The ligands cis- and trans-1,2-(NCHC6H5)2C6H10 (trans, 1; cis, 2) underwent cyclometalation of one phenyl substituent on reaction with [Pt2Me4(μ-SMe2)2] (3) to give CH4 and chiral platinum(II) complexes [PtMe{1-(NCHC6H4)-2-(NCHC6H5)C6H10}], containing new N,N,C-donor tridentate ligands (trans, 4; cis, 5); complex 4 was structurally characterized. Reaction of racemic 4 and 5 with primary alkyl halides (iodomethane, iodoethane, 1-iodopropane, and benzyl bromide) gave platinum(IV) products by oxidative addition, often with a high degree of stereoselectivity. The absolute configuration of the major stereoisomer was determined for the following representative platinum(IV) products by X-ray crystal structure analyses: [PtIMe2{cis-1-(NCHC6H4)-2-(NCHC6H5)C6H10}] (7a), [PtIMeEt{trans-1-(NCHC6H4)-2-(NCHC6H5)C6H10}] (8a), and [PtBrMeBz{trans-1-(NCHC6H4)-2-(NCHC6H5)C6H10}] (13a; Bz = benzyl). In other cases, structural assignment was made by NMR (including NOE effects in selected cases) and supported by molecular mechanics calculations. The first step of the oxidative addition occurs primarily at the face syn and anti to the cyclohexyl group in 4 and 5, respectively, and the initial oxidative addition is trans. In the case of methyl iodide addition, but in no others, subsequent isomerization can occur to give products which appear to arise from cis oxidative addition, and the basis for this trend is elucidated. Hydrolysis of [PtIMeEt{cis-1-(NCHC6H4)-2-(NCHPh)C6H10}] (9b) gives PhCHO and [PtIMeEt{cis-1-(NCHC6H4)-2-(NH2)C6H10}], with a change in stereochemistry at platinum.
The degree of stereoselectivity in intramolecular oxidative addition of aryl−halogen bonds to platinum(II) has been determined using the following diimine ligands based on trans- or cis-1,2-diaminocyclohexane, trans-1,2-(NCHC6H4X)2C6H10 (X = Br, Cl), and cis-1,2-(NCHC6H4Br)2C6H10. Reaction of trans-1,2-(NCHC6H4X)2C6H10 (X = Br, Cl) with [Pt2Me4(μ-SMe2)2] gave the isomerically pure binuclear complexes [Pt2Me4X2(μ-SMe2){trans-1,2-(NCHC6H4)2C6H10}], which on further reaction gave the complexes [Pt2Me4(μ-X)2{trans-1,2-(NCHC6H4)2C6H10}] by loss of Me2S. The products are formed by oxidative addition of both aryl−halogen bonds of the diimine, yielding a novel chiral tetradentate N2C2-donor ligand. The ligand cis-1,2-(NCHC6H4Br)2C6H10 reacted with [Pt2Me4(μ-SMe2)2] to give mononuclear platinum(IV) products of the type [PtBrMe2{cis-1,2-(NCHC6H4)(NCHC6H4Br)C6H10}] with approximately 90% stereoselectivity and [PtBrMe2(SMe2){cis-1,2-(NCHC6H4)(NCHC6H4Br)C6H10}], and these reactions involve oxidative addition of only one of the aryl−halogen bonds. 1H NMR spectroscopy was used to characterize the new complexes and to establish diastereomeric ratios. The structures of complexes [Pt2Me4Br2(μ-SMe2){trans-1,2-(NCHC6H4)2C6H10}] (3a), [Pt2Me4(μ-Br)2{trans-1,2-(NCHC6H4)2C6H10}] (4a), and [PtBrMe2{cis-1,2-(NCHC6H4)(NCHC6H4Br)C6H10}] (6a‘) have been established by X-ray structure determinations.
