Article pubs.acs.org/Organometallics
Reactivity of Cycloplatinated Amine Complexes: Intramolecular C−C Bond Formation, C−H Activation, and PPh2 Migration in Coordinated Alkynylphosphines Kien-Wee Tan, Xiang-Yuan Yang, Yongxin Li, Yinhua Huang, Sumod A. Pullarkat, and Pak-Hing Leung* Division of Chemistry and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore 637371 S Supporting Information *
ABSTRACT: The monomeric ortho-platinated complexes [Pt{R1CH(1-C6H4)NMe2-C,N}{Ph2PCCR2}Cl] (R1 = Me, Et; R2 = Me, Ph) with trans-N,P geometries were obtained regiospecifically from the reaction between the dimeric [Pt2(μCl)2{R1CH(1-C6H4)NMe2-C,N}2] and the corresponding alkynylphosphines in high yields. The phosphine complexes are highly stable in the solid state and in solution. However, in the presence of additional Pt(II) ions, an intramolecular coupling reaction occurred in which a new carbon−carbon bond was formed between the aromatic γ-carbon of the orthoplatinated chiral phenylamine and the α-carbon of the (Ph2P)−CαCβ−(R2) ligand. The (Ph2P) moiety migrated to the neighboring β-carbon during the coupling reaction. By the judicious selection of the substituents on the alkynylphoshine along with deliberate introduction of selected chirality on the ortho-platinated phenylamine, the coupling reaction and the (Ph2P) migration were found to proceed via an associative intramolecular mechanism that involves a Pt-vinylidene intermediate.
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INTRODUCTION The past decade has witnessed a rapid development of chiral cyclometalated reagents. Of particular interest is the contribution from Dunina et al. on the structural design of new chiral cyclopalladated complexes.1 To date, many cyclopalladated complexes are routinely used as resolving agents and for the determination of optical purity of chiral ligands.2 We have applied a series of chiral ortho-palladated benzylamine and naphthylamine complexes and their platinum analogues as reaction promoters, in stoichiometric quantities, for the synthesis of chiral functionalized phosphines via cycloaddition and hydrophosphination reactions.3 We have also highlighted in many earlier reports that aromatic carbon−metal bonds in these ortho-metalated species offer strong and predictable trans electronic influences to the metal centers, thus rendering them as excellent activators for various organic substrates.3a Recently, we found that several of these cyclopalladated complexes can be used in catalytic quantities for the highly efficient asymmetric hydrophosphination reactions at low temperatures, thus providing efficient and direct access to hitherto unattainable tertiary phosphines.4 In all the documented applications of chiral cyclometalated complexes, it is evident that the organometallic ligand is, as desired, a reliable stabilizer for the metal ion, and furthermore it provides an efficient and tunable stereochemical control in asymmetric synthetic scenarios. From the literature reviews, it is also clear that these chiral auxiliaries are stable and that they are not directly involved in any chemical transformations and thus © 2012 American Chemical Society
retain their structural and stereochemical integrity. In our recent effort to search for new chiral cyclometalated complexes as catalysts for asymmetric hydrophosphination reactions, however, we observed that the aromatic carbon−palladium bonds in ortho-palladated benzylamine complexes are indeed chemically reactive, as they undergo insertion reactions in the presence of coordinated alkynylphosphines.5,6 On the other hand, the carbon−platinum bonds in the analogous orthoplatinated complexes are completely unreactive toward similar insertion reactions.6 Upon closer examination of the chemical behavior of these ortho-platinated benzylamine complexes, interestingly, we discovered that the organometallic chelates themselves can be activated by the presence of external platinum ions toward the aforementioned carbon−carbon bond formation. In this paper we report the first instance in the literature of a coupling reaction between the ortho-platinated N,N, α-trimethylbenzylamine and alkyl-substituted alkynylphosphines. The reaction involved the activation of an aromatic proton adjacent to the platinum−carbon bond and the formation of a vinylidene intermediate, which also triggered the unexpected migration of the P−C bonds during the coupling reaction. Received: October 19, 2012 Published: November 12, 2012 8407
dx.doi.org/10.1021/om300976y | Organometallics 2012, 31, 8407−8413
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RESULTS AND DISCUSSION Regiospecific Synthesis and the Thermo-Stability of Monoalkynylphosphine Platinum(II) Complexes. As illustrated in Scheme 1, the reaction between the dimeric Scheme 1. Synthesis of Precursor Complexes (S)-2 and (S)-3
