On the regioselective mechanism of novel rearrangements of 1,6-enynes catalyzed by PtCl2: a DFT study

Journal of Molecular Structure: THEOCHEM 717 (2005) 21–32 www.elsevier.com/locate/theochem

On the regioselective mechanism of novel rearrangements of 1,6-enynes catalyzed by PtCl2: a DFT study Rong-Xing Hea,b, Ming Lib,*, Xiang-Yuan Lia a College of Chemical Engineering, Sichuan University, Chengdu 610065, China Department of Chemistry, Southwest-China Normal University, Chongqing 400715, China

b

Received 8 September 2004; accepted 25 October 2004 Available online 8 January 2005

Abstract In the present work, the density functional theory is employed to study qualitatively the mechanism of the rearrangement reaction of 1,6-enynes catalyzed by PtCl2 in gas phase. An efficient and reliable strategy has been adopted to search for the transition states of the reaction involved. As demonstrated, the rearrangement reaction process mainly involves two possible regioalternative mechanisms: cycloisomerization and the formation of cyclopropanes. Four reaction paths for the mechanism of cycloisomerization and one reaction path for the formation of cyclopropanes are investigated at the levels of B3LYP/LANL2DZ* and B3LYP/LANL2MB. The results of calculations show that the rate-determining step for the cycloisomerization is the hydrogen transfer from Pt to C7, and the rate-determining step for the formation of cyclopropanes is the formation of 3-member ring involving C6 and C7. This conclusion is different from the mechanism conjectured by Echavarren. According to our calculations, the result obtained is in agreement with the experiment. The coordination of Pt with the unsaturated bonds releases a large amount of heat, which provides the thermodynamical driving force for the reaction. q 2004 Elsevier B.V. All rights reserved. Keywords: Mechanism; DFT; Rearrangement; PtCl2; 1,6-enyne

1. Introduction The rearrangement reaction of 1,6-enynes is considered to be a novel and important synthetical method to product dienes or polycyclic compounds. Since the pioneering studies of Murai et al. [1–4], four different reaction modes have been discovered which convert the substrates into different species [1–7]. Up to now, a number of rearrangement reactions of enynes catalyzed by transition metal chlorides and other transition metal complexes have been examined [1–14]. These transition metals include palladium, rhodium, ruthenium, platinum, and so on. The results showed that cycloisomerization of enynes catalyzed by different transition metal complexes containing disubstituted alkenes gave selectively dienes [15]. For example, the cis-olefins were obtained using the cationic Rh(I) complexes as a catalyst to trigger the rearrangement of enynes [16,17]. * Corresponding author. E-mail address: [email protected] (M. Li). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.10.067

On the other hand, the mechanism of these reactions is still unclear. Echavarren et al. [8] investigated a great deal of experiments and tried to make clear the stereo- and regioselectivity of these reactions, PtCl2 was used as a catalyst to induce the isomerization of enyne 1 (Scheme 1). The information came from the experiment showed that the reaction of enyne 1 with PtCl2 in acetone gave a 1.1:1 mixture of cycloisomerization product 1M7 and cyclopropane 2M7 [9,18,19]. Interestingly, cis-1M7 was the major stereoisomer (2.8:1 cis/trans). Moreover, Echavarren et al. observed that the formation of cyclopropanes takes place only with heteroatom-tethered enynes. To get further knowledge about the mechanism of this reaction, Echavarren et al. conjectured the process of this regioselective reaction based on its synthesis experiment. The suggested isomerization process of enyne 1 with catalysts PtCl2 mainly involves two possible regioalternatives (Scheme 2). One is the cycloisomerization. This mechanism includes the following steps: (1) coordination of the transition metal Pt both with two unsaturated bonds to form the ligand 1M3, (2) formation of key metalacycle

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Scheme 1. The rearrangement reaction of 1,6-enynes catalyzed by transition metal chlorides.

1M4 from complex 1M3 which suffer an oxidative cyclometalation, (3) evolvement of 1M4 by b-hydrogen elimination from an alkyl substituent to form 1M5, and (4) the reductive elimination of PtCl2 leads to cycloisomerized products 1M7 with the regeneration of the catalysts. The other possible path is the formation of cyclopropanes: (1) the catalysts PtCl2 coordinate only with alkyne to form the platinum complex 2M3, (2) complex 2M3 could suffer a 1,2-hydrogen migration to form an alkenyl platinum carbene 2M4 stabilized by the donor heteroatom [20], (3) the platinum carbene 2M4 undergo an intramolecular [2C2] cycloaddition with the alkene to form a fourcoordinated cyclobutane 2M6 [21–23], (4) reductive elimination of 2M6 to form cyclopropanes 2M7.