A series of enantiopure C1-symmetric metallocenes, [(SiMe2)2[eta5-C5H(CHMe2)2][eta5-C5H2((S)-CHMeCMe3)]]ZrCl2, (S)-2, [(SiMe2)2[eta5-C5H(CHEt2)2][eta5-C5H2((S)-CHMeCMe3)]]ZrCl2, (S)-6, and [(SiMe2)2[eta5-C5HCy2][eta5-C5H2((S)-CHMeCMe3)]]ZrCl2, (S)-7 (Cy = cyclohexyl), zirconocene dichlorides that have an enantiopure methylneopentyl substituent on the "upper" cyclopentadienyl ligand, and diastereomerically pure precatalysts, [(SiMe2)2[eta5-C5H((S)-CHMeCy)(CHMe2)][eta5-C5H3]]ZrCl2, (S)-8a and (S)-8b, which have an enantiopure, 1-cyclohexylethyl substituent on the "lower" cyclopentadienyl ligand, has been synthesized for use in the polymerization of chiral alpha-olefins. When activated with methylaluminoxane, these metallocenes show unprecedented activity for the polymerization of bulky racemic monomers bearing substitution at the 3- and/or 4-positions. Due to the optically pure nature of these single site catalysts, they effect kinetic resolution of racemic monomers: the polymeric product is enriched with the faster reacting enantiomer, while recovered monomer is enriched with the slower reacting enantiomer. The two components are easily separated. For most olefins surveyed, a partial kinetic resolution was achieved (s = k(faster)/k(slower) approximately 2), but, in one case, the polymerization of 3,4-dimethyl-1-pentene, high levels of separation were obtained (s > 15). (13)C NMR spectroscopy of poly(3-methyl-1-pentene) produced with (S)-2 indicates that the polymers are highly isotactic materials. X-ray crystal structure determinations for (S)-2, [(SiMe2)2[eta5-C5H(CHMe2)2][eta5-C5H2((S)-CHMeCMe3)]]Zr(SC6H5)2, (S)-6, and (S)-7 have been used in combination with molecular mechanics calculations to examine the prevailing steric interactions expected in the diastereomeric transition states for propagation during polymerization. Precatalysts (S)-8a and (S)-8b are less selective polymerization catalysts for the kinetic resolution of 3-methyl-1-pentene than are (S)-2, (S)-6, and (S)-7.
The complexes [PtMe 2 (DMEP)], 1, and [PtMe 2 (DMPP)], 2 [DMEP, DMPP ) Me 2 N(CH 2 ) n Nd CH-2-C 5 H 4 N, n ) 2, 3, respectively], contain ligands that chelate to platinum through the imine and pyridyl groups, with the tertiary amine group not coordinated. Methyl iodide reacted with complex 1 to give [PtIMe 3 (DMEP)] but reacted with 2 to give a mixture of [PtIMe 3 (DMPP)] and [PtMe 3 (DMPP)]I, indicating the greater ability of DMPP to act as a fac-tridentate ligand. Complex 2 reacted with MeO 3 SCF 3 to form [PtMe 3 (DMPP)][CF 3 SO 3 ] only. The primary reaction of 1 and 2 with HX (X ) Cl, O 2 CCF 3 , or O 3 SCF 3 ) occurred by protonation of the pendant amine, while with excess acid, a methylplatinum bond trans to imine was cleaved selectively. Two products of protonolysis, namely [PtMe(DMEP)][O 3 SCF 3 ], containing mer-tridentate DMEP, and [PtMe(O 2 CCF 3 )(DMEPH)][O 2 CCF 3 ], with a protonated bidentate DMEP ligand, have been characterized by structure determinations, and the reaction pathways have been deduced by monitoring the reactions by 1 H NMR at varying temperature. In the reaction of 2 with HCl, an intermediate hydridoplatinum(IV) complex [PtHClMe 2 (DMPPH)][Cl] was detected and determined to be stable up to -30 °C, when it decomposed to give methane and [PtClMe(DMPPH)][Cl]. NMR spectrum gave the 195 PtH couplings 3 J(PtH a ) )
The bis(bidentate) ligands trans- and cis-1,2-C6H10(NCH-2-C5H4N)2 (1 and 2) yield the diplatinum(II) complexes trans- and cis-1,2-[C6H10{NCH-2-C5H4N(PtMe2)}2] (3 and 4, respectively). Reaction of 3 and 4 with [H]+[HOB(C6F5)3]- in MeCN or with HBF4 in the presence of excess CO gave the corresponding electrophilic binuclear complexes trans- and cis-1,2-[C6H10{NCH-2-C5H4N(PtMeL)}2][X]2 (5, trans, L = MeCN, X = [HOB(C6F5)3]; 6, cis, L = MeCN, X = [HOB(C6F5)3]; 7, trans, L = CO, X = BF4; 8, cis, L = CO, X = BF4). The electrophilic complexes 5−8 are formed by selective methyl group protonolysis, and the stereochemistries were confirmed for complexes 7 and 8 by X-ray structure determinations.
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