ARTICLE pubs.acs.org/Organometallics
Predicting Selectivity in Oxidative Addition of C S Bonds of Substituted Thiophenes to a Platinum(0) Fragment: An Experimental and Theoretical Study T€ulay A. Ates-in, Sabuj Kundu, Karlyn Skugrud, Katherine A. Lai, Brett D. Swartz, Ting Li, William W. Brennessel, and William D. Jones* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
bS Supporting Information ABSTRACT: Exchange reactions of 2- and 3-cyanothiophene, 2- and 3-methylthiophene, and 2- and 3-methoxythiophene, either with thiophene in the thiaplatinacycle Pt(dippe)(k2-C,S-C4H4S) or with norbornene in Pt(dippe)(nor)2, were performed to probe the kinetic and thermodynamic selectivity of the C S bond activation reactions. Kinetic data were collected by following these reactions by 31P{1H} NMR spectroscopy. The ground-state energies of the two possible products and the transition-state energies leading to the formation of these products were calculated using density functional theory. The comparison of the predicted selectivities from calculations with the experimentally observed selectivities showed good agreement for thermodynamic selectivity, but only moderate agreement for kinetic selectivity. The reactions with 2-cyanothiophene, 3-cyanothiophene, and 3-methoxythiophene gave kinetic products that were less favored thermodynamically. All of the other substituted thiophenes gave kinetic products that were also preferred thermodynamically. These results indicate that the selectivities seen in the C S bond activation reactions of substituted thiophenes with the [Pt(dippe)] fragment are initially under kinetic control.
’ INTRODUCTION The insertion of metal atoms into the C S bonds of thiophene has been extensively studied with homogeneous model compounds1 in order to better understand the reaction mechanism by which these bonds may break during the heterogeneous HDS process.2 Most of the studies reported to date use thiophene, benzothiophene, or dibenzothiophene substrates, although a few examples report on methyl-substituted derivatives. Steric effects appear to dominate C S bond activation selectivities. The effects of other heteroatom-containing functional groups on thiophene C S bond activation remained largely unexplored, until recently. Our group has examined the selectivity of C S bond activation of substituted thiophenes by reacting the coordinatively unsaturated [(C5Me5)Rh(PMe3)] fragment, which is both an experimentally3 and theoretically4,5 well-studied homogeneous model system, with asymmetrically substituted thiophenes possessing a variety of substituents that modify electronic and steric properties (Scheme 1).3,6 The reactions of 2-cyano and 2-methoxythiophene with the [(C5Me5)Rh(PMe3)] fragment gave only one product, resulting from the insertion of the rhodium atom into the more hindered substituted C S bond, whereas the reaction of 2-methylthiophene gave the product resulting from the activation of the unsubstituted C S bond. These results are consistent with the calculated energy differences between the ground states of the two possible products (7.6, 2.6, and 5.8 kcal mol 1, respectively). The experimentally observed ∼1:1 ratio of r 2011 American Chemical Society
products with 3-cyano- and 3-methyl-substituted thiophenes is also consistent with the small differences in the calculated ground-state energies of the two insertion products (0.4 and 0.8 kcal mol 1). The initially observed insertion product ratios did not change over time with heating. For all of the thiophene derivatives, the experimentally observed selectivities were found to be different from those predicted on the basis of the calculated activation barriers for C S bond cleavage, but consistent with those predicted from the calculated ground-state energies. These results suggest that the C S bond cleavage reactions with the [(C5Me5)Rh(PMe3)] fragment are under thermodynamic control; that is, the kinetic products were apparently too short-lived to permit the kinetic product distribution to be observed experimentally. This is a common problem in establishing thermodynamic versus kinetic selectivity, as only when the ratio of products changes over time can the observed selectivities be undoubtedly assigned to kinetic and themodynamic selectivity. The reaction of 2-cyanothiophene with the [Pt(dippe)] fragment has also been examined starting either with Pt(dippe)(k2C,S-C4H4S) (1) and generating the [Pt(dippe)] fragment in situ by reductive elimination of thiophene at elevated temperature or with [Pt(dippe)]2(cod) via dissociation of cyclooctadiene.7 The kinetic product formed from the cleavage of the unsubstituted C S bond. Further heating resulted in its conversion to the Received: April 26, 2011 Published: August 15, 2011 4578
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Organometallics Scheme 1. Summary of Selectivities in Thiophene C S Bond Activation by the [(C5Me5)Rh(PMe3)] Fragmenta
a
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Scheme 2. Reactions of the [Pt(dippe)] Fragment with Substituted Thiophenes
From ref 5.
