Dicationic Palladium(II) Complexes as Active Lewis Acid Catalysts for

Jul 12, 2010 - Polarized Nazarov Cyclization. Jing Zhang, Tulaza Vaidya, William W. Brennessel, Alison J. Frontier,* and. Richard Eisenberg*. Departme...
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Organometallics 2010, 29, 3341–3349 DOI: 10.1021/om100274g

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Dicationic Palladium(II) Complexes as Active Lewis Acid Catalysts for Polarized Nazarov Cyclization Jing Zhang, Tulaza Vaidya, William W. Brennessel, Alison J. Frontier,* and Richard Eisenberg* Department of Chemistry, University of Rochester, Rochester, New York 14627 Received April 6, 2010

Two new bidentate phosphonamidite ligands - 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1phosphapentaleno)ethane, L1, and 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)benzene, L2 - were prepared from (S)-R,R-diphenylprolinol and either 1,2-bis(dichlorophosphino)ethane or 1,2-bis(dichlorophosphino)benzene, respectively. Reactions of L1 and L2 with one equivalent of Pd(PhCN)2Cl2 in dichloromethane result in the formation of the neutral complexes Pd(L1-P,P)Cl2, 2, and Pd(L2-P,P)Cl2, 3, in good yields (85% and 92%, respectively). Complexes 2 and 3 were fully characterized by NMR spectroscopy, elemental analyses, and X-ray crystallography. Chloride abstraction from 1 and 2 using two equivalents of AgSbF6 in the presence of excess diethylisopropylidene malonate (DIM) leads to the formation of the respective dicationic palladium complexes [Pd(L1-P,P)(DIM)][SbF6]2, 4, and [Pd(L2P,P)(DIM)][SbF6]2, 5. As a result of the cationic nature of 4 and 5 and the labile coordination of DIM, these complexes exhibit high activity as electrophilic catalysts for the Nazarov cyclization of electronically polarized substrates. For those substrates in which a quaternary carbon center is generated upon Nazarov cyclization, spirocycle formation occurs via a Wagner-Meerwein rearrangement following cyclization. In these cases, addition of NaBAr4f as a cofactor to the reaction system improves the selectivity to spirocycle formation relative to the simple Nazarov product.

Introduction Cationic palladium(II) complexes of the type of [Pd(diphosphine)(solvent)]2þ have been studied extensively over the past decade as electrophilic and/or Lewis acidic catalysts for important transformations including hydroarylation1 and hydroamination2 of olefins, Friedel-Crafts alkylation,3 Diels-Alder cycloddition,4 aldol condensation,5 enantioselective glyoxylateand ketone-ene reactions,6 asymmetric addition of organometallic reagents to R,β-unsaturated carbonyl compounds,7 and *To whom correspondence should be addressed. E-mail: eisenberg@ chem.rochester.edu, [email protected]. (1) Cucciolito, M. E.; D’Amora, A.; Tuzi, A.; Vitagliano, A. Organometallics 2007, 26, 5216. (2) Phua, P. H.; White, A. J. P.; de Vries, J. G.; Hii, K. K. Adv. Synth. Catal. 2006, 348, 587. (3) Doherty, S.; Knight, J. G.; Smyth, C. H.; Harrington, R. W.; Clegg, W. Organometallics 2007, 26, 5961. (4) Pignat, K.; Vallotto, J.; Pinna, F.; Strukul, G. Organometallics 2000, 19, 5160. (5) (a) Sodeoka, M.; Ohrai, K.; Shibasaki, M. J. Org. Chem. 1995, 60, 2648. (b) Nishikata, T.; Kobayashi, Y.; Kobayshi, K.; Yamamoto, Y.; Miyaura, N. Synlett 2007, 3055. (c) Hamashima, Y.; Sodeoka, M. Chem. Rec. 2004, 4, 231. (d) Umebayashi, N.; Hamashima, Y.; Hashizume, D.; Sodeoka, M. Angew. Chem., Int. Ed. 2008, 47, 4196. (6) (a) Becker, J. J.; Van Orden, L. J.; White, P. S.; Gagne, M. R. Org. Lett. 2002, 4, 727. (b) Luo, H. K.; Khim, L. B.; Schumann, H.; Lim, C.; Jie, T. X.; Yang, H. Y. Adv. Synth. Catal. 2007, 349, 1781. (c) Mikami, K.; Kawakami, Y.; Akiyama, K.; Aikawa, K. J. Am. Chem. Soc. 2007, 129, 12950. (d) Aikawa, K.; Hioki, Y.; Mikami, K. J. Am. Chem. Soc. 2009, 131, 13922. (7) (a) Nishikata, T.; Yamamoto, Y.; Gridnev, I. D.; Miyaura, N. Organometallics 2005, 24, 5025. (b) Nishikata, T.; Yamamoto, Y.; Miyaura, N. Chem. Lett. 2007, 36, 1442. (c) Nishikata, T.; Kiyomura, S.; Yamamoto, Y.; Miyaura, N. Synlett 2008, 2487. r 2010 American Chemical Society

copolymerization of carbon monoxide and olefins.8 These complexes are characterized by defined square-planar coordination geometries, often with adjacent coordination sites occupied by labile solvent molecules and poorly coordinating ligands. Additionally, these cationic complexes are often salts of noncoordinating anions. Through the extraordinary variety of ligands that have been studied for Pd(II) catalysis, the complexes have been shown to be electronically and sterically tunable. In this paper, we report new cationic palladium complexes that contain a new chiral chelating ligand and show that these complexes are catalytically active for Nazarov cyclization and for a particular set of polarized Nazarov substrates, spirocycle formation that occurs following ring closure. Our interest in Nazarov cyclization chemistry of polarized substrates, i.e., divinyl and aryl vinyl carbonyl substrates having one electron-rich and one electron-poor double bond, was stimulated by (1) the fact that the Nazarov reaction can be used in the synthesis of natural products and bioactive molecules containing highly functionalized five-membered carbocycles9 and (2) the discovery that the cationic iridium(III) complex [Ir(Me)(CO)(dppe)(DIB)][BArf4]2, 1 (where dppe = 1,2bis(diphenylphosphino)ethane, DIB=1,2-diiodobenzene, and BArf4- = B(3,5-(CF3)2C6H3)4), is an active catalyst for the (8) Bajracharya, G. B.; Koranne, P. S.; Tsujihara, T.; Takizawa, S.; Onitsuka, K.; Sasai, H. Synlett 2009, 310. (9) (a) Janka, M.; He, W.; Frontier, A. J.; Flaschenriem, C.; Eisenberg, R. Tetrahedron 2005, 61, 6193. (b) He, W.; Huang, J.; Sun, X.; Frontier, A. J. J. Am. Chem. Soc. 2008, 130, 300. (c) Malona, J. A.; Cariou, K.; Frontier, A. J. J. Am. Chem. Soc. 2009, 131, 7560. Published on Web 07/12/2010

