C2-Symmetric (SO)N(SO) Sulfoxide Pincer Complexes of Mg and Pd

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

C2‑Symmetric (SO)N(SO) Sulfoxide Pincer Complexes of Mg and Pd: Helicity Switch by Ambidentate S/O‑Coordination and Isolation of a Chiral Pd-Sulfenate Falk W. Seidel,† Sibylle Frieß,† Frank W. Heinemann, Ahmed Chelouan, Andreas Scheurer, Alexander Grasruck, Alberto Herrera, and Romano Dorta* Department Chemie und Pharmazie, Anorganische und Allgemeine Chemie, Friedrich−Alexander−Universität Erlangen−Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Quinine-based (R)-tert-butylsulfinate 3 reacts with tris-lithiated bis-arylamide 2 to afford gram-quantities of optically pure (S*O)N(S*O) sulfoxide pincer ligand (R,R)-4. Deprotonation of (R,R)-4 and p-Tol-substituted analogue (S,S)-5 with MgPh2 and BnK yields respective Mg and K amido-bissulfoxides 6−9. In Mg complexes 6 and 7, the sulfoxide functions are O-coordinated, thereby imparting a pronounced helicity to the ligand backbone. Transmetalation of 6 and 7 with [PdCl2(NCPh)2] affords the S,S-coordinated C2-symmetric and the O,S-coordinated C1-symmetric chlorido complexes 10 and 11, respectively, and reaction of potassium amides 8 and 9 with [PdCl(CH3)(COD)] leads to methyl-palladium pincer complexes 12 and 13, respectively. The crystal structures of 6, 7, 12, and 13 reveal a chameleonic ligand system with predictable behavior: (R)-configured 4 induces pronounced λ backbone helicity in the O-coordinated Mg-complex and weaker δ helicity in S-coordinated Pd-complexes, while (S)-configured ligand 5 mirrors this stereochemistry. S-Coordination induces stronger, C2-symmetric, steric crowding in the head-on quadrants compared to O-coordination. When (R,R)-4 is reacted with 2 equiv of [Pd(CH3)2(tmeda)], crystalline chiral Pd-sulfenate complex 16 forms by elimination of iso-butene and methane with inversion of configuration at the sulfenate S atom.

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deprotonated sulfenate anion8 are highly reactive species, which may be observed as fleeting intermediates at best. Sulfenate anions are eliciting considerable interest as “traceless”, catalytically active species,9 and they are postulated intermediates in the preparation of sulfoxides,10 notably in Pd-catalyzed coupling reactions.11 However, sulfenate coordination chemistry is not well documented,12 and structurally characterized Pd-sulfenate complexes are rare.13

INTRODUCTION Chiral sulfoxides have emerged as competent ligands for organocatalysis and metal-catalyzed organic transformations,1 not the least because effective chiral induction appears to be achieved also through electrostatic effects.2 In particular, chiral sulfoxide pincer designs prove promising,3 and in this context, our laboratory is developing new C2-symmetric sulfoxide (SO)−N−(SO) pincer architectures.4 Here we wish to communicate the enantioselective synthesis of a bulky, t-Bu-substituted and electronrich variant of this ligand type and present our insights into the coordination chemistry of such ligands with magnesium and palladium. Structurally characterized chiral sulfoxide complexes of Mg(II)5 and Pd(II)6 are very scarce, and in this study, we confirm the C2-symmetric coordination modes and the amphiphilic character of the sulfoxide function,7 i.e., O-coordination to the hard Mg(II) and mostly S-coordination to soft Pd(II). The switching of the coordination modes from oxygen to sulfur thereby has a deep impact on the stereochemistry of the complexes by altering the helicity of the ligand backbones and strongly modifying the steric maps around the metal centers. This is a vital aspect for assessing the potential of these ligands for enantioselective applications. Furthermore, we wish to present the isolation and crystallographical characterization of a chiral palladium sulfenate complex. Sulfenic acid and its © XXXX American Chemical Society

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RESULTS AND DISCUSSION

The synthesis of the t-butyl substituted ligand (R,R)-4 is outlined in Scheme 1 and follows the previously established protocol for the p-tolyl derivative (S,S)-5 (see Scheme 2 below).4 Halogenlithium exchange of bis(2-bromo-4-(t-butyl)phenylamine (1)14 with 3 equiv of BuLi affords sparingly soluble off-white trilithium salt 2 in quantitative isolated yields. For the introduction of the chiral t-Bu sulfoxide moiety three reagents were tested: Ellman’s thiosulfinate ester,15 The glucose-derived Alcudia− Fernández−Khiar sulfinate,16 and the quinine-based Lu− Senanayake sulfinate 3.17 While all three reagents lead to the desired product, in our hands, sulfinate 3 turned out to be the Received: January 30, 2018

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DOI: 10.1021/acs.organomet.8b00038 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Stereoselective Synthesis of the Amino Bis-sulfoxide (R,R)-4

Interestingly, of the three molecules, two display (aR) axial chirality (the left molecule in Figure 1 is one of them) and one has (aS) axial chirality (right molecule). In both conformers, the back of the sulfinyl moiety is sterically shielded while the top is openly exposed for nucleophilic attack, which in part explains the high enantioselectivities achieved with this reagent. Slow addition of 2 equiv of 3 to 2 at low temperature followed by two chromatographic purification steps (one to remove traces of meso-4, another on C2-modified silica gel to remove colored impurities) and drying of (R,R)-4 in benzene solution over CaH2 affords multigram quantities of the optically and analytically pure ligand as an off-white solid. The crystal structures of (R,R)-4 and meso-4 are depicted in Figures 2 and 3, respectively. The former shows approximate C2 symmetry and the expected inversion of configuration at the sulfur atoms with respect to 3, while the latter features an N−H1···O2 contact of 1.983 Å.21 The S−C(sp2) and the slightly longer S−C(sp3) bond distances in both isomers are in the expected ranges (1.788(3)−1.800(3) Å and 1.853(3)−1.863(3) Å, respectively). The sum of angles around the S atoms varies only slightly between 311.54 and 313.23° with a notable flattening of S2 in the H-bonded sulfoxide in meso-4 to 317.56°. The N−H proton in (R,R)-4 resonates as a singlet at 8.47 ppm in CDCl3, while in the meso-form it is shifted to 9.14 ppm. H-bonding in meso-4 is facilitated by the sterically demanding t-Bu groups pointing into opposite directions, but its ground state energy appears to be only marginally lower than that of (R,R)-4.22 Deprotonation of (R,R)-4 and (S,S)-5 with 0.5 equiv of Ph2Mg in benzene gives magnesium complexes (all-S)-6 and (all-R)-7, respectively, in quantitative yields as yellow solids (see Scheme 2).23 Likewise, potassium salts 8 and 9 are

Scheme 2. Syntheses of the Enantiopure Sulfoxide Pincer Complexes of Magnesium and Potassium 6−9

reagent of choice for giving the cleanest reactions even though it comes at the price of the lowest atom economy.18 An optimized protocol for 3 gives us access to 40 g batches of material of high purity (see Experimental Section), which also allowed us to grow single crystals for the structural elucidation of such important a reagent.19 The unit cell of 3 contains three symmetryindependent molecules with the expected (R)-configured sulfur atoms and with “closed” conformations of the quinine moiety.20

Figure 1. Molecular structures of the (aR,RS)-diastereomer (to the left) and of the (aS,RS)-diastereomer (to the right) in the crystal of (RS)-3 (ORTEPs drawn at 50% probability; third independent (aR,RS)-diastereomer is not shown. H atoms are omitted for clarity). Selected bond lengths (Å) and angles (deg): S1−O1 1.464 (3), S1−O2 1.644 (3), S1−C1 1.828 (4), O2−C5 1.465 (4), O1−S1−O2 109.20 (17), O1−S1−C1 107.29 (19), O2−S1−C1 94.61 (17), C6−C5−C16−N2 51.13, C30−C29−C40−N4 49.34. B


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Organometallics

Figure 2. Molecular structure of (R,R)-4 in the crystal (ORTEP drawn at 50% probability; the position of H1 was derived from a difference Fourier synthesis, all other H atoms are omitted). Selected bond lengths (Å) and angles (deg): S1−O1 1.496(2), S1−C2 1.798(3), S1−C11 1.863(3), S2−O2 1.500(2), S2−C16 1.799(3), S2−C25 1.859(3), N1−H1 0.83(4), N1−C1 1.414(4), N1−C15 1.390(4), C1−C2 1.399(4), C15−C16 1.402(4), O1−S1−C2 106.12(14), O1−S1−C11 104.71(14), C2−S1−C11 100.71(14), O2−S2−C16 105.34(14), O2−S2−C25 106.29(14), C16−S2−C25 101.60(14), C15−C16−S2 121.7(2), C1−C2−S1 120.5(2), C1−N1−C15 125.7(3).

Figure 4. Molecular structure of (all-λR)-6 in the crystal (ORTEP drawn at 50% probability; H atoms are omitted). Selected bond lengths (Å) and angles (deg): Mg1−O1 2.047(2), Mg1−O2 2.0470(19), Mg1−O3 2.051(2), Mg1−O4 2.055(2), Mg1−N1 2.151(2), Mg1−N2 2.164(2), S1−O1 1.5107(19), S2−O2 1.5102(18), S3−O3 1.5091(19), S4−O4 1.5141(19), S1−C2 1.780(3), S1−C11 1.856(3), N1−C1 1.374(3), O1−Mg1−O2 178.25(9), N1−Mg1−N2 177.06(9), O1−O2−C15−C1 43.93, O3−O4−C43−C29 44.42.

