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Reaction of Metal Carbonyls with Naked Enolates to Make a Metallocarbene Enolate and Alkylcarbonylmetalates
No full textThe KH-mediated deprotonation of acetophenone and propiophenone in THF led to the ion-pair and naked forms of the following crystalline enolates in good yield: ArC(CH2)0 ... K-(18-crown-6) [Ar = Ph, 4; 2-MeOC6H4, 5; 1,3,5-Me3C6H2,6], [PhC(CH2)0][K(kryptofix-2,2,2)], 7, and [PhC(--CHMe)O ... K(18-crown-6)], 8. The reaction of ion-pair 4 with [Cr(CO)5(THF)] led to the isolation of an alkylcarbonylmetalate, 9, [PhC(O)CH2Cr(CO)4CO ... K(18-crown-6)]n, a crystalline polymer in which the [PhC(O)CH2Cr(CO)5]- anion is bridged by [K(18-crown-6)]+ via the oxygen of the enolate and one of the carbonyl oxygens. The protonation of the Fischer carbene [(CO)5Cr=C(OMe)Me], 10, led to an alkylcarbonylmetalate analogue of 9. The X-ray analysis of [CH2(OMe)CCr(CO)4CO ... K(18-crown-6)], 11, revealed a decrease in the Cr-C(carbene) bond distance, and electron transfer from the anionic carbene to the trans CO. The trans Cr-C-0 has an increased Cr-C bond length, a decreased C-O bond length, and a Cr-C-O angle of 159.7(6)-degrees. The reaction of 4 with [Cr(CO)6] led to a Fischer-type metallacarbene, 12, from attack of the enolate on carbon monoxide; 12 was deprotonated by starting material to a dianionic metallacarbene enolate [(CO)5Cr--C(O-)CH=C(O-)Ph], 13. The alkylation and silylation of 13 led to the corresponding alkylated and silylated forms of 13, [(CO)5Cr=C(OMe)-CH=C-(OMe)Ph], 16, and [(CO)5Cr=C(OSiR2R')-CH=C(OSiR2R')Ph] [R = R' = Me, 17; R = Me, R' = Bu(t), 18]. An analogous metallacarbene enolate and its silylated form have been obtained for tungsten, [(CO)5W=C(O-)CH=C(O-)Ph ... {K(18-crown-6)}2], 19, and [ (CO)5W=C(OSiMe3)-CH=C(OSiMe3)Ph], 20. The reaction of the naked enolate 7 and [Cr(CO)6] led to a metallacarbene enolate which, due to the absence of stabilization by ion-pair formation, is much less stable than 13 or 19. Crystallographic details are as follows: 4 is monoclinic, space group P2(1)/n, a = 10.109(1) angstrom, b = 9.041(1) angstrom, c = 24.935(2) angstrom, beta = 96.94(l)-degrees, Z = 4, and R = 0.053; 7 is monoclinic, space group P2/n, a = 17.473(2) angstrom, b = 10.615(1) angstrom, c = 16.133(2) angstrom, beta = 99.75(1)-degrees, Z = 4, and R 0.040; 8 is monoclinic, space group P2(1)/c, a = 14.285(3) angstrom, b = 16.714(2) angstrom, c = 10.381(4) angstrom, = 107.88(2)-degrees, Z = 4, and R = 0.0 51; 9 is triclinic, space group P1BAR, a = 10.649(l) angstrom, b = 14.122(1) angstrom, c = 10.635(1) angstrom, alpha = 94.64(l)-degrees, beta = 98.04(l)-degrees, gamma = 108.81(l), Z = 2, and R = 0.031; 11 is triclinic, space group P1BAR, a 14.029(1) angstrom, b = 12.355(1) angstrom, c = 8.995(1) angstrom, a = 91.82(l)-degrees, beta = 91.17(l)-degrees, gamma = 91.89(l)-degrees, Z = 2, and R = 0.051
The alpha-Sulfinyl Ligand-Transition Metal Bond. 2. alpha-Sulfinyl Palladium(II) Complexes and Their Reactivity in Insertion Reactions