platinum(II) complex (S)-1 and the diphenylphosphinoprop-1yne, Ph2PCCMe, gave the mononuclear complex (S)-2 regiospecifically. Before purification, the 31P{1H} NMR spectrum of the crude product in CDCl3 exhibited only one singlet at δ −7.2 accompanied by the expected 195Pt satellites indicative of a strong Pt−P coupling (JPt−P = 4379 Hz). After crystallization from dichloromethane/methanol, the neutral complex was isolated as white needles in 85% yield. The regiospecific coordination of Ph2PCCMe to the organometallic unit is attributed to the electronic directing effects originating from the π-accepting aromatic carbon and the σdonating nitrogen atom on the platinum. Such electronic control imparted by the cyclometalated ligands has been commonly observed with palladium(II) ions.3a Clearly, the same effects are also operating effectively on the softer platinum(II) ion in the current work. Similarly, the reaction between (S)-1 and diphenylphosphinophenylacetylene Ph2PCCPh gave the monomeric complex (S)-3 regiospecifically as white prisms in 83% isolated yield. The 31P{1H} NMR spectrum of (S)-3 (CDCl3) exhibited only one singlet at δ −6.3 (JPt−P = 4387 Hz). The molecular structure of complex (S)-3 and the trans N−Pt−P coordination geometry were confirmed by single-crystal X-ray crystallography, as shown in Figure 1. Selected bond lengths and angles are listed in Table 1. The Ph2PCCPh ligand coordinated to platinum as a simple monodentate phosphorus ligand. The C(11)−C(12) bond length is 1.211(8) Å, which exhibits clearly the triple-bond character. Both in the solid state and in solution, there are no observable intra- or intermolecular interactions between the platinum ion and the CC triple bond. Both complexes (S)-2 and (S)-3 are highly stable in the solid state and in solution. In contrast to their palladium analogues, the aromatic carbon−platinum bonds in both complexes do not undergo the possible intra- or intermolecular insertion reaction with the alkynyl moieties. Indeed, the 31P{1H} NMR spectra of both complexes remained unchanged even after being refluxed in 1,2-dichloroethane for 30 days. Coupling Reaction between Coordinated Alkynylphosphines and ortho-Platinated Benzylamine. Interestingly, the chemically stable organometallic complexes mentioned earlier could however be activated if they are refluxed in 1,2-dichloroethane in the presence of an external platinum(II) ion. As illustrated in Scheme 2, when a mixture of the Ph2PC CMe coordinated complex (S)-2 and 0.5 equivalent of the dimeric complex (S)-1 was refluxed in 1,2-dichloroethane for 4 days, a new N−C−P tridentate complex, (S)-4, was generated quantitatively. Prior to purification, the 31P{1H} NMR spectrum of the crude reaction mixture in CDCl3 exhibited a
Figure 1. ORTEP plot of complex (S)-3 (50% probability level). Hydrogen atoms except that at the C(7) stereocenter have been omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Platinum Complex 3 Pt(1)−C(1) Pt(1)−N(1) Pt(1)−P(1) Pt(1)−Cl(1) C(1)−C(2) C(7)−N(1) C(9)−N(1) C(10)−N(1) C(11)−C(12) C(11)−P(1) C(19)−P(1) C(25)−P(1)
2.006(6) 2.152(5) 2.208(2) 2.380(2) 1.401(8) 1.481 (8) 1.477(7) 1.487(7) 1.211(8) 1.758(6) 1.839 (6) 1.822(5)
C(1)−Pt(1)−N(1) C(1)−Pt(1)−P(1) N(1)−Pt(1)−P(1) C(1)−Pt(1)−Cl(1) N(1)−Pt(1)−Cl(1) P(1)−Pt(1)−Cl(1) C(11)−P(1)−C(25) C(11)−P(1)−C(19) C(25)−P(1)−C(19) C(11)−P(1)−Pt(1) C(25)−P(1)−Pt(1) C(19)−P(1)−Pt(1)
80.9(2) 96.3(2) 173.6(1) 171.0(2) 90.4(1) 92.6(1) 103.3(3) 102.2(3) 102.1(3) 113.6(2) 116.2(2) 117.5(2)
Scheme 2. Pt(II)-Promoted Coupling Reaction
sole singlet at δ 8.6 (JPt−P = 3901 Hz). The complex could be further crystallized from dichloromethane/n-hexane to form yellow prisms in 51% isolated yield, [α]D = +26.0 (c 0.5, CH2Cl2). Similar treatment of the Ph2PCCPh coordinated complex (S)-3 with (S)-1 in 1,2-dichloroethane for 6 days produced the corresponding new complex (S)-5 in quantitative yield. The 31P{1H} NMR spectrum of the crude reaction mixture in CDCl3 recorded only one singlet signal at δ 10.7 (JPt−P = 3892 Hz). The new tridentate complex could be induced to crystallize as yellow prisms in dichloromethane/nhexane, [α]D = +28.6 (c 0.5, CH2Cl2). As illustrated in Figure 2, 8408
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reaction could not be induced in the presence of the analogous palladium(II) compounds. The amount of (S)-1 could be reduced from 0.5 to 0.1 equivalent to generate the coupled products in similar yields, but a significantly longer reaction time (up to 10 days) was required. Increasing the amount of (S)-1 to 1 equivalent did not accelerate the reaction any further. For the purpose of providing mechanistic insights, the analogous ethyl-substituted dimer (S)-6 and the precursor complexes (S)-7 and (S)-8, respectively, were subsequently designed and prepared. The Pt-coordinated alkynylphosphine complex (S)-7 exhibited similar stability and reactivity to its counterparts (S)-2 and (S)-3. Accordingly, (S)-7 could be converted quantitatively to the new N−C−P tridentate complex (S)-9 by refluxing with (S)-1 or (S)-6 in 1,2dichloroethane (Scheme 3). Similar treatment also led to the Scheme 3. Coupling Reaction of the Ethyl-Labeled Complexes Figure 2. ORTEP plot of the coupling product (S)-5 (50% probability level). Hydrogen atoms except that at the C(7) atom have been omitted for clarity.