Figure 3. Molecular structure of meso-4 in the crystal showing the N1−H1···O2 interaction (ORTEP drawn at 50% probability; the position of H1 was derived from a difference Fourier synthesis, all other H atoms are omitted). Selected bond lengths (Å) and angles (deg): S1−O1 1.4963(19), S1−C2 1.800(3), S1−C11 1.861(3), S2−O2 1.5085(19), S2−C16 1.788(3), S2−C25 1.853(3), N1−H1 0.87(3), N1−C1 1.398(3), N1−C15 1.388(3), H1−O2 1.983, H1−S1 2.708, H1−S2 2.684, O2−S2−C16 107.77(12), O2−S2−C25 106.67(13), C16−S2−C25 103.12(13).

Figure 5. Molecular structure of (all-δS)-7 in the crystal (ORTEP drawn at 50% probability; H atoms are omitted). Selected bond lengths (Å) and angles (deg): Mg1−O1 2.082(4), Mg1−O2 2.065(4), Mg1−O3 2.086(4), Mg1−O4 2.061(4), Mg1−N1 2.146(4), Mg1−N2 2.162(4), S1−O1 1.505(4), S2−O2 1.508(4), S3−O3 1.511(4), S4−O4 1.515(4), S1−C1 1.799(5), S1−C8 1.778(5), N1−C13 1.371(7), N1−Mg1−N2 176.98(19), O2−Mg1−O1 177.92(18), O1−O2−C18−C13−43.66, O3−O4−C52−C47−41.07.

obtained as yellow powders in excellent yields by reaction of 4 and 5 with 1 equiv of benzyl potassium in benzene or THF solution, respectively, and are generally less soluble than their Mg counterparts. The formation of amido complexes 6−9 is monitored by the disappearance of the amine proton resonance by 1 H NMR or of the N−H stretching bands by IR spectroscopy. The characteristic S−O IR stretching frequencies are not strongly affected by complexation when compared to the free ligands, for example going from 1034 cm−1 in 4, to 1026 cm−1 in 6, and to 1007 and 1042 cm−1 in 8. This observation points to metal−oxygen coordination. Single crystals of the Mg salts 6 and 7 were grown from pentane, and X-ray diffraction analyses reveal in both cases octahedral coordination environments around the Mg centers. The two respective pincer ligands coordinate in a meridional fashion through their sulfoxide O atoms (Figures 4 and 5). The amido functions in both structures are

essentially planar (the sums of angles around N are comprised between 359.72° and 359.95°), and the S atoms show the expected configurations, i.e., (R) in 6 and (S) in 7. The aryl rings of the ligand backbones are strongly twisted out of plane and in opposite directions in the two complexes (by 66.8(1) and 62.9(1)° in 6 and by 73.5(2) and 72.5(2)° in 7) due to the steric demand of the bicyclic six-membered chelate rings. This in turn imparts λ (or (M)) and δ (or (P)) helicity in the respective ligand backbones of complexes 6 and 7. As an approximate measure of this backbone helicity, we define the torsion angle ε vide inf ra between the vector connecting the two oxygen donor atoms and the vector connecting the N-bound ipso-C atoms of a same ligand. In the case of compound 6, C


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Organometallics Scheme 3. Synthetic Routes to the Chlorido and Methanido Palladium Pincer Complexes 10−13

the average value for ε is +44.2°, and for 7, it is −42.4°. The Mg−O bond distances in 7 (2.061(4)−2.086(4) Å) are slightly longer than those in 6 (2.047(2)−2.055(2) Å), and all are within the range found in Mg(II)-DMSO complexes.24 The S−O bond lengths in 6 (1.509(2)−1.514(2) Å) and in 7 (1.505(4)−1.515(4) Å) are elongated by ca. 0.01 Å when compared to those of their respective free ligands, 4 and 5 (and similar to the distances of the H-bonded S−O functions in meso-4 and in (S,S)-5),4 while the S−CAr bonds to the backbone (1.774(3)−1.784(2) Å in 6; 1.766(5)−1.786(5) Å in 7) are contracted by about 0.02 Å. As observed in the H-bonded sulfoxide in meso-4, the geometry of the S atoms is flattened upon O-coordination to the Mg(II) centers (sum of angles around S: 317.5−318.7° in 6; 315.61−319.58° in 7), mainly through opening of the CBackbone−S−O angles by about 5° when compared to the free ligands. Unfortunately, potassium salts 8 and 9 so far could not be crystallized. Metathetical exchange of Mg-complexes (all-R)-6 and (all-S)-7 with 2 equiv of [PdCl2(PhCN)2] in THF solution in the presence of 1,4-dioxane (to drive the quantitative precipitation of the MgCl2) affords complexes (S,S)-10 and (R,S)-11 as turquoise and dark green solids, respectively (see Scheme 3). These complexes form deep blue solutions in common aprotic solvents. Emerald green colors of similar selenide pincer complexes of the type (SeNSe)PdCl have recently been observed by Ritch and co-workers.25 NMR spectra of (S,S)-10 are consistent with a C2-symmetric species in solution, while those for 11 are rather inexpressive showing a pattern of broadened signals indicative of a break of molecular symmetry and the presence of two or more species, which are assumed to be S/S-, S/O-, and O/O-coordinated linkage isomers. Unfortunately, more detailed NMR studies were hampered by the poor solubility of the complex, especially at low temperatures. An alternative route to 11 is the stoichiometric addition of HCl (0.9 M in Et2O) to the methylated complex (R,R)-13 (vide inf ra), which in benzene solution affords quantitatively identical dark green crystalline needles of 11. The crystal structure of (S,S)-10 reveals three symmetry independent, approximately C2-symmetrical molecules, one of which is depicted in Figure 6. The ligand is anchored to the square planar Pd-center through an essentially planar amido function, and the sulfoxide pincer arms are S-bound. The S atoms show the expected (S) configurations,26 which induce δ (or P) helicity in the backbone of the bicyclic five-membered

Figure 6. Crystal structure of one of three symmetry-independent molecules of (δ,S,S)-10 (ORTEP drawn at 50% probability; H atoms and cocrystallized solvent molecules are omitted). Selected bond lengths (Å) and angles (deg): Pd1−Cl1 2.3060(9), Pd1−S1 2.2816(9), Pd1−S2 2.2765(8), Pd1−N1 2.022(3), S1−O1 1.475(2), S1−C2 1.779(3), S1−C11 1.853(3), S2−O2 1.474(2), N1−C1 1.393(4), S1−Pd1−Cl1 96.02(3), S2−Pd1−S1 167.71(3), N1−Pd1−Cl1 179.25(8), N1−Pd1−S1 83.97(8), N1−Pd1−S2 84.38(8), O1−S1−Pd1 121.54(9), O2−S2−Pd1 123.80(10), C11−S1−Pd1 113.30(10), C25−S2−Pd1 109.57(10), C15−N1−C1 125.2(3), S1−S2−C15−C1−23.40.

chelate ring system. This stands in contrast to the λ (or M) helicity of the backbone in (all-R)-6 where the sulfoxide motive is rotated around the S−CAr bonds in order to accommodate for the larger bite angle of the O-coordination mode (see also Scheme 4). Even though the three molecules in the unit cell are structurally very similar, there is a noticeable flexibility of the ligand backbone with dihedral angles between the aryl rings varying between 34.6(2) and 46.6(1)°. The X-ray diffraction analysis of prism-shaped green crystals of 1127 shows square planar coordination of the Pd-center and both, S- and O-bound sulfoxide arms (Figure 7).28 The former has the S1−O1 bond notably contracted to 1.464(2) Å (free ligand: ∼1.50 Å), while the latter, O-bound sulfoxide has the S2−O2 bond elongated to 1.542(2) Å. In addition, oxygen coordination leads to a flattened trigonal pyramid around the sulfoxide group with angles summing 315.53°, whereas the angles in the opposite side average 322.46°. The Pd1−S1 bond length of 2.1799(8) Å is remarkably short in the present context and compared to that of 13 (Pd1−S1:2.241(2) Å, vide inf ra). This is mainly due to D


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Organometallics Scheme 4. Backbone Helicity ε as a Function of S- vs O-Coordination in Complexes 6, 7, 10, and 13a

with coexisting C2-symmetrical S,S- or O,O-coordinated and C1-symmetrical S,O-coordinated isomers, comparable to those of (R,S)-11. The two isomers show characteristic resonances of the methyl protons at 1.24 and 1.32 ppm, respectively, in a ca. 3:2 ratio over wide temperature range.30 However, complex 13 crystallizes exclusively as the S,S-coordinated isomer, and its structure is depicted in Figure 8. The molecule has

a For the definition of ε, see text. Where applicable, averaged values are given.