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Nazarov reaction, as well as for cationic polymerizations of electron-rich olefins.10 Over the past decade, several transition metal complexes have been reported for promoting the Nazarov cyclization,11 and with chiral ligands present, modest to good enatioselectivities have been obtained. The latter reports include catalysts such as Sc(pybox)(OTf)3,12 Cu(pybox)(OTf)2,13 and Ni(bis{(R)-1-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}cyclohexylphosphine)(THF)][ClO4]2.14 For complex 1, however, unselective binding of the substrate carbonyl oxygen atoms to the two adjacent but inequivalent binding sites at the iridium center generates two isomers, A and B, as observed by low-temperature 1H NMR spectroscopy.9a,15 The binding selectivity might explain why the resulting asymmetric induction is poor despite the use of a chiral bisphosphine ligand such as (R)-(þ)-BINAP (where (R)-(þ)-BINAP=(R)-(þ)-2,20 -bis(diphenylphosphino)-1,10 -binaphthyl).16

The notion that square-planar palladium(II) complexes of the type [Pd(PP*)(solvent)2]2þ (PP* = chiral bidentate phosphine ligand) would have C2 symmetry with two adjacent and equivalent binding sites at the metal center suggested that such a complex could improve the enantioselectivity of Nazarov cyclization catalysis, thus stimulating the present study. We describe herein the synthesis and characterization of two new dicationic palladium(II) complexes containing new bidentate phosphonamidite ligands, [Pd(L1-P,P)(DIM)][SbF6]2 (4) (L1=1,2-bis((3RS)-3,3diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)ethane) and [Pd(L2-P,P)(DIM)][SbF6]2 (5) (L2=1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)benzene), and their catalytic properties in the Nazarov cyclization reaction and spirocycle formation with polarized substrates.

Zhang et al.

P-chiral monodentate and bidentate ligands have been reported by Reetz,17 Dorta,18 and Schmalz,19 while rhodium complexes containing some of these ligands have been investigated for asymmetric hydrogenation of olefins17a and hydroboration of styrene.17b,19 Following a procedure similar to that used for the preparation of 1-methyl-3,3-diphenyltetrahydro-3H-pyrrolo[1,2-c][1,3,2]oxazaphosphole,17b (S)-R,R-diphenylprolinol was reacted with half an equivalent of 1,2-bis(dichlorophosphino)ethane in the presence of excess triethylamine at 0 °C to result in the formation of 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa6R-aza-1-phosphapentaleno)ethane (L1) in 55% yield (eq 1). The 31P NMR spectrum of L1 shows a singlet at δ 167.7, indicating that the two phosphorus atoms of L1 are magnetically equivalent. The resonance is shifted downfield by 22 ppm in comparison to the corresponding resonance in 1,2-bis(dichlorophosphino)ethane (δ 189.8). The 1H NMR spectrum of L1 exhibits a group of signals containing one doublet (δ 7.47, 2 H) and two triplets (δ 7.11, 2 H and 7.05, 1 H) for one phenyl group, and another doublet (δ 7.28, 2 H) and two triplets (δ 6.99, 2 H and 6.92, 1H) for the second phenyl group, indicating inequivalence of the two prolinol phenyl groups. Ligand L1 was also characterized by 13 C{1H} NMR spectroscopy and elemental analysis. Similarly, when (S)-R,R-diphenylprolinol was reacted with half an equivalent of 1,2-bis(dichlorophosphino)benzene in the presence of excess triethylamine at 0 °C for 6 h, 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)benzene (L2) was obtained as an analytically pure solid in 35% yield (eq 2). Ligand L2 has a relatively rigid o-phenylene backbone instead of an ethylene group as in L1. The 31P{1H} NMR spectrum of L2 shows a singlet at δ 145.3, which corresponds to an upfield shift of ∼7 ppm relative to that of 1,2-bis(dichlorophosphino)benzene and confirms the equivalence of the two phosphorus atoms in the ligand. The identity of L2 was further confirmed by its 1H NMR and 13C{1H} NMR spectra as well as by its elemental analysis.

Results and Discussion Synthesis and Characterization of New Chiral Bidentate Ligands 1,2-Bis((3rS)-3,3-diphenyltetrahydro-2-oxa-6r-aza-1phosphapentaleno)ethane (L1) and 1,2-Bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6a-aza-1-phosphapentaleno)benzene (L2). (S)-R, R-Diphenylprolinol is a commercially available compound that can also be prepared easily from L-proline. Its derived (10) (a) Albietz, P. J., Jr.; Cleary, B. P.; Paw, W.; Eisenberg, R. J. Am. Chem. Soc. 2001, 123, 12091. (b) Albietz, P. J., Jr.; Cleary, B. P.; Paw, W.; Eisenberg, R. Inorg. Chem. 2002, 41, 2095. (11) (a) He, W.; Herrick, I. R.; Atesin, T. A.; Caruana, P. A.; Kellenberger, C. A.; Frontier, A. J. J. Am. Chem. Soc. 2008, 130, 1003. (b) He, W.; Sun, X.; Frontier, A. J. J. Am. Chem. Soc. 2003, 125, 14278. (c) Huang, J.; Frontier, A. J. J. Am. Chem. Soc. 2007, 129, 8060. (d) Malona, J. A.; Colbourne, J. M.; Frontier, A. J. Org. Lett. 2006, 8, 5661. (e) Fujiwara, M.; Kawatsura, M.; Hayase, S.; Nanjo, M.; Itoh, T. Adv. Synth. Catal. 2009, 351, 123. (f) Kawatsura, M.; Higuchi, Y.; Hayase, S.; Nanjo, M.; Itoh, T. Synlett 2008, 1009. (g) Walz, I.; Bertogg, A.; Togni, A. Eur. J. Org. Chem. 2007, 2650. (12) (a) Liang, G.; Gradl, S. N.; Trauner, D. Org. Lett. 2003, 5, 4931. (b) Liang, G.; Trauner, D. J. Am. Chem. Soc. 2004, 126, 9544. (13) Aggarwal, V. K.; Belfield, A. J. Org. Lett. 2003, 5, 5075. (14) Walz, I.; Togni, A. Chem. Commun. (Cambridge, U. K.) 2008, 4315. (15) Janka, M.; He, W.; Frontier, A. J.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 6864. (16) (a) Atesin, A. C.; Zhang, J.; Vaidya, T.; Brennessel, W. W.; Frontier, A. J.; Eisenberg, R. Inorg. Chem. 2010, 49, 4331. (b) Vaidya, T.; Atesin, A. C.; Herrick, I. R.; Frontier, A. J.; Eisenberg, R. Angew. Chem., Int. Ed. 2010, 49, 3363.