a single-crystal X-ray diffraction analysis of (S)-5 confirmed that, during the reaction, a new carbon−carbon bond, C(2)− C(11), was formed between the aromatic γ-carbon of the orthoplatinated chiral phenylamine and the CC moiety of the original monodentate (Ph2P)−CαCβ−(Ph) phosphine ligand. Following the conversion of the alkynyl group into the CC bond, the C(11)−C(12) bond distance increased to 1.346(11) Å. Surprisingly, the structural analysis also revealed that the original α-PPh2 and β-Ph alkynyl substituents in the precursor complex (S)-3 are both bonded to the same α-carbon C(12) in the product complex (S)-5. In the solid state, the bond angles around the disubstituted C(12) atom [C(11)− C(12)−C(13), 120.9(7)°; C(11)−C(12)−P(1), 120.1(6)°; and C(13)−C(12)−P(1), 119.0(6)°] are typical for trigonal geometry. However, the four substituents on the C(11) C(12) moiety showed a dihedral angle of 8.17°. The nonplanar arrangement is indicative of the ring strain in the newly formed six-membered metallacycle. Selected bond lengths and angles are listed in Table 2. It is important to reiterate that in the absence of complex (S)-1 the precursor complexes (S)-2 and (S)-3 did not undergo any coupling reaction under similar or stronger heating conditions. Clearly the platinum(II) complex (S)-1 is required as the reaction activator. It is also noteworthy that the coupling
transformation of (S)-8 into complex (S)-10. It should be noted that the activator complexes (S)-1 and (S)-6 could be replaced by less soluble achiral platinum(II) species such as [Pt(MeCN)2Cl2] to generate the respective N−C−P tridentate complexes, but in significantly lower yields. The application of the chiral reaction activators, however, provided a unique opportunity for the detailed mechanistic investigation of this novel coupling reaction. Intramolecular Reaction Mechanism. It is important to note that, as the coupled reaction required the presence of an external platinum(II) ion, the kinetic stability of the P→Pt bonds in the precursor complexes and in the reaction intermediates becomes a primary concern in the mechanistic study of this reaction. If the P→Pt bond is kinetically labile under the reaction conditions, the ligand redistribution process would allow the coupling reaction to proceed via an intermolecular pathway on the original Pt center. On the other hand, if the P→Pt bond is kinetically inert throughout the course of the coupling reaction, an intramolecular reaction mechanism must be involved. We decided to take advantage of the stereochemistry of the reacting species to establish the kinetic stability of the P→Pt bonds during the course of the coupling reaction. Therefore, the coupling reaction of (S)-2 was repeated in which the chiral activator complex (S)-1 was replaced by its equally accessible enantiomeric counterpart, (R)-1 (Scheme 4). Should the P→Pt bonds in the precursor complex and in the reaction intermediates be labile; the product complex would be obtained as a mixture of (R)-4 and (S)-4. On the other hand, if the P→Pt bonds remain kinetically inert throughout the coupling reaction, the optically pure product (S)-4 would be the sole product. Interestingly, we found that only the optically pure (S)-4 was obtained from this experiment. As a further confirmation, the same (S)-enantiomer was generated even when a large excess (2 equivalents) of (R)-
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Coupling Product 5 Pt(1)−C(1) Pt(1)−N(1) Pt(1)−P(1) Pt(1)−Cl(1) C(1)−C(2) C(7)−N(1) C(9)−N(1) C(10)−N(1) C(11)−C(12) C(12)−P(1) C(19)−P(12) C(25)−P(1)