To verify the rationality of the suggested mechanism, Echavarren [8] et al. investigated the properties of analogues of the structures 1M3, 1M4, 1Ts1, 2M3, 2M4 and 2Ts1 by means of the density functional theory (DFT). Up to now, however, the detailed quantum chemical studies on the mechanism of the stereo- and regioselective cycloisomerization of enynes 1 catalyzed by PtCl2 have not been reported. In addition, the reliability of the mechanism presumed by Echavarren et al. needs to be testified. In the present work, therefore, the model computations of the stereo- and regioselective rearrangement of 1,6-enynes catalyzed by PtCl2 are performed by means of DFT for the purpose of investigating the mechanism in detail.

2. Models and computations In the present computations, real reaction system is used as theoretical computation model. According to the computational results obtained, two reaction pathways shown in Schemes 3 and 4 are designed. As for the reaction of cycloisomerization, there are two unsaturated bonds in enynes 1; hence the coordination of the PtCl2 to the unsaturated ligands may result in two types of

Scheme 2. Catalyzed cycle of conjectured by Echavarren et al.

R.-X. He et al. / Journal of Molecular Structure: THEOCHEM 717 (2005) 21–32

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Scheme 3. Catalyzed cycle of cycloisomerization according to the computational results obtained.

transition metal complexes. One is generated from the coordination of the catalysts PtCl2 both at the two unsaturated bonds sites of enyne 1 such as 1M3 shown in Scheme 3, and the other is from the coordination of PtCl2 only at the alkyne site of enyne, for example 1M3*. Selected modeling molecules for the intermediates and transition states are illustrated in Fig. 1. As shown in Scheme 3, in the reaction of cycloisomerization, the cis-product is generated from the paths marked by ‘a’ and the trans-product from the paths marked by ‘b’. In this work, the geometrical parameters of all stationary points and transition states (denote by TS) for the rearrangement reaction of 1,6-enynes have been optimized in vacuo, employing analytic energy gradients by means of the Becke-type three-parameter hybrid density functional model B3LYP [24,25] implemented in GAUSSIAN03 package [37]. All theoretical calculations have been carried out employing the LANL2MB [26–30] basis set, and the basis set constructed by adding one set of d polarization functions to the LANL2DZ [28–31] basis set (hereafter denoted LANL2DZ*). The exponents of the polarization functions [32,33] for carbon, oxygen, chlorine and platinum are 0.63, 1.154, 0.514 and 0.993, respectively. The optimized B3LYP/LANL2MB structures have been used as the starting point for the optimizations at B3LYP/LANL2DZ* level.

Scheme 4. Suggested catalyzed cycle for formation of cyclopropanes.

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Fig. 1. Optimized structures of selected intermediates and transition states.

R.-X. He et al. / Journal of Molecular Structure: THEOCHEM 717 (2005) 21–32

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Fig. 1 (continued)

The vibrational analysis and the natural bond orbital (NBO) analysis [34–36] were performed for all structures at the B3LYP/LANL2MB (denoted by MB) and B3LYP/ LANL2DZ* (denoted by DZ*) levels. All the fully optimized TS structures have been confirmed by the existence of a sole imaginary frequency, whereas the intermediates of the optimal structures possess only real frequencies. Throughout this work, E is used to denote total energy after zero-point Energy (ZPE) correction. Gibbs free energy, G, has also been calculated adding ZPE, the thermal contributions and entropy at 298 K. The corrected parameters, E and G, formation energy DE and DG, and reaction activation energies DEs and DGs are summarized in Table 1.

3. Discussions 3.1. Cycloisomerization According to Echavarren’s catalytic cycle, the rearrangement reaction of 1,6-enynes catalyzed by PtCl2 gave a 1.1:1 mixture of cycoisomerization product 1M7 and cyclopropane 2M7 [9,18,19]. In Echavarren’s experiment, the species involved in the reaction are: the complex of 1,6enynes with the catalyst PtCl 2, the complex from

metalacycle through a b-hydrogen elimination, the TS leading to the C2–C6 bond, and the TS for the hydrogen atom transfer. The crucial steps for stereo- and regioselective rearrangement are the complexation of the transition metal to the unsaturated ligands and the hydrogen atom transfer; thus a correct calculation of complex and TS geometries and energies is particularly important. 3.1.1. Intermediate The enyne 1 has two unsaturated bonds, alkene and alkyne functional groups. Therefore the coordination with PtCl2 (Pt (II)) may result in two types of transition metal complexes, 1M3 and 1M3* (1M3 and 1M3* have two structures, respectively, such as 1M3a, 1M3b and 1M3*a, 1M3*b). 1M3 is generated from the complexation of PtCl2 both at the two unsaturated bond sites of enyne 1, and 1M3* is formed from the coordination of the PtCl2 only at the alkyne site of enyne 1. 1M3a denotes the complex in which PtCl2 coordinate at the trans positions of the two unsaturated bonds with the phenyl group, but in 1M3b PtCl2 coordinate at the cis site. 1M3*b is formed through the 1808 rotation of the single s bond between C2 and C3 of 1M3*a. Therefore, the optimized structure of 1M3*a is almost the same as that of 1M3*b. The energies given in Table 1 show that the complexation of 1,6-enynes with PtCl2 is an exothermic process. As shown