Synthesis of Complexes Pd(L1-P,P)Cl2 (2) and Pd(L2-P,P)Cl2 (3). Reaction of Pd(PhCN)2Cl2 with one equivalent of 1,2bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)ethane, L1, in dichloromethane results in the formation of (17) (a) Reetz, M. T.; Mehler, G.; Bondarev, O. Chem. Commun. (Cambridge, U. K.) 2006, 2292. (b) Bondarev, O. G.; Goddard, R. Tetrahedron Lett. 2006, 47, 9013. (18) Mariz, R.; Briceno, A.; Dorta, R. Organometallics 2008, 27, 6605. (19) (a) Kranich, R.; Eis, K.; Geis, O.; Muhle, S.; Bats, J. W.; Schmalz, H. G. Chem.-Eur. J. 2000, 6, 2874. (b) Blume, F.; Zemolka, S.; Fey, T.; Kranich, R.; Schmalz, H. G. Adv. Synth. Catal. 2002, 344, 868.

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Pd(L1-P,P)Cl2, 2, in 85% yield (eq 3). The 31P{1H} NMR spectrum of 2 exhibits one singlet at δ 176.2, indicating a downfield shift of 8 ppm relative to the free ligand L1. Colorless crystals suitable for X-ray diffraction were obtained by slow diffusion of hexanes into a concentrated solution of 2 in dichloromethane at room temperature. Crystallographic data, intensity data collection, and refinement parameters are shown in Table 1, while complete structural information can be found from the CIF supplied in the Supporting Information. An ORTEP diagram of the structure of 2 is shown in Figure 1, with selected bond distances and angles given in Table 2.

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Table 1. Crystal Data, Data Collection, and Structure Refinement Parameters for Pd(L1-P,P)Cl2, 2, and Pd(L2-P,P)Cl2, 3 empirical formula fw temperature wavelength cryst syst space group unit cell dimens

C40H46Cl10N2O2P2Pd 1109.63 100.0(1) K 0.71073 A˚ orthorhombic P212121 a = 19.357(3) A˚ b = 22.915(3) A˚ c = 31.461(3) A˚

volume Z density (calcd) absorp coeff F(000) cryst color, morphology cryst size

13955(3) A˚3 12 1.584 Mg/m3 1.079 mm-1 6744 colorless, block

C40.5H39Cl3N2O2P2Pd 860.43 100.0(1) K 0.71073 A˚ monoclinic P21 a = 10.642(1) A˚ b = 24.796(3) A˚ c = 15.569(2) A˚ β = 101.667(2)° 4023.2(9) A˚3 4 1.421 Mg/m3 0.776 mm-1 1756 colorless, plate

0.24  0.24  0.20 mm3 1.52 to 33.14°

0.22  0.16  0.05 mm3 1.57 to 28.28°

-29 e h e 29, -35 e k e 34, -47 e l e 47 202 837 52 835 [R(int) = 0.1012] 35 381 99.4% multiscan 0.8131 and 0.7817

-14 e h e 14, -33 e k e 33, -20 e l e 20 56 617 19 941 [R(int) = 0.0960] 13 599 99.9% multiscan 0.9622 and 0.8478

52 835/0/1540

19 941/1/910

1.009

0.936

R1 = 0.0697, wR2 = 0.1517 R1 = 0.1179, wR2 = 0.1764 -0.02(2)

R1 = 0.0562, wR2 = 0.0957 R1 = 0.0898, wR2 = 0.1060 -0.01(2)

4.011 and -1.487 e A˚-3

0.853 and -0.675 e A˚-3

θ range for data collection index ranges reflns collected indep reflns

The molecular structure of 2 shows a square-planar coordination geometry of the Pd(II) center with two cis phosphine donors of L1 and two chloride ligands. The complex possesses approximate C2 symmetry with the two phosphorus atoms of L1 in S configuration. The bond distances and angles in 2 are close in value to related Pd(II) dichloride di(phosphine) complexes such as Pd(dppe)Cl220 and Pd((R)-(þ)-BINAP)Cl2 (2.245 A˚).21 The analogous complex with L2 as the ligand, Pd(L2-P,P)Cl2 (3), was obtained by the reaction of L2 with one equivalent of Pd(PhCN)2Cl2 in 92% yield (eq 4). The 31P{1H} NMR spectrum of 3 exhibits one singlet at δ 162.7, suggesting a downfield chemical shift of 17 ppm relative to free L2. Colorless crystals suitable for X-ray diffraction were obtained by slow diffusion of pentane into a concentrated solution of 3 in dichloromethane at room temperature. Table 1 contains crystallographic data, intensity data collection, and refinement parameters, with complete structural details in the CIF format (see Supporting Information). An ORTEP drawing of the structure of 3 is shown in Figure 2, with selected bond distances and angles listed in Table 3.

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obsd reflns completeness to θ absorp corr max. and min transmn data/restraints/ params goodness-of-fit on F2 final R indices [I > 2σ(I )] R indices (all data) absolute struct param largest diff peak and hole

Figure 1. ORTEP diagram of 2 with thermal ellipsoids drawn at the 50% level. All hydrogen atoms and cocrystallized solvent molecules (CH2Cl2) have been removed for clarity.

The molecular structure of 3 confirms a Pd(II) square-planar coordination geometry defined by the two phosphorus atoms (20) Steffen, W. L.; Palenik, G. J. Inorg. Chem. 1976, 15, 2432. (21) Ozawa, F.; Kubo, A.; Matsumoto, Y.; Hayashi, T.; Nishioka, E.; Yanagi, K.; Moriguchi, K. Organometallics 1993, 12, 4188.

of L2 and two chloride atoms. The P1-Pd1-P2 angle of 87.41(5)° in 3 is essentially equivalent to that of 2 (86.85(5)°). The ligand L2 possesses an approximate C2 symmetry with the two phosphorus donor atoms in S configuration. The bond distances are similar to those of 2 and related Pd(PP)Cl2 complexes noted above.