Figure 8. Molecular structure of (λ,R,R)-13 in the crystal showing crystallographic C2 symmetry (ORTEP drawn at 50% probability; most H atoms and cocrystallized solvent molecules are omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd1−C18 2.037(7), Pd1−S1 2.2411(16), Pd1−N1 2.082(5), S1−O1 1.470(5), S1−C2 1.767(6), S1−C11 1.771(7), N1−C1 1.372(6), C18−Pd1−S1 95.28(4), C18−Pd1−N1 180.0, S1−Pd1−S1A 169.45(7), N1−Pd1−S1 84.72(4), S1−S1A−C1A−C1 19.06.

crystallographic C2 symmetry, and the ligand is coordinated to the approximately square planar palladium center through the central amido function and the S-bound sulfoxide pincer arms, which are as expected (S)-configured.26 The Pd−C bond measures 2.037(7) Å and is shorter than in a comparable pincer PNP−Pd complex bearing basic P-donors (2.061(2) Å),31 whereas the Pd−N bond at 2.082(5) Å is within the expected range. The backbone of the bicyclic five-membered chelate ring system shows λ (or M) helicity, in contrast to the δ (or P) helicity of the same O-coordinated ligand in (all-R)-7. The dihedral angle between pairs of the three refined positions of the disordered aryl rings ranges from 26.5(9) to 32.4(8)°. The structure suffers from disorder in most parts of the molecule. Symmetric and asymmetric IR stretching frequencies of the S−O functions in purely S,S-coordinated complexes 10 and 12 show characteristic high frequency shifts to 1107 and 1169 cm−1 and to 1126 and 1165 cm−1, respectively, when compared to those of free ligand 4. Scheme 4 summarizes the stereochemical behavior of deprotonated ligands 4 and 5 as observed in the crystal structures of complexes 6, 7, 10, and 13. The following rule may be devised for such (SO)−N−(SO) pincer architectures: (R)-Configured sulfur atoms will give rise to λ helicity in a O-coordinated complex and δ helicity in S-coordinated complexes and vice versa for (S)-configured ligands. Moreover, the extent of helicity expressed by the torsion angle ε will decrease substantially when going from O- (over 40°) to S-bound complexes (around 20°). The angle ε is measured between the vector connecting the donor atoms O−O or S−S and the C−C vector of the N-bound ipso-carbon atoms, which approximates the deviation of the trigonal plane of the sp2-hybridized N from

Figure 7. Crystal structure of (R,S)-11 (ORTEP drawn at 50% probability; H atoms and cocrystallized solvent molecules are omitted). Selected bond lengths (Å) and angles (deg): Pd1−Cl1 2.3033(8), Pd1−S1 2.1799(8), Pd1−O2 2.071(2), Pd1−N1 2.032(2), S1−O1 1.464(2), S1−C1 1.782(3), S1−C8 1.760(3), S2−O2 1.542(2), S2−C23 1.758(3), S2−C28 1.784(3), S2−O2−Pd1 116.26(11), C8−S1−C1 102.96(13), C23−S2−C28 105.23(14).

the weak trans effect of the O-bound sulfoxide, which shows a Pd1−O2 contact of 2.071(2) Å. The Pd−N distances in complexes 10 and 11 at 2.020(3) Å (average of three values) and 2.032(2) Å, respectively, are in line with Ozerov’s similarly configured PNP Pd−chlorido pincer complex (2.0258(19) Å), as are the Pd−Cl distances.29 Methylation of chlorido complexes 10 and 11 with LiMe or methyl-Grignard reagents, unfortunately, does not afford analytically pure Pd−methyl complexes under a variety of conditions, presumably due to lithium and magnesium salt contamination. We speculate that the Li+ and Mg2+ cations are effectively retained by the amido function. The problem is circumvented by using [PdCl(CH3)(COD)] and potassium salts 8 and 9 in metathetical exchange reactions in benzene solution, which affords 12 and 13, respectively, in analytically pure form and good yields (Scheme 3). Both complexes form deep red solutions in benzene. The NMR spectra of 12 are consistent with C2 molecular symmetry and show a characteristic singlet resonance of the methyl protons at 1.19 ppm in C6D6. In contrast, complex 13 exhibits NMR spectra that are consistent E


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Organometallics

Pd-complex 14 bearing neutral protonated ligand 5 (NMR data suggest S,S- and S,O-coordination isomers in a 1:1 ratio). Treatment of 14 with 4 equiv of CH3CN in benzene solution causes the immediate precipitation of copious amounts of a yellow solid along with free ligand 5 (see eq 1). The precipitate is identified by NMR spectroscopy as cationic methyl complex 15. These observations indicate that the amide function in 13 is more basic than the methyl group, which corroborates also with the fact that chlorides 10 and 11 do not show clean metathetical exchange reactions with Li- and Mg-organyls and AgBF4. In these latter cases, we suspect that the amide function effectively retains the Li+, Mg2+, and Ag+ cations.

the coordination plane of Pd. Even though the helicity found in the O-bound magnesium complexes is much more pronounced than that in the S-bound palladium complexes, steric maps of complexes 6 and 10 generated with Cavallo’s SambVca 2 web tool32 show a different picture (see Figure 9). It becomes

CH3CN

C6H6

(R , R )‐13 + HBF4 ·Et 2O ⎯⎯⎯⎯⎯→ [(5)Pd(CH3)][BF4 ] ⎯⎯⎯⎯⎯⎯⎯→ 14

(S , S)‐5 + [Pd(CH3)(NCCH3)3 ][BF4 ] 15

(1)

Finally, we were also wondering whether the direct reaction of neutral N−H ligands (R,R)-4 and (S,S)-5 with [Pd(CH3)2(tmeda)] under elimination of 1 equiv of methane might be a more efficient route to complexes 12 and 13, respectively. While mixtures of (R,R)-4 and [Pd(CH3)2(tmeda)] in benzene do not react at RT, heating to 50 °C produces clear solutions from which yellow needles of 16 readily crystallize upon cooling (see Scheme 5); the mother liquors contain iso-butene. The diminished yields are at the expense of some Pd-black formation, but the reaction is reproducible on a preparative scale affording analytically pure material. The crystal structure of 16 is shown in Figure 10 and reveals a dinuclear complex bearing a chiral dianionic SNS sulfenate pincer ligand. In an attempt to improve the yield and as a mechanistic probe, a benzene solution of 1 equiv of [Pd(CH3)2(tmeda)] and methyl palladium complex (S,S)-12 were heated to 50 °C. No reaction took place apart from very slow decomposition to Pd-mirror, which means that complex 12 is not an intermediate that forms en route to 16. In a separate experiment, heating a benzene solution of free ligand 4 above 50 °C gave iso-butene and decomposition products,34 which is in line with Perrio’s11d and Walsh’s9b recent discovery of “traceless” aryl-sulfenate anions generated from t-butyl aryl sulfoxides in the presence of a base by elimination of iso-butene. We therefore propose that complex 16 forms in two steps: First, (R,R)-4 decomposes to 1 equiv of iso-butene and the corresponding monosulfenic acid intermediate (or the ammonium-sulfenate zwitterion), which then is trapped by 2 equiv of [Pd(CH3)2(tmeda)] acting as base to form 16 and 2 equiv of CH4 (see Scheme 6). Also in accordance with the exclusion of complex 12 as a possible intermediate to 16 is the observation that ligand (S,S)-5 does not react with [Pd(CH3)2(tmeda)] to form methyl pincer complex 13. The crystal structure of (R,R)-16 reveals two square planar Pd(II) centers that are bridged by the chiral sulfenate anion of the meridional dianionic SNS pincer ligand. Formally, the t-Bu

Figure 9. Steric maps based on the crystal structures of complexes (all-R)-6 (left) and (S,S)-10 (right) generated with SambVca 2.32 Sphere radii are 3.5 Å, and bondi radii are unscaled.33 Metal atoms and secondary ligands were excluded and H atoms were included in the calculations. Note that the helicities appear inverted compared to Scheme 4 due to the opposed point of view. Red: more bulk; blue: less bulk.

strikingly evident how the coordination mode determines the steric pressure around the respective metal centers: In the O-coordinated Mg-complex the larger chelate ring size moves the chirogenic centers away from the metal. Moreover, due to the large torsion in the backbone the tert-butyl groups are pointing backward away from the metal thus reducing the steric pressure in the relevant head-on quadrants. In the S-coordinated Pd-complex, in contrast, the smaller chelate ring size allows the tert-butyl and sulfoxide oxygen groups to push in front of the metal plane, therefore creating a pronounced C2-symmetric chiral pocket in trans position to the amide function. Thereby, the oxygen atoms are well positioned to serve as chiral H-bond anchors for incoming substrates. By comparing the two coordination modes we conclude that S-coordination leads to a more effective desymmetrization of the relevant coordination quadrants. Therefore, complexes of soft thiophilic metals should prove more effective chirality transducers than O-coordinated complexes of hard oxophilic metals. Attempts to generate the corresponding cationic palladium pincer complexes as tetrafluoroborate salts by abstracting the chlorides in complexes 11 and 12 with AgBF4 or by reacting potassium salts 8 and 9 with [Pd(NCCH3)4][BF4]2 only afforded impure or intractable material under a variety of conditions. Surprisingly, when trying to selectively protolyze the methyl function in 13 by slow addition of an equimolar amount of dry HBF4 in benzene solution, a green solid is isolated and formulated as

Scheme 5. Chiral Palladium Sulfenate Complex (R,S)-16 Does Not Form via (S,S)-12

F


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Organometallics

The decisive driver of O- versus S-coordination, however, is the nature of the metal: The hard, oxophilic Mg(II) ion commands O-coordination even at the cost of less favorable six-membered chelation rings and exposure of the sulfur lone pairs. Soft Pd(II), in contrast, prefers S-coordination. Furthermore, switching between the S- and O-coordination modes leads to a predictable inversion of the helicity in the ligand backbones and to a significant alteration of the steric map around the metal centers. Preliminary reactivity studies indicate a pronounced basicity of the amido function in these Pd−pincer complexes, and the new t-Bu-derived ligand 4 is shown to decompose at temperatures above 50 °C to iso-butene and a “traceless” sulfenate species. However, this fleeting species may be trapped by Pd(II) fragments in the form of the first crystallographically characterized chiral palladium sulfenate complex, 16. The S−O distances measured in this study compare as follows: S-bound Pd−sulfoxide (146−147 pm) < Pd−sulfenate (150 pm) ≈ free sulfoxide < O-bound Mg− and Pd−sulfoxides (152−154 pm). The generation of clean cationic Pd(II) pincer complexes bearing ligands such as 4 and 5 for applications in asymmetric catalysis and as chirality sensors is the subject of ongoing studies in our laboratory, and results will be reported in due course.

