Flyover Compounds and Bridging Bent Benzene Derivatives as

Article pubs.acs.org/Organometallics

Flyover Compounds and Bridging Bent Benzene Derivatives as Intermediates in the Cobalt Carbonyl Cyclotrimerization of Alkynes Peng Wu,† Yi Zeng,† Qunchao Fan,† Hao Feng,*,†,‡ Yaoming Xie,§ R. Bruce King,*,§ and Henry F. Schaefer, III§ †

Research Center for Advanced Computation, School of Physics and Chemistry, Xihua University, Chengdu, Sichuan 610039, China Institute of Atomic and Molecular Physics, Sichuan University, Chengdu, Sichuan 610065, China § Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, United States ‡

S Supporting Information *

ABSTRACT: The so-called flyover complexes (η3,1,η3,1-μ-R3R′3C6)Co2(CO)4 are important because of their role as intermediates in the cobalt carbonyl-catalyzed cyclotrimerization of alkynes. A density functional study of such flyover complexes has led to the discovery of isomers with bent benzene rings bridging a pair of cobalt atoms. Such complexes are likely to be involved in the process of forming a carbon−carbon bond in closing the six-carbon chain in the flyover complexes to give the corresponding arene derivatives. The relative energies of the flyover complexes and the bridging bent benzene derivatives depend on the substituents on the six-carbon chain. Thus, for [C6(CF3)6]Co2(CO)4 and [1,3,6-tBu3C6H3]Co2(CO)4 with bulky CF3 and tert-butyl substituents at the ends of the six-carbon chain, the experimentally known flyover complexes are preferred energetically over the isomeric bridging bent benzene ring structure by 23.3 and 1.1 kcal/mol, respectively. However, for the less sterically hindered (C6R6)Co2(CO)4 (R = H, CH3) and 1,3,6-(CF3)3C6H3]Co2(CO)4 derivatives the bridging bent benzene ring structures are preferred energetically over the flyover structures by 3.6 to 14.3 kcal/mol, respectively.

1. INTRODUCTION One of the most extensively studied reactions catalyzed by cobalt carbonyls is the cyclotrimerization of alkynes to give benzene derivatives.1 Such trimerizations of unsymmetrical alkynes RCCR′ (R ≠ R′) are of interest in providing routes to 1,2,4- rather than 1,3,5-trisubstituted benzenes. Certain alkynes with bulky substituents do not undergo cyclotrimerization with cobalt carbonyl derivatives. Instead they give the socalled flyover compounds Co2(CO)4(RC2R′)3 as stable products through alkyne dicobalt hexacarbonyl and cobaltacyclopentadiene intermediates (Figure 1).2−7 Thermal decomposition of the flyover compounds then leads to the substituted benzenes. This suggests the involvement of related unstable Co2(CO)4(RC2R′)3 flyover complexes as reaction intermediates in the mechanism for the formation of substituted benzenes in the reactions of alkynes and Co2(CO)8.8 We have investigated such flyover compounds using density functional theory. Such investigations have resulted in not only a greater understanding of the thermochemistry and structures of flyover compounds but also the discovery of a new type of isomeric Co2(CO)4(RC2R′)3 derivative with a central bent benzene ring bridging the two cobalt atoms. Such bent benzene derivatives appear to be likely intermediates in the conversion of flyover compounds to the corresponding benzene derivatives. The six-carbon-atom unit bridging the two cobalt atoms in the flyover compounds Co2(CO)4(RC2R′)3 (Figure 1) arises from the oligomerization of the three alkyne molecules. © 2014 American Chemical Society

Figure 1. Conversion of Co2(CO)8 to the flyover compounds Co2(CO)4(μ-C6R6)3 via the alkyne complexes Co2(CO)6(μ-C2R2) and the cobaltacyclopentadiene complexes Co2(CO)5(η4,η1-μ-C4R4). The R and R′ groups are omitted for clarity.

Although it is possible to consider this six-carbon chain as a bridging group supplying eight electrons, we prefer to describe the central structural unit as a heterocyclic eight-membered Received: March 13, 2014 Published: April 30, 2014 2352

dx.doi.org/10.1021/om500270s | Organometallics 2014, 33, 2352−2357

Organometallics

Article

Figure 2. The optimized (C6H6)Co2(CO)4 structures indicating their symmetry point groups and relative energies in kcal/mol.