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Zhang et al. Scheme 1

Figure 2. ORTEP diagram of 3 with thermal ellipsoids drawn at the 50% level. All hydrogen atoms and cocrystallized solvent molecules (CH2Cl2) have been removed for clarity. Table 2. Selected Bond Lengths and Angles of Pd(L1-P,P)Cl2, 2 Bond Length (A˚) Pd(1)-P(1) Pd(1)-Cl(1) P(1)-O(11) P(2)-O(12)

2.222(1) 2.364 (1) 1.614(4) 1.612(4)

Pd(1)-P(2) Pd(1)-Cl(2) P(1)-N(11) P(2)-N(12)

2.214(1) 2.357(1) 1.676(5) 1.636(5)

Bond Angles (deg) P(1)-Pd(1)-P(2) P(2)-Pd(1)-Cl(2) Cl(2)-Pd(1)-P(1)

86.85(5) 89.66(5) 173.37(5)

P(1)-Pd(1)-Cl(1) Cl(1)-Pd(1)-Cl(2) Cl(1)-Pd(1)-P(2)

89.44(5) 94.55(5) 173.43(5)

Catalytic Studies of Nazarov Cyclization and Spirocycle Formation of Aryl Vinyl β-Ketoesters Using 4 and 5. Initial catalytic studies were carried out using the aryl vinyl β-ketoester 6a. Using 10 mol % of either 4 or 5 in dichloromethane at room temperature, 6a was converted to the Nazarov product 6b in 90% or 85% yield, respectively, after 24 h (eq 5). The catalytic activity of 4 and 5 for this reaction is thus similar to that of Cu(OTf)2 but slower than that of dicationic Ir(III) dppe complex 1.9a,11a In view of the chiral nature of L1 and L2, the enantioselectivity of 6b in each reaction was measured and found to be 5% and 3% for catalysis by 4 or 5, respectively.

Table 3. Selected Bond Lengths and Angles of Pd(L2-P,P)Cl2, 3 Bond Length (A˚) Pd(1)-P(1) Pd(1)-Cl(1) P(1)-O(11) P(2)-O(12)

2.207(1) 2.362(2) 1.602(4) 1.608(4)

Pd(1)-P(2) Pd(1)-Cl(2) P(1)-N(11) P(2)-N(12)

2.213(2) 2.350(1) 1.646(4) 1.646(5)

Bond Angles (deg) P(1)-Pd(1)-P(2) P(2)-Pd(1)-Cl(2) Cl(2)-Pd(1)-P(1)

87.41(5) 89.75(5) 177.13(6)

P(1)-Pd(1)-Cl(1) Cl(1)-Pd(1)-Cl(2) Cl(1)-Pd(1)-P(2)

89.68(5) 93.14(5) 176.70(5)

Synthesis and Characterization of Dicationic Complexes [Pd(L1-P,P)(DIM)][SbF6]2 (4) and [Pd(L2-P,P)(DIM)][SbF6]2 (5) (DIM = diethylisopropylidene malonate). In order to obtain dicationic palladium(II) complexes having adjacent labile sites, chloride abstraction from Pd(L1-P,P)Cl2, 2, using two equivalents of AgSbF6 was carried out in the presence of DIM in dichloromethane. The resultant complex [Pd(L1-P,P)(DIM)][SbF6]2, 4, was obtained in 71% yield (eq 3). The 31P{1H} NMR spectrum of 4 exhibits a singlet at δ 181.0, which indicates a downfield shift of 5 ppm compared to that of 2. The presence of DIM in 4 was confirmed by its 13C{1H} NMR spectrum with a singlet at δ 174.0, corresponding to the carbonyl carbons of the ligand and indicating chelation to Pd(II) by their equivalence. The chemical shift of the carbonyl carbon of free DIM is δ 165.9. Likewise, reaction of Pd(L2-P,P)Cl2, 3, with two equivalents of AgSbF6 in the presence of excess DIM results in the formation of [Pd(L2-P,P)(DIM)][SbF6]2, 5, in 76% yield (eq 4). The 31P{1H} NMR spectrum of 5 exhibits a singlet at δ 160.2, which is 2.5 ppm upfield that of 3, and the 13 C{1H} NMR spectrum shows only one singlet at δ 174.0 for the two carbonyl carbon atoms, consistent with the proposed C2 symmetric complex.

Nazarov Cyclization Catalysis Leading to Quaternary Carbon Generation and Spirocycle Formation. β-Ketoester alkylidene substrates having a methyl substituent at the C1 position were next examined (eq 6). It has previously been reported that for substrates having a methyl substituent at C1, spirocycle formation can occur in competition with simple Nazarov cyclization.11c In eq 6, substrate a reacts to form the Nazarov cyclization product b or one of two spirocycle products, c or d.11c,16a Scheme 1 illustrates the branching point in the reaction pathway of the palladium-bound oxyallyl cation intermediate formed upon conrotation. Proton elimination generates the Nazarov product exclusively, while two types of spirocyclic products can form depending on the nature of the R group. The formation of spirocycle products has been previously analyzed and is thought to occur via a WagnerMeerwin rearrangement upon ring closure that involves a C-C bond shift and subsequent migration of a R or H group from the C5 position to C1.11c,16a The mechanism of this single-pot transformation and migratory aptitude of (22) Spek, A. L. PLATON: A multipurpose crystallographic tool; version 30016; Utrecht University: Utrecht, The Netherlands: 2003. (23) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579.

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various R groups have been elucidated in detail using Cu(II)24 and dicationic Ir(III) catalysts.11c,16a

When 7a is treated with catalytic amounts of 4 or 5 (10 mol %), a single diastereomer from Nazarov cyclization, 7b, is isolated as the final product with the relative configurations at C1 and C5 corresponding to conrotation of the Z isomer of 7a in high yield (Table 4, entries 1 and 2). This observation is in agreement with E/Z isomerization of 7a prior to cyclization and results obtained using Cu(OTf)2 as the catalyst.11a This transformation does not proceed without a catalyst.11a In order to assess the activity of a more common chiral di(phosphine) palladium complex, Pd((R)(þ)-BINAP)Cl2 was prepared and treated with two equivalents of AgSbF6 in the presence of excess DIM in dichloromethane. After removal of the precipitate (AgCl), the resulting yellow solution was used in situ as the catalyst for the cyclization of 7a. The Nazarov product 7b was isolated after 12 h at room temperature in 90% yield and an ee of 3%. Despite the presence of chiral ligands L1, L2, or (R)-(þ)BINAP in these palladium complexes, the resultant asymmetric induction was minimal. These observations suggest that the site of cyclization (C1, C5) that leads to the formation of the carbon stereocenter may be too far from the chiral phosphine ligand to cause significant levels of enantioinduction. Interestingly, when substrate 8a, with a p-methoxyphenyl substituent at the C5 position, is reacted in the presence of 10 mol % of 4, Nazarov product 8b is obtained in only 19% yield, while spirocycle 8d, in which the p-methoxyphenyl group has migrated from C5 to C1, is formed in 57% yield (entry 5). It is worth noting that the formation of 8d from 8a as reported by Huang and Frontier employed a stoichiometric amount of Cu(SbF6)2.11c Similarly, catalyst 5 promotes the cyclization of 8a, affording 8b and 8d in yields of 29% and 58%, respectively (entry 6). In these studies, the Nazarov and spirocycle products are identified by their 1H NMR spectra and ESI mass spectrometric data, and the relative amounts are determined using the NMR data. All of the products have been fully characterized previously.11c,16a While 9a yields only the Nazarov product upon reaction (entry 7), substrate 10a, with a cinnamyl group at the C5 position, yields both Nazarov and spirocycle products with relative amounts of each product depending on the catalyst. With 4 as the catalyst, 10b is obtained as the major product (71%) accompanied by a smaller amount of spirocycle 10d (24%), in which the cinnamyl group has shifted from C5 to C1 (entry 8). In contrast, the reaction of 10a with 5 under identical conditions leads predominantly to spirocycle 10d (24) APEX2, 3.0 ed.; Bruker AXS: Madison, WI, 2009.