Figure 10. Molecular structure of (R,R)-16 in the crystal (ORTEP drawn at 50% probability). H atoms and cocrystallized solvent molecules are omitted; only one conformation for the TMEDA ligand is shown). Selected bond lengths (Å) and angles (deg): Pd1−S2 2.2691(9), Pd1−S1 2.2522(9), Pd1−N1 2.071(3), Pd1−C25 2.061(4), Pd2−S2 2.2367(9), Pd2−N2 2.137(10), Pd2−N3 2.151(16), Pd2−C32 2.022(4), S1−O1 1.482(2), S2−O2 1.508(3), S2−C16 1.794(4), S1−C2 1.780(3), S1−C11 1.858(4), S1−Pd1−S2 169.65(3), C25−Pd1−S2 91.35(12), C25−Pd1−S1 98.69(12), C32−Pd2−S2 86.33(12), O2−S2−Pd1 113.11(11), O2−S2−Pd2 112.90(11), O2−S2−C16 107.28(16), O1−S1−Pd1 124.28(11), O1−S1−C2 109.10(15).

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EXPERIMENTAL SECTION

All reactions were carried out under anaerobic and anhydrous conditions, using standard Schlenk and glovebox techniques, unless otherwise stated. Quinine (abcr, 98%) was used as received. THF, Et2O, and benzene were distilled from purple Na/Ph2CO solutions, and other reagents were distilled as follows: toluene from Na; pentane, C6D6, and THF-D8 from Na2K alloy; CH3CN, CH2Cl2, and CD2Cl2 from CaH2; NEt3 and 1,4-dioxane from K. CDCl3 was degassed with three freeze−pump−thaw cycles and then kept over activated molecular sieves (4 Å) in a glovebox. Commercial 2.5 M BuLi in hexanes was dissolved in hexane to 1.43 M, filtered (Whatman GF/B glass fiber), and titrated with the Suffert method before use.35 1,14 (S)-menthyl p-tolylsulfinate,36 MgPh2,37 KBn,38 [PdCl2(PhCN)2],39 [PdCl(CH3)(COD)],40 and [Pd(CH3)2(tmeda)]41 were prepared according to published procedures. Air-sensitive samples for elemental analysis samples were handled in a glovebox. NMR spectra were recorded on a Jeol Lambda/Eclipse 400 MHz spectrometer, and the solvent residual signals were used as internal reference for the 1H NMR-spectra.42 Elemental analyses (EA) were performed on a Euro EA 3000 analyzer. Infrared spectra were recorded on a Shimadzu IRTracer-100 spectrometer. Bis(2-lithio-4-tert-butylphenyl)lithiumamide (2). A cold solution of n-BuLi in hexanes (1.43 M, 250 mL, 358 mmol) was added dropwise over 80 min to a vigorously stirred solution of 1 (50.09 g, 114.0 mmol) in hexane (350 mL) at −30 °C. During addition, the mixture turns gradually yellow, then orange, until finally a white precipitate starts to form. The mixture was stirred for 17 h and allowed to reach RT. The solid was separated by filtration (glass frit porosity 4, for smaller amounts separation by centrifugation is preferrable), thoroughly washed with hexane (3 × 200 mL), and dried in vacuo to yield an off-white powder (35.6 g, 99%). EAs were hampered by the high air sensitivity of the compound, and the solubility in common deuterated solvents is too low for meaningful NMR spectra to be recorded. However, careful hydrolysis of a weighed sample in C6D6 (0.6 mL) with a slight excess of dry MeOH afforded a yellow solution and a white precipitate (supposedly LiOMe), which was separated by centrifugation (2000 rpm, 3 min). The 1H NMR spectrum of the supernatant solution is in accordance with bis(4-(tert-butyl)phenyl)amine and 0.16 equiv of hexanes, thus confirming quantitative ortholithiation at both aryl rings. Improved Synthesis of (1S)-(6-Methoxyquinolin-4-yl)(5-vinylquinuclidin-2-yl)methyl-(R)-2-tert-butylsulfinate (3)43. SOCl2 (10.2 mL, 141 mmol) was added to chilled THF (700 mL, −80 °C) in a 2 L two-necked Schlenk flask equipped with a mechanical stirrer

Scheme 6. Sulfenate Anion Generation from t-Bu-arylsulfoxide and Its Stabilization by Pd-Complexation

group on S2 has been replaced by the Pd(CH3)(tmeda (lower case)) moiety with inversion of configuration at S2; therefore, O1 and O2 are pointing into the same coordination hemisphere. The Pd1−S2 bond is 0.03 Å longer than the Pd1−S2 bond due to a stronger trans effect of S1. The sulfenate S2−O2 bond distance of 1.508(3) Å is similar to the free sulfoxide but significantly longer than the sulfoxide S1−O1 bond (1.482(2) Å). As in the other complexes described here, N1 is planar (sum of angles around N1: 359.45°). The Pd−CH3 groups are positioned cis to the sulfenate function and may model intermediates prior to reductive elimination of alkyl-arylsulfoxides in catalytic processes. Heating 16 at 50 °C in the presence of 4 equiv of PPh3, however, did not afford a clean elimination product. To summarize, t-Bu-sulfinate 3 is the reagent of choice for the stereoselective introduction of the t-BuS(O) group onto diaryl amide framework 2, producing (SO)−N−(SO) pincer 4 in enantiopure form. This ligand and p-tolyl variant 5 are cleanly deprotonated by MgPh2 and KBz to afford corresponding magnesium and potassium salts 6−9. Metathetical exchange of magnesium salts 6 and 7 with [PdCl2(PhCN)2] gives palladium chlorido complexes 10 and 11, whereas potassium salts 8 and 9 react selectively with [PdCl(CH3)(COD)] to afford corresponding methylated complexes 12 and 13. Due to reduced steric bulk of p-Tol bearing ligand 5 when compared to ligand 4, ambidentate behavior of the sulfoxide function is observed in the solid state in complex 11 and in solution in complex 13. G


Article

Organometallics

69.17; H, 8.90; N, 2.73; S, 12.85. Calcd for C28H43NO2S2·(C6H6)0.05: C, 68.85; H, 8.84; N, 2.84; S, 12.99. [α]D + 183.7° (c 1.0, THF, 22 °C). 1H NMR (400 MHz, CDCl3) δ 8.47 (s, 1H), 7.47 (d, J = 2.2 Hz, 2H), 7.31 (dd, J = 8.6, 2.3 Hz, 2H), 7.13 (d, J = 8.6 Hz, 2H), 1.30 (s, 18H), 1.28 (s, 18H).13C NMR (68 MHz, CDCl3) δ 144.11, 141.60, 128.99, 128.46, 126.09, 125.36, 119.13, 57.74, 34.51, 31.36, 23.36. 1H NMR (400 MHz, C6D6) δ 9.31 (s, 1H), 7.63 (br, 2H), 7.10 (dd, J = 8.6, 2.3 Hz, 2H), 7.05 (d, J = 8.6 Hz, 2H), 1.19 (s, 18H), 1.18 (s, 18H). 13C NMR (68 MHz, C6D6) δ 143.77, 142.40, 128.85, 128.59, 125.82, 119.65, 57.43, 34.33, 31.30, 23.24. The spectrum shows the presence of ca. 0.05 equiv benzene. X-ray-quality single crystals of (R,R)-4 were grown from benzene. meso-4: 1H NMR (400 MHz, C6D6) δ 9.94 (s, 1H), 7.66 (s, 2H), 7.27 (d, J = 8.6 Hz, 2H), 7.18− 7.14 (m, 2H), 1.18 (s, 18H), 1.12 (s, 18H). 1H NMR (400 MHz, CDCl3) δ 9.14 (s, 1H), 7.44 (s, 2H), 7.30 (dd, J = 8.6, 2.3 Hz, 2H), 7.23 (d, J = 8.6 Hz, 2H), 1.31 (s, 18H), 1.24 (s, 18H). 13C NMR (68 MHz, CDCl3) δ 143.59, 141.43, 128.75, 125.40, 117.57, 57.65, 34.42, 31.34, 23.41. X-ray-quality single crystals of meso-4 were grown from a mixture of n-hexane and EtOAc. Improved Synthesis of (S,S)-Bis(4-(tert-butyl)-2-(ptolylsulfinyl)phenyl)amine ((S,S)-5).4 BuLi (106 mL, 1.42 M in hexanes, 150 mmol) was added slowly via cannula to a cooled solution (−50 °C) of 1 (22.0 g; 50.0 mmol) in Et2O (300 mL). The resulting yellow solution was stirred for another 30 min at −50 °C, allowed to gradually warm up to RT. Stirring for 3 h caused the precipitation of a white solid. This slurry was then cooled to −84 °C, and a solution of (S)-menthyl p-tolylsulfinate (29.5 g, 100 mmol) in Et2O (700 mL) was added dropwise over a period of 45 min. While the golden yellow reaction mixture was allowed to warm to RT slowly inside the cooling bath, the precipitate dissolved, and the color of the solution turned orange. After stirring for 24 h, the volatiles were removed in vacuo, fresh Et2O (600 mL), and H2O (400 mL) added, and the pH adjusted to ∼8 with NH4Cl. The organic layer was separated and washed with brine (2 × 400 mL) and H2O (100 mL) and the aqueous washings re-extracted with Et2O (100 mL). The combined organic phases were dried over Na2SO4, filtered, and evaporated to dryness. Column chromatography of the crude brown oil (Merck silica gel G60, hexane/ EtOAc 9:1−1:1 v/v), followed by washing with hexane and recrystallization from dry THF/pentane (1:2 v/v), afforded white crystalline material (17.4 g, 62%). [α]D − 98° (c 1.00, THF, 294 K). 1 H and 13C NMR spectra correspond. (all-R)-Magnesium-di(bis(4-(tert-butyl)-2-(tert-butylsulfinyl)phenyl)amide) ((all-R)-6). Benzene (10 mL) was added to (R,R)-4 (989 mg, 2.02 mmol) and Ph2Mg·(Et2O)0.44·(1,4-dioxane)0.013 (215 mg, 1.01 mmol), and the yellowish mixture was stirred for 16 h, during which time it turned orange. Removing the volatiles in vacuo yielded a yellow powder (1.01 g, 99%). EA found: C, 67.55; H, 8.53; N, 2.70; S, 12.54. Calcd for C56H84MgN2O4S4·(C6H6)0.25: C, 67.62, H, 8.44; N, 2.74; S, 12.56. 1H NMR (400 MHz, C6D6) δ 7.59 (d, J = 8.7 Hz, 4H), 7.12 (d, J = 2.4 Hz, 4H), 7.09 (dd, J = 8.7, 2.6 Hz, 4H), 1.22 (s, 36H), 1.15 (s, 36H). 13C NMR (68 MHz, C6D6) δ 159.15, 135.40, 129.43, 126.64, 123.18, 120.81, 58.16, 33.97, 31.81, 24.56. The spectrum indicates the presence of ca. 25% of cocrystallized benzene. X-ray quality single crystals were grown from a saturated and filtered pentane solution. Up-Scaled Synthesis and Crystallization of (all-R)-7.4 A solution of (S,S)-5 (5.00 g, 8.85 mmol) in benzene (20 mL) was added to a slurry of MgPh2 (939 mg, 4.43 mmol) in benzene (10 mL), and the bright yellow solution was stirred for 20 h. Lyophilization in vacuo left a fine, yellow powder (5.03 g, 99%). Spectroscopic data corresponded. X-ray quality single crystals were grown from a saturated and filtered pentane solution. (R,R)-Potassium-bis(4-(tert-butyl)-2-(tert-butylsulfinyl)phenyl)amide ((R,R)-8). (R,R)-4 (502 mg, 1.02 mmol) and BnK (134 mg, 1.02 mmol) were stirred in benzene (10 mL) for 16 h during which time the red mixture dissolved and turned yellow. Evaporation of the volatiles in vacuo afforded a yellow powder (520 mg, 97% yield). EA found: C, 63.98; H, 8.34; N, 2.52; S, 11.86. Calcd for C28H42KNO2S2·(C6H6)0.05: C, 63.92; H, 8.02; N, 2.63; S, 12.06. 1H NMR (270 MHz, THF-d8) δ 7.30 (m, 2H), 7.01 (d, J = 8.8 Hz, 2H),