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Table 1 Total energies E, Gibbs free energies G, formation energy DE and DG, and activation energy DEs and DGs DZ* DEs(DGs)

DEs(DGs)

G

DE(DG)

K610.309814 K149.007859 K759.463874 K759.463665 K759.503074 K759.503387

K610.355060 K149.038987 K759.514472 K759.514653 K759.553027 K759.553471

K383.85 (K316.18) K383.30 (K316.65) K102.92 (K101.23) K104.29 (K101.92)

141.29 (139.60) 115.54 (116.09) 45.31 (46.08)

K759.490484 K759.491312 K610.378996 K610.379132 K759.426832 K759.426742 K759.420256 K759.422415 K759.418185

K759.544220 K759.541986 K610.420658 K610.422174 K759.482825 K759.481940 K759.471571 K759.472594 K759.468550

K767.258417

47.60 (45.89)

K759.437605

K759.490513

172.71 (165.30)

K767.228803 K767.230770 K767.211273 K767.212831 K767.281614 K767.281543 K767.302204 K618.130790 K767.196610 K767.257878 K767.244974

78.52 88.76 35.47 31.35

K759.414900 K759.414437 K759.509127 K759.509125 K759.547498 K610.406412 K759.383031 K759.484135 K759.480085

K759.469266 K759.468540 K759.563591 K759.563576 K759.597798 K610.446322 K759.440104 K759.535395 K759.529580

31.33 (35.60) 32.31 (35.18)

G

K618.032124 K149.073712 K767.202618 K767.200890 K767.225038 K767.226150 K767.207223 K767.212411 K767.225081 K767.219407 K618.083395 K618.083195 K767.17.691 K767.170380 K767.148805 K767.156882 K767.207782

K618.077429 K149.104469 K767.252091 K767.250930 K767.274818 K767.275894 K767.257838 K767.261625 K767.275472 K767.269487 K618.125292 K618.125575 K767.227569 K767.227733 K767.198922 K767.206714 K767.257268

K767.208022 K767.177315 K767.178603 K767.157180 K767.158440 K767.225554 K767.225554 K767.253443 K618.090607 K767.139926 K767.207262 K767.196571

K244.34 (K184.29) K239.81 (K181.24) K58.86 (K59.67) K66.32 (K65.54) 46.77 (44.58) 36.07 (37.46) K46.89 (K46.30) K18.37 (K20.64) 178.47 (120.01) 164.09 (103.56) K160.52 (K119.91) K159.70 (K120.33)

(76.23) (81.01) (42.79) (39.13)

K144.04 (K141.90) K73.22 (K54.25) 233.96 (175.76) 80.77 (81.28) 48.03 (62.13) 149.32 (150.26)

E and G are in a.u. but DE, DG, DEs and DGs in kJ/mol. a At the DZ* level, the data denote activation energies of 1TS2.At MB level, the data denote activation energies of *1TS2.

33.06 (23.12) 31.70 (30.15) 272.08 (220.05) 273.89 (212.21) K286.60 (K233.09) K286.36 (K230.76) 114.52 (112.64) 108.30 (110.43) 222.88 (221.79)

K216.07 (K212.05) K100.75 (K89.85) 349.79 (295.34) 115.00 (112.16) 65.61 (73.99) 176.99 (179.11)

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1 2 1M3a 1M3b 1M4a 1M4b 1M5a 1M5b 1M6a 1M6b 1M7a 1M7b 1M3*a 1M3*b 1TS1a 1TS1b 1TS2aa (*1TS2a)a 1TS2ba (*1TS2b)a 1TS3a 1TS3b 1TS1*a 1TS1*b 2M4 2M5 2M6 2M7 2TS1 2TS2 2TS3

MB DE (DG)

E

E

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in Table 1, after the contributions to the free energy are considered, the stability of cis mode and trans mode is decreased. Moreover, the coordination energies are very high. It is understandable that there are high coordination energies because the coordination takes place between Pt atom with empty d orbits and the rich electrons unsaturated bonds. The formation of complex 1M3 has important effect on the electronic structures of enyne 1 and catalyst, which is responsible for their high activity. Its consequence is that the positive charges on C2 and the negative charges on C6 and platinum atom of 1M3 increase remarkably compared with free enyne and PtCl2. This result shows the interaction between C2 and C6 will be strengthened, which is of advantage to the formation of the C2–C6 bond. Furthermore, in 1M3 and 1M3*, the alkene and alkyne functional groups are significantly elongated because of the coordination of the transition metal to unsaturated bonds. The optimized geometries (see Fig. 1) show the expected square-planar coordination around the metal. The Cl–Pt–Cl angle remains around 908 for complex 1M3 and 1M3*. In 1M3 the alkene is almost perpendicular to the coordination plane, whereas the alkyne ligand is less tilted. In addition, the alkyne is arranged tipsily to the coordination plane in 1M3*. In complex 1M3*, PtCl2 coordinates with the C6bC7, the nucleophilic alkyne group. In Echavarren’s experiments, a solvent molecule, e.g. H2O, was employed as an additional ligand. Due to the computational capacity, the solvent effect