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(47%, entry 9). For this reaction, 10b is only a minor product component (4%). Substrate 11a, with a cycloheptenyl ring instead of a cyclohexenyl ring, undergoes cyclization more rapidly with catalysts 4 and 5 than does 10a. The product distribution is, however, dependent on the catalyst. When complex 4 is used, the only product is the Nazarov cyclization product 11b (95%), with no spirocycle products formed (entry 10). On the other hand, when 5 is used, a mixture of 11b and 11d in a nearly 1:1 ratio (40% and 45%, respectively) is obtained (entry 11). In a recent study of Nazarov cyclization and spirocycle formation with substrates 7a-11a catalyzed by cationic Ir(III) complexes,16a it was found that the addition of certain salts can influence the relative product distributions significantly. In particular, the addition of a stoichiometric equivalent (with respect to the substrate) of NaBAr4f to the reaction solutions was observed to alter the product distribution in favor of spirocycle formation. The rationale for the change in product distribution with added NaBAr4f was that Naþ ions bind to the substrate (and product) carbonyl oxygen atoms to inhibit the reversible proton loss and addition needed to complete the Nazarov cyclization. Instead, if Hþ loss is inhibited, a C-C shift occurs that results ultimately in spirocycle formation following R or H migration from C5 to C1.16a On the basis of the previously reported observations and analysis, the effect of NaBAr4f on product distribution was examined using 4 and 5 as catalysts. When 7a was treated with 10 mol % of 5 and one equivalent of NaBAr4f at room temperature for 5.5 h, spirocycle 7c was formed in good yield (85%), while Nazarov product 7b was obtained in just 7% yield (entry 4). Catalyst 5 was better than 4 for spirocycle formation in terms of yield and enantioselectivity (compare entries 3 and 4). Similarly, the formation of spirocycle 11d from the cyclization of 11a was improved significantly by the addition of one equivalent of NaBAr4f (65%), while the yield of Nazarov product 11b was reduced to 11% (entry 12). The influence of NaBAr4f on the product distribution using the Pd(II) cationic 5 is more pronounced than what was previously reported with the Ir(III) catalyst [Ir(CO)(Me)(DIM)(R-(þ)-BINAP)]2þ(X)2 (where X- = SbF6-, BAr4f-)16a and may therefore expand the utility of the transformations seen for the synthesis of more complex natural products and bioactive molecules.

Conclusion Two neutral palladium(II) dichloride complexes, 2 and 3, containing new bidentate phosphonamidite ligands L1 and L2 have been synthesized and fully characterized. X-ray diffraction of 2 and 3 indicates the two phosphorus atoms in L1 and L2 are in the S configuration. The corresponding dicationic palladium(II) complexes [Pd(L1-P,P)(DIM)][SbF6]2 (4) and [Pd(L2-P,P)(DIM)][SbF6]2 (5), with the weakly coordinating ligand DIM and a hexafluoroantimonate(VI) counteranion, were prepared from 2 and 3, respectively. Both 4 and 5 exhibit Lewis acidity and are effective catalysts for the Nazarov cyclization of polarized substrates. For those substrates in which a quaternary C is generated upon conrotation, competing formation of a spirocyclic product is also observed. The product distribution is also found to be influenced by the addition of NaBArf, with spirocycle formation being favored significantly as the amount of NaBArf is increased relative to substrate. The effect is based on the presence of Naþ and is explained on the basis of proton loss from the

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Table 4. Catalysis of Nazarov Cyclization and Spirocycle Formation Using 4 and 5

a Reaction conditions: substrate (5 mM) and catalyst (0.5 mM) in dichloromethane. b Isolated yields. c NaBAr4f (5 mM) was used as additive. d 80% of 9a was recovered; TMP = 2,4,6-trimethoxyphenyl; PMP = 4-methoxyphenyl; PNP = 4-nitrophenyl.

Article

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Nazarov ring-closed intermediate versus a C-C shift to form a spirocycle.

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Anal. Calcd for C36H38N2O2P2: C, 72.96, H, 6.46, N, 4.73. Found: C, 72.92, H, 6.39, N, 4.61.

Experimental Section General Methods. Unless otherwise stated, all the reactions and manipulations were performed in oven-dried glassware under a purified nitrogen atmosphere using either standard Schlenk techniques or an inert-atmosphere glovebox. All solvents (dichloromethane, THF, toluene, hexanes, and diethyl ether) were purified and dried under nitrogen by conventional methods. Dichloromethane-d2 and benzene-d6 were purchased from Cambridge Isotope Laboratories, degassed, and stored in dry glovebox. (S)-R,R-Diphenylprolinol, 1,2-bis(dichlorophosphino)ethane, and 1,2-bis(dichlorophosphino)benzene were purchased from Strem Chemicals, Inc. NaBAr4f 23 and substrates 6a-11a11c were prepared according to the reported procedures. Elemental analyses were done at the University of Rochester or Quantitative Technologies Inc. (QTI). 1H NMR, 31P{1H} NMR, and 13C{1H} NMR spectra were recorded on a Bruker Avance 400 MHz or a Bruker Avance 500 MHz spectrometer. 1 H and 13C{1H} chemical shifts are reported in parts per million after referencing to the appropriate solvent resonances. 31P{1H} chemical shifts are reported after an external calibration to an 85% solution of phosphoric acid. The assignment of NMR resonances for L1, L2, 2, and 3 were verified using 2D NMR experiments (HSQC). A few broad 13C NMR resonances were observed for complexes 4 and 5, while some signals overlapped.