and a dropping funnel. A solution of (−)-quinine (45.8 g, 141 mmol) and NEt3 (30.0 mL, 212 mmol) in THF (440 mL) was transferred to the dropping funnel via cannula and then added dropwise over the course of 7 h to the reaction vessel under stirring (200 rpm). The temperature should always be kept below −70 °C, and mechanical stirring is strongly advised because a thick white slurry forms that eventually stops magnetic stir bars from stirring. Stirring was continued for an additional hour at −75 °C. Then, a solution of t-BuMgCl(Et2O)1.1 (59.0 g, 297 mmol) in THF (300 mL) was added dropwise over the course of 1.5 h to the reaction mixture, which was kept at −75 °C, turning dark orange. After stirring for 30 min at −75 °C, the reaction was quenched at this temperature by dropwise addition of saturated aqueous NH4Cl (200 mL) over the course of 30 min. The suspension was allowed to reach room temperature and additional water (200 mL) was added. The phases were separated, and the aqueous phase was extracted with EtOAc (3 × 200 mL). The combined organic phases were washed with brine (2 × 200 mL), and the aqueous solution was extracted with EtOAc (200 mL). The organic phases were dried over MgSO4, and the solvent was removed to afford the crude product as a yellow oil, which was purified by dry column vacuum chromatography44 (EtOAc/MeOH: Gradient MeOH 0−1% in 0.125%steps; 1−5% in 0.25% steps). To remove the remaining t-Bu2SO, the product was recrystallized from dry Et2O (0.5 g/mL, in a glovebox), which afforded a white microcrystalline solid (46 g; 70%). [α]D + 69.6° (c 1.00, THF, 21 °C); Rf = 0.13 with 95:5 EtOAc/ MeOH. 1H NMR (400 MHz, CDCl3) δ 8.74 (d, J = 4.4 Hz, 1H), 8.00 (d, J = 9.6 Hz, 1H), 7.42 (s, 1H), 7.34 (d, J = 7.3 Hz, 2H), 5.81 (ddd, J = 17.7, 10.4, 7.6 Hz, 1H), 5.67 (br, 1H), 5.03−4.93 (m, 2H), 3.92 (s, 3H), 3.50−3.37 (m, 1H), 3.13−2.87 (m, 2H), 2.70−2.52 (m, 2H), 2.24 (br, 1H), 2.04−1.80 (m, J = 35.1, 27.8 Hz, 2H), 1.73−1.42 (m, 3H), 1.19 (s, J = 7.3 Hz, 9H). X-ray quality single crystals were grown from a saturated filtered solution of 3 in dry Et2O in a glovebox. Di-tert-butylsulfoxide. t-BuMgCl (1.4 M in Et2O, 48 mL) was added to a stirred solution of SOCl2 (2.44 mL, 33.6 mmol) in Et2O (10 mL) over the course of 20 min at −100 °C. The resulting yellow mixture was allowed to reach RT, stirred for 13 h, and quenched by addition of water (100 mL). The mixture was extracted with EtOAc (2 × 100 mL), the combined organic phases washed with brine (100 mL), and dried over Na2SO4. Evaporation of the volatiles left a pale yellow oil, which was purified by flash column chromatography (150 mL silica; 100 mL EtOAc, then 200 mL 95:5 EtOAc/MeOH, then 100 mL 4:1 EtOAc/MeOH). This yielded 715 mg (13%) of a white solid. Rf = 0.31 with 95:5 EtOAc/MeOH. 1H NMR (270 MHz, CDCl3) δ 1.34 (s, 18H). 13C NMR (68 MHz, CDCl3) δ 57.22, 25.60. (R,R)-Bis(4-(tert-butyl)-2-(tert-butylsulfinyl)phenyl)amine ((R,R)-4). A solution of 3 (26.6 g 62.0 mmol) in THF (60 mL) was added dropwise via gas-tight syringe over 20 min to a vigorously stirred suspension of 2 (9.29 g, 31.0 mmol) in Et2O (210 mL) at −82 °C. The reaction mixture was allowed to reach RT over the course of 12 h with the cooling bath in situ and was further stirred for 30 h at RT, during which time the color of the reaction mixture changed from yellow to red. The volatiles were removed in vacuo to give a brown solid foam, which was treated with saturated aqueous NH4Cl (300 mL, pH 8) and extracted with CH2Cl2 (1 × 300 mL followed by 3 × 150 mL). The combined organic phases were washed with brine (600 mL) and dried over Na2SO4 (50 g) to give a brown solid (50.6 g) after evaporation of the volatiles in vacuo. A first flash column chromatography (300 mL silica gel 60, 40−63 μm, 6:4 petroleum ether/EtOAc) is necessary to remove the meso-4 (characterized by the amine proton resonance at 9.13 ppm in CDCl3 or 9.94 ppm in C6D6; Rf = 0.26 in 1:1 petroleum ether: EtOAc). The resulting pale brown solid was then submitted to a second flash column chromatography on deactivated C2-silica45 (163 g silica, 7:3 petroleum ether/ EtOAc) to remove colored impurities. The product was then timely transferred to a glovebox, slurried over CaH2 (800 mg) in benzene (10 mL) for 2 h, filtered, and stripped in vacuo to an off-white solid (7.30 g, 48%). The product should be stored at −30 °C to avoid decomposition. Rf = 0.31, 1:1 petroleum ether/EtOAc. EA found: C, H


