez, M. V.; Meli, A,; Moneti, S.;Vizza, F. J . Organomet. Chem., in press. (b)Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Moneti, S.;Herrera, V.; Sanchez-Delgado, R. A. J . Am. Chem. Soc. 1994, 116, 4370. (22)Bianchini, C.; Jimenez, M. V.; Meli, A,; Moneti, S.;Vizza, F.; Herrera, V.; Sanchez-Delgado, R. A. Organometallics, in press.
or DBT to give the complexes (triphos;--(H)(? S,C)SC,HJ (x = 8, y = 6; x = 12, y = 8) as kinetic products.21~22The T analog (triphos)Ir(H)(t12(S,C)SC4H4) was prepared by hydride addition to the iridathiabenzene complex [(triphos)Ir(llz(S,C)-SC4H4)1BPh4 (Scheme 4L3 From these T and BT hydrido iridathiacycles, the formation of the thermodynamically more stable butadienethiolate and 2-vinylthiophenolate products proceeds as a thermal step upon hydride migration to the a-carbon of the vinyl moiety of the metalated C4H4S and CsH6S systems (Scheme 4). As previously discussed for the formation of 2 and the lack of detectable intermediates in the course of the transformation of 1 into 3-9 can be ascribed to the higher energy of activation required t o promote the reductive elimination of Hz from 1, as compared to the energies necessary t o accomplish both the C-S bond scission and the migration of the terminal hydride to the a-carbon of the metallathiacycle. In this paper, we have shown that variation of the substituents in the thiophene gives rise to insertion selectivities which are exclusively subject to steric control. Indeed, both electron-donating (i.e. methoxy) or electron-withdrawing (i.e. carboalkoxy)groups in the thiophene do not alter the direction of insertion, which invariably occurs away from the substituent. Such a rigid control of the regioselectivity of the C-S bond scission has never been observed and is most likely due to the steric repulsions between the bulky PPhz groups in the triphos ligand and the substituent in either the 2- or 3-position. In fact, unsaturated metal fragments with minor steric demand do not exhibit the same selectivity of insertion: the 16-electronfragments [(C&les)Rh(PMe3)I6Joand [Fe(drn~e)21~~ react with 3-MeT to give isomeric mixtures of the 1,2- and 1,5-insertion products (Scheme 1). Within this context, the lack of reactivity of 2,5-MezT with [(triphos)RhHlcan reasonably be ascribed to steric repulsion between the six phenyl rings of triphos and the methyl groups in the thiophene. Insertion into the more hindered side of the thiophene has been observed uniquely in the reaction of [(CsMedRh(PMe3)l with 2-OMeT and explained in terms of a
Bianchini et al.
3202 Organometallics, Vol. 14, No. 7, 1995
showing that there is preferential C-S cleavage of the stabilizing contribution of a delocalized resonance form unsubstituted thiophene (2:l product ratio).6J0 (methoxycarbene moiety).ll This remarkable electronic control on the insertion, driven by the methoxy subThe order of competitive reactivity of the present substituted thiophenes toward [(triphos)RhHlis as folstituent in the 2-position, is likely responsible for the unsuccessful reaction of 1 with 2-OMeT. The steric lows: 2-COzEtT > 2-COMeT > 3-COMeT >> 3-OMeT hindrance of the metal fragment in favor of 1,5-insertion > T z 2-MeT > 3-MeT. From this trend, one may is an antithesis to the electronic preference of the readily conclude that (i) the insertion of rhodium into substrate for 1,2-insertion. the C-S bond is facilitated by thiophene substituents Although electronic effects are not the determining exhibiting electron-withdrawing ability; (ii) for identical factors at work in the selectivity of the C-S insertions substituents, the C-S opening is easier for 2-substituted described in this paper, the electronic properties of the thiophenes than for 3-substituted thiophenes. substituent in the thiophene remarkably affect the A qualitative explanation of the role exerted by the chemistry of the butadienethiolate products. electronic nature of the substituents on the competitive In CH2Cl2 or CHCl3, the complexes containing electronreactions is provided by assuming that the mechanism withdrawing substituents (COMe, C02Et) are fairly reported by Jones for the insertion that the mechanism stable, whereas those containing electron-donating subreported by Jones for the insertion [(CsMes)Rh(PMes)] stituents rapidly decompose. Although the nature of the into a thiophene C-S bond is operative also for [(tridecomposition products has not been studied, we strongly phos)RhHl (this assumption is acceptable, as the two suspect that compounds 3 , 4 , 7, and 9 react by nucleometal systems are isoelectronic and isolobal and generphilic attack of the thiolate sulfur atom at the electroally exhibit the same chemistry). In Jones’s mechanism, philic carbon atom of the chlorinated solvent. C-S insertion proceeds by S-coordination of the thioThe remarkable nucleophilic character of