Synthesis of 1,2-Bis((3aS)-3,3-diphenyltetrahydro-2-oxa-6aaza-1-phosphapentaleno)ethane, L1. An oven-dried 50 mL Schlenk flask was charged with (S)-R,R-diphenylprolinol (0.506 g, 2 mmol) and triethylamine (0.606 g, 6 mmol) in THF (10 mL). The colorless solution was cooled to 0 °C, and 1,2-dichlorophosphinoethane (0.232 g, 1 mmol) was added dropwise maintaining the temperature below 5 °C. Triethylammonium chloride precipitated immediately as a white solid. After 2 h at 0 °C, the mixture was warmed to and stirred at room temperature for an additional 6 h. The precipitate was then removed by filtration, and the colorless filtrate was concentrated under vacuum. The residue was dissolved in THF (3 mL), and hexanes (15 mL) were added to precipitate a white solid. After removing the solid, the filtrate was concentrated to about 9 mL and then cooled at -25 °C for 2 days. The solid that precipitated was separated by filtration and washed with hexanes (3  1.5 mL). Ligand L1 was isolated as a white solid in 55% yield (0.325 g). 31P{1H} NMR (162 MHz, C6D6, δ): 167.7 (s). 1H NMR (400 MHz, C6D6, δ): 0.73-0.79 (m, 2H, P-CHH-), 1.18-1.23 (m, 2H, H4), 1.44-1.55 (m, 6H, P-CHH-, N3, H4), 1.62-1.67 (m, 2H, H3), 3.00-3.02 (m, 2H, H5), 3.15-3.17 (m, 2H, H5), 4.32-4.34 (m, 2H, H2), 6.92 (t, 2H, JHH = 7.2 Hz, Ph-para-H), 6.99 (t, 4H, JHH = 7.2 Hz, Ph-meta-H), 7.05 (t, 2H, JHH = 7.2 Hz, Ph-para-H), 7.11 (t, 4H, JHH = 7.2 Hz, Ph-meta-H), 7.28 (d, 4H, JHH = 7.2 Hz, Ph-ortho-H), 7.47 (d, 4H, JHH = 7.2 Hz, Ph-ortho-H). 13C{1H} NMR (100 MHz, C6D6, δ): 25.7 (s, pyrrol-C4), 29.6 (s, prolinolC3), 30.3 (dd, JPC=33.1 Hz, JPC=26.5 Hz, P-CH2), 51.8 (t, JPC= 14.0 Hz,, prolinol-C5), 71.0 (s, prolinol-C2), 92.7 (t, JPC = 5.7 Hz, C-O-P), 126.9 (s, phenyl-C), 127.2 (s, phenyl-C), 127.3 (s, phenylC), 127.8 (s, phenyl-C, overlapped with C6D6), 128.3 (s, phenyl-C, overlapped with C6D6), 144.7 (s, phenyl-C), 147.3 (s, phenyl-C).

Synthesis of 1,2-Bis((3rS)-3,3-diphenyltetrahydro-2-oxa-6aaza-1-phosphapentaleno)benzene, L2. To an oven-dried 50 mL Schlenk flask were added (S)-R,R-diphenylprolinol (0.506 g, 2 mmol), triethylamine (0.606 g, 6 mmol), and THF (10 mL). The colorless solution was cooled to 0 °C, and a solution of 1,2bis(dichlorophosphino)benzene (0.280 g, 1 mmol) in THF (3 mL) was added dropwise so that the temperature remained within 0 to 5 °C. Triethylammonium chloride was observed immediately as a white precipitate. The reaction mixture was allowed to stir at 0 °C for 2 h and then warmed to room temperature. After 4 h of stirring, the precipitate was isolated after vacuum filtration and the pale yellow filtrate was taken to dryness under vacuum. The residue was dissolved in THF (6 mL), and then hexanes (24 mL) were added. The precipitate so obtained was isolated by filtration, washed with hexanes (3  1.5 mL), and dried under vacuum for several hours. Ligand L2 was obtained in a pure form in 31% yield (0.2 g). 31P{1H} NMR (162 MHz, CD2Cl2, δ): 145.3 (s). 1H NMR (400 MHz, CD2Cl2, δ): 0.58-0.65 (m, 2H, H4), 1.54-1.61 (m, 2H,, H3,), 1.82-1.87 (m, 2H, H3), 2.06-2.15 (m, 2H, H4), 3.32-3.36 (m, 2H, H5), 3.49-3.55 (m, 2H, H5), 4.04 (br d, 2H, JHH =6 Hz, H2), 6.67 (dd, 2H, JPH=3.2 Hz, JHH=5.6 Hz, phenyl-H)), 7.16-7.27 (m, 18H, phenyl-H), 7.39 (d, 4H, JHH =7.6 Hz, phenyl-H). 13C{1H} NMR (100 MHz, CD2Cl2, δ): 25.6 (s, pyrrol-C4), 29.1 (s, prolinol-C3), 50.9 (t, JPC =14.5 Hz, prolinol-C5), 71.1 (s, prolinolC2), 93.6 (t, JPC=4.9 Hz, C-O-P), 127.16 (s, phenyl-C), 127.23 (s, phenyl-C), 127.27 (s, phenyl-C), 127.4 (s, phenyl-C), 128.0 (s, phenyl-C), 128.1 (s, phenyl-C), 129.7 (t, JPC = 6.1 Hz, phenylC), 144.8 (s, phenyl-C), 146.1 (s, phenyl-C), 148.6 (t, JPC = 9.4 Hz, phenyl-C). Anal. Calcd for C40H38 N2O2P2: C, 74.99, H, 5.98, N, 4.37. Found: C, 74.28, H, 5.46, N, 4.33. Synthesis of [(L1-P,P)PdCl2], 2. Ligand L1 (0.059 g, 0.1 mmol), Pd(PhCN)2Cl2 (0.038 g, 0.1 mmol), and CH2Cl2 (5 mL) were added to an oven-dried 20 mL vial. Stirring at room temperature for 8 h led to a change in the color of the reaction mixture from orange to pale yellow. Upon removal of the solvent under vacuum, the residue was redissolved in CH2Cl2 (3 mL). The addition of hexanes (12 mL) provided a yellow solid, which was separated after filtration and washed with hexanes (3  1.5 mL). Complex Pd(L1-P,P)Cl2 (2) was obtained in 85% yield (0.062 g). 31 P{1H} NMR (162 MHz, CD2Cl2, δ): 176.2 (s). 1H NMR (400 MHz, CD2Cl2, δ): 0.80-0.91 (m, 4H, (-P-CH2)2-), 1.11-1.17 (m, 2H, prolinol-H3), 1.75-1.82 (m, 2H, prolinol-H4), 1.84-1.92 (m, 2H, prolinol-H3), 2.09-2.17 (m, 2H, prolinol-C4), 3.24-3.31 (m, 2H, prolinol-H5), 4.17-4.24 (m, 2H, JHH=9.6 Hz, JHH =10.8 Hz, prolinol-H5), 4.49 (dd, 1H, JHH =4.8 Hz, JHH = 9.8 Hz, prolinol-H2), 4.55 (dd, 1H, JHH = 4.8 Hz, JHH = 9.8 Hz, prolinol-H2), 7.24 (t, 2H, JHH=7.2 Hz, phenyl-H), 7.32 (t, 10H, JHH= 7.2 Hz, phenyl-H), 7.45 (d, 4H, JHH = 7.2 Hz, phenyl-H), 7.50-7.52 (m, 4H, phenyl-H). 13C{1H} NMR (100 MHz, C6D6, δ): 26.1 (s, pyrrol-C4), 30.7 (s, prolinol-C3), 35.4 (t, JPC = 32.0 Hz, P-CH2), 51.5 (br s, prolinol-C5), 70.5 (s, prolinol-C2), 93.1 (t, JPC = 2.1 Hz, C-O-P), 126.2 (s, phenyl-C), 127.5 (s, phenyl-C), 128.0 (s, phenyl-C), 128.4 (s, phenyl-C), 128.9 (s, phenyl-C), 129.2 (s, phenyl-C), 141.9 (t, JPC = 3.3 Hz, phenyl-C), 143.6 (s, phenylC). Anal. Calcd for C36H38N2O2P2PdCl2: C, 56.16, H, 4.97, N, 3.64. Found: C, 55.83, H, 4.92, N, 3.54.