No full textThe deprotonation of ArSOCH3 (Ar = C6H5, 1; p-MeC6H4,2) with KH in THF, followed by reaction with cis-[(PPh3)2PdCl2], led to the Pd-C-bonded alpha-sulfinyl complexes trans-[(PPh3)2(Cl)PdCH2SOAr] (Ar = C6H5, 3; Ar = p-MeC6H4, 4). As expected, the ''CH2'' unit appears as an AB part of an ABX2 system (the two heterotopic protons being coupled with both P atoms). The structure of 3 was confirmed by X-ray analysis. The reaction of 3 and 4 with Bu(t)NC afforded complexes [(PPh3)(Bu(t)NC)(Cl)PdC(NBu(t))C(HNBu(t))=CHSOAr] (Ar = C6H5, 5; Ar = p-MeC6H4, 6) via the replacement of PPh3 by Bu(t)NC and insertion of two molecules of the isocyanide into the Pd-C bond. The organic fragment resulting from the insertion of Bu(t)NC was isolated in the beta-enamino sulfoxide form and confirmed by an X-ray analysis on 5. The ionization of the Pd-Cl bond in 3 and 4 in MeCN led to the corresponding solvated cationic forms cis-[(PPh3)2(MeCN)PdCH2SOAr] (Ar = C6H5, 10; Ar = p-MeC6H4, 11). The attempt to dehydrohalogenate 3 with the Wittig reagent Ph3P-CH2 led rather to the isolation of a zwitterionic complex where one of the PPh3 groups had been replaced by Ph3PCH2, [(Cl)(PPh3)(CH2PPh3)(CH2SOPh)Pd], 12, containing two different alpha-functionalized carbanions bonded to Pd(II). Crystallographic details: 3 is triclinic, space group P1BAR, a = 12.517(2) angstrom, b = 16.153(2) angstrom, c = 12.420(2) angstrom, alpha = 104.16(2)degrees, beta = 119.40(2)degrees, gamma = 92.60(2)degrees, Z = 2, and R = 0.046; 5 is monoclinic, space group C2/c, a = 43.577(4) angstrom, b = 9.214(l) angstrom, c = 20.752(2) angstrom, alpha = gamma = 90-degrees, beta = 101.20(l)degrees, Z = 8, and R = 0.037; 10 is triclinic, space group P1BAR, a = 15.111 (1) angstrom, b = 12.873(1) angstrom, c = 12.332(1) angstrom, alpha = 99.89(l)degrees, beta = 101.44(1)degrees, gamma = 98.08(1)degrees, Z = 2, and R = 0.049; 12 is triclinic, space group P1BAR, a = 11.897(1) angstrom, b = 14.891(2) angstrom, c = 11.357(1) angstrom, alpha = 104.24(1)degrees, beta = 98.49(l)degrees, gamma = 96.87(l)degrees, Z = 2, and R = 0.025
Terminal and Bridging Bonding Modes of the Acetophenone Enolate to Palladium(II): The Structural Evidence and the Insertion of Isocyanides
No full textThe oxidative addition of PhCOCH2Cl to Pd(PPh3)4 led to a high yield of trans-[(PPh3)2(Cl)Pd(CH2COPh)], 3, in which the enolate is C-bonded to Pd(II), as shown by an X-ray analysis. Complex 3 reacts with ButNC., and depending on the stoichiometric ratio, two compounds were isolated. With a 1:1 stoichiometry, the insertion of ButNC into the PdC bond of 3 was observed and the crystalline solid 5 isolated, trans-[(PPh3)2(Cl)PdC-(NHBut)CHC(O) Ph], which contains the enolic form of a β-keto imino fragment. The reaction of 3 with 2 mol of ButNC led to a good yield (>70%) of an organic compound 7, which results from double insertion of ButNC on the PdC bond of 3, followed by homolytic cleavage of the PdC bond and dimerization of the resulting organic fragments. The ionization of the PdCl bond by AgCF3SO3 in acetonitrile led not to the cationic solvated form of 3, but rather to the dimer [{cis-(PPh3)2Pd}2(μ-CH2COPh)2]2+·2[CF3SO3]−, 9, containing the enolato anion bridging two palladium atoms in the oxoallyl form. In coordinating solvents, 9 is a source of the expected monomeric cationic form. Crystallographic details: 3 is monoclinic, space group Pn, a = 9.845(1) Å, b = 12.119(2) Å, c = 15.432(2) Å, α = γ = 90°, β = 95.29°, Z = 2, and R = 0.024; 5 is triclinic, space group P1̄, a = 12.040(1) Å, b = 18.473(2) Å, c = 11.922(1) Å, α = 97.25(1)°, β = 109.29(1)°, γ = 81.31°, Z = 8, and R = 0.072; 9 is monoclinic, space group I2/a, a = 21.550(2) Å, b = 34.773(3) Å, c = 22.592(2) Å, α = γ = 90°, β = 95.88(1)°, Z = 8, and R = 0.072