thermodynamically preferred product formed from the cleavage of the substituted C S bond (eq 1). Density functional theory (DFT) calculations gave a solvation-corrected energy difference of 5.3 kcal mol 1 between the transition states leading to the formation of products (favoring C S bond cleavage away from the cyano substituent). There is also a 6.8 kcal mol 1 energy difference between their ground-state energies (favoring cleavage adjacent to the cyano substituent), matching well with the observed experimental results. These results indicate that the C S bond activation reactions are ultimately under thermodynamic control if the product mixture is subjected to high enough temperatures over the course of several weeks. Unlike the C S bond activation reactions with the [(C5Me5)Rh(PMe3)] fragment, the reactions with the [Pt(dippe)] fragment give an observable initial kinetic product ratio that is different from the thermodynamic product ratio.
In this report, the reactions of 2- and 3-cyanothiophene, 2- and 3-methylthiophene, and 2- and 3-methoxythiophene with the [Pt(dippe)] fragment are examined to complete the comparison with our earlier studies using the [(C5Me5)Rh(PMe3)] fragment. DFT calculations using the [Pt(dmpe)] fragment as a model for the [Pt(dippe)] fragment are also reported. The product distributions observed from these reactions are compared with the transition-state energies of all the possible bond activation reactions. The thermodynamic aspect of the C S bond activation reactions is also investigated by comparing the equilibrium ratios and the ground-state energies of the two possible insertion products.
’ RESULTS AND DISCUSSION Reaction with 3-Cyanothiophene. As mentioned above, the reaction of Pt(dippe)(k2-C,S-C4H4S) with 2-cyanothiophene at
160 °C leads to the formation of the kinetic product resulting from insertion into the least hindered C S bond, which then rearranges to the thermodynamic product resulting from insertion into the more hindered C S bond. Simulation of the kinetic data gave an initial kinetic selectivity of 5:1 for the formation of 1B over 1A, and the observed thermodynamic product ratio was 1:49 (Scheme 2). These observations, i.e., the change in the initial product ratio to a final product ratio, allowed us to establish kinetic product preferences versus thermodynamic product preferences. For comparison, the reaction of 3-cyanothiophene with the [Pt(dippe)] fragment was examined using Pt(dippe)(nor)2 (nor = norbornene) as a source of Pt0. Upon heating a C6D6 solution of Pt(dippe)(nor)2 with 20 equivalents of 3-cyanothiophene at 100 °C for 6 days, the appearance of two C S cleavage products, 2A and 2B, was observed in a 1:1.3 ratio. The isomers showed distinct Pt P couplings in the 31P{1H} NMR spectrum, with 2A showing two doublets for the inequivalent phosphorus nuclei, at δ 63.55 (JP P = 5.0 Hz, JPt P = 1809 Hz) and 62.91 (JP P = 5.0 Hz, JPt P = 2867 Hz), and 2B showing two doublets, at δ 62.73 (JP P = 4.4 Hz, JPt P = 1713 Hz) and 63.23 (JP P = 4.4 Hz, JPt P = 3032 Hz). Upon heating a second sample at 160 °C in p-xylene-d10, the ratio of 2A:2B reached an equilibrium ratio of 1.7:1, corresponding to a ΔG433 of 0.5 kcal mol 1 for the 2A h 2B isomerization. The structure of one of the products was determined by single-crystal X-ray diffraction, as shown in Figure 1. The same crystal was dissolved in benzene, and its 31 1 P{ H} NMR spectrum showed coupling constants identifying the isomer as 2B, allowing this product to be unambiguously identified. A crystal of 2A was also obtained from the mother liquor, and its structure was also determined. Reaction with 2-Methylthiophene. The C S bond activation of 2-methylthiophene was examined by treating a solution of (dippe)Pt(nor)2 in C6D6 with 50 equivalents of 2-methylthiophene and heating at 100 °C for 54 days (Scheme 2). The reaction was 4579
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Figure 1. Thermal ellipsoid plots of (a) 2A and (b) 2B resulting from the insertion of the [Pt(dippe)] fragment into the C S bond of 3-cyanothiophene. Ellipsoids are shown at the 50% probability level.