Article

Organometallics 6.87 (dd, J = 8.8 Hz, J = 2.5 Hz, 2H), 1.24 (s, 18H), 1.21 (s, 18H). 13 C NMR (68 MHz, THF-d8) δ 155.31, 132.34, 127.77, 126.15, 123.38, 115.41, 56.75, 34.19, 32.00, 24.21. The spectrum indicates the presence of ca. 5% of cocrystallized benzene. (S,S)-Potassium-bis(4-(tert-butyl)-2-(p-tolylsulfinyl)phenyl)amide ((S,S)-9). A solution of (S,S)-5 (5.00 g, 8.85 mmol) in THF (20 mL) was added dropwise to a solution of BnK (1.17 g, 8.85 mmol) in THF (20 mL) forming a bulky, yellow precipitate. The mixture was stirred for 20 h. Then, the slurry was allowed to settle, and the supernatant was decanted off. The solid was washed with pentane (2 × 10 mL) and dried in vacuo to give a fine yellow powder (4.96 g, 92%). EA found: C, 68.90; H, 6.28; N, 2.35; S, 10.57. Calcd for C34H38KNO2S2: C, 68.53; H, 6.43; N, 2.35; S, 10.76. 1H NMR (270 MHz, C6D6): δ 7.65−7.35 (bs, 6H), 7.30−7.00 (bs, 4H), 7.00−6.85 (bs, 4H), 2.01 (s, 6H), 1.19 (s, 18H). (S,S)-(Bis(4-(tert-butyl)-2-(tert-butylsulfinyl)phenyl)amidoκ3S,N,S′)(chlorido)palladium(II) ((S,S)-10). A solution of 6 (500 mg; 0.499 mmol) in THF (8 mL) was added dropwise to a stirred mixture of [PdCl2(PhCN)2] (382 mg; 0.998 mmol) and 1,4-dioxane (229 mg, 2.60 mmol) in THF (2 mL). After complete addition a dark green solution was obtained, which was stirred for 48 h. Volatiles were evaporated, and benzene (8 mL) and 1,4-dioxane (229 mg; 2.60 mmol) were added. The slurry was stirred for additional 24 h. After centrifugation (15 min, 3900 rpm), the green supernatant was decanted off, and the remaining residue was extracted further with benzene (5 mL). The combined extracts were evaporated to dryness to give a dark green-blue solid, which was washed thoroughly with Et2O (4 × 10 mL). A turquoise powder was obtained and dried in vacuo (367 mg, 58%). EA found: C, 53.93; H, 6.81; N, 2.21; S, 9.87. Calcd for C28H42ClNO2PdS2·(C6H6)0.05: C, 53.57, H, 6.72; N, 2.21; S, 10.11. 1H NMR (270 MHz, C6D6) δ 7.61 (d, J = 2.3 Hz, 2H), 7.24 (d, J = 8.7 Hz, 2H), 7.07 (dd, J = 8.8, 2.3 Hz, 2H), 1.33 (s, 18H), 1.10 (s, 18H). 13C NMR (68 MHz, C6D6) δ 149.88, 142.13, 133.97, 132.03, 125.52, 115.09, 67.99, 34.14, 31.17, 23.12. X-ray quality single crystals of 10 were grown from a filtered C6D6 solution. (R,S)-(Bis(4-(tert-butyl)-2-(p-tolylsulfinyl)phenyl)amidoκ3S,N,O)(chlorido)palladium(II) ((R,S)-11). A solution of (S,S)-7 (300 mg, 0.264 mmol) in benzene (3 mL) was added to a stirred suspension of [PdCl2(PhCN)2] (203 mg, 0.528 mmol) in benzene (2 mL) to afford a dark green slurry. Stirring for 24 h followed by solvent evaporation in vacuo gave a dark green powder, to which fresh benzene (10 mL) was added. The mixture was centrifuged (3500 rpm, 30 min), and the dark green-blue supernatant was decanted off and evaporated to dryness. The crude was washed with pentane (3 × 5 mL), dried, and recrystallized from a saturated benzene solution to afford dark green needles, which were dried in vacuo (213 mg, 56%). X-ray-quality single crystals were grown from benzene. EA found: C, 58.93; H, 5.36; N, 2.27; S, 8.38. Calcd for C34H38ClNO2PdS2·0.26C6H6: C, 59.41; H, 5.55; N, 1.95; S, 8.92. Recording of meaningful NMR spectra was hampered by the very low solubility of this compound. (S,S)-(Bis(4-(tert-butyl)-2-(tert-butylsulfinyl)phenyl)amidoκ3S,N,S′)(methanido)palladium(II) ((S,S)-12). A solution of (R,R)-8 (300 mg, 0.568 mmol) in benzene (6 mL) was added dropwise to a slurry of [PdCl(CH3)(COD)] (151 mg, 0.568 mmol) in benzene (4 mL). The resulting dark red solution turned turbid within 10 min and was stirred for 24 h. The mixture was separated by centrifugation and the supernatant solution evaporated to an orange solid, which was washed and slurried with pentane (3 × 10 mL) and dried in vacuo (270 mg, 78%). EA found: C, 57.02; H, 7.36; N, 2.14; S, 10.40. Calcd for C29H45NO2S2Pd: C, 57.08; H, 7.43; N, 2.30; S, 10.51. 1H NMR (400 MHz, C6D6): δ 7.72 (d, J = 2.3 Hz, 2H), 7.41 (d, J = 8.8 Hz, 2H), 7.18−7.15 (m, 2H), 1.20 (s, 18H), 1.18 (s, 3H), 1.16 (s, 18H). 13 C NMR (100.6 MHz, C6D6): δ 150.4, 139.6, 134.5, 131.8, 125.5, 114.8, 66.2, 34.4, 31.7, 23.4, −1.32. (R,R)-Bis(4-(tert-butyl)-2-(p-tolylsulfinyl)phenyl)amido)(methanido)palladium(II) ((R,R)-13). A suspension of (S,S)-9 (500 mg, 0.820 mmol) in benzene (10 mL) was added dropwise to a slurry of [PdCl(CH3)(COD)] (217 mg, 0.820 mmol) in benzene

(5 mL). The resulting dark red solution turned turbid within 1 h and was stirred for 24 h. The slurry was centrifuged and the orange-red supernatant decanted off and lyophilized to leave an orange powder. Washing with pentane (2 × 5 mL) followed by recrystallization from toluene/pentane (3 mL/15 mL) at −30 °C afforded dark red crystals that were also suitable for X-ray diffraction analysis (429 mg, 77%). EA found: C, 62.97; H, 6.29; N, 2.01; S, 8.99. Calcd for C35H41NO2S2Pd· 0.28C7H8: C, 63.05; H, 6.19; N, 1.99; S, 9.11. While the solid-state structure shows exclusively S/S-coordination, in solution the S/O-isomer is also observed: 1H NMR (270 MHz, C6D6) of major symmetrical isomer (60%): δ 7.97 (d, J = 8.3 Hz, 4H), 7.75 (d, J = 2.2 Hz, 4H), 7.20 (dd, J = 9.0 Hz, J = 2.2 Hz, 2H), 6.65 (d, J = 8.3 Hz, 4H), 1.78 (s, 6H), 1.24 (s, 3H), 1.09 (s, 18H), minor S/O-isomer isomer (40%): δ 7.79− 7.68 (m, 1H), 7.63 (d, J = 9.0 Hz, 1H), 7.56−7.44 (m, 2 + 1H), 7.36 (d, J = 8.9 Hz, 2H), 7.28 (dd, J = 10.8 Hz, J = 2.5 Hz, 1H), 7.12 (d, J = 7.4 Hz, 1H), 7.02 (dd, J = 8.1 Hz, J = 2.2 Hz, 1H), 6.87 (d, J = 8.0 Hz, 2H), 6.79 (d, J = 8.2 Hz, 2H), 1.95 (s, 3H), 1.73 (s, 3H), 1.32 (s, 3H), 1.23 (s, 9H), 1.09 (s, 9H). 13C NMR (67.8 MHz, C6D6) of major isomer: δ 148.6, 141.0, 140.5, 139.6, 137.1, 130.3, 122.1, 114.2, 32.5, 29.6, 19.4, − 2.02, minor isomer (40%): δ 154.8, 150.6, 142.2, 140.2, 140.0, 139.3, 137.5, 136.4, 135.9, 131.8, 129.7, 118.5, 116.4, 32.4, 32.3, 29.8, 29.3, 19.5, 19.3, − 0.14. Addition of HBF4·Et2O to (R,R)-13. HBF4·Et2O (24.2 mg, 0.145 mmol) was added to a red solution of (R,R)-13 (100 mg, 0.145 mmol) in benzene (3 mL). The color disappeared, and after 20 h of stirring a green solution was obtained. Volatiles were evaporated in vacuo to yield a light green powder (111 mg, 99% calcd based on [(5)Pd(CH3)][BF4] (14)). In CD2Cl2 a 1:1 mixture of coordination isomers (S,S and S,O coordination) is observed. 1H NMR (270 MHz, CD2Cl2): δ 8.76 (s, 1H), 8.14 (s, 1H), 7.95−7.15 (m, 12H + 12H), 6.85−6.00 (m, 2H + 2H), 2.56 (s, 3H), 2.48 (s, 6H), 2.41 (s, 3H), 1.43 (s, 9H), 1.33 (s, 9H), 1.20 (s, 18H), 1.13 (bs, 6H). Addition of CH3CN (4 equiv) to a solution of 14 in benzene precipitates [(CH3CN)3Pd(CH3)][BF4] (15) quantitatively: 1H NMR (270 MHz, CD2Cl2): δ 2.35 (s, 6H), 2.21 (s, 3H), 1.06 (s, 3H, Pd−CH3). Addition of CH3CN (4 equiv) to a solution of 14 in CD2Cl2 gives 1 equiv of free ligand 5 (1H NMR corresponds) and 1 equiv of 15. (R,R)-(4-tert-Butyl-2-(tert-butylsulfinyl)phenyl)(4′-tert-butyl2′-(1:2μ-sulfenato)phenyl)amido-1-κ3S,N,S′)-(N,N,N′,N′-tetramethylethane-1,2-diamine-2κ 2 N,N′)-bis((methanido)palladium(II) ((R,R)-16). A solution of Pd(CH3)2(TMEDA) (110 mg, 0.435 mmol) and (R,R)-4 (108 mg; 0.218 mmol) in benzene (2 mL) was stirred at 50 °C for 20 h during which time a yellow precipitate formed. The solvent was then evaporated and the residue extracted with bromobenzene (5 mL) by filtration through a Celite plug (black residue). The clear orange solution was evaporated to a yellow-orange solid, which was washed with n-pentane (3 × 5 mL) and dried in vacuo to yield a bright yellow powder (62 mg, 36%). EA found: C, 48.97; H, 6.67; N, 5.02; S, 7.69. Calcd for C32H55N3O2Pd2S2: C, 48.60, H, 7.01; N, 5.31; S, 8.11. Measurement of meaningful NMR spectra was hampered by the very low solubility of 16. X-ray quality single crystals were grown from a benzene solution of [Pd(CH3)2(tmeda)] and (R,R)-4 (ca. 0.2 M), which was heated to 50 °C for 5 h and then slowly cooled to RT. Crystallographic Information. CCDC-1812095 (3), CCDC1812096 ((R,R)-4), CCDC-1812097 (meso-4), CCDC-1812098 ((R)-6), CCDC-1812099 ((all-S)-7), CCDC-1812100 ((S,S)-10), CCDC1812101 ((R,S)-11), CCDC-1812102 ((R,R)-13), and CCDC-1812103 ((R,R)-16) contain the supplementary crystallographic data for this paper.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00038. Crystallographic experimental details, crystallographic data, details on data collection and structure refinement, I