alpha-Sulfinyl Transition Metal Bond. 1. Naked alpha-Sulfenyl Anions and their Reactivity with Metal Carbonyls: Synthesis of Metal-Carbon Bonded alpha-Sulfenyls and alpha-Sulfenylcarbenes
No full textDeprotonation of ArSOCH3 [Ar = C6H5,1; Ar = 4-MeC6H4, 2] with KH in THF in the presence of 18-crown-6 and Kryptofix-2,2,2 led to the isolation of the respective ion-contact and ionseparated pairs: [ArS(CH2)O⋯K(18-crown-6)] [Ar = C6H5, 3; Ar = 4-MeC6H4,4]; [ArS(CH2)O⋯K-(kryptofix-2,2,2)] [Ar = C6H5, 5; Ar = 4-MeC6H4,6]. The complexation of the potassium cation in both 4 and 6 causes the complete racemization of the α-sulfinyl anion. The structures of 3 and 5 have been determined with an X-ray analysis. The reaction of 3 and 4 with [Cr(CO)5-(THF)] led to the formation of α-sulfinyl carbonylmetalates containing a CrC σ bond, in [ArS(O)CH2Cr(CO)5]−[K(18-crown-6)]+[Ar = C6H5, 7; Ar = 4-MeC6H4, 8]. The high nucleophilicity of 3 is demonstrated in the reaction with a weakly electrophilic CO in [Cr(CO)6]. The reaction gives an unprecedented metallacarbene–sulfinyl anion complex, [{(CO)5CrC(O)CHS(O)Ph}⋯{K(18-crown-6)}2], 10, in which the carbene sulfoxide fragments migrate to a carbon monoxide, forming an α-sulfinylcarbene metallacycle, [formula omitted]⋯{K(18-crown-6)}2], 11. Exhaustive silylation of the peripheral oxygens of 11 led to the corresponding silylated form 12. Crystallographic details: 3 is monoclinic, space group P21, with a = 9.025(1) Å, b = 15.799(2) Å, c = 8.579(1) Å, α = γ = 90°, β = 107.76(1)°, Z = 2, and R = 0.040; 5 is triclinic, space group P1̅, with a = 10.713(1) Å, b = 10.825(1) Å, c = 14.877(1) Å,α = 112.07(1)°, β = 112.83(1)°, γ = 80.39(1)°, Z = 2, and R = 0.067; 11 is triclinic, space group P1̅, with a = 15.251(4) Å, b = 16.774(2) Å, c = 11.833(2) Å, α = 109.75(1)° β = 110.30(2)°, γ = 85.94(1)°, Z = 2, and R = 0.049
Titanocene-Based Acetophenone Enolates: Synthesis and Structural Characterization of Mono, Bis, and Cationic Forms of Titanium Enolates and Some Aspects of Their Reactivity
No full textThe reaction of [cp2TiCl2] with potassium enolates derived from 2-MeOC6H4-COMe, 2, 4, 6-Me3C6H2COMe, and KH led to the metal–enolates [2-MeOC6H4C(CH2)OTi-(Cl)cp2](1), [2, 4, 6-Me3C6H2C(CH2)CTi(Cl)cp2] (2), and [{2-MeOC6H4C(CH2) 0}2Ticp2] (3) where the enolate is η1-O-bonded to the metal and the o-methoxy group does not interact with the metal. In order to increase the acidity and the template properties of the metal, complex 2 was ionized with CF3SO3Ag and AgBF4. The cationic form of 2 readily reacts with cyclohexanone to give [2, 4, 6-Me3C6H2C(O)CH=C6H10] (6) and a titanium oxo species. The cationic form, however, was isolated as an isocyanide adduct with ButNC, [2, 4, 6-Me3C6H2C(CH2)OTi-(CNBut)cp2]+X−(X− = CF3SO3, 8; X− = BF4, 9). The increased reactivity of the enolato functionality in the cationic form of 2 is demonstrated in the reaction with acetonitrile and benzonitrile, where