172295-97-5Relevant academic research and scientific papers
The methylene-transfer reaction: Synthetic and mechanistic aspects of a unique C-C coupling and C-C bond activation sequence
Cohen, Revital,Van Der Boom, Milko E.,Shimon, Linda J. W.,Rozenberg, Haim,Milstein, David
, p. 7723 - 7734 (2007/10/03)
Oxidative addition of aryl iodides ArI (Ar = (a) C6H5, (b) C6H4CF3, (c) C6H3(CF3)2, (d) C6H4CH3, (e) C6H4OCH3), to the PCP-type complex Rh(PPh3)[CH2C6H(CH3)2(CH2PPh2)2] (1), yields the complexes Rh(Ar)[CH2C6H(CH3)2(CH2PPh2)2](I) (2a-e). Compounds 2a-e undergo intramolecular methylene transfer from the bis-chelating ligand to the incoming aryl under mild conditions (room temperature) giving Rh(CH2-Ar)[C6H(CH3)2(CH2PPh2)2](I) (3a-e). The methylene transfer, which is a unique sequence of sp2-sp3 C-C bond reductive elimination and sp2-sp3 C-C bond activation, was investigated kinetically (reaction 2a → 3a), yielding the activation parameters ΔH(+) = 17 ± 3 kcal/mol, ΔS(+) = -23 ± 4 eu. The rate-determining step of this reaction is the C-C reductive elimination rather than the C-C activation step. X-ray structural analysis of 2a and 3b demonstrates that the Rh atom is located in the center of a square pyramid with the aryl (2a) and the benzyl (3b) trans to the vacant coordination site. Reaction of the complex Rh(CH2C6H4CF3)[C6H3(CH2PPh2)2](Br) (7c) with carbon nucleophiles (MeLi, PhLi, BzMgCl) leads to a competitive sp2-sp3 and sp3-sp3 C-C coupling, resulting in migration of a methylene or benzylidene into the bis-chelating ring and formation of the corresponding organic products, sp2-sp3 C-C coupling was shown to be kinetically preferred over the sp3-sp3 one, and the more electron-rich the benzyl ligand, the better the migratory aptitude observed. X-ray structural analysis of two benzyl migration products, complexes Rh(PPh3)[CH(C6H4CF3)C6H3(CH2PPh2)2] (11) and Rh(PPh3)[CH(C6H5)C6H(CH3)2(CH2PPh2)2] (16), demonstrates that the rhodium atom is located in the center of a square planar arrangement where the PPh3 ligand occupies the position trans to the methyne carbon of the benzylidene bridge. The methylene and benzylidene migration reaction is an important transformation for the regeneration of the methylene-donating moiety in the methylene-transfer process.
Carbon-carbon bond activation by Rhodium(I) in solution. Comparison of sp2-sp3 vs sp3-sp3 C-C, C-H vs C-C, and Ar-CH3 vs Ar-CH2CH3 activation
Van Der Boom, Milko E.,Liou, Shyh-Yeon,Ben-David, Yehoshoa,Gozin, Michael,Milstein, David
, p. 13415 - 13421 (2007/10/03)
Reaction of [RhCIL2]2 (L = cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6(CH2P(t)Bu2)2C6H3} (1) in toluene results in a selective metal insertion into the strong Ar-Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp3-sp3 ArCH2-CH3 bond. The identity of complex Rh(Et){2,6-(CH2P(t)Bu2)2C6H3}Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(η(I)-N2){2,6-(CH2P(t)Bu2)2C6H3} (7) with EtI. It is possible to direct the bond activation process toward the benzylic C-H bonds of the aryl- alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms ((t)Bu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH2-CH3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar-CH2CH3 bond activation process. Thus, aryl-C activation by Rh(I) is kinetically preferred over activation of the alkyl-C bond in this system. Moreover, cleavage of an Ar-CH2CH3 bond, followed by β-H elimination, may be preferred over sp2-sp3 C-C activation of an Ar-CH3 group.
Alkyl- and aryl-oxygen bond activation in solution by rhodium(I), palladium(II), and nickel(II). Transition-metal-based selectivity
Van Der Boom, Milko E.,Liou, Shyh-Yeon,Ben-David, Yehoshoa,Shimon, Linda J. W.,Milstein, David
, p. 6531 - 6541 (2007/10/03)
Reaction of [RhCl(C8H14)2]2 (C8H14 = cyclooctene) with 2 equiv of the aryl methyl ether phosphine 1 in C6D6 results in an unprecedented metal insertion into the strong sp2-sp3 aryl-O bond. This remarkable reaction proceeds even at room temperature and occurs directly, with no intermediacy of C-H activation or insertion into the adjacent weaker ArO-CH3 bond. Two new phenoxy complexes (8 and 9), which are analogous to the product of insertion into the ArO-CH3 bond (had it taken place) were prepared and shown not to be intermediates in the Ar-OCH3 bond cleavage process. Thus, aryl-O bond activation by the nucleophilic Rh(I) is kinetically preferred over activation of the alkyl-O bond. The phenoxy Rh(I)-η1-N2 complex (8) is in equilibrium with the crystallographically characterized Rh(I)-μ-N2-Rh(I) dimer(12). Reaction of [RhClC8H14)2]2 with 2 equiv of the aryl methyl ether phosphine 2, PPh3, and excess HSiR3 (R = OCH2CH3, CH2CH3) results also in selective metal insertion into the aryl-O bond and formation of (CH3O)SiR3. Thus, transfer of a OCH3 group from carbon to silicon was accomplished, showing that hydrosilation of an unstrained aryl-O single bond by a primary silane is possible. The selectivity of C-O bond activation is markedly dependent on the transition-metal complex and the alkyl group involved, allowing direction of the C-O bond activation process at either the aryl-O or alkyl-O bond. Thus, contrary to the reactivity of the rhodium complex, reaction of NiI2 or Pd(CF3CO2)2 with 1 equiv of 1 in ethanol or C6D6 at elevated temperatures results in exclusive activation of the sp3-sp3 ArO-CH3 bond, while reaction of the analogous aryl ethyl ether 4 and Pd(CF3CO2)2 results in both sp3-sp3 and sp2-sp3 C-O bond activation. The resulting phenoxy Pd(II) complex (18) is fully characterized by X-ray analysis. Heating the latter under mild dihydrogen pressure results in hydrodeoxygenation to afford an aryl-Pd(II) complex (19).
Carbon-Carbon Activation by Rhodium in Solution; sp2-sp3 is Preferred Over sp3-sp3 Bond Cleavage
Liou, Shyh-Yeon,Gozin, Michael,Milstein, David
, p. 1965 - 1966 (2007/10/02)
Reaction of the ethyl-aromatic phosphine 1 with PhRh(PPh3)3 results in C-H activation, yielding complex 2, which upon treatment with H2 undergoes selective cleavage of the sp2-sp3 hybridized C-C bond forming ethane and complex 3; Ar-