Figure 2. Thermal ellipsoid plot of 3B resulting from the insertion of the [Pt(dippe)] fragment into the C S bond of 2-methylthiophene. Ellipsoids are shown at the 50% probability level. Selected bond lengths (Å): Pt S(1), 2.282(1); C(5) Pt, 2.080 (4); S(1) C(2), 1.730(5). Selected angles (deg): P(1) Pt S(1), 172.36(4); P(2) Pt C(5), 175.06(11).
Figure 3. X-ray single-crystal structure of 4B resulting from the C S bond activation of 3-methylthiophene with the [Pt(dippe)] fragment. The structure is disordered (54:46) with interchange of the locations of S1 and C4, but only 4B is present. Selected bond lengths (Å): Pt S(1), 2.307(3); C(4) Pt, 2.023(16); S(1) C(1), 1.719(15); C(2) C(5), 1.517(12). Selected angles (deg): P(2) Pt S(1), 172.80(17); P(1) Pt C(4), 175.4(9).
monitored by using 31P{1H} NMR spectroscopy. The doublets at δ 63.13 (JP P = 4.7 Hz, JPt P = 2990 Hz) and 62.41 (JP P = 4.7 Hz, JPt P = 1668 Hz) for the dominant product (94%) were assigned to the isomer 3B. A second reaction mixture heated in p-xylene-d10 at 160 °C for 24 days resulted in the formation of 3B as the major product and also 3A as a minor product, which has a doublet at δ 69.38 (JP P = 4.1 Hz, JPt P = 1701 Hz) and 63.58 (JP P = 4.1 Hz, JPt P = 2986 Hz). At the end of the reaction, the ratio of 3A:3B was 1:11. The structure of 3B was confirmed by single-crystal X-ray diffraction (Figure 2). Reaction with 3-Methylthiophene. A solution of (dippe)Pt(nor)2 in C6D6 was treated with 50 equivalents of 3-methylthiophene and heated at 100 °C for 64 days (Scheme 2). The reaction was monitored by using 31P{1H} NMR spectroscopy. The major product, 4B, has two doublets at δ 63.09 (JP P = 5.2 Hz, JPt P = 1677 Hz) and 61.67 (JP P = 4.7 Hz, JPt P = 2990 Hz), and the minor product, 4A, has two doublets at δ 63.59 (JP P = 4.9 Hz, JPt P = 1698 Hz) and 61.52 (JP P = 4.4 Hz, JPt P = 2968 Hz).