Co2C6 ring twisted into the shape of a figure “8”. However, the central carbon−carbon bond of this six-carbon chain is not perpendicular to the cobalt−cobalt bond and thus is not a bridging group in the sense that diphenylacetylene bridges the metal atoms in Co2(CO)6C2Ph29 and in Ni2(C5H5)2C2Ph2.10 The angle is only 28°, so that a better description of the arrangement is a “flyover” found at a superhighway interchange rather than a “bridge” crossing another highway or a river. The metal−carbon distances show that each cobalt atom is involved in bonding with four of these six carbon atoms. This bonding mode can be interpreted as a bis(trihaptoallyl) structure plus two cobalt−carbon σ bonds. The conversion of the Co2(CO)4(RC2R′)3 flyover compounds with an acyclic six-carbon chain (Figure 1) to the corresponding benzene derivatives requires formation of a new carbon−carbon bond joining each end of the six-carbon chain. Our density theory studies have revealed novel stable Co2(CO)4(μ-C6R3R′3) structures with a bridging bent benzene ring as such an intermediate. This can represent the metal complex obtained by joining the end carbon atoms in the sixcarbon chain in the flyover compounds. Furthermore, we find that for flyover compounds with relatively small R and R′ groups such as H and CH3, the bridging benzene complexes Co2(CO)4(μ-C6R3R′3) are of lower energies than the isomeric flyover compounds. These are the systems leading directly to alkyne trimerization to the corresponding benzene without isolation of an intermediate flyover compound. However, the situation is reversed with more bulky R and R′ substituents such as tert-butyl and CF3 when the flyover compounds are of lower energies than the isomeric bridging benzene complexes. These are the systems for which the Co2(CO)4(RC2R′)3 flyover compounds have been isolated as stable compounds. These flyover compounds thus represent frozen intermediates in the cobalt carbonyl-catalyzed trimerization of alkynes. This paper presents the details of these studies.

metallic compounds.15−21 Three DFT methods were used in our present study. The first method uses the hybrid B3LYP functional, which incorporates Becke’s three-parameter functional (B3) with the Lee, Yang, and Parr (LYP) correlation functional.22,23 The second approach is the BP86 method, which combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 correlation functional.24,25 The M06-L functional was constructed using three strategies, namely, constraint satisfaction, modeling the exchange−correlation hole, and empirical testing.26 Since the new generation M06-L functional is recommended for organometallic and inorganic thermochemistry both by Truhlar et al. and by Gusev in a recent study,27 only the M06-L geometries and energies are discussed in the present paper. However, results from the B3LYP and BP86 methods used in most of our previous work essentially agree with the M06-L results and are listed in the Supporting Information. The geometries of all structures were fully optimized. The harmonic vibrational frequencies and the corresponding infrared intensities were evaluated analytically. All of the computations were carried out with the Gaussian 09 program,28 For the BP86 and B3LYP methods, the fine (75, 302) grid is used for numerical evaluation of the integrals, whereas for the M06-L method the finer (99, 590) grid is used. All of the optimized structures are genuine minima with no imaginary vibrational frequencies. In the present paper each structure is designated as X-Y-n where X is the substituent on the C6X6 ligand, e.g., CH3 indicating the methyl group; Y indicates the conformer of the C6X6 ligand, e.g., b indicating a ring benzene and c indicating a C6H6 chain; and n orders the structures according to their M06-L relative energies. Thus, the lowest energy (C6H6)Co2(CO)4 structure is designated H-b-1. Triplet spinstate structures are not considered in this paper since they were found to lie at considerably higher energies than the corresponding singlet structures

3. RESULTS AND DISCUSSION 3.1. (C6H6)Co2(CO)4 (R = R′ = H) Structures. Three (C6H6)Co2(CO)4 structures, namely, H-b-1, H-b-2, and H-c-3, were obtained (Figure 2), in which all carbonyls are terminal groups. The C2 structure H-b-1, with a bent bridging benzene ring, is the global minimum. Each cobalt atom is connected to four benzene carbon atoms. The cobalt atoms are in cis positions with respect to the benzene ring, placing them within a bonding distance of 2.446 Å. This can be interpreted as the formal single Co−Co bond to give each cobalt atom the favored 18-electron configuration after considering the η4,η4-μC6H6 bridging benzene ring as a four-electron donor to each cobalt atom. The (C6H6)Co2(CO)4 structure H-b-2 with C2h symmetry lies 5.8 kcal/mol in energy above H-b-1 (Figure 2). The bridging benzene ring in H-b-2 is planar, in contrast to the bent benzene ring in H-b-1. The two cobalt atoms in H-b-1 are located in trans positions with respect to the benzene ring, so there is no possibility for a Co−Co bond. The benzene ring is