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Linden, A.; Dorta, R. Synthesis, Structure, and Catalytic Studies of Palladium and Platinum Bis-Sulfoxide Complexes. Organometallics 2014, 33, 627−636. (7) For amphphilic behavior of chiral phosphino−t-Bu-sulfinamide hybrid ligands in late transition metal complexes, see (a) Achard, T.; Benet-Buchholz, J.; Riera, A.; Verdaguer, X. Cationic Rhodium (I) Complexes of N-Phosphino-tert-butylsulfinamide Ligands: Synthesis, Structure, and Coordination Modes. Organometallics 2009, 28, 480. (b) Achard, T.; Benet-Buchholz, J.; Escudero-Adán, E.; Riera, A.; Verdaguer, X. N-Benzyl-N-phosphino-tert-butylsulfinamide and Its Coordination Modes with Ir(I), Cu(I), Pd(II), and Pt(II): P,S or P,O? Organometallics 2011, 30, 3119. (8) For a review, see O’Donnell, J. S.; Schwan, A. L. Generation, structure and reactions of sulfenic acid anions. J. Sulfur Chem. 2004, 25, 183. (9) (a) Zhang, M.; Jia, T.; Yin, H.; Carroll, P. J.; Schelter, E. J.; Walsh, P. J. A New Class of Organocatalysts: Sulfenate Anions. Angew. Chem. 2014, 126, 10931. (b) Zhang, M.; Jia, T.; Sagamanova, I. K.; Pericás, M. A.; Walsh, P. J. tert-Butyl Phenyl Sulfoxide: A Traceless Sulfenate Anion Precatalyst. Org. Lett. 2015, 17, 1164−1167. (10) (a) Lohier, J.-F.; Foucoin, F.; Jaffrès, P.-A.; Garcia, J. I.; Sopkova-de Oliveira Santos, J.; Perrio, S.; Metzner, P. An Efficient and Straightforward Access to Sulfur Substituted [2.2]Paracyclophanes: Application to Stereoselective Sulfenate Salt Alkylation. Org. Lett. 2008, 10, 1271−1274. (b) Gelat, F.; Jayashankaran, J.; Lohier, J.-F.; Gaumont, A.-C.; Perrio, S. Organocatalytic Asymmetric Synthesis of Sulfoxides from Sulfenic Acid Anions Mediated by a CinchonaDerived Phase-Transfer Reagent. Org. Lett. 2011, 13, 3170−3173. (c) Söderman, S. C.; Schwan, A. L. The Diastereoselective Alkylation of Arenesulfenate Anions Using Homochiral Electrophiles. Org. Lett. 2011, 13, 4192−4195. (d) Söderman, S. C.; Schwan, A. L. Sulfenate Substitution as a Complement and Alternative to Sulfoxidation in the Diastereoselective Preparation of Chiral β-Substituted β-Amino Sulfoxides. J. Org. Chem. 2013, 78, 1638−1649. (e) Zong, L.; Ban, X.; Kee, C. W.; Tan, C.-H. Catalytic Enantioselective Alkylation of Sulfenate Anions to Chiral Heterocyclic Sulfoxides Using Halogenated Pentanidium Salts. Angew. Chem., Int. Ed. 2014, 53, 11849−11853. (f) Dornan, P. K.; Kou, K. G. M.; Houk, K. N.; Dong, V. M. Dynamic Kinetic Resolution of Allylic Sulfoxides by Rh-Catalyzed Hydrogenation: A Combined Theoretical and Experimental Mechanistic Study. J. Am. Chem. Soc. 2014, 136, 291−298. (11) (a) Bernoud, E.; Le Duc, G.; Bantreil, X.; Prestat, G.; Madec, D.; Poli, G. Aryl Sulfoxides from Allyl Sulfoxides via [2,3]-Sigmatropic Rearrangement and Domino Pd-Catalyzed Generation/Arylation of Sulfenate Anions. Org. Lett. 2010, 12, 320−323. (b) Izquierdo, F.; Chartoire, A.; Nolan, S. P. Direct S-Arylation of Unactivated Arylsulfoxides Using [Pd(IPr*)(cin)Cl]. ACS Catal. 2013, 3, 2190− 2193. (c) Jia, T.; Zhang, M.; Jiang, H.; Wang, C. Y.; Walsh, P. J. Palladium-Catalyzed Arylation of Alkyl Sulfenate Anions. J. Am. Chem. Soc. 2015, 137, 13887−13893. (d) Gelat, F.; Lohier, J.-F.; Gaumont, A.-M.; Perrio, S. tert-Butyl Sulfoxides: Key Precursors for PalladiumCatalyzed Arylation of Sulfenate Salts. Adv. Synth. Catal. 2015, 357, 2011−2016. (e) Jiang, H.; Jia, T.; Zhang, M.; Walsh, P. J. PalladiumCatalyzed Arylation of Aryl Sulfenate Anions with Aryl Bromides under Mild Conditions: Synthesis of Diaryl Sulfoxides. Org. Lett. 2016, 18, 972−975. For enantioselective sulfoxide generation, see (f) Maitro, G.; Vogel, S.; Sadaoui, M.; Prestat, G.; Madec, D.; Poli, G. Enantioselective Synthesis of Aryl Sulfoxides via Palladium-Catalyzed Arylation of Sulfenate Anions. Org. Lett. 2007, 9, 5493−5496. (g) Jia, T.; Zhang, M.; McCollom, S. P.; Bellomo, A.; Montel, S.; Mao, J.; Dreher, S. D.; Welch, C. J.; Regalado, E. L.; Williamson, R. T.; Manor, B. C.; Tomson, N. C.; Walsh, P. J. Palladium-Catalyzed Enantioselective Arylation of Aryl Sulfenate Anions: A Combined Experimental and Computational Study. J. Am. Chem. Soc. 2017, 139, 8337−8345. (12) (a) For structural reports on Ru sulfenate complexes, see Shiu, K.-B.; Chen, J.-Y.; Yu, S.-J.; Wang, S.-L.; Liao, F.-L.; Wang, Y.; Lee, G.H. Synthesis and X-ray structures of hydridotris(1-pyrazolyl)borate carbonyl complexes of ruthenium. J. Organomet. Chem. 2002, 648, 193−203. (b) Sellmann, D.; Hein, K.; Heinemann, F. W. Ruthenium-

crystallographic comments, and UV−vis spectra of complex 10 (PDF) Accession Codes

CCDC 1812095−1812103 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frank W. Heinemann: 0000-0002-9007-8404 Andreas Scheurer: 0000-0002-2858-9406 Romano Dorta: 0000-0001-5986-9729 Author Contributions †

F.W.S. and S.F. contributed equally to this publication.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Christina Wronna for carrying out the elemental analyses and Friedrich−Alexander University for financial support.

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REFERENCES

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Organometallics

conditions using the plane-wave self-consistent field software from the Quantum ESPRESSO distribution. PBE GGA exchange-correlation functional, ultrasoft pseudopotentials, and semiempirical van der Waals correction were used. (23) Reactions of 4 or 5 with 1.0 equiv of MgPh2 does not give the respective L*Mg−Ph species, but 5 or 7 plus 0.5 equiv of unreacted MgPh2. Similarly, MgBn2 does not react with 5 or 7 to afford L*Mg− Bn. (24) (a) Jaenschke, A.; Paap, J.; Behrens, U. Synthesis and Structure of [Mg(dmso)6]2+[C5H5]−2 and [Mg(η1-C5H5)(η5-C5H5)(thf)2] (dmso = dimethylsulfoxide, thf = tetrahydrofuran). Organometallics 2003, 22, 1167. (b) Bremer, M.; Nöth, H.; Warchhold, M. The Structure of Some Amine Solvates of Magnesium Bis(tetrahydroborate) and DFT Calculations on Solvates of Lithium Tetrahydroborate. Eur. J. Inorg. Chem. 2003, 2003, 111−119. (c) Harrowfield, J. M.; Richmond, W. R.; Skelton, B. W.; White, A. H. Solid-State Models of Ion Solvation: Crystal Structures of Dimethyl Sulfoxide Solvates of Alkaline Earth Cations. Eur. J. Inorg. Chem. 2004, 2004, 227−230. (d) Jaenschke, A.; Olbrich, F.; Behrens, U. Structures of Polar Magnesium Organyls: Synthesis and Structure of Base Adducts of Magnesium Indenide. Z. Anorg. Allg. Chem. 2009, 635, 2550−2557. (25) Charette, B. J.; Ritch, J. S. A Selenium-Containing Diarylamido Pincer Ligand: Synthesis and Coordination Chemistry with Group 10 Metals. Inorg. Chem. 2016, 55, 6344−6350. (26) It should be noted that the absolute configuration of the S atom is unchanged and that the (S) label is merely the consequence of the Cahn-Ingold-Prelog priority rules. (27) Besides the analyzed prism-shaped crystals, the bulk crystallizes as needles, which contain the same S/O-coordinated complex. (28) For structurally authenticated ambidentate S/O-coordination of sulfoxide ligands in Pd(II)−DMSO complexes, see (a) Diao, T.; White, P.; Guzei, I.; Stahl, S. S. Characterization of DMSO Coordination to Palladium(II) in Solution and Insights into the Aerobic Oxidation Catalyst, Pd(DMSO)2(TFA)2. Inorg. Chem. 2012, 51, 11898−11909. For those in Pd−BINASO complexes, see ref 6b. (29) Fan, L.; Foxman, B. M.; Ozerov, O. V. N−H Cleavage as a Route to Palladium Complexes of a New PNP Pincer Ligand. Organometallics 2004, 23, 326−328. (30) Coalescence of the methyl resonances of Pd−Me and the p-Tol groups takes place at around 55 °C in THF-D8 and 95 °C in tolueneD8. The ratio of the isomers does not significantly change even when cooling to −95 °C. (31) Ozerov, O. V.; Guo, C.; Fan, L.; Foxman, B. M. Oxidative Addition of N−C and N−H Bonds to Zerovalent Nickel, Palladium, and Platinum. Organometallics 2004, 23, 5573−5580. (32) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. SambVca 2. A Web Tool for Analyzing Catalytic Pockets with Topographic Steric Maps. Organometallics 2016, 35, 2286−2293. (33) Burried volumes with bondi radii scaled at the usual 1.17 are 68.1% for 10. (34) 1H NMR spectra indicate the onset of decomposition at 50 °C, but 70 °C is needed for appreciable rates. (35) Suffert, J. Simple direct titration of organolithium reagents using N-pivaloyl-o-toluidine and/or N-pivaloyl-o-benzylaniline. J. Org. Chem. 1989, 54, 509−510. (36) Klunder, J. M.; Sharpless, B. K. Convenient synthesis of sulfinate esters from sulfonyl chlorides. J. Org. Chem. 1987, 52, 2598. (37) Tang, H.; Richey, H. G. J. Reactions of Diorganocadmium Compounds with Other Dialkylmetal Compounds and Macrocycles: Synthesis of Organocadmate Anions. Organometallics 2001, 20, 1569. (38) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Henderson, L. C.; Herber, C.; Liddle, S. T.; Loroño-González, D.; Parkin, A.; Parsons, S. The First Structural Characterisation of a Group 2 Metal Alkylperoxide Complex: Comments on the Cleavage of Dioxygen by Magnesium Alkyl Complexes. Chem. - Eur. J. 2003, 9, 4820−4828. (39) Doyle, J. R.; Slade, P. E.; Jonassen, H. B.; Rhoda, R. N. Inorg. Synth. 2007, 6, 216−219.