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is not considered at present. This subject will be considered in future work. The platinacycle 1M4 are generated following the formation of C2–C6 bond in complex 1M3 (Scheme 3). The complexation energies obtained in this work indicate that the formation of 1M4a and 1M4b are exothermic (Table 1). The optimized structures of these Pt (IV) complex show an octahedral arrangement of the ligands around the d6 metal center, with two vacant coordination sites. The alkene and alkyne functional groups are also greatly elongated compared with corresponding bonds of 1M3 and 1M3* (see Table 2). From the analysis above, it is clear that the alkene and alkyne bonds have been reduced because of the formation of C2–C6 bond as well as the coordination with PlCl2 at alkyne sites. According to the calculation results obtained, the Cl–Pt–Cl angle changes evidently compared with that of 1M3a and 1M3*a. In both 1M4a and 1M4b, the Cl–Pt–Cl angle is around 1638, this means that the cyclometalation takes place by an initial shift of the alkene from its location at the coordination plane in complex 1M4 to an axial position with concomitant formation of the C–C bond and new s-alkenyl-Pt and alkyl-Pt bonds. In order to investigate the properties of complex 1M4a and 1M4b in detail, the NBO analysis is performed in this work. In the NBO analysis, E (2) describes the delocalization trend of electrons from a donor bond to an acceptor bond. It is evident from the calculation results that there is a trend of the electron transfer from other bonding orbitals to

Table 2 Geometric data for selected intermediary states and transition states Cycloisomerization: B3LYP/LANL2DZ*(B3LYP/LANL2MB)a 1M3a 1M3b 1M3*a 1M3*b 1M4a 1M4b 1TS1a 1TS1b 1TS1*a 1TS1*b 1TS2a [*1TS2a]b 1TS2b [*1TS2b]b 1TS3a 1TS3b

C1–C2

C6–C7

0.1409(0.1425) 0.1411(0.1425) 0.1350(0.1348) 0.1350(0.1349) 0.1548(0.1565) 0.1541(0.1562) 0.1450(0.1458) 0.1459(0.1464) 0.1365(0.1363) 0.1361(0.1366) 0.1500 [0.1525] 0.1503 [0.1543] 0.1526 0.1530

0.1250(0.1252) 0.1250(0.1249) 0.1300(0.1242) 0.1290(0.1300) 0.1329(0.1331) 0.1330(0.1331) 0.1290(0.1283) 0.1287(0.1280) 0.1300(0.1300) 0.1297(0.1300) 0.1331 [0.1387] 0.1333 [0.1392] 0.1349 0.1351

C2–C6

Cl–Pt–Cl

0.2989(0.3092) 0.2930(0.3086)

0.1512(0.1540) 0.1512(0.1538) 0.1947(0.2058) 0.1945(0.2060) 0.2688(0.2760) 0.2770(0.2770) 0.1547 [0.1581] 0.1546 [0.1571] 0.1527 0.1517

88.6(89.7) 88.2(89.2) 83.7(89.4) 83.6(82.0) 162.2(164.2) 162.4(164.3) 87.0(88.4) 85.6(86.6) 170.4(173.4) 169.4(173.1) 171.6 [171.4] 171.0 [173.8] 163.4 163.1

C8–HC8

Pt–H

0.1458 [0.1357] 0.1517 [0.1397]

0.1701

HC8–C7

[0.1465] 0.1667 [0.1449] 0.1615 0.1611

Formation of cyclopropanes: B3LYP/LANL2DZ*(B3LYP/LANL2MB)

2M4 2M6 2TS1 2TS2 2TS3 a b

C1–C2

C6–C7

0.1352(0.1356) 0.1555(0.1584) 0.1352(0.1350) 0.1404(0.1386) 0.1477(0.1521)

0.1401(0.1386) 0.1448(0.1417) 0.1315(0.1315) 0.1460(0.1431) 0.1375(0.1360)

C2–C7 0.1538(0.1554) 0.1927(0.2164) 0.1523(0.1540)

The data in parentheses denote the data obtained at MB level. The data in bracket is the geometric data of the transition states *1TS2.