The assignments for major product 4B and minor product 4A (3.4:1 ratio) were confirmed by 13C{1H} and HSQC NMR spectra. In the 13C{1H} NMR spectrum, the methyl group in 4A appears as a doublet with platinum satellites at δ 32.6 (JP C = 12.5 Hz, JPt C = 40.5 Hz), whereas in 4B the methyl appears as a singlet at δ 27.9. Colorless crystals of 4B were obtained that were suitable for single-crystal X-ray diffraction (Figure 3). A similar reaction heated in p-xylene-d10 at 160 °C for 24 days to achieve equilibrium showed 4B as the major product, with a 1:1.3 ratio of 4A:4B. Reaction with 2-Methoxythiophene. A solution of (dippe)Pt(nor)2 in C6D6 was treated with 20 equivalents of 2-methoxythiophene and heated at 100 °C for 64 days (Scheme 2). The reaction was monitored by using 31P{1H} NMR spectroscopy and showed doublets at δ 66.22 (JP P = 6.2 Hz, JPt P = 3124 Hz) and 56.04 (JP P = 5.5 Hz, JPt P = 1673 Hz) for one isomer (5A) and a singlet at δ 63.47 (JPt P = 3147, 1625 Hz) for a second isomer (5B, with coincident 31P chemical shifts) in a 1.1:1 ratio. Upon heating a 4580
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Figure 4. X-ray single-crystal structure of 5A resulting from the C S bond activation of 2-methoxythiophene with the [Pt(dippe)] fragment. Selected bond lengths (Å): C(5) Pt, 2.031(18); S(1) Pt, 2.288(4); S(1) C(2), 1.72(2); C(5) O, 1.40(2). Selected angles (deg): P(1) Pt S(1), 171.51(17); P(2) Pt C(5), 177.5(5).
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Figure 6. Change in concentration of Pt(dippe)(k2-C,S-C4H4S) (29 mM) (9), 2A ([), and 2B (2) during the reaction with 3-cyanothiophene (580 mM) in THF at 160 °C. The solid line reflects the simulated data.
Scheme 3. Kinetic Scheme for Associative 3-Cyanothiophene Exchange
Figure 5. X-ray single-crystal structure of 6B resulting from the C S bond activation of 3-methoxythiophene with the [Pt(dippe)] fragment. Selected bond lengths (Å): Pt S(1), 2.2972(6); Pt C(5), 2.041(2); S(1) C(2), 1.735(2); C(3) O(1), 1.395(3). Selected angles (deg): P(1) Pt S(1), 174.36(2); P(2) Pt C(5), 174.63(6).
second sample at 160 °C in p-xylene-d10 for 24 days, 5A and 5B were observed in a 10:1 ratio. The assignments of 5A and 5B were confirmed by 13C{1H} spectroscopy, in which the 13C{1H} NMR spectrum of 5A shows the methoxy group as a triplet at δ 54.2 (JP C = 4.1 Hz, JPt C = 49 Hz), whereas 5B shows the methoxy group as a singlet at δ 57.6. Crystals of 5A were isolated, and a singlecrystal X-ray structure confirmed these assignments (Figure 4). Reaction with 3-Methoxythiophene. A solution of (dippe)Pt(nor)2 in C6D6 was treated with 20 equivalents of 3-methoxythiophene and heated at 100 °C for 36 days (Scheme 2). The reaction was monitored by using 31P{1H} NMR spectroscopy. A single dominant product (96%) observed as doublets at δ 63.51 (JP P = 6.2 Hz, JPt P = 1829 Hz) and 61.70 (JP P = 4.6 Hz, JPt P = 2911 Hz) was assigned to be 6B on the basis of the single-crystal X-ray structure (Figure 5). A second sample, examined in p-xylene-d10 using 50 equivalents of
3-methoxythiophene while heating at 160 °C for 24 days, showed 6B as the major product along with a second isomer, 6A, with doublets at δ 63.04 (JP P = 4.6 Hz, JPt P = 2955 Hz) and 65.54 (JP P = 4.6 Hz, JPt P = 1729 Hz) in a 2.2:1 ratio. On the basis of the series of reactions shown above, it can be concluded that the C S bond activation reactions of 2- and 3-cyanothiophenes with the [Pt(dippe)] fragment have the fastest reaction rates. Since the nitrile group is a strongly electron-withdrawing group, and the reaction mechanism is proposed to involve the bimolecular displacement of an η2bound thiophene (or norbornene),7 the greater π-acidity of the cyano-substituted thiophenes allows them to bind tightly. Steric effects play a role in these reactions, giving rise to kinetic products that involve insertion into the less-hindered C S bond. With extended heating, this kinetic distribution of products approaches a thermodynamic equilibrium ratio. Kinetic Studies. Several of these reactions were monitored over time to learn about the mechanism of the reaction. To monitor the exchange reaction of thiophene in Pt(dippe)(k2-C, S-C4H4S) with 3-cyanothiophene (20 equiv), a sample prepared in THF was heated at 160 °C in a sealed NMR tube and monitored periodically. Figure 6 shows the distribution of Pt(dippe)(k2-C,S-C4H4S), 2A, and 2B as a function of time. An associative mechanism was used to model the exchange results based on the analogous 2-cyanothiophene exchange reaction 4581
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Table 1. Summary of Simulated Rate Constants for the Reaction of Pt(dippe)(j2-C,S-C4H4S) with 3-Cyanothiophene at 160 °C in THFa rate constant
k, M
1
s
1
1
rate constant
k, s
2.14 (1) 10
7
4.27 (3) 10
7
k2
5.09 (1) 10
4
k4
k3
5.76 (3) 10
5
k
4
a
Simulation parameters: k 2 and k 3 were set to 0. k1 and k 1 were taken from ref 7. All other rate constants were allowed to vary independently. Errors are indicated in parentheses as standard errors.