(II) and Ruthenium(III) Complexes Containing the [pyS4]2‑ Ligand [pyS42‑ = 2,6-Bis(2-mercaptophenylthio)dimethylpyridine(2-)]. Eur. J. Inorg. Chem. 2004, 2004, 3136−3146. (c) Petzold, H.; Xu, J.; Sadler, P. J. Metal and Ligand Control of Sulfenate Reactivity: Arene Ruthenium Thiolato-Mono-S-Oxides. Angew. Chem., Int. Ed. 2008, 47, 3008−3011. For reports on Ir complexes, see (d) Aucott, S. M.; Milton, H. L.; Robertson, S. D.; Slawin, A. M. Z.; Woollins, J. D. The preparation and characterisation of bimetallic iridium(II) complexes containing derivatised bridging naphthalene-1,8-disulfur or 4,5-dithiolato acephenanthrylene ligands. Dalton Trans. 2004, 3347−3352. (e) GameroMelo, P.; Melo-Trejo, P. A.; Cervantes-Vasquez, M.; MendizabalNavarro, N. P.; Paz-Michel, B.; Villar-Masetto, T. I.; Gonzalez-Fuentes, M. A.; Paz-Sandoval, M. A. Chemistry of Iridium(I) Cyclooctadiene Compounds with Thiapentadienyl, Sulfinylpentadienyl, and Butadienesulfonyl Ligands. Organometallics 2012, 31, 170. (13) Pd-sulfenates may be obtained by (a) oxidation of Pd-thiolato complexes: Tuntulani, T.; Musie, G.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1995, 34, 6279. or (b) by oxidative addition of sulfoxide to Pd(0): Lee, E.; Yandulov, D. V. Synthesis and characterization of Pd(IMe) 2, and its reactivity by CeS oxidative addition of DMSO. J. Organomet. Chem. 2011, 696, 4095−4103. (14) Dodds, D. L.; Boele, M. D. K.; van Strijdonck, G. P. F.; de Vries, J. G.; van Leeuwen, P. W. N. M.; Kamer, P. C. J. Design, Testing and Kinetic Analysis of Bulky Monodentate Phosphorus Ligands in the Mizoroki−Heck Reaction. Eur. J. Inorg. Chem. 2012, 2012, 1660− 1671. (15) Weix, D. J.; Ellman, J. A. Improved Synthesis of tertButanesulfinamide Suitable for Large-Scale Production. Org. Lett. 2003, 5, 1317−1320. (16) (a) Khiar, N.; Fernández, I.; Alcudia, F. Asymmetric synthesis of optically pure tert-butyl sulfoxides using the “DAG methodology. Tetrahedron Lett. 1994, 35, 5719−5722. (b) Chelouan, A.; Recio, R.; Alcudia, A.; Khiar, N.; Fernández, I. DMAP-Catalysed Sulfinylation of Diacetone-D-Glucose: Improved Method for the Synthesis of Enantiopure tert-Butyl Sulfoxides and tert-Butanesulfinamides. Eur. J. Org. Chem. 2014, 2014, 6935−6944. (17) (a) Lu, B. Z.; Jin, F.; Zhang, Y.; Wu, X.; Wald, S. A.; Senanayake, C. H. New General Sulfinylating Process for Asymmetric Synthesis of Enantiopure Sulfinates and Sulfoxides. Org. Lett. 2005, 7, 1465−1468. (b) Zhang, Y.; Chitale, S.; Goyal, N.; Li, G.; Han, Z. S.; Shen, S.; Ma, S.; Grinberg, N.; Lee, H.; Lu, B. Z.; Senanayake, C. H. Asymmetric Synthesis of Sulfinamides Using (−)-Quinine as Chiral Auxiliary. J. Org. Chem. 2012, 77, 690−695. (18) Ellmann’s reagent gives poor diastereselectivities (dr = 63:37 (meso)), whereas the DAG-based reagent is highly diastereoselective (dr > 99:1). However, separation of the DAG auxiliary from the ligand is difficult. (19) In contrast to previous reports on the synthesis of 3 we repeatedly observed the formation of the byproduct (t-Bu)2SO, which is identified in the 1H NMR spectrum by a singlet at 1.31 ppm in CDCl3 corresponding to an independently prepared sample of (tBu)2SO. The presence of this impurity prevents crystallization of 3. (20) Conformations are termed closed when the torsion angle around the three bonds connecting the quinoline moiety with the Natom of the quinuclidine part is around 50° (corresponding to C5− C6−C16−N2 in the left molecule in Figure 1). The corresponding angle of the third molecule, which is not shown in Figure 1, measures 58.29°. Open conformations typically show 150−290°. The only other quinine sulfinate structure we are aware of has a torsion angle of 62.48°.6b (21) In (S,S)-4, the distances of H1 to the S atoms are 2.72 and 2.78 Å, and those to the O atoms are 4.11 and 4.12 Å. It is noteworthy that substitution of the bulky t-Bu groups on S in (S,S)-4 for sterically sleeker p-Tol moieties does lead to N−H···O−S bonding in the crystal of (R,R)-5 (see ref 4). (22) DFT calculations show meso-4 to be only 2.9 kcal/mol more stable; therefore, thermodynamics should not be the driving force for the epimerization of (R,R)-4. The geometries of (R,R)-4 and meso-4 were optimized in the crystal environment, applying periodic boundary K


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Organometallics (40) Ladipo, F. T.; Anderson, G. K. Convenient method for the synthesis of chloro-bridged methyl- and acetylpalladium(II) dimers. Organometallics 1994, 13, 303. (b) Rülke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van Leeuwen, P. W. N. M.; Vrieze, K. NMR study on the coordination behavior of dissymmetric terdentate trinitrogen ligands on methylpalladium(II) compounds. Inorg. Chem. 1993, 32, 5769−5778. (41) De Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; Van Koten, G. Dimethyl(N,N,N′,N′-tetramethylethanediamine)palladium(II) and dimethyl[1,2-bis(dimethylphosphino)ethane]palladium(II): syntheses, x-ray crystal structures, and thermolysis, oxidative-addition and ligand-exchange reactions. Organometallics 1989, 8, 2907−2917. (42) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist. Organometallics 2010, 29, 2176−2179. (43) (a) Lu, B. Z.; Jin, F.; Zhang, Y.; Wu, X.; Wald, S. A.; Senanayake, C. H. New General Sulfinylating Process for Asymmetric Synthesis of Enantiopure Sulfinates and Sulfoxides. Org. Lett. 2005, 7, 1465−1468. (b) Zhang, Y.; Chitale, S.; Goyal, N.; Li, G.; Han, Z. S.; Shen, S.; Ma, S.; Grinberg, N.; Lee, H.; Lu, B. Z.; Senanayake, C. H. Asymmetric Synthesis of Sulfinamides Using (−)-Quinine as Chiral Auxiliary. J. Org. Chem. 2012, 77, 690−695. (44) Pedersen, D. S.; Rosenbohm, C. Dry Column Vacuum Chromatography. Synthesis 2001, 16, 2431. (45) (a) Panne, P.; DeAngelis, A.; Fox, J. M. Rh-Catalyzed Intermolecular Cyclopropanation with α-Alkyl-α-diazoesters: Catalyst-Dependent Chemo- and Diastereoselectivity. Org. Lett. 2008, 10, 2987−2989. (b) Ma, Z.; Lu, J.; Wang, X.; Chen, C. Revisiting the Kinnel−Scheuer hypothesis for the biosynthesis of palau’amine. Chem. Commun. 2011, 47, 427.

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C2-Symmetric (SO)N(SO) Sulfoxide Pincer Complexes of Mg and Pd

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