the binding of the nitrile prompted attack by the nucleophilic enolate on the nitrile. The reaction led to β-keto enamine derivatives N, O-bonded to the [cp2Ti]2+ fragment, [2-MeOC6H4C(O) CH=C(Me) NHTicp2]+CF3SO3-(13) and [2, 4, 6-Me3C6H2C(O)CHC(R)-NHTicp2]+CF3SO3-(R = Me, 14; R = Ph, 15). Crystallographic details: 1 is orthorhombic, space group P212121, a = 7.690(1) Å, b = 14.113(2) Å, c = 15.685(2) Å, a = β = γ = 90°, Z = 4, and R = 0.034; 3 is orthorhombic, space group Aba2, a = 9.472(1) Å, b = 20.257(2) Å, c = 12.378(1) Å, α = β = γ = 90°, Z = 4, and R = 0.045; 14 is monoclinic, space group P21/n, a = 7.765(6) Å, b = 18.386(3) Å, c = 17.677(3) Å, α = γ = 90°, β = 100.6(3)°, Z = 4, and R = 0.088
Zirconium-Assisted Aldol Condensation Reactions of Amido Enolates: Structural and Kinetic Analysis of the Reaction of N,N-Diphenylacetamide and N,N-Diphenylpropionamide Enolates with Benzaldehyde and p-Substituted Acetophenones
No full textThe deprotonation of N,N-diphenyl amides with LDA and subsequent reaction with [(cp)(2)ZrCl2] (cp = eta(5)-C5H5) allowed the enolate complexes [{Ph(2)NC(CH2)O}Zr(cp)(2)(Cl)] (5) and [{Ph(2)NC(CHCH3)O}Zr(cp)(2)(Cl)] (6) to be isolated. The crystal structure of 6 is reported. Reaction of 5 with [Cr(CO)(5)]. THF gave [Ph(2)NC{CH2Cr(CO)(5)}{OZr(Cl)(cp)(2)}] (7), an 0- and C-bonded dimetallic amido enolate. Reaction of 5 and 6 with benzaldehyde gave the corresponding aldol products 8 and 9; according to previous reports, the conversion of 6 into 9 is syn selective. Reaction of 6 with acetophenone followed by addition of silver triflate gave the complex [Ph(2)NC(=O)CH(CH3)C(Me)(Ph)OZr(cp)(2)(Cl)(OSO2CF3)] (11) as a diastereoisomeric mixture in the ratio anti:syn = 65:35. The crystal structure of sym-11 is reported. This cyclic complex mimics the postulated cyclic transition state of the aldol reaction mediated by zirconium amide enolates. A kinetic investigation of the reaction of 6 with acetophenone was performed and gave the following activation parameters: Delta H-double dagger = 38.3 +/- 0.9 kJ mol(-1); Delta S-double dagger greater than or equal to -181 +/- 3 J mol(-1) K-1; Delta G(298)(double dagger) = 92.2 +/- 1.2 kJ mol(-1). A similar study with 4-fluoroacetophenone gave Delta H-double dagger = 43.5 +/- 1.3 kJ mol(-1), Delta S-double dagger greater than or equal to -167 +/- 4 J mol(-1) K-1, and Delta G(298)(double dagger) = 93.3 +/- 1.8 kJ mol(-1). The reaction rate at 320 K determined with 4-chloro-, 4-methyl-, and 4-nitroacetophenone allowed the determination of a Hammett plot with rho = 0.41. This value is implicit of a carbon-carbon bond-forming, rate-limiting step
Titanium and Zirconium Ferrocene-Substituted Enolates and Their Reaction Products with Benzaldehyde and Acetophenone: Structural and Kinetic Studies of the Aldol Condensation Pathway