2. THEORETICAL METHODS In this work double-ζ plus polarization (DZP) basis sets were used. For carbon and oxygen, one set of pure spherical harmonic d functions is added with orbital exponents αd(C) = 0.75 and αd(O) = 0.85 to the Huzinaga−Dunning standard contracted DZ sets, and they are designated (9s5p1d/4s2p1d).11,12 For hydrogen, a set of p-polarization functions αp(H) = 0.75 is added to the Huzinaga−Dunning DZ sets. For cobalt, in our loosely contracted DZP basis set, Wachters’ primitive set is used but is augmented by two sets of p functions and one set of d functions, contracted following Hood et al. and designated as (14s11p6d/10s8p3d).13,14 Electron correlation effects have been included by employing density functional theory (DFT) methods, which have been suggested as a practical and effective computation tool, especially for organo2353


Organometallics

Article

Figure 3. The three optimized C6(CH3)6Co2(CO)4 structures indicating their symmetry point groups and relative energies in kcal/mol. The CH3 substituents are omitted for the sake of clarity.

Figure 4. The three optimized C6(CF3)6Co2(CO)4 structures indicating their symmetry point groups and relative energies in kcal/mol. The CF3 substituents are omitted for the sake of clarity.

bonded as a trihapto η3,η3-ligand to each cobalt atom. This gives each cobalt atom a 16-electron configuration. The (C6H6)Co2(CO)4 structure H-c-3, with a chain C6H6 ligand, lies 14.3 kcal/mol above the global minimum H-b-1 (Figure 2). Structure H-c-3 contains a C2 axis so that the two cobalt atoms are symmetry equivalent. The predicted Co−Co distance of 2.445 Å can correspond to a formal single bond to give each cobalt atom the favored 18-electron configuration. The C6H6 chain can be regarded as comprising two linked allyl groups with C−C distances within each allyl group (C1−C2 or C5−C6) of 1.412 Å and C−C−C angles of 109.9°. The distance between the two terminal carbon atoms of the C6H6 chain of 3.117 Å clearly indicates no direct bond between the two terminal C atoms. 3.2. [C6(CH3)6]Co2(CO)4 (R = R′ = CH3) Structures. When all six hydrogen atoms in the C6H6 structural unit are replaced by methyl groups, three analogous [C6(CH3)6]Co2(CO)4 structures, namely, CH3-b-1, CH3-b-2, and CH3-c-3, are obtained (Figure 3). Similar to the (C 6 H 6 )Co 2 (CO) 4 structures, all four CO groups in all three structures are terminal ligands. The C2 structure CH3-b-1 is the lowest energy structure. In CH2-b-1 each cobalt atom is connected to four carbon atoms in the benzene ring. The cobalt atoms are in cis positions with respect to the benzene ring, placing them within a bonding distance of 2.443 Å, thereby giving each cobalt atom the favored 18-electron configuration similar to H-b-1. The two higher energy C6(CH3)6Co2(CO)4 structures are analogous to the C6H6Co2(CO)4 structures (Figure 3). Thus, the Ci structure CH3-b-2, lying 5.7 kcal/mol in energy above CH3-b-1, has the two cobalt atoms in trans positions relative to a planar bridging η3,η3-C6(CH3)6 benzene ring. The bonding of