Figure 7. Change in concentration of Pt(dippe)(k2-C,S-C4H4S) (9, 0.026 M) and 3B ([) during the reaction with 20 equivalents of 2-methylthiophene (0.520 M) in THF at 160 °C as a function of time. The solid line reflects the simulated data with the rate constants kf = 4.77(1) 10 6 M 1 s 1 and kr = 1.66(6) 10 5 M 1 s 1. Errors are indicated in parentheses as standard errors.
investigated in detail previously7 (Scheme 3). In this earlier study, the rate constants k1 and k 1 for thiophene C S bond formation and cleavage were determined to be 1.30(1) 10 5 and 3.5(17) 10 4 s 1, respectively, at 160 °C.8 Here, reversible C S elimination in Pt(dippe)(k2-C,S-C4H4S) to form an η2-C, C adduct is followed by bimolecular reaction with 3-cyanothiophene, giving kinetic products (via k2, k3) that then slowly interconvert (via k4, k 4). Simulations for the associative pathway were carried out using the KINSIM/FITSIM programs to obtain best fit rate constants for the overall process given in Scheme 3.9 The reverse rate constants k 2 and k 3 were set to zero on the basis of the results of a competition experiment between thiophene and 3-cyanothiophene, which showed no reaction with thiophene (i.e., thiophene does not displace 3-cyanothiophene). All other rate constants were allowed to vary independently. The calculated rate constants for this process are given in Table 1, while the calculated fits of concentration versus time are shown by the solid lines in Figure 6. In comparison to the related exchange with 2-cyanothiophene, k2 is about half as fast, whereas k3 is 5 times as fast, leading to the preference for formation of 2A. k4 and k 4 are both about 3 times larger, but the ratio is such that 2A is preferred at equilibrium. Note also that the kinetic selectivity (k2/k3) is opposite that measured at 100 °C in C6D6 solvent. The reaction of Pt(dippe)(k2-C,S-C4H4S) with 50 equivalents of 2-methylthiophene in THF in a sealed NMR tube at 160 °C showed an equilibrium strongly favoring the formation of one of the products, 3B, which results from the activation of the unsubstituted
Figure 8. (a) Change in concentration of Pt(dippe)(k2-C,S-C4H4S) (1) as a function of reaction time at different concentrations of 2-methylthiophene (0.17 M: b, 0.23 M: 2, 0.47 M: [, and 0.97 M: 9) (THF, 160 °C). The curvature at long times is due to the approach to equilibrium as opposed to completion. (b) Plot of kobs vs [2-MeTP].
C S bond (Scheme 2). Under these conditions, the ratio of Pt(dippe)(k2-C,S-C4H4S):3B reaches an equilibrium ratio of 1:7, corresponding to a ΔG433 of 1.0 kcal/mol for the exchange (Keq = 0.303).10 As in the case of 3-cyanothiophene, an associative mechanism is proposed through which this exchange reaction can occur (eq 2). Simulations for the approach to equilibrium were carried out using the KINSIM/FITSIM programs to obtain best fit rate constants for the mechanism shown in eq 2.9 Both rate constants kf and kr were allowed to vary independently. The calculated fits of concentration versus time are shown by the solid lines in Figure 7.
