Me2AlCH2PMe2: a new, bifunctional cocatalyst for the Ni(II)-catalyzed oligomerization of PhSiH3

The role of methylaluminoxane (MAO) in the Ni-catalyzed dehydrogenative homologation of PhSiH3 has been investigated with a view to designing new cocatalysts possessing well-defined chemical compositions and structures. These studies show that species such as the bifunctional reagent (Me2PCH2AlMe2)2, 3, should act as co-catalyst for the Si-Si bond formation reactions. Thus, it was found that the combination of (1-Me-indenyl)Ni(PPh3)Me, 2a, and 3 (Ni/Al ratio of 1:1) converts PhSiH3 to cyclic oligomers (PhSiH)n with a turnover frequency (TOF) of >500 h(-1), 50 times faster than with 2a alone. Detailed NMR studies have indicated that this acceleration is due to the formation of the intermediate (1-Me-indenyl)Ni(Me)(Me2PCH2AlMe2), 4. Coordination of the PMe2 moiety in this complex to the Ni center allows the tethered AlMe2 moiety to interact with the Ni-Me moiety in such a way that promotes fairly slow Al-Me/Ni-CD3 exchange (t(1/2) ca. 12 h) but accelerates the Si-H bond activation and Si-Si bond formation reactions. The catalysis promoted by 2a/3 proceeds even faster in the presence of NEt3 or THF (TOF > 1600 h(-1)), because these Lewis bases favor the monomeric form of 3, which in turn favors the formation of 4. On the other hand, the much more nucleophilic base quinuclidine suppresses the catalysis (TOF < 300 h(-1)) by hindering the Ni.R.Al interactions. These observations point to an emerging strategy for using bifunctional reagents such as 3 to place geometrically constrained Lewis acid moieties adjacent to metal centers, thereby activating certain metal-ligand bonds.     

Polyhedral Ruthenaborane Chemistry Preparation,Molecular Structure,and N.M.R. Properties of the Mixed-Ligand Eleven-Vertex isocloso-Type Compound [1,1-(PMe3) (PPh3)-closo-1-RuB10H8-2,3-(OEt)2]

Treatment of [1,1-(PPh₃)₂-closo-1-RuB₁₀H₈-2, 3-(OEt)₂]¹) 1 with the more basic ligand PMe₃ readily results in the displacement of PPh₃ to form the hitherto unreported mixed ligand compound [1,1-(PMe₃) (PPh₃)-closo-1-RuB₁₀H₈-2, 3-(OEt)₂] 2 , which is characterized by singlecrystal X-ray diffraction analysis. Crystals are monoclinic, space group P2₁, with a = 1059.2(2), b = 1291.5(2), c = 1187.6(2) pm, β = 91.56(1)°, Z = 4; the final R factor is 0.0198 for 3303 observed reflections [I>2.0 s?(I)]. Compound 2 is also characterized by detailed multielement, multiple resonance, and multidimensional n.m.r. spectroscopy (¹H, ¹H-{¹¹B}, [¹H–¹H]{¹¹B}-COSY, ¹¹B, ¹¹B-{¹H}, and ³¹P).     

Liquid chromatographic determination of lysine, methionine, and threonine in pure amino acids (feed grade) and premixes: collaborative study

A total of 17 laboratories (including one author's laboratory) participated in a collaborative study for determination of lysine, methionine, and threonine in trade products or concentrated amino acid premixes. Thirteen samples, 4 pure amino acids and 6 premixes, including 3 Youden matched pairs, were analyzed. The applied liquid chromatographic (LC) method using cation-exchange resin and post-column derivatization with ninhydrin or o-phthaldialdehyde was shown to be accurate and specific for the analytes. Titration procedures, normally used for the assay of pure amino acids, are unspecific and the accuracy of the results can be affected by impurities. Repeatability relative standard deviations, RSDr, ranged from 0.84 to 1.17% for pure amino acids and from 0.50 to 1.68% for premixes; reproducibility relative standard deviations RSDR, ranged from 1.52 to 2.31% for pure amino acids and from 1.48 to 2.59% for premixes. Recoveries were between 97.5 and 102.8% of the expected amino acid assays. The method has been adopted Official First Action status by AOAC INTERNATIONAL.