1.992(7) 2.152(5) 2.196(2) 2.384(2) 1.436(10) 1.507(7) 1.494(7) 1.474(9) 1.346(11) 1.831(8) 1.807(7) 1.819(8)
C(1)−Pt(1)−N(1) C(1)−Pt(1)−P(1) N(1)−Pt(1)−P(1) C(1)−Pt(1)−Cl(1) N(1)−Pt(1)−Cl(1) P(1)−Pt(1)−Cl(1) C(19)−P(1)−C(25) C(19)−P(1)−C(12) C(25)−P(1)−C(12) C(19)−P(1)−Pt(1) C(25)−P(1)−Pt(1) C(12)−P(1)−Pt(1)
82.2(2) 95.5(2) 176.1(2) 174.4(2) 92.7(2) 89.6(1) 106.2(4) 102.7(4) 101.7(4) 115.7(3) 113.2(3) 115.6(3) 8409
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Scheme 4. Study on the Kinetic Stability of the P→Pt Bond during the Coupling Reaction
Scheme 5. The Proposed Reaction Intermediates
1 was used as the reaction activator. Similar results were observed from the treatment of the precursor complex (S)-3 with (R)-1: in this instance the enantiomerically pure complex (S)-5 was the sole product under various reaction conditions. These carefully designed experiments leveraging on the stereochemical handle therefore established unequivocally that the P→Pt bonds are kinetically inert and the coupling reactions proceed via a reaction mechanism in which both the orthoplatinated benzylamine and the Ph2P moiety remained coordinated on the same platinum ion throughout the course of the coupling reaction. The coupling reaction between the coordinated alknylphosphine and the ortho-platinated benzylamine involved several interesting and well-defined processes: (i) the activation of the aromatic γ-proton on the chiral phenylamine auxiliary, (ii) the activation of the CC moiety in the Pt-coordinated Ph2PC CR (R = Me, Ph) phosphine ligands, (iii) the striking 1,2rearrangement of the PPh2 or the R groups in the reaction intermediates, and (iv) the formation of a new carbon−carbon bond and hence a new six-membered P−C chelate. Electronically, the ortho-platination activates the entire aromatic ring in the chelating benzylamine. Therefore the γ-aromatic proton could be activated intrinsically as a readily available agostic proton once the cyclometalated ring is formed. In contrast, the strong P→Pt coordination does not directly activate the CC moiety in the Ph2PCCR ligand toward the carbon−carbon bond formation. Therefore, the precursor complexes (S)-2 and (S)-3 showed no chemical reactivity when they were heated in 1,2-dichloroethane for 30 days. On the other hand, we believe that in the presence of the external platinum(II) ion, the CC moiety in the Pt-coordinated Ph2PCCR ligand forms the η2alkyne →Pt species 11, as illustrated in Scheme 5. Upon heating in solution, the η2-alkyne →Pt moiety rearranged to give the η1-vinylidene intermediate 12. The η2- to η1rearrangement simultaneously triggered an interesting migration of one of the sterically bulkier alkynyl substituents to the neighboring carbon. Interestingly, two possible pathways could be envisaged that can ultimately lead to the formation of intermediate 12: (A) migration of the β-(R) group in the