The first-order decay of Pt(dippe)(k2-C,S-C4H4S) to equilibrium (kobs) was followed at increasing concentrations of 2-methylthiophene (Figure 8a). As in the 2-cyanothiophene reaction, when the concentration of 2-methylthiophene was gradually increased from 0.17 M (5 equiv) to 0.94 M (40 equiv), the rate of reaction also increased, but this time an almost linear dependence on the 2-methylthiophene concentration was found (Figure 8b). The small nonzero intercept may be indicative of a minor dissociative exchange pathway. Computational Results. To simplify the calculations, the i-Pr groups in the [Pt(dippe)] fragment were substituted by methyl 4582
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groups. This simplification is assumed to have no steric outcome on the calculations, as the X-ray single-crystal structures showed no interaction between the methyl groups of the dippe ligand and the thiophene ring, as indicated in earlier calculations on the mechanism of C S bond activation of thiophene by the Table 2. Dependence of Rate Constants for Approach to Equilibrium (eq 2) on the Concentration of 2-Methylthiophene for the Reaction of Pt(dippe)(j2-C,S-C4H4S) with 2-Methylthiophenea 1.
run
[2-MeTP]
kobs, s
1
0.1175
1.40(2) 10
6
2
0.235
2.06(1) 10
6
3
0.47
3.37(5) 10
6
0.94
5.46(4) 10
6
4
[Pt]0 = 26 mM, T = 160 °C, THF. Data were fit to Ct = (C0 C∞) exp( kobst) + C∞ using Excel Solver with Keq = 0.3027. Errors are indicated in parentheses as standard errors. See Supporting Information for details. a
[Pt(dmpe)] fragment.11 The optimization of all of the groundstate and transition-state structures was done in the gas phase starting with the optimized structures of the parent thiophene complex by adding substituents to this structure. The optimized ground-state structures of the substituted and unsubstituted C S bond activation products of 2- and 3-cyanothiophene, 2- and 3-methylthiophene, and 2- and 3-methoxythiophene and the transition-state structures leading to the formation of these products are shown in Figure 9 ([Pt(dmpe)] + X-thiophene). The structural parameters for all of these structures as well as the energetics relative to the free fragments are given in Table 3. The energies in gas phase are the Gibbs free energies calculated at 298 K, and those calculated with the polarizable continuum model (PCM, THF) correction are the total free energies in solution. In all of the ground-state structures, the C1 C2 and C3 C4 bond lengths are indicative of a CdC double bond, and those of C2 C3 are indicative of a C C single bond. The six-membered metallacycles are almost planar, with a maximum puckering angle of 7.1° between the P Pt P and S Pt C1 planes in S3A.