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Synthesis of [(L2-P,P)PdCl2], 3. An oven-dried 20 mL vial was charged with L2 (0.064 g, 0.1 mmol), Pd(PhCN)2Cl2 (0.038 g, 0.1 mmol), and CH2Cl2 (5 mL). The orange solution was stirred at room temperature for 12 h to provide a yellow color. Removal of the solvent under vacuum led to a residue, which was redissolved in CH2Cl2 (2.5 mL). The addition of hexanes (20 mL) furnished a yellow precipitate, which was isolated upon filtration and washed with hexanes (3  1.5 mL). Pd (L2-P,P) Cl2 (3) was isolated in 92% yield (0.075 g). 31P{1H} NMR (162 MHz, CD2Cl2, δ): 162.7 (s). 1H NMR (400 MHz, CD2Cl2, δ): 1.51-1.54 (m, 2H, prolinol-H4), 1.95-2.07 (m, 4H, prolinol-H4 and H3), 2.26-2.30 (m, 2H, prolinol-H3), 3.20-3.25 (m, 2H, prolinol-H5), 4.28-4.35 (m, 2H, JHH = 9.1 Hz, JHH= 17.2 Hz, prolinol-H5), 4.93-5.01 (m, 2H, prolinol-H2), 6.49 (br m, 2H, phenyl-H), 6.88-6.90 (m, 2H, phenyl-H), 7.23 (t, 2H, JHH = 7.2 Hz, Phenyl-H), 7.29-7.31 (m, 10H, phenyl-H), 7.51-7.52 (m, 4H, phenyl-H), 7.58 (d, 4H, JHH = 7.6 Hz, phenyl-H). 13C{1H} (100 MHz, CD2Cl2, δ): 27.0 (s, pyrrolC4), 31.7 (s, prolinol-C3), 48.4 (br s, prolinol-C5), 72.5 (s, prolinolC2), 94.1 (t, JPC = 3.7 Hz, C-O-P), 126.4 (s, phenyl-C), 126.8 (s, phenyl-C), 127.7 (s, phenyl-C), 128.2 (s, phenyl-C), 128.8 (s, phenylC), 129.0 (s, phenyl-C), 129.6 (br d, JPC=32.1 Hz, phenyl-C), 129.9 (d, JPC =27.5 Hz, phenyl-C), 132.7 (br s, phenyl-C), 142.1 (br s, phenyl-C), 143.2 (s, phenyl-C), 143.8 (s, phenyl-C), 144.4 (s, phenylC), 144.9 (s, phenyl-C). Anal. Calcd for C40H38Cl2N2O2P2Pd: C, 58.73, H, 4.68, N, 3.42. Found: C, 58.85, H, 4.57, N, 3.41. Synthesis of [(L1-P,P)Pd(DIM)][SbF6]2, 4. To an oven-dried 20 mL vial were added [(L1-P,P)PdCl2] (60 mg, 0.078 mmol), diethylisopropylidene malonate (DIM) (156 mg, 0.78 mmol), and CH2Cl2 (30 mL). After AgSbF6 (55 mg, 0.16 mmol) was added to the yellow solution, the mixture was rapidly stirred at room temperature for 16 h. The precipitate thus formed was removed by vacuum filtration using a pad of Celite. The pale yellow filtrate was concentrated to about 5 mL, and then hexanes (20 mL) were added slowly to precipitate a pale yellow solid. This solid was collected, washed with hexanes (3  1.5 mL), and dried under vacuum for several hours to yield 4 in a 71% yield (76 mg). 31P{1H} NMR (162 MHz, CD2Cl2, δ): 181.0 (s). 1H NMR (400 MHz, CD2Cl2, δ): 1.24 (t, 6H, JHH = 7.2 Hz), 1.22-1.33 (m, 2H), 1.45-1.48 (m, 2H), 1.62-1.65 (m, 2H), 2.00-2.13(m, 4H), 2.22 (br s, 6H), 3.48-3.54 (m, 4H), 4.05-4.20 (m, 4H), 4.22-4.30 (m, 2H), 4.70-4.78(m, 2H), 7.26-7.40 (m, 12H, JHH = 7.2 Hz), 7.44 (d, 4H, JHH = 7.2 Hz), 7.53 (m, 4H, JHH = 6.0 Hz). 13C{1H} NMR (125 MHz, CD2Cl2, δ): 13.7 (s), 25.3 (s), 26.0 (s), 30.9 (s), 34.5 (d, JPC = 36.2 Hz), 34.9 (d, JPC = 33.2 Hz), 50.2 (br s), 67.3 (s), 72.2 (s), 95.7 (br s), 125.3 (s), 125.5 (s), 127.0 (s), 128.7 (s), 128.9 (s), 129.4 (s), 129.9 (s), 140.5 (br s), 141.8 (s), 168.7 (s), 174.0 (s). Anal. Calcd for C46H54F12N2O6P2PdSb2 3 1/4CH2Cl2: C, 39.91, H, 3.94, N, 2.01. Found: C, 39.88, H, 3.95, N, 2.19. The presence of 1/4 molecule of dichloromethane in the sample used for elemental analysis was confirmed by 1H NMR spectroscopy. Synthesis of [(L2-P,P)Pd(DIM)][SbF6]2, 5. An oven-dried 20 mL vial was charged with [(L2-P,P)PdCl2] (58.4 mg, 0.071 mmol) and DIM (143 mg, 0.71 mmol) in CH2Cl2 (30 mL). The yellow mixture was treated with AgSbF6 (49 mg, 0.14 mmol) followed by rapid stirring at room temperature for 16 h. The precipitate formed was removed via filtration through a pad of Celite, and the yellow filtrate was evaporated down to about 4 mL. Hexanes (20 mL) were added slowly to precipitate a yellow solid. The solid was collected, washed with hexanes (3  1.5 mL), and then dried under vacuum for several hours to yield 5 in 76% yield (78 mg). 31P{1H} NMR (202 MHz, CD2Cl2, δ): 159.5 (s). 1 H NMR (500 MHz, CD2Cl2, δ): δ 1.16 (t, 6H, JHH = 7.5 Hz), 1.58-1.63 (m, 2H), 2.01-2-04 (m, 2H), 2.27-2.33 (m, 4H), 2.28 (br s, 6H), 3.45-3.50 (m, 2H), 3.71-3.75 (m, 2H), 3.93-3.97 (m, 2H), 4.03-4.09 (m, 2H), 5.09-5.16(m, 2H), 6.69-6.71 (m, 2H), 7.26-7.58 (m, 22H). 13C{1H} NMR (125 MHz, CD2Cl2, δ): 13.6 (s), 25.1 (s), 27.0 (s), 31.6 (s), 47.2 (br s), 67.5 (s), 73.4 (s), 96.2 (br s), 125.3 (s), 125.5 (s), 125.7 (s), 126.4 (s), 128.7 (s), 129.1 (s), 129.5 (s), 129.9 (s), 129.7 (d, JPC = 1.9 Hz), 130.5 (s), 135.6 (br s), 140.9 (s), 143.1 (s), 167.1 (s), 174.0 (s). Anal. Calcd for