Cl–Pt–Cl 158.3(169.1) 160.8(162.9) 171.7(173.2) 171.5(172.7) 161.2(159.6)

C5–HC5

HC5–C6

C1–C7

0.1090(0.1100) 0.1287(0.1328)

0.1435(0.1483) 0.1084(0.1095) 0.2062(0.2330)

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the Pt–C8 antibonding orbital and a trend of the electron transfer from other bonding orbitals to the C8–HC8 antibonding orbital. Therefore, the Pt–C8 and C8–HC8 bonds of 1M4 are weakened. The HC8 transfer to the Pt atom of 1M4a and 1M4b and generate the complex 1M5a and 1M5b, respectively. As shown in Table 1, the formation of 1M5 is an endothermic reaction. It must be emphasized that we do not obtain the structures of 1M5 by using MB level. This involves a different reaction mechanism, and we will discuss it in the following sections. Considering the contributions to the free energy, the coordination energy is somewhat stabilized with respect to the internal energy calculation, from 46.77 to 44.58 kJ/mol for cis mode 1M5a. A slightly unfavorable shift is observed for trans coordination free energy of 1M5b, which changes from 36.07 to 37.46 kJ/mol. In 1M5, HC8 atom transfers to Pt. At DZ* level, complex 1M6 is generated by 1M5 through TS 1TS3, and at MB level, 1M6 is formed from 1M4. According to the energies obtained as shown in Table 1, the formation of 1M6 through 1M5 is an exothermic reaction using DZ* level. But the formation reaction of 1M6 is an endothermic using MB method. 3.1.2. Transition state The TS search method has been employed. In a complex system, we perform a simple method to find conformational TS. This method consists in iteratively converging the molecular geometry to the closest point in which the first derivatives of potential energy vanish (which corresponds to a stationary point). At the stationary point, the Hessian matrix has a negative eigenvalue and this means the point is a first-order saddle point. However, the found TS structure employing above method is much closer to the ‘reactant’ than to the desired structure of TS. Therefore, the only way to have a greater chance of success is to use a better initial guess for the TS, that is, a structure closer to it. This requires the determination of the leading parameters of the reaction. Here we give the example for searching the 1TS1 to illustrate the method. Firstly, we obtain a ‘product’ (here is 1M4) using a lower method (HF/LANL2MB). The complex 1M4 must distort its geometry from that of local minimum to allow C2 and C6 to reach a proper distance without raising the energy too much. Therefore, the reaction coordinate is described by freezing the C2/C6 distance. Then we can draw a grid in which each point is an optimized structure which is obtained by using HF/LANL2MB level with frozen C2/C6 distance. Finally, from a PES constructed, the appropriate geometries are selected and used to start the TS search. Here we use the HF structure as the initial guess and search the TS in higher levels, e.g. MB and DZ*. However, an outstanding disadvantage of such an approach is the huge computational cost. In fact, there is no necessary to build all the PES and to perform a large number of calculations, because it is reasonable to assume that the different conformations of 1TS1 (1TS1a and 1TS1b) should

have almost same C2/C6 distance and have similar PES shape. In general, an appropriate C2/C6 distance is assumed based on experience to perform the geometry optimization with some bond lengths fixed. As shown in Scheme 3, 1TS1 is the TS intermediated between 1M3 to 1M4, and 1TS1* is one between 1M3* and 1M4. 1TS1 and 1TS1* have two structures, respectively, such as 1TS1a and 1TS1b, 1TS1*a and 1TS1*b. The TS have been characterized by analyzing their vibrational modes, and it is shown that there is only a sole imaginary frequency that corresponds to the stretching vibration of the C2–C6 bond for all these TS structures. The energies from computation suggest the activation energy barriers (DEs, DGs) of 1TS1a and 1TS1b are very high (see Table 1). It is predicted that the formation of complex 1M4 is very difficult because of the high activation energy barrier of the 1TS1. Moreover, the DEs and DGs (DZ*) of 1TS1a are higher than corresponding values of 1TS1b. This means the formation of 1M4 mainly passes through the path of 1M3b/1TS1b/1M4b, which leads to trans products. However, this result is opposite to the experimental observation of Echavarren et al. [8]. On the other hand, according to the calculated results of DEs and DGs for these TS, there will be a great decrease for the energy barriers of 1TS1* than that of 1TS1. Therefore, the reaction should proceed through the mechanism of 1M3*/ 1TS1*/1M4 rather than that of 1M3/1TS1/1M4. The optimized structures of the 1TS1 and 1TS1* are illustrated in Fig. 1. The C2/C6 distance is listed in Table 2. It is obvious that between atoms C2 and C6 exists a slight interaction. This result indicates a new C2–C6 bond is being formed. In these optimized structures of 1TS1 using DZ* and MB levels, the alkene and alkyne functional groups are also significantly elongated compared with corresponding bonds of 1M3 (as shown Table 2), and the corresponding Mulliken overlap populations are decreased. It is clear that the alkene and alkyne functional groups are weakened, which is of advantage to generate complex 1M4. The Cl–Pt–Cl angle almost has no change compared with 1M3. Here we must indicate that TSab is the TS that the single s C2–C3 bond of 1M3*a rotates 1808 and forms 1M3*b. At DZ* level, the activation energy barriers DEs of TSab is 1.99 kJ/mol. In complex 1M4, the hydrogen atom of methyl group transfer will pass through the 1TS2. It must be emphasized that there are two different reaction mechanisms if the DZ* and MB methods are employed, which has been mentioned in the section of intermediate 1M5. Briefly, at the DZ* level, the hydrogen atom (the transfer hydrogen for cis mode is HC8 and for trans mode is HC9 because of the spatial effect coming from the phenyl group at C5 site) of complex 1M4 will transfer towards to Pt atom, which passes through 1TS2 and leads to intermediate 1M5, that is, the path 1M4/ 1TS2/1M5. But HC8 of complex 1M4 will directly transfer towards to C7 atom if the MB level is used,