coordinated (Ph2P)−CαCβ−(R) ligand to the α-carbon or (B) the shift of the coordinated α-(Ph2P) group to the βcarbon. It is our judgment that, in the current coupling reaction, pathway B is involved, as the P−C(alkyne) bonds are known to be chemically reactive7 but the Ph−C and Me−C bonds are generally considered as much more stable. A subsequent C−C bond formation between the highly reactive η1-vinylidene intermediate 12 and the γ-carbon of ortho-platinated benzylamine generated the unexpected N−C−P tridentate product complexes. The experiments described in Scheme 4 established that the P→Pt bonds remained intact throughout the coupling reaction. On the other hand, it has been previously reported that coordinated alkynylphosphines in metal clusters undergo P− C(alkyne) bond cleavage upon heating to generate distinct phosphido M−P−M bridges and η2-coordinated acetylenic (CC) fragments.7 In order to determine if the migration of the coordinated α-(Ph2P) group and the formation of the η1vinylidene intermediate 12 involve a concerted or a stepwise mechanism, a mixture of complexes (S)-2 and (S)-8 was refluxed in 1,2-dichloroethane in the presence of either (S)-1 or (S)-6 as the reaction activators (Scheme 6). If a stepwise P− C(alkyne) bond cleavage occurs during the course of the reaction, a random redistribution of the resulting individual alkynic fragments, [η2-(CC−Me)Pt] and [η2-(CC−Ph)Pt], during the subsequent P−C bond formation step would consequentially generate a mixture of the four possible N−C−P tridentate products, viz., (S)-4, (S)-5 (S)-9, and (S)-10. However, all the control experiments produced only complexes (S)-4 and (S)-10, which are expected to be formed directly from the corresponding precursors (S)-2 and (S)-8, respectively. The random distribution products (S)-5 and (S)-9 were not detected during these reactions. The experiments thus revealed that no distinct bond cleavage occurred throughout the course of the new C−C bond formation. Accordingly, the migration of the coordinated α-(Ph2P) group and the formation of the η1-vinylidene intermediate 12 involve an associative and concerted reaction mechanism. 8410
dx.doi.org/10.1021/om300976y | Organometallics 2012, 31, 8407−8413
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The alkynylphosphine ligands diphenylphosphinoprop-1-yne,8 2diphenylphosphino-1-phenylethyne,9 N,N-dimethyl-(S)-1-ethylphenylamine,10 and N,N-dimethyl-(S)-1-propylphenylamine10 were prepared according to literature methods. Synthesis of Bis(μ-chloro)-bis{(S)-1-(dimethylamino)ethyl]phenyl-C2,N}diplatinum(II), (S)-1. The optically pure amine (S)MeCH(1-C6H4)NMe2 (3.37 g, 22.6 mmol) was mixed with 800 mL of dichloromethane followed by the addition of PtCl2 (3.00 g, 11.3 mmol). The reaction mixture was left stirring for 10 d at room temperature. Removal of solvent followed by purification using silica gel column chromatography with dichloromethane as eluent gave complex (S)-1, which was subsequently crystallized out from dichloromethane/n-hexane as fluffy yellow crystals: 2.78 g, 65%; mp 214−216 °C (dec); [α]D = +17.3 (c 0.5, CH2Cl2); 1H NMR (CDCl3) δ 1.52 (d, 6H, 3JHH = 6.8 Hz, CHMe), 2.80 (s, 6H, NMeeq), 3.07 (s, 6H, NMeax), 3.97 (q, 2H, 3JHH = 4JPH = 6.7 Hz, CHMe), 6.78−7.09 (m, 8H, aromatics). Anal. Calcd for C20H28N2Pt2Cl2: C, 31.7; H, 3.8; N, 3.7. Found: C, 31.7; H, 3.8; N, 3.8. The enantiomer (R)-1 was prepared similarly using (R)-MeCH(1-C6H4)NMe2 as the starting material. Synthesis of Chloro{(S)-1-[1-(dimethylamino)ethyl]phenylC2,N]}{diphenylphosphinoprop-1-yne}platinum(II), (S)-2. Complex (S)-1 (0.50 g, 0.66 mmol) was dissolved in dichloromethane (100 mL) followed by addition of Ph2PCCMe (0.30 g, 1.32 mmol). The resulting solution was stirred at room temperature for 5 h. Removal of solvent followed by recrystallization from dichloromethane/methanol gave complex 2 as white needles: 0.68 g, 85%; mp 160−164 °C (dec); [α]D = +9.8 (c 0.5, CH2Cl2); IR (KBr) 3047(m), 2972 (m), 2899 (m), 2197 (s, CC), 1435 (s), 1101 (s), 