this bridging benzene to each cobalt atom as a trihapto allylic ligand gives each Co atom a 16-electron configuration. The C6(CH3)6Co2(CO)4 structure CH3-c-3, lying 9.8 kcal/mol above the global minimum CH3-b-1, has a predicted Co−Co distance of 2.449 Å, considered to be a formal single bond. This gives each cobalt atom the favored 18-electron configuration. The C6(CH3)6 chain can be regarded as comprising two linked allyl groups with C−C distances within each allyl group (C1− C2 or C5−C6) of 1.419 Å and C−C−C angles of 109.6°. The distance between the two terminal carbon atoms of the C6(CH3)6 chain of 3.155 Å clearly indicates no direct bond between the two terminal C atoms. 3.3. [C6(CF3)6]Co2(CO)4 (R = R′ = CF3) Structures. Three structures for [C6(CF3)6]Co2(CO)4, namely, CF3-b-2, CF3-c-1, and CF3-b-3, were obtained in which all carbonyls are terminal groups (Figure 4). The [C6(CF3)6]Co2(CO)4 system differs from the (C6X6)Co2(CO)4 (X = H, CH3) systems discussed above since the lowest energy structure is the chain structure CF3-c-1 rather than the bridging bent benzene structure CF3-b2. This chain structure CF3-c-1 has a predicted Co−Co distance of 2.443 Å, considered to be a formal single bond to give both cobalt atoms the favored 18-electron configurations. The C−C distance within each allyl group of the C6(CF3)6 chain in CF3-c-1, namely, C1−C2 or C5−C6, is 1.415 Å, and the C−C−C angles within the allyl groups are 107.8°. The distance between the two terminal carbon atoms of the C6(CF3)6 chain is 3.106 Å, which is clearly too long for a direct C−C bond. The higher energy C6(CF3)6Co2(CO)4 structure CF3-b-2, lying 8.4 kcal/mol in energy above CF3-c-1, has a bent η4,η4-μC6(CF3)6 ring bridging the Co2 unit by bonding as a tetrahapto 2354


Organometallics

Article

Figure 5. The three optimized [1,3,6-(CF3)3C6H3]Co2(CO)4 structures indicating their symmetry point groups and relative energies in kcal/mol. The CF3 substituents are omitted for the sake of clarity.

Figure 6. The three optimized 1,3,6-tBu3C6H3Co2(CO)4 structures indicating their symmetry point groups and relative energies in kcal/mol. The tBu substituents are omitted for the sake of clarity.

lying 6.1 kcal/mol above tri-CF3-b-1, has the two cobalt atoms in trans positions relative to a nearly planar substituted benzene ring functioning as a bis(trihaptoallylic) μ-η3,η3-(CF3)3C6H3 ligand. This gives each cobalt atom in tri-CF3-b-3 a 16-electron configuration. 3.5. [1,3,6-tBu3C6H3]Co2(CO)4 (R = tBu) Structures. The [1,3,6-tBu3C6H3]Co2(CO)4, system was also studied for comparison with the experimental work of Krüerke and coworkers on this system.2,29 The three structures tri-tBu-b-2, tritBu-b-3, and tri-tBu-c-1 were obtained analogous to the other (C6R6)Co2(CO)4 systems studied in this work (Figure 6). The C1 structure tri-tBu-c-1 synthesized by Krüerke and coworkers2 is the lowest energy structure in accord with the experimental work. The predicted Co−Co distance of 2.426 Å is considered to be a formal single bond, thereby giving each cobalt atom the favored 18-electron configuration. The C−C distances within each allyl group (C1−C2 or C5−C6) are 1.417 Å, and the C−C−C angles within the allyl groups are 113.2°. The 3.220 Å distance between the two terminal carbon atoms of the tBu3C6H3 chain is too long for a C−C bond. The C1 structure tri-tBu-b-2, lying only 1.1 kcal/mol in energy above tri-tBu-c-1, has a bent bis(tetrahapto) μ-η4,η4tBu3C6H3 ring bridging the cobalt atoms, located in cis positions relative to this ring. The predicted Co−Co distance of 2.462 Å can be considered as a formal single bond to give each cobalt atom the favored 18-electron configuration. The C1 structure tri-tBu-b-3, lying 10.7 kcal/mol above tri-tBu-c-1, has the two cobalt atoms in trans positions relative to a nearly