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which generate complex 1M6, the TS is *1TS2, that is, the path 1M4/*1TS2/1M6. 1TS2 has two conformations, cis mode 1TS2a and trans mode 1TS2b; and correspondingly *1TS2 also has *1TS2a and *1TS2b. Vibrational analysis shows they have a sole imaginary frequency, which of 1TS2 corresponds to the stretching vibration of the Pt–H–C bond and of *1TS2 corresponds to the stretching vibration of the C8–H–C7 bond. Comparing these activation energies as shown in Table 1, it is clearly that the activation energies of *1TS2 are too high almost to make the reaction proceed. Of course, this result depends on the approach applied. From an inspection of TS geometries, the distances of Pt–H and C8–H (C9–H for 1TS2b) for 1TS2a and 1TS2b are, respectively, 0.1701, 0.1458 nm and 0.1667, 0.1517 nm. This result implies that there is a trend to bond between Pt and H, and a trend to break down at the C–H bond. Comparing with the complex 1M4, the corresponding Mulliken overlap populations are increased for Pt–H bond and which are decreased for C–H bond of 1TS2. The distances of C1–C8 bond for the 1TS2a and the C1–C9 bond for the 1TS2b are 0.1387 and 0.1379 nm, which implies that the C1–C8 and C1–C9 are almost C–C double bond. In addition, in the 1TS2a, 1TS2b, *1TS2a and *1TS2b, the alkene and alkyne functional groups are all lengthened and they have almost changed into normal s C–C single bond and CaC double bond. The NBO analysis for the 1TS2a, 1TS2b, *1TS2a and *1TS2b is carried out. The stabilization interaction energies obtained based on the second-order perturbative theory [34–36] are calculated. The analysis results show that in 1TS2 the electron will transfer from other bonding orbitals to the C8–HC8 antibonding orbital for 1TS2a and from other bonding orbitals to the C9–HC9 antibonding orbital for 1TS2b. This indicates that the bond of C8–HC8 and C9–HC9 will break down. Similarly, in *1TS2 the trend that the electron transfer from other bonding orbitals to the C7–HC7 antibonding orbital is very large, which lead to the C7–HC7 bond is weakened. As shown in Scheme 3, the transfer of H from Pt to C7 passes through the 1TS3. 1TS3 has two structures such as 1TS3a and 1TS3b. The vibrational analysis shows there is a sole imaginary frequency, which corresponding to the stretching vibration of the Pt–HPt–C7 bond. Their activation energies DEs are 78.52 and 88.76 kJ/mol, respectively (in Table 1). It is clear that the DEs of 1TS3a is smaller than that of 1TS3b. Comparing the activation energies of 1TS3 with that of 1TS2, an interesting fact will be found, that is, the activation energies of 1TS2 is far smaller than that of 1TS3. This implies the hydrogen transfer from C8 (or C9) to Pt atom is easier than it transfer from Pt atom to C7. In fact, transition metal Pt atom has empty d orbits, which means it tends to obtain a hydrogen atom with negative charge but not to lose it. For the Pt–HPt bonds of 1TS3a and 1TS3b, the NBO analysis shows the stabilization interaction energies have