Figure 9. Left: Optimized ground-state structures of the C S bond activation products of 2- and 3-cyanothiophene, 2- and 3-methylthiophene, and 2and 3-methoxythiophene by the [Pt(dmpe)] fragment. Right: Optimized structures of the C S bond activation transition states of 2- and 3-cyanothiophene, 2- and 3-methylthiophene, and 2- and 3-methoxythiophene by the [Pt(dmpe)] fragment. 4583
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Table 3. Gas Phase Optimized Structures (interatomic distances in Å) of the C S Bond Activation Products of 2- and 3-Cyanothiophene, 2- and 3-Methylthiophene, and 2- and 3-Methoxythiophene by the [Pt(dmpe)] Fragment and Their Energy of Formations (ΔG/kcal mol 1). a ΔGd
ΔGe
Pt S
Pt C1
S C1
C1 C2
C2 C3
C3 C4
C4 S
Pt Pb
Pt Pc
S1Af
2.345
2.060
3.211
1.377
1.429
1.361
1.726
2.365
2.336
40.1
S1Bf
2.333
2.022
3.204
1.365
1.433
1.368
1.752
2.381
2.299
32.9
45.6
S2A
2.334
2.020
3.197
1.374
1.449
1.355
1.735
2.374
2.305
33.8
46.1
52.4
S2B
2.339
2.026
3.194
1.359
1.451
1.371
1.720
2.380
2.300
34.0
46.6
S3A S3B
2.344 2.338
2.063 2.022
3.226 3.191
1.365 1.362
1.442 1.439
1.353 1.361
1.738 1.750
2.382 2.379
2.324 2.294
20.1 28.1
34.6 40.8
S4A
2.334
2.028
3.200
1.366
1.448
1.356
1.736
2.375
2.298
27.3
40.1
S4B
2.332
2.024
3.178
1.362
1.447
1.360
1.738
2.375
2.298
27.9
40.8
S5A
2.343
2.037
3.225
1.366
1.441
1.354
1.741
2.370
2.312
34.7
47.8
S5B
2.342
2.023
3.207
1.362
1.439
1.363
1.748
2.381
2.288
31.5
44.7
S6A
2.339
3.221
3.221
1.364
1.446
1.352
1.737
2.361
2.296
32.3
43.8
S6B
2.330
2.025
3.178
1.359
1.444
1.359
1.745
2.372
2.297
31.0
42.3
TS1Af TS1Bf
2.423 2.376
2.153 2.089
1.977 2.144
1.429 1.408
1.396 1.398
1.387 1.394
1.732 1.767
2.358 2.239
2.356 2.338
2.6 4.4
5.7 10.9
TS2A
2.389
2.076
2.144
1.414
1.428
1.367
1.759
2.379
2.334
5.3
11.7
TS2B
2.396
2.111
2.070
1.401
1.415
1.392
1.735
2.374
2.349
0.8
5.1
TS3A
2.406
2.128
2.103
1.407
1.408
1.380
1.743
2.374
2.352
6.3
2.0
TS3B
2.403
2.116
2.080
1.399
1.411
1.380
1.760
2.374
2.350
4.9
1.3
TS4A
2.396
2.113
2.113
1.402
1.414
1.378
1.745
2.373
2.355
4.8
1.9
TS4B
2.401
2.114
2.085
1.400
1.415
1.382
1.747
2.376
2.349
3.9
0.5
TS5A TS5B
2.611 2.421
2.074 2.144
1.905 2.035
1.441 1.396
1.408 1.415
1.380 1.380
1.740 1.757
2.350 2.364
2.321 2.355
0.5 5.0
2.5 3.3
TS6A
2.404
2.114
2.073
1.406
1.408
1.379
1.744
2.367
2.367
5.0
2.6
TS6B
2.407
2.114
2.067
1.400
1.412
1.386
1.749
2.378
2.346
1.5
0.4
a Atom numbering around the platinacycle begins with C1 attached to platinum and ends with C4 attached to sulfur. b P atom trans to C atom. c P atom trans to S atom. d Gibbs free energies in gas phase. e Total free energies in THF solution. f Data from ref 7.
Steric factors due to the presence of the R-methyl substitution were shown to be responsible for the observed ring deformations.12 In the transition-state structures on the other hand, there is considerable electron delocalization in the thiophenic ring between the C1, C2, C3, and C4 atoms, as there interatomic distances are nearly equal, as seen in Table 3 (1.378 1.441 Å). There is an average puckering angle of 64° between the P Pt P and S Pt C1 planes. Energetics of the C S bond activation products and the transition states of the parent thiophene and substituted thiophenes by the [Pt(dmpe)] fragment relative to the total energies of fragments are shown in Figures 10 and 11, respectively. The earlier results with 2-cyanothiophene7 are included for comparison purposes. As seen in Figure 10, there is very good agreement between the experimentally observed thermodynamic product ratios (Scheme 2) and the calculated ground-state energies of S1A S6B. Cases where the alternative isomers are separated by several kcal mol 1 also show strong experimental selectivities in the same direction. Cases where the two regioisomers are separated by