the unsubphene, followed by attack by the electron-rich Rh(1) stituted derivatives 2 and 10 has recently been demmetal on the adjacent carbon atom (via donation into onstrated experimentally and t h e o r e t i ~ a l l y . ~ActuJ~~ the C-S antibonding orbital). In this mechanistic ally, each complex molecule contains two nucleophilic picture, electron-donating substituents in the thiophene sites: the sulfur atom and the C2 carbon atom of the are expected to destabilize the transition state,6s26and distal olefinic moiety.16a Alkyl halides such as Me1 as a consequence, insertion would be disfavored, conselectively react with the sulfur atom t o give S-methsistent with what we observe experimentally. The ylated adducts of the formula [(triphos)M(y3-(Me)anomalous position of 3-OMeT in the order of competiSCH=CHCH=CH2)]BPb (M = Rh,16a11-9.An increase tive reactivity is ascribed t o the capability of the OMe of the nucleophilic character of the butadienethiolate substituent t o stabilize the insertion product via a ligand, produced by replacing a hydrogen with alkyl or delocalized methoxycarbene resonance form.” methoxy groups, would make the sulfur atom sufIn conclusion, the electronic control exerted by the ficiently nucleophilic to attack the carbon atom of CH2thiophene substituent on the transition state for the C12.23 As a matter of fact, the methylated adducts C-S bond cleavage brought about by the [(triphos)RhH] [(triphos)Rh(y3-(Me)SCR=CRCH=CH2)IBPh4 ( R = H, fragment seems to be more determinant than the R = Me, Et; R = H, R = Me, OMe), independently electronic influence on the nucleophilicity of the thioprepared by treatment of 3,4,7, or 9 with MeI, are fully phene. In fact, it is well-known that electron-donating stable in chlorinated solvents.24 substituents in the thiophene strengthen the donor Our observation that electron-donating substituents ability of the s u l f ~ rthus , ~ favoring the formation of the in the starting thiophene increase the basicity, and thus rl(S) intermediate that precedes the C-S insertion the reactivity of the butadienethiolate products, nicely reaction. fits with previous studies in which the relative HDS Apparently, our conclusions are in contrast with those reactivity of alkyl-substituted thiophenes on Co-Mo/ of previous heterogeneous studies, according to which A1203 catalysts increases in the order T < 2-MeT < the greater the number of alkyl substituents in the 3-MeT < 2,5-MezT. Indeed, as widely s u g g e ~ t e d ,if~ ~ , ~thiophene, ~ the easier the adsorption on the catalyst heterolytic splitting of H2 is operative in hydrotreating surface and the higher the HDS r e a ~ t i v i t y . ~This ~-~~ catalysis, the major reactivity of alkyl-substituted contrast may simply mean that our HDS model is poor thiophenes may be explained in terms of a more facile indeed. On the other hand, it has also been suggested attack by H+ at either the sulfur or the C2 carbon that the assumption of vl(S) binding of thiophene, made atom3J6a,21a of the C-S insertion product once the latter in the adsorption studies, may be incorrect and that y5 has been reduced to butadienethiolate by H-. coordination of thiophene may be a better form of Competitive Reactivity of Substituted Thioadsorption on the HDS catalysts i n v e ~ t i g a t e d . ~ ~ ~ ~ - ~ ~ phenes with [(triphos)RhHl. Studies which compare C-S bond scission reactions between a unique metal Acknowledgment. We thank the Progetto Stratesystem and thiophenes substituted at different positions g i c ~“Tecnologie Chimiche Innovative”, CNR, Rome, by electronically different groups are limited indeed. Italy, for financial support. A postdoctoral grant to Jones has examined the thermal reaction of [(CsMedM.V.J. from the Ministerio de Educatih y Ciencia of Rh(PMed1 with a 1:l mixture of T and 2,5-Me2T, Spain is gratefully acknowledged. (23)Merola, J.S.;Grieb, A.; Ladipo, F. T.; Selnau, H. E. Symposium on Mechanism of HDSIHDNReactions, 206th National Meeting of the American Chemical Society, Chicago, IL, August 22-27,1993;Division of Petroleum Chemistry, Inc., American Chemical Society: Washington, DC, 1993;p 674. (24)Bianchini, C.; Meli, A., to be submitted for publication. ( 2 5 ) ( a )Anderson, A. B.; Al-Saigh, Z. Y.; Hall, W. K. J . Phys. Chem. 1988,92, 803. (b) Lacroix, M.; Yuan, S.; Breysse, M.; DoremieuxMorin, C.; Fraissard, J. J . Catal. 1992,138,409.( c )Neurock, M.; van Santen, R. A. J . Am. Chem. SOC.1994,116,4427.
OM950054V (26)Harris, S.:Chianelli, R. R. J . Catal. 1984,86,400. (27)( a )Zdrazil, M.Collect. Czech. Chem. Commun. 1977,42,1484. ( b ) Zdrazil, M. Catal. Today 1988,3,269. (28)Zdrazil, M.Collect. Czech. Chem. Commun. 1975,40,3491. (29)Desikan, P.:Amberg, C. H. Can. J . Chem. 1963,41, 1966. (30)Angelici, R.J. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley: New York, 1994:Vol. 3, p 1433.