29

great increase comparing with that of complex 1M5. There is a significant trend that the electron transfers from other bonding orbitals to the Pt–HPt antibonding orbital. This result leads to the decomposition of Pt–HPt bond and to form complex 1M6. 3.2. Formation of cyclopropanes According to the mechanism conjectured by Echavarren et al., we design a catalytic cycle that the rearrangement reaction of 1,6-enynes catalyzed by PtCl2, are shown in Scheme 4. As we know from above, the 1,6-enynes coordinate with PtCl2 (Pt (II) may result in two types of transition metal complexes, such as 1M3 and 1M3* as shown in Scheme 3. Based on the discussion above, the cycloisomerized products 1M7 may be formed through complex 1M3*. But after a large number of investigations, Echavarren et al. have observed that the formation reaction of cyclopropanes takes place only with heteroatom-tethered enyne, such as our system 1,6-enyne 1 (the heteroatom is oxygen). Therefore, complex 1M3* will also occur a reaction of cyclopropanes. In the plausible mechanism, HC5 of platinum complex 1M3* first suffers a 1,2-hydrogen migration to form an alkenyl platinum carbene 2M4 stabilized by the donor oxygen atom. The platinum carbene 2M4 might then undergo an intramolecular [2C2] cycloaddition with the alkene to generate a platina (IV)cyclobutane 2M6. Finally, 2M6 could evolve by reductive elimination of PtCl2 to form the products cyclopropanes 2M7. In our work, 1M3*a is used as the investigated object for the reaction of cyclopropanes formation. It is reasonable because the result obtained using 1M3*b is the same as that from 1M3*a. 3.2.1. Intermediate The HC5 of the complex 1M3*a will transfer to the C6 atom and form platinum carbene 2M4. The energetic data obtained in this work for HC5 transfer indicate the reaction is exothermic, which leads to a stable complex 2M4. Considering the effect of free energy, its contribution is destabilization for the formation reaction of 2M4 (Table 1). From an inspection of geometries, it is obvious that the unsaturated alkyne bond (C6bC7) has been reduced (as shown in Table 2). Comparing with 1M3*a, the formation of complex 2M4 has important effect on the electronic structures. In 1M3*a, the net charges of Pt and O are, 0.492 and K0.246 at DZ* level, 0.074 and K0.163 at MB level, whereas those for 2M4 are 0.363 and K0.143 at DZ* level, K0.033 and K0.049 at MB level. Obviously, the formation of complex 2M4 leads to the transfer of electron partially from O atom to Pt atom and stabilizes the platinum carbene 2M4. It is the reason why the formation of cyclopropanes places only heteroatom-tethered enyne. The platinum carbene 2M5 is an isomer of 2M4 through an electron rearrangement reaction. In complex 2M5, the C1 atom has a net negative charge and Pt atom has empty d

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orbits. Therefore, in the sequential reaction, an intramolecular electrophilic attack on the alkene might take place and form the key platina (IV)cyclobutane 2M6. Pt atom of 2M5 coordinate at the Cl site to form platina(IV) -cyclobutane 2M6. 2M6 is a bicyclic complex as shown in Scheme 4. The coordination of Pt with C1 is an exothermic reaction (see Table 2). In the Pt–C1–C2–C7 4-membered ring, the Pt–C1, Pt–C7, C1–C2 and C2–C7 bonds are 0.2116, 0.2141, 0.1555 and 0.1538 nm, respectively (at DZ* level). The corresponding bonds are 0.2144, 0.2411, 0.1584 and 0.1554 nm at MB level. The C1–Pt–C7 and C1–C2–C7 angles are: 69.4 and 103.18 at DZ* level; 67.3 and 107.78 at MB level. The dihedral of two planes is: 144.48 at DZ* level and 130.38 at MB level. In the sequential reaction, a reductive elimination leads to cyclopropane products 2M7 and regenerates catalytically active PtCl2. 3.2.2. Transition state As shown in Scheme 4, the transfer of HC5 from C5 to C6 passes through 2TS1. 2TS1 has been characterized by analyzing its vibrational modes, which shows a sole imaginary frequency, corresponding to the stretching vibration of the C5–HC5–C6 bond. In the optimized structure of 2TS1, the distances of C5/HC5 and HC5/C6 at DZ* level are 0.1287 and 0.1435 nm, the distances are 0.1328 and 0.1483 nm at MB level. Obviously, the C5–HC5 bond is lengthened comparing with that of 1M3*a, and the corresponding Mulliken overlap population is decreased. In 2TS1, since the HC5 with negative charges will transfer toward C6, the C5 atom will have partial positive charges. Thus, the weak electron-withdrawing oxygen atom will afford electron to C5 to increase the stabilization of system. On the other hand, because of the presence of vacant coordination sites of Pt, the electron of O atom will continue to transfer toward to Pt, which leads to 2M4 finally. As shown in Scheme 4, 2TS2 is the TS which generates the platina (IV) cyclobutane 2M6. The vibrational analysis shows it has only one imaginary frequency, corresponding to the C2/C7 bond stretching vibration. Based on our computational results, the corresponding activation energies are not high for 2TS2 (see Table 1). If the thermal contribution of free energy is introduced, the activation energy will increase slightly both at DZ* and MB levels. In 2TS2, the distance of C2/C7 bond is 0.1927 nm (DZ*) or 0.2164 nm (MB). The corresponding Mulliken overlap population is increased comparing with that of 2M5. With the formation of C2–C7 bond, atom Pt will also simultaneously coordinate with the rich electron carbon C8 because of the existence of vacant coordination sites at Pt atom. In 2TS3, a sole imaginary frequency is obtained, which corresponds to the C1/C7 bond stretching vibration. The corresponding activation energies are very high (as shown in Table 1). The distance of C1/C7 bond is

