Mechanistic Studies on the Insertion of Carbonyl Substrates into Cu-H: Different Rate-Limiting Steps as a Function of Electrophilicity.

We report mechanistic studies on the insertion reactions of [(NHC)Cu(µ-H)]2 complexes with carbonyl substrates by UV-vis and 1 H NMR spectroscopic kinetic studies, H/D isotopic labelling, and X-ray crystallography. The results of these comprehensive studies show that the insertion of Cu-H with an aldehyde, ketone, activated ester/amide, and unactivated amide consist of two different rate limiting steps: the formation of Cu-H monomer from Cu-H dimer for more electrophilic substrates, and hydride transfer from a transient Cu-H monomer for less electrophilic substrates. We also report spectroscopic and crystallographic characterization of rare Cu-hemiacetalate and Cu-hemiaminalate moieties from the insertion of an ester or amide into the Cu-H bond.

TEMPO-Mediated Catalysis of the Sterically Hindered Hydrogen Atom Transfer Reaction between (C5Ph5)Cr(CO)3H and a Trityl Radical.

We have demonstrated the ability of TEMPO to catalyze H· transfer from (C5Ph5)Cr(CO)3H to a trityl radical (tris( p- tert-butylphenyl)methyl radical). We have measured the rate constant and activation parameters for the direct reaction, and for each step in the catalytic process: H· transfer from (C5Ph5)Cr(CO)3H to TEMPO and H· transfer from TEMPO-H to the trityl radical. We have compared the measured rate constants with the differences in bond strength, and with the changes in the Global Electrophilicity Index determined with high accuracy for each radical using state of the art quantum chemical methods. We conclude that neither is a major factor in determining the rates of these H· transfer reactions and that the effectiveness of TEMPO as a catalyst is largely the result of its relative lack of steric congestion compared to the trityl radical.

Insertion of Isonitriles into the M-C Bonds of Group 4 Dialkyl Complexes.

The 2,3-dimethylbutadiene complexes of Group 4 metals with constrained geometry (cg) ligands have been prepared and found to adopt a supine orientation with σ2,π bonding. Treatment of cgTi(2,3-dimethylbutadiene) (1-Ti) with tBuNC leads to the formation of a titana-aziridine (3) with a coordinated cyclopentenimine that arises from the formal [4+1] addition of the diene to the isonitrile. In contrast, the reactions of cgZr(2,3-dimethylbutadiene) (1-Zr) or cgHf(2,3-dimethylbutadiene) (1-Hf) with 2 equiv of tBuNC or XyNC proceeded in a more sophisticated manner to yield unsymmetrical 2,5-diazametallacyclopentane derivatives (4, 6-Zr, and 6-Hf) or symmetrical 2,5-diazametallacyclopentene complexes (7-Zr and 7-Hf). The unsymmetrical products contain coordinated cyclopropanes; the strength of the interaction is measured by the reduction in the 1 JCC of the C-C bond that is coordinated. A detailed mechanistic analysis has been possible with the related cgM(Me)2 (M = Ti and Hf) complexes. The first insertion is too fast to monitor, but allows complete conversion to an alkyl iminoacyl intermediate. The second isonitrile (RNC) may react with that intermediate by either of two different mechanisms, reductive elimination and coordination/insertion. In the first mechanism (Ti), rate-determining C-C coupling gives a titana-aziridine, followed by fast coordination of the isonitrile. In the second mechanism (Hf), coordination is the slow step; insertion to form a bis(iminoacyl) Hf complex is rapid.