No full textThe reaction of acetyl- and propionylferrocene, [(RCOcp)(cp)Fe] (R = Me (1), Et (2)), with KH led to the isolation of the corresponding ion-pair enolates 3 and a in the solid state. When the deprotonation of 1 is carried out in the presence of 18-crown-6, the naked enolate 5, [(CH(2)COcp)(cp)Fe]K--(18-crown-6)(+)], was isolated. The potassium enolate 3 has been converted into the corresponding titanium and zirconium enolates via the reaction with (cp)(2)MCl(2) (M = Ti, Zr), while only 4 was converted into the analogous zirconium enolate. The following metal enolates have been isolated in good yield and as crystalline solids: [(cp)-Fe(cpC(CH2)O)M(cp)(2)(Cl)] (M = Ti (6), Zr (7)) and [(cp)Fe(cpC(CHMe)O)Zr(cp)(2)(Cl)] (8). Compounds 6 and 7 have been characterized by H-1 and C-13 NMR, and an X-ray crystal structure of 7 was obtained; compound 8 was characterized by H-1 NMR. The aldol reaction of 6 and 7 with benzaldehyde led to the corresponding metal aldol derivatives [(cp)Fe(cpC-(O)CH2C(H)(Ph)O)M(cp)(2)(Cl)] (M = Ti (9), Zr(10)). For compound 9 the solid-state structure and solution data are reported. Complex 9 undergoes a facile Ti-Cl ionization, leading to the cationic complex [(cp)Fe(cpC(O)CH2C(H)(Ph)O)Ti(cp)(2)](+)BPh(4)(-) (11). In complex 11, the aldol fragment forms a metallacycle where both oxygens are bonded to titanium. This structure mimics the bond connectivity of the generally proposed ''transition state'' of aldol reactions. The isolation of structurally well-defined titanium and zirconium enolates allowed us to carry out a kinetic investigation into the reaction of 7 with acetophenone. The reaction was carried out in C6D6 at temperatures from 283 to 340 K. The reaction is second order (first order in each reactant), and the following activation parameters have been obtained: Delta H-double dagger = 44.4 +/- 1.7 kJ mol(-1), Delta S-double dagger greater than or equal to -150 +/- 6 J mol(-1) K-1, and Delta G(298)(double dagger) = 89.0 +/- 2.4 kJ mol(-1). A similar study with 4-fluoroacetophenone gave Delta H-double dagger = 33.0 +/- 4.6 kJ mol(-1), Delta S-double dagger greater than or equal to -189 +/- 15 J mol(-1) K-1, and Delta G(298)(double dagger) = 89.2 +/- 6.4 kJ mol(-1). The reaction rate at 320 K determined with 4-chloro-, 4-methyl-, and 4-nitroacetophenone allowed the determination of a Hammett plot with rho = 0.42 +/- 0.9. This value is implicit for a carbon-carbon bond-forming, rate-limiting step. Complexes 7 and 9 have been characterized by X-ray analysis
beta-Keto Amino Enolates Binding to Transition Metals: Synthesis and Structure of the Ion-Pair Form and Its Mono- and Bidentate Coordination to Zirconium and Nickel
No full textThe 2-piperidinoacetophenone, 1, was deprotonated in THF by KH, and the resulting beta-keto amino enolate-potassium ion pair, 2, complexed with 18-crown-6 to give 3, in which the enolate group chelates the [K(18-crown-6)]+ moiety. The reaction of 2 with cp2-ZrCl2 [cp = eta5-C5H5] afforded complex 4 in which the beta-keto amino enolate exhibits an eta1-O bonding mode to zirconium(IV). The enolate 2 acts as a N-O bidentate ligand complexing Ni(II) in the form of a square planar, diamagnetic complex. Crystallographic details: 3 is monoclinic, space group P2(1)/n, with a = 11.836(2) angstrom, b = 16.757(3) angstrom, c = 13.899(2) angstrom, beta = 95-55(1)degrees, Z = 4, and R = 0.081; 5 is triclinic, space group P1BAR, with a = 9.415(1) angstrom, b = 11.533(1) angstrom, c = 17.892(3) angstrom, alpha = 102.81(2)degrees, beta = 92.00(2)degrees, gamma = 67.28(2)degrees, Z = 3, and R = 0.057
(eta6-Acetophenone)Cr(CO)3 Enolates Complexed to Bis(cyclopentadienyl) titanium(IV) and -zirconium(IV)