0.2062 nm (DZ*) or 0.1965 nm (MB). The formation of 2TS3 leads to the decomposition of Pt–C7 bond, and then the Pt coordinates with C5 and C6 because of the double bond C5aC6 is a rich electron unsaturated bond. With the reaction proceeding, the product 2M7 is formed and the catalyst PtCl2 is regenerated. 3.3. Mechanism of reaction At the basis sets of B3LYP/LANL2DZ* and B3LYP/LANL2MB levels, the properties of all intermediates and TS of the rearrangement reaction of 1,6-enynes have been discussed. As demonstrated above, there are two possible regioalternative mechanisms: cycloisomerization and formation of cyclopropanes. As for the cycloisomerization reaction employed at DZ* level, there are four paths. The first two paths are 1/1M3x/1M4x/1M5x/ 1M6x/1M7x (x denote a or b), and they pass through the TS 1TS1x, 1TS2x and 1TS3x. The second two paths are 1/1M3*x/1M4x/1M5x/1M6x/1M7x, and pass through the TS 1TS1*x, 1TS2x and 1TS3x. By careful analysis, it is obvious that the activation energy barriers of 1TS1 are the highest among these activation energies. This implies the rearrangement reaction of 1,6-enynes proceeds mainly through path 1/1M3*x/1M4x/1M5x/ 1M6x/1M7x, and involves TS 1TS1*x, 1TS2x and 1TS3x (x denote a or b). Further, among these TS, we can find that the activation energies of 1TS3 are the highest. Therefore, the transfer of hydrogen from Pt to C7 is the ratedetermining step for the cycloisomerization reaction of 1,6enynes. Moreover, in 1TS3, the activation energy of 1TS3b is higher (10.24 kJ/mol) than that of 1TS3a. This result indicates the cycloisomerization reaction proceeds mainly through the following path, 1/1M3*a/1M4a/1M5a/ 1M6a/1M7a, which leads to cis products. This conclusion is in agreement with the Echavarren’s experiment [8], that is, cis mode is the main product (2.8:1 cis/trans). However, the present mechanism is different from that proposed by Echavarren [8]. According to Echavarren’s mechanism, the regioselective rearrangement reaction should proceed through the path 1/1M3a/1M4a/1M5a/1M6a/ 1M7a, and the key step of this reaction should be the complexation of the transition metal to the unsaturated ligands. If his mechanism were right, the trans mode will be the main product in our work because of the activation energy of 1TS1a is higher than that of 1TS1b. At the MB level, the result obtained is always opposed to the Echavarren’s experiment. The energy relation of this cycloisomerization reaction is illustrated in Fig. 2. As for the formation of cyclopropanes, the reaction path is 1/1M3*/2M4/2M5/2M6/2M7. The result obtained is similar both at DZ* and MB levels, that is, the formation of C1–C7 bond which leads to the C1–C2–C7 3-membered ring is the rate-determining step for the formation of cyclopropanes. 2TS3 is the TS to form

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Fig. 2. Energy relationship for the cycloisomerization at DZ* level.

3-membered ring. The activation energies of 2TS3 are very high both at DZ* and MB levels. Based on the data obtained, it is obvious that the activation energy barrier of 2TS3 is higher than that of 1TS3. This result shows the cycloisomerizated products 1M7 is predominant in this rearrangement reaction of 1,6enynes catalyzed by PtCl2, which is in agreement with the Echavarren’s experiment [8] (1.1:1 cycloisomerization/ cyclopropanes).

4. Conclusions The possible regioalternatives mechanism of cycloisomerization and formation of cyclopropanes have been discussed. For the mechanism of cycloisomerization, four possible reaction paths have been studied at the MB and DZ* levels, respectively. Several conceivable intermediates and TS, 1M5(cis/trans), 1M6(cis/trans), 1TS2(cis/ trans) and 1TS3(cis/trans), have been found stable at the DZ* level. However, none of them are stable at the MB level. The HC8 of complex 1M4 will directly transfer towards to C7 at MB level through *1TS2. However, the result obtained employing MB method is opposed to the experimental observation. According to our computations, a reasonable reaction path, 1/1M3*a/1M4a/1M5a/ 1M6a/1M7a, has been confirmed at DZ* level. This reaction path leads to cis product. But our mechanism is different from that suggested by Echavarren et al.

The mechanism of formation of cyclopropanes has been investigated. Comparing the energy data, it is found that the product of cycloisomerization is dominant. This result is in correspondence to the experimental observation. A systematic search for TS has been performed by an efficient and reliable strategy, which has been successfully employed for the search of all TS in this reaction. In general this reaction is exothermic. The results obtained are concerned to the basis set levels employed. Because of considering the polarization functions, the results from DZ* level are more credible than that from MB level. As we know, the solvent effect is important for this reaction. This subject will be studied in a future publication.

Acknowledgements This work is supported by the Science Foundation of National Education Ministry, People’s Republic of China (No. 104263).

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