Zhang et al. C50H54F12N2O6P2PdSb2: C, 42.33, H, 3.84, N, 1.97. Found: C, 42.33, H, 4.19, N, 2.04. General Procedures for Catalytic Reactions with Complexes 4 and 5. (a) In a glovebox, a J-Young tube was charged with either catalyst 4 or 5 (0.005 mmol) and substrate 6a (0.05 mmol) in CD2Cl2 (0.7 mL) and sealed with a Teflon cap. After subjecting the tube to the desired temperature, the reaction progress was monitored by 1H NMR spectroscopy. Upon completion, the product was purified by flash column chromatography and confirmed by 1H NMR spectroscopy relative to that reported previously for Nazarov cyclization and spirocycle products. (b) In a glovebox, an oven-dried 20 mL vial was charged with either complex 4 or 5 (0.005 mmol) and the appropriate substrate (7a-11a) (0.05 mmol) (Table 2) in CH2Cl2 (10 mL) and sealed with a Teflon cap. For those reactions examining the influence of NaBAr4f on product distribution, 0.05 mmol of NaBAr4f was added at this point. The solution was stirred at room temperature for several hours, and the ensuing reaction was monitored by thin-layer chromatography. After the starting substrate was consumed, the solution was taken to dryness and the residue was redissolved and passed through a small silica gel column to remove the metal complexes. The organic products were collected and their identities were confirmed by comparison of their 1H NMR spectra with those of previously reported samples.16a The yields of the Nazarov and spirocycle products were obtained by comparison of their 1H NMR resonances versus mesitylene (10 μL) used as an internal standard. Each experiment was repeated at least two times. Crystal Structure Determinations of Complexes 2 and 3. Each crystal was placed onto the tip of a glass fiber and mounted on a Bruker SMART Platform diffractometer equipped with an APEX II CCD area detector.24 All data were collected at 100.0(1) K using Mo KR radiation (graphite monochromator). For each sample a preliminary set of cell constants and an orientation matrix were determined from reflections harvested from three orthogonal wedges of reciprocal space. Full data collections were carried out with frame exposure times of 60-120 s at a detector distance of 4 cm. Randomly oriented regions of reciprocal space were surveyed for each sample: four major sections of frames were collected with 0.50° steps in ω at four different j settings and a detector position of -38° in 2θ. The intensity data were corrected for absorption,25 and final cell constants were calculated from the xyz centroids of approximately 4000 strong reflections from the actual data collection after integration.26 Structures were solved using SIR9727 and refined using SHELXL-97.28 Space groups were determined on the basis of systematic absences, intensity statistics, and Cambridge Structural Database frequencies.29 Direct-method solutions were calculated, which provided most non-hydrogen atoms from the difference Fourier map. Full-matrix leastsquares (on F2)/difference Fourier cycles were performed, which located the remaining non-hydrogen atoms. Non-hydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. Absolute configuration for both structures was determined by anomalous dispersion effects. All refinements were run to mathematical convergence. (25) Sheldrick, G. M. SADABS, 2008/1 ed.; University of G€ottingen: Germany, 2008. (26) SAINT, 7.60A ed.; Bruker AXS: Madison, WI, 2008. (27) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR97: A new program for solving and refining crystal structures; Istituto di Cristallografia, CNR: Bari, Italy, 1999. (28) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. (29) Allen, F. H. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380.

Article The crystal structure of 2 contains three independent palladium molecules and four independent cocrystallized dichloromethane molecules in the asymmetric unit, with all atoms in general positions, while the crystal structure of 3 has two independent palladium molecules and one cocrystallized dichloromethane molecule in the asymmetric unit, with all atoms in general positions. In the crystal structure of 3, there is a solvent channel parallel to the a-axis that contains highly disordered solvent that could not be modeled satisfactorily. The reflection contributions from this solvent were removed using the “Squeeze” function of program PLATON, which determined there to be 143 electrons in 434 A˚3 removed per unit cell.22 Since the exact identity and quantity of this solvent are unknown, it was not included in the molecular formula, and

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thus, calculations that derive from the molecular formula (F(000), density, etc.) are known to be incorrect.

Acknowledgment. The authors thank the National Science Foundation (Grant CHE 0847851) for supporting this work. Supporting Information Available: Experimental details and H NMR and MS data for previously reported organic compounds obtained from catalytic experiments using complexes 4 and 5 are included. X-ray crystallographic data of 2 and 3 are provided in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. 1