690 (s) cm−1; 31P NMR (CDCl3) δ −7.2 (s, 1P, JPtP = 4379 Hz); 1H NMR (CDCl3) δ 1.69 (d, 3H, 3JHH = 6.4 Hz, CHMe), 2.08 (d, 3H, 4JPH = 3.6 Hz, CMe), 2.84 (d, 3H, 4 JPH = 3.2 Hz, NMe), 2.91 (d, 3H, 4JPH = 2.8 Hz, NMe), 3.97 (qn, 1H, 3 JHH = 4JPH = 5.6 Hz, CHMe), 6.62−7.89 (m, 14H, aromatics). Anal. Calcd for C25H27NPPtCl: C, 49.8; H, 4.5; N, 2.3. Found: C, 49.8; H, 4.4; N, 2.2. Synthesis of Chloro{(S)-1-[1-(dimethylamino)ethyl]phenylC2,N]}{diphenylphosphino-1-(phenyl)ethyne}platinum(II), (S)3. Using a similar method to that adopted for the synthesis of 2 described earlier, a stirring solution of (S)-1 (0.50 g, 0.66 mmol) in dichloromethane (100 mL) was treated with Ph2PCCPh (0.38 g, 1.33 mmol). The resulting mixture was stirred at room temperature for 5 h. The solvent was subsequently reduced, and recrystallization from dichloromethane/methanol gave 3 as white crystals: 0.73 g, 83%; mp 173−175 °C (dec).; [α]365 = −61.4 (c 0.5, CH2Cl2); IR (ν) 3049(m), 2974 (m), 2918 (m), 2174 (s, CC), 1435 (s), 1099 (s), 691 (s) cm−1; 31P NMR (CDCl3) δ −6.3 (s, 1P, JPtP = 4387 Hz); 1H NMR (CDCl3) δ 1.71 (d, 3H, 3JHH = 6.0 Hz, CHMe), 2.87 (d, 3H, 4JPH = 2.8 Hz, NMe), 2.95 (d, 3H, 4JPH = 1.6 Hz, NMe), 4.01 (qn, 1H, 3JHH = 6.4 Hz, 4JPH = 6.0 Hz, CHMe), 6.71−8.05 (m, 19H, aromatics). Anal. Calcd for C30H29NPPtCl·0.25CH2Cl2: C, 49.6; H, 4.2; N, 1.9. Found: C, 49.8; H, 4.4; N, 2.2. Coupling Reaction: Generation of the Tridentate Complex (S)-4. Complex 2 (0.40 g, 0.66 mmol) was dissolved in 1,2dichloroethane (100 mL) followed by addition of 0.5 molar equivalent of complex 1 (0.25 g, 0.33 mmol). The reaction mixture was stirred and refluxed at 83 °C for 4 d with daily monitoring using 31P NMR spectroscopy. The crude reaction product was purified via silica gel-60 column chromatography using dichloromethane/n-hexane (3:1) as eluent. Removal of solvent followed by slow addition of n-hexane to the purified product gave complex (S)-4 as yellow crystals: 0.20 g, 51%; mp 199−201 °C (dec); [α]D = +26.0 (c 0.5, CH2Cl2); 31P NMR (CDCl3) δ 8.6 (s, 1P, JPtP = 3901 Hz); 1H NMR (CDCl3) δ 1.52 (d, 3H, 3JHH = 6.4 Hz, CHMe), 1.78 (dd, 3H, 4JHH = 1.2 Hz, 3JPH = 9.6 Hz, =CMe), 2.87 (d, 3H, 4JPH = 2.8 Hz, NMe), 2.89 (d, 3H, 4JPH = 2.4 Hz, NMe), 3.89 (qn, 1H, 3JHH = 4JPH = 5.6 Hz, CHMe), 6.94 (d, 1H, 3JPH = 7.2 Hz, CH), 7.01−7.83 (m, 13H, aromatics). Anal. Calcd for C25H27NPPtCl: C, 49.7; H, 4.5; N, 2.3. Found: C, 49.1; H, 4.4; N, 2.7. Synthesis of the Tridentate Complex (S)-5. A solution of complex (S)-3 (0.40 g, 0.60 mmol) in 1,2-dichloroethane (100 mL) was treated with 0.5 molar equivalent of complex (S)-1 (0.23 g, 0.30
Scheme 6. Study on the Mechanism of the (Ph2P) Group Migration
In conclusion, we have observed that ortho-platinated benzylamine complexes are highly stable both in the solid state and in solution. Although the aromatic γ-proton on the chiral phenylamine auxiliary can be activated by metal complexation, the organometallic rings remain intact under the prolonged heating conditions employed. The coupling reaction between the activated aromatic carbon and the coordinated alkynylphosphine ligands occurred via an intramolecular mechanism with all the reacting species remaining coordinated on the cycloplatinated unit. In subsequent work, it will be shown that the information obtained in the current investigation is crucial to the rational development of chiral cycloplatinated complexes and in their application as catalysts for stereochemically demanding organic transformations.
