Hydrocarbon activation via reversible 1,2-RH-elimination from ((t)Bn(3)SiNH)(3)ZrR: Synthetic, structural, and mechanistic investigations

被引:191
作者
Schaller, CP [1 ]
Cummins, CC [1 ]
Wolczanski, PT [1 ]
机构
[1] CORNELL UNIV, DEPT CHEM, BAKER LAB, ITHACA, NY 14853 USA
关键词
D O I
10.1021/ja950745i
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Hydrocarbyl complexes, ((t)Bu(3)SiNH)(3)ZrR (1-R), were prepared via metatheses of ((t)Bu(3)SiNH)(3)ZrCl (1-Cl) with RMgX or RLi (R = Me, Et, Cy, CH(2)Ph, allyl, CH=CH2, Ph, CH(2)(t)Bu, C=CPh, C=C(t)Bu), through addition of isobutylene, H2C=C=CMe(2), and acetylene to 1-H (R = (i)Bu, dma, or CH=CH2), and by CH-bond activation; thermal 1,2-RH-elimination from 1-R produced putative ((t)Bu(3)SiNH)(2)Zr=NSi(t)Bu(3) (2), which was subsequently trapped by R'H. Thermolysis of 1-R (similar to 100 degrees C, R = Me or Cy) in the presence of H-2, c-C3H6, and CH4 in cyclohexane or neat C6H6, mesitylene, and toluene afforded 1-R (R = ii, Pr-c, Me, Ph, CH2-3,5-Me(2)C(6)H(3)) and a mixture of 1-CH(2)Ph and 1-C(6)H(4)Me, respectively. Exposure of 1-Cy to C2H4 or C6H6 in cyclohexane provided 1-CH=CH2 or 1-Ph, respectively, but further reaction produced 1(2)-(trans-HC=CH) and 1(2)-(p-C6H4) through double CH-bond activation. Thermolysis of ((t)Bu(3)SiND)(3)ZrCH3 (1-(ND)(3)-CH3) in C6H6 or C6D6 yielded CH3D, and 1C(6)H(5) or 1-(ND)(3)C6D5, through reversible benzene activation. Thermolysis of l-Cy in neat cyclohexane, and with C2H6 Or CMe(4) present, gave cyclometalation product ((t)Bu(3)SiNH)(2)ZrNHSi(t)Bu(2)CMe(2)CH(2) (3) and 1-NHSi(t)Bu(3). In THF, thermolysis of 1-CH3 afforded ((t)Bu(3)SiNH)(2)-(THF)Zr=NSi(t)Bu(3) (2-THF); at 25 degrees C, 1-H lost H-2 in the presence of L (L = THF, Et(2)O, NMe(3), PMe(3)) generating 2-L; 2-L (L = Et(2)O py) was also prepared via ligand exchange with 2-THF. Single crystal X-ray diffraction studies of 2-THF revealed a pseudotetrahedral core, with a long Zr=N bond distance (1.978(8) Angstrom), normal Zr-N(H) bond lengths (2.028(8), 2.031(8) Angstrom, similar amide (154.7(5), 158.1(5)degrees) and imide (156.9(5)degrees) bond angles, and little O(p pi) --> Zr(d pi) bonding. Crystal data: monoclinic, P2(1)/n, a = 13.312(5) Angstrom, b = 18.268(6) Angstrom, c = 20.551(7) Angstrom, beta = 92.30(3)degrees, Z = 4, T = 25 degrees C. 2-Et(2)O thermally eliminated C2H4 to give 1-OEt through gamma-CH activation. Kinetic isotope effects (KIE) on 1,2-RD-elimination from 1-(ND)(3)-R (95.7 degrees C, R = CH3, z(Me) = 6.3(1); CH(2)Ph, z(Bz) = 7.1(6); Ph, z(Ph) = 4.6(4)) and CD3H loss from 1-CD3((CH3)/k(CD3) = (z'(Me))(3) = 1.32) revealed a symmetric H-transfer in a loose transition state. 1,2-RH-elimination rates follow: (96.7 degrees C, k(R) (x10(4) s(-1)) = 22.6(2), Ph; 15.5(2), Pr-c; 13.2(4), CH=CH2; 10.4(2), Cy; 3.21(6), Et; 3.2(1), (i)Bu; 1.3(1), dma; 1.51(6), H; 1.42(4), CH(2)(t)Bu; 1.06(2), Me; 0.34(2), CH2-3,5-Me(2)C(6)H(3); 0.169(3), CH(2)Ph). Competition for ((t)Bu(3)SiNH)(2)Zr=NSi(t)Bu(3) (2) by RH/R'H and equilibria provided information about the stabilities of 1-R relative to 1-Pr-c (R = CPr (0.0 kcal/mol) < Ph (0.3) < CH(2)Ph (0.7) < Me (1.2) < CH(2)(t)Bu (greater than or equal to 7.6) < Et (greater than or equal to 7.8) < Cy (greater than or equal to 10.9)). Transition state energies afforded relative C-H bond activation selectivities (Delta Delta G double dagger relative to Pr-c-H): (PrH)-Pr-c approximate to ArH (0.0 kcal/mol) > MeH (3.4) > PhCH(2)H (4.0) > cyclometalation (greater than or equal to 8.5) > EtH(greater than or equal to 8.9) > (t)BuCH(2)H (greater than or equal to 9.3) > CyH (greater than or equal to 11.2). A correlation of Delta G double dagger(1,2-RH-elimination) with D(R-H) indicated generally late transition states but suggested an earlier composition for the alkyls, as rationalized through a Hammond analysis. Correlation of Delta G double dagger(1,2-RH-elimination) with RH proton affinity implicated tight binding of RH in the transition state and possible RH-binding intermediates (2-RH). 1,2-HC=CR-elimination from 1-C=CR was not observed, but second-order exchanges of 1-C=CPh with (t)BuC=CH, and 1-C=C(t)Bu with HC=CPh were indicative of an associative pathway. All data can be accommodated by the following mechanism: 1-R + R'H reversible arrow 2-RH + R'H reversible arrow 2-R'H + RH reversible arrow 1-R' + RH; a variant where 2 mediates reversible 2-RH + R'H exchange is less likely.
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页码:591 / 611
页数:21
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