No full textThis is a synthetic and structural report on substituted acetophenone-Cr(CO)(3) enolate complexes of titanium(IV) and zirconium(IV). The deprotonation of[(eta(6)-2,4,6-R'R''R'''C6H2-COMe)Cr(CO)(3)] [R' = R'' = R''' = H, 1; R' = Me, R'' = R''' = H, 2; R' = R'' = Me, R''' = H, 3, R' = R'' = R''' = Me, 4] with KH in the presence of 18-crown-6 in THF gave the ion-pair enolates [{eta(6)-2,4,6-R'R''R'''C6H2-C(CH2)O-K(18-crown-6)} Cr(CO)(3)] [R' = R'' = Me, R''' = H, 5; R' = R'' = R''' = Me, 6]. When the deprotonation of 1-4, carried out using LiNPr2i in THF, was followed by reaction with (cp)(2)MCl(2) [M = Ti, Zr], dimetallic enolates were isolated and characterized [eta(6)-2,4,6-R'R''R'''C6H2-C(CH2)O-M(Cl)(cp)(2)][R' = R'' = R''' = H, M = Ti, 7; R' Me, R'' = R''' = H, M = Zr, 8; R' = R'' = Me, R''' = H, M = Zr, 9;R' = R'' = R''' Me, M = Ti, 10; R' = R'' = R''' = Me, M = Zr, 11]. In all of these complexes, the enolato functionality is O-bonded to the oxophilic [(cp)(2)M(Cl)] fragment. Complexes 10 and 11 have been isolated as an 85:15 diastereoisomeric mixture with a significant diastereoselection for the most hindered form. The two diastereoisomers cannot be interconverted thermally because of the very high barrier to free rotation around the phenyl-C(enolate) bond, as from our calculations. An X-ray crystallographic analysis has been carried out on the most hindered diastereoisomer of 11 and compared with that of the starting complex 4, where the oxygen is directed to the opposite face of the arene ring. In order to clarify the mechanism of the diastereoisomerization, the protolysis of the potassium and lithium enolates, including that of the pyridine adduct 12[{eta(6)-2,4,6-Me(3)C(6)H(2)-C(CH2)OLiPy(2)}Cr(CO)(3)], has been carried out. The protolysis, which produces exclusively the starting diastereoisomer of 4 having the oxygen syn to the [Cr(CO)(3)] group, supports strongly the fact that there is no change in the conformation at the level of the enolato formation. Then the comparison between results from the reaction of the lithium enolate of 4 and 12 with Me(3)SiCl, MeBu(2)(t)SiCl, Me(3)SiOSO(2)-CF3, and (cp)(2)ZrCl2 shows that the diastereoselection depends mostly on steric,rather than on electronic; factors. Further support for the steric argument comes when some steric hindrance has been introduced in the starting ketone, i.e. using [(eta(6)-2,4,6-R'R''R'''C6H2COCH2-CH3)Cr(CO)(3)]. The corresponding lithium enolate reacted with (cp)(2)ZrCl2 gave a diastereoselection with a 67:33 ratio. The only plausible hypothesis for explaining the diastereoisomerization occurring at the stage of the reaction of the enolate with the electrophile is an intermolecular migration of the [Cr(CO)(3)] fragment from the more to the less hindered face of the arene ring. Crystallographic details: 4 is monoclinic, space group P2(1)/c, a 6.858(1) Angstrom, b = 13.977(2) Angstrom, c 14.557(2) Angstrom, alpha = gamma = 90 degrees, beta = 94.46(1)degrees, Z = 4, and R = 0.034; 11 is monoclinic, space group P2(1)/c, a = 11.238(1) Angstrom, b = 14.144(1) Angstrom, c = 14.808(1) Angstrom, alpha = gamma = 90 degrees, beta = 95.12(1)degrees, Z = 4, and R = 0.033
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