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EXPERIMENTAL SECTION
Reactions involving air-sensitive compounds were performed under a positive pressure of purified nitrogen. A Bruker Avance 300 spectrometer was used to record the 1H (300 MHz) and 31P{1H} (121 MHz) NMR spectra. Optical rotations were measured on the specified solution in a 1 dm cell at 25 °C with a Perkin-Elmer model 341 polarimeter. Infrared spectra were measured on a Shimadzu IR Prestige-21 instrument. Elemental analyses were performed by the Elemental Analysis Laboratory of the Department of Chemistry at the National University of Singapore and the Division of Chemistry and Biological Chemistry at the Nanyang Technological University. Melting points were determined on a SRS-Optimelt MPA-100 apparatus. 8411
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Organometallics
Article
mmol). The reaction mixture was stirred and refluxed at 83 °C with daily monitoring using 31P NMR spectroscopy and was found to be complete in 6 d. The crude reaction product was purified over silica gel-60 column chromatography using ethyl acetate/n-hexane (3:4) as eluent. Removal of solvent followed by slow addition of n-hexane to the purified product gave complex (S)-5 as yellow crystals: 0.19 g, 48%; mp 233−237 °C (dec); [α]D = +28.6 (c 0.5, CH2Cl2); 31P NMR (CDCl3) δ 10.7 (s, JPtP = 3892 Hz, 1 P); 1H NMR (CDCl3) δ 1.54 (d, 3H, 3JHH = 6.4 Hz, CHMe), 2.89 (d, 3H, 4JPH = 3.2 Hz, NMe), 2.92 (d, 3H, 4JPH = 2.4 Hz, NMe), 3.92 (qn, 1H, 3JHH = 4JPH = 6.0 Hz, CHMe), 6.75 (d, 1H, 3JPH = 7.4 Hz, =CH), 6.98−7.81 (m, 18H, aromatics). Anal. Calcd for C30H29NPPtCl: C, 54.2; H, 4.4; N, 2.1. Found: C, 54.2; H, 4.0; N, 2.6. Synthesis of Bis(μ-chloro)-bis{(S)-1-(dimethylamino)propyl]phenyl-C2,N}diplatinum(II), (S)-6. The optically pure amine (S)EtCH(1-C6H4)NMe2 (3.20 g, 19.6 mmol) was mixed with 800 mL of dichloromethane. Thereafter, a stoichiometric amount of PtCl2 (2.60 g, 9.8 mmol) was then added and left to stir at room temperature for 9 d. Removal of solvent followed by purification using silica gel column chromatography using dichloromethane as eluent gave the pure complex (S)-6. Subsequently crystallization of the product from dichloromethane/n-hexane deposited (S)-6 as pale yellow crystals: 2.54 g, 66%; mp 218−219 °C (dec); [α]D = +169.0 (c 0.6, CH2Cl2); 1 H NMR (CDCl3) δ 0.94 (t, 6H, 3JHH = 7.4 Hz, CHMe), 2.06−2.16 (m, 1H, HCHMe), 2.17−2.28 (m, 1H, HCHMe), 2.82 (s, 6H, NMe), 2.99 (s, 6H, NMe), 3.24 (q, 2H, 3JHH = 4JPH = 4.3 Hz, CHEt), 6.84− 7.12 (m, 8H, aromatics). Anal. Calcd for C22H32N2Pt2Cl2: C, 33.6; H, 4.1; N, 3.6. Found: C, 33.7; H, 3.8; N, 3.8. Synthesis of Chloro{(S)-1-[1-(dimethylamino)propyl]phenylC2,N]}{diphenylphosphinoprop-1-yne}platinum(II), (S)-7. A dichloromethane solution of (S)-6 (0.79 g, 1.00 mmol in 100 mL of CH2Cl2) was treated with Ph2PCCMe (0.45 g, 2.00 mmol), and the resulting mixture was stirred at room temperature for approximately 5 h. Removal of solvent followed by recrystallization of the product from dichloromethane/n-hexane gave complex (S)-7 as yellow crystals: 1.07 g, 87%; mp 227−229 °C (dec); [α]D = +51.7 (c 0.6, CH2Cl2); IR (ν) 3042(m), 2953 (m), 2922 (m), 2205 (s, CC), 1436 (s), 1103 (s), 690 (s) cm−1; 31P NMR (CDCl3) δ −7.5 (s, 1P, JPtP = 4404 Hz); 1H NMR (CDCl3) δ 0.97 (t, 3H, 3JHH = 7.4 Hz, CH2Me), 2.09 (d, 3H, 4 JPH = 3.5 Hz, CMe), 2.30 (qn, 2H, 3JHH = 7.4 Hz, CH2Me), 2.74 (d, 3H, 4JPH = 2.2 Hz, NMe), 2.99 (d, 3H, 4JPH = 3.5 Hz, NMe), 3.47 (q, 1H, 3JHH = 4JPH = 6.2 Hz, CHEt), 6.60 − 7.94 (m, 14H, aromatics). Anal. Calcd for C26H28NPPtCl·0.25CH2Cl2: C, 49.4; H, 4.7; N, 2.2. Found: C, 49.8; H, 4.4; N, 2.2. Synthesis of Chloro{(S)-1-[1-(dimethylamino)propyl]phenylC2,N]}{diphenylphosphino-1-(phenyl)ethyne}platinum(II), (S)8. Complex (S)-6 (0.77 g, 0.98 mmol) was dissolved in dichloromethane (100 mL) followed by the addition of Ph2PCCPh (0.56 g, 1.96 mmol). The resulting solution was stirred at room temperature for 5 h. The solvent was removed, and the compound was further dried in vacuo for 6 h: 1.17 g, 88%; mp 160−164 °C (dec); [α]D = +63.2 (c 0.5, CH2Cl2); IR (ν): 3050 (m), 2974 (m), 2920 (m), 2174 (s, CC), 1437 (s), 1099 (s), 691 (s) cm−1; 31P NMR (CDCl3) δ −6.6 (s, 1P, JPtP = 4413 Hz); 1H NMR (CDCl3) δ 0.97 (t, 3H, 3JHH = 5.6 Hz, CH2Me), 2.30 (dq, 2H, 3JHH = 7.4 Hz, CH2Me), 2.76 (d, 3H, 4 JPH = 1.4 Hz, NMe), 3.30 (d, 3H, 4JPH = 2.5 Hz, NMe), 3.48 (q, 1H, 3 JHH = 4JPH = 5.4 Hz, CHEt), 6.66−8.08 (m, 19H, aromatics). Anal. Calcd for C31H31NPPtCl·CH2Cl2: C, 50.2; H, 4.4; N, 1.8. Found: C, 49.8; H, 4.4; N, 2.2. Coupling Reaction: Generation of the Tridentate Complex (S)-9. Complex (S)-7 (0.53 g, 0.86 mmol) was dissolved in 1,2dichloroethane (100 mL) followed by the addition of 0.5 molar equivalent of (S)-6 (0.34 g, 0.43 mmol). The reaction mixture was stirred and refluxed at 83 °C with daily monitoring using 31P NMR and was found to be complete in 5 d. The crude reaction product was purified over silica gel-60 column chromatography using dichloromethane/n-hexane (2:1) as eluent. Slow crystallization of the pure column fraction using dichloromethane/diethyl ether afforded (S)-9 as yellow crystals: 0.26 g, 50%; mp 211−212 °C (dec); [α]D = +122.8 (c 0.6, CH2Cl2); 31P NMR (CDCl3) δ 8.7 (s, 1P, JPtP = 3925 Hz); 1H
NMR (CDCl3) δ 0.76 (t, 3H, 3JHH = 6.4 Hz, CHMe), 1.78 (dd, 3H, JHH = 1.2 Hz, 3JPH = 9.5 Hz, CMe), 1.84−1.92 (m, 1H, HCHMe), 2.07−2.15 (m, 1H, HCHMe), 2.78 (d, 3H, 4JPH = 1.9 Hz, NMe), 2.97 (d, 3H, 4JPH = 3.2 Hz, NMe), 3.49 (qn, 1H, 3JHH = 4JPH = 6.0 Hz, CHEt), 6.92 (d, 1H, 3JPH = 7.4 Hz, CH), 6.99−7.86 (m, 13H, aromatics). Anal. Calcd for C26H29NPPtCl·0.25CH2Cl2: C, 49.4; H, 4.7; N, 2.3. Found: C, 49.1; H, 4.4; N, 2.7. Synthesis of the Tridentate Complex (S)-10. Complex (S)-8 (0.34 g, 0.50 mmol) was dissolved in 1,2-dichloroethane (100 mL) followed by the addition 0.5 molar equivalent of [(S)-6 (0.20 g, 0.25 mmol). The reaction mixture was stirred and refluxed at 83 °C with daily monitoring using 31P NMR and was found to be complete in 8 d. The crude reaction product was purified over silica gel-60 column chromatography using ethyl acetate/n-hexane (1:3) as eluent. Removal of solvent followed by slow addition of n-hexane to the purified product gave (S)-10 as yellow crystals: 0.17 g, 49%; mp 253−254 °C (dec); [α]D = +135.6 (c 0.5, CH2Cl2); 31P NMR (CDCl3) δ 10.8 (s, JPtP = 3917 Hz, 1P); 1H NMR (CDCl3) δ 0.80 (t, 3H, 3JHH = 7.4 Hz, CH2Me), 1.89−1.97 (m, 1H, HCHMe), 2.09−2.18 (m, 1H, HCHMe), 2.84 (d, 3H, 4JPH = 1.9 Hz, NMe), 3.00 (d, 3H, 4JPH = 3.3 Hz, NMe), 3.56 (qn, 1H, 3JHH = 4JPH = 5.6 Hz, CHEt), 6.77 (d, 1H, 3JPH = 8.1 Hz, CH), 7.00−7.84 (m, 18H, aromatics). Anal. Calcd for C31H31NPPtCl: C, 54.8; H, 4.6; N, 2.1. Found: C, 54.2; H, 4.0; N, 2.6. 4
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ASSOCIATED CONTENT
S Supporting Information *
Crystals and refinement data for complexes 3 and 5 and experimental details of the single-crystal X-ray diffraction structure determinations are given in Table 3. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Nanyang Technological University for a scholarship to X.-Y.Y. and the financial support for this project under the grant M58110016.
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