ligand to each cobalt atom (Figure 4). The predicted Co−Co distance of 2.499 Å is interpreted as a formal single bond to give each cobalt atom the favored 18-electron configuration. The highest energy C6(CF3)6Co2(CO)4 structure CF3-b-3, lying 23.7 kcal/mol in energy above CF3-c-1, has the two cobalt atoms in trans positions relative to a bis(trihapto) nearly planar bridging η3,η3-μ-C6(CF3)6 ring. This gives each cobalt atom a 16-electron configuration. 3.4. [1,3,6-C6H3(CF3)3]Co2(CO)4 (R = CF3, R′ = H) Structures. Three structures for [1,3,6-[C6 H3 (CF 3 ) 3]Co2(CO)4], namely, tri-CF3-b-1, tri-CF3-b-3, and tri-CF3-c-2, were obtained (Figure 5). All four carbonyl groups in each of these structures are terminal groups. The C1 structure tri-CF3b-1, with a bridging bent μ-η4,η4-C6H3(CF3)3 ring and cis orientation of the cobalt atoms, is the lowest energy structure. This is analogous to the lowest energy structure for the [C6(CH3)6]Co2(CO)4 system, rather than that for the [C6(CF3)6]Co2(CO)4 system. The predicted Co−Co distance of 2.459 Å suggests a formal single bond, thereby giving each cobalt atom the favored 18-electron configuration. The C1 structure tri-CF3-c-2, with a bridging chain C6H3(CF3)3 ligand, lies 3.6 kcal/mol above tri-CF3-b-1 (Figure 5). The predicted Co−Co distance of 2.446 Å in tri-CF3-c-2, considered as a formal single bond, gives each cobalt atom the favored 18-electron configuration. The C−C distances within each allyl group (C1−C2 or C5−C6) are 1.409 Å, and the C− C−C angles within the allyl groups are 109.6°. The distance between the two terminal carbon atoms of the chain is too long (3.114 Å) to be a C−C bond. The C1 structure tri-CF3-b-3, 2355


Organometallics

Article

flyover complexes as isolable stable intermediates appears to require bulky substituents at the ends of the six-carbon bridge to inhibit cyclization to benzene derivatives. This is a good example of how otherwise reactive catalytic intermediates can be stabilized by the use of bulky substituents.

planar bis(trihapto) μ-η3,η3-tBu3C6H3 ring without a Co−Co bond. This gives each Co atom a 16-electron configuration.

4. CONCLUSION We find three (RC2R′)3Co2(CO)4 isomers for each set of R and R′ groups in which the three original RC2R′ alkyne units have coupled to form a C6R3R′3 structural unit bridging the pair of cobalt atoms. In the flyover complexes, which can be designated as (η3,1,η3,1-μ-R3R′3C6)Co2(CO)4, this structural unit is an open chain bridge with nonbonding C···C distances greater than 3 Å between the end carbon atoms of the chain. The other two types of (RC2R′)3Co2(CO)4 isomers have a substituted C6R3R′3 benzene ring bridging the pair of cobalt atoms. In one of these isomers the cobalt atoms are on the same side of the benzene ring and form a Co−Co bond, leading to a cis-(η4,η4-μ-C6R3R′3)Co2(CO)4 structure with a central bent benzene ring. The third type of (RC2R′)3Co2(CO)4 isomer also has a bridging but nearly planar benzene ring with the cobalt atoms on opposite sides of the ring and thus a trans-(η3,η3-μ-C6R3R′3)Co2(CO)4 structure. This isomer, which has no Co−Co bond and two independent (allylic)Co(CO)2 units with 16-electron configurations for the cobalt atoms, is found to be the highest energy isomer in all of the (RC2R′)3Co2(CO)4 systems studied in this work. This relatively high-energy structure is not likely to be an intermediate in the cyclotrimerization of alkynes catalyzed by cobalt carbonyls. The cyclotrimerization of alkynes by cobalt carbonyls can involve both the flyover complexes (η3,1,η3,1-μ-C6R3R′3)Co2(CO)4 and the bridging bent benzene complexes cis(η4,η4-μ-C6R3R′3)Co2(CO)4 as intermediates (Figure 7). Thus,

■

ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S9: Atomic coordinates of the optimized structures for the (RC2R′)3Co2(CO)4 (R = R′ = H, CH3, CF3) complexes; Tables S10−S15: Atomic coordinates of the optimized structures for the (RC2R′)3Co2(CO)4 (R = H, while R′ = tBu; R = H, while R′ = CF3) complexes; Tables S16−S24: Harmonic vibrational frequencies and infrared intensities for the (RC2R′)3Co2(CO)4 (R = R′ = H, CH3, CF3) complexes; Tables S25−S30: Harmonic vibrational frequencies and infrared intensities for the (RC2R′)3Co2(CO)4 (R = H, while R′ = tBu; R = H while R′ = CF3) complexes; Tables S31−S35: Total energies, relative energies, Co−Co bond lengths, predicted by three different functionals for the different structures; complete Gaussian09 reference (ref 28). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge research support by the Program for New Century Excellent Talents in University, Grant No. NCET-100949, the Chinese National Natural Science Foundation (Grant No. 11174236), and the U.S. National Science Foundation (Grants CHE-1057466 and CHE-1054286).

■

REFERENCES

(1) Hübel, W.; Wender, I.; Pino, P. Organic Syntheses via Metal Carbonyls; J. Wiley and Sons: New York, 1968. (2) Krüerke, U.; Hübel, W. Chem. Ber. 1961, 94, 2829−2856. (3) Dickson, R. S.; Fraser, P. J.; Gatehouse, B. M. J. Chem. Soc., Dalton Trans. 1972, 2278−2282. (4) Dickson, R.; Yawney, D. Aust. J. Chem. 1969, 22, 533−541. (5) Dickson, R.; Yawney, D. Aust. J. Chem. 1967, 20, 77−84. (6) Baxter, R. J.; Knox, G. R.; Pauson, P. L.; Spicer, M. D. Organometallics 1998, 18, 197−205. (7) Baxter, R. J.; Knox, G. R.; Moir, J. H.; Pauson, P. L.; Spicer, M. D. Organometallics 1998, 18, 206−214. (8) Whitesides, G. M.; Ehmann, W. J. J. Am. Chem. Soc. 1969, 91, 3800−3807. (9) Sternberg, H. W.; Greenfield, H.; Friedel, R. A.; Wotiz, J.; Markby, R.; Wender, I. J. Am. Chem. Soc. 1954, 76, 1457−1458. (10) Tilney-Bassett, J. F.; Mills, O. S. J. Am. Chem. Soc. 1959, 81, 4757−4758. (11) Dunning, T. H. J. Chem. Phys. 1970, 53, 2823−2833. (12) Huzinaga, S. J. Chem. Phys. 1965, 42, 1293−1302. (13) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033−1036. (14) Hood, D. M.; Pitzer, R. M.; Schaefer, H. F. J. Chem. Phys. 1979, 71, 705−712. (15) Ziegler, T.; Autschbach, J. Chem. Rev. 2005, 105, 2695−2722. (16) Bühl, M.; Kabrede, H. J. Chem. Theory Comput. 2006, 2, 1282− 1290.

Figure 7. Conversion of a flyover compound to a bridged bent benzene ring compound as an intermediate step in the cyclotrimerization of alkynes by cobalt carbonyls.

coupling the three alkyne units in a dicobalt system can first lead to the flyover compounds where the six-carbon unit is still an open chain, albeit one bridging the two cobalt atoms to form a Co2C6 eight-membered ring. Forming a C−C bond between the end carbon atoms of the C6 chain in the flyover compounds can then generate the benzene ring in the bridging bent benzene derivatives, in which the Co−Co bond forces the benzene ring to bend. The relative energies of the flyover (η3,1,η3,1-μ-R3R′3C6)Co2(CO)4 complexes and bridging bent benzene complexes cis(η4,η4-μ-R3R′3C6)Co2(CO)4 depend on the substituents in the six-carbon chain. For relatively small substituents R and R′ such as hydrogen and methyl, the bridged bent benzene complexes have lower energies. To date, no flyover complexes have been isolated as stable compounds. However, for larger substituents, such as trifluoromethyl and particularly tert-butyl, the flyover complexes have lower energies and have been isolated experimentally as stable compounds.2 The stabilization of 2356


Organometallics

Article

(17) Brynda, M.; Gagliardi, L.; Widmark, P. O.; Power, P. P.; Roos, B. O. Angew. Chem., Int. Ed. 2006, 45, 3804−3807. (18) Sieffert, N.; Bühl, M. J. Am. Chem. Soc. 2010, 132, 8056−8070. (19) Schyman, P.; Lai, W.; Chen, H.; Wang, Y.; Shaik, S. J. Am. Chem. Soc. 2011, 133, 7977−7984. (20) Adams, R. D.; Pearl, W. C.; Wong, Y. O.; Zhang, Q.; Hall, M. B.; Walensky, J. R. J. Am. Chem. Soc. 2011, 133, 12994−12997. (21) Lonsdale, R.; Oláh, J.; Mulholland, A. J.; Harvey, J. N. J. Am. Chem. Soc. 2011, 133, 15464−15474. (22) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (23) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789. (24) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824. (25) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100. (26) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157−167. (27) Gusev, D. Organometallics 2013, 32, 4239−4243. (28) Frisch, M. J.; et al. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009 (see Supporting Information for the full reference). (29) Krüerke, U.; Hoogzand, C.; Hübel, W. Chem. Ber. 1961, 94, 2817−2820.

2357


Flyover Compounds and Bridging Bent Benzene Derivatives as

Recommend Documents