Nickel-triad catalysts

The use of Ni(0) catalysts for the hydrosilation of phenyl acetylene with PhsSiH and of various internal alkynes and conjugated diynes with Ph2SiH2, PhMc2SiH and EtsSiH has also been reported.It is of interest to note that although the Ni equivalent of Karstedt s catalyst has been prepared, dehydrogenative silylation of alkenes leading to unsaturated products is the dominant reaction and hydrosilation a secondary pathway with this Ni catalyst. 

The ability of Pd complexes to catalyze tandem cyclization-hydrosilation reactions, that could provide a synthetic route to carbocycles of interest in the fields of biochemistry and medicine, was recently demonstrated. Widenhoefer s group has developed phenanthroline-based complexes 11 and others that quite efficiently bring about this dual transformation for a wide variety of functionalized dienes, with high regio- and stereoselectivities [e.g., Eq. (10)].  

Amongst catalyst modifications that achieve purely rate effects, two reports are noteworthy. One is the hydrosilation rate enhancement in the presence of j -cyclodextrins, for Karstedt s catalyst Pt2(s ym-tetramethyldivinyldisiloxane)3] and Lamoreaux s catalyst. Lamoreaux s catalyst (prepared from H2PtCl6 and 1-octanol), showed the most dramatic rate accelerations. The second instance of rate moderation is reported by Endo and collaborators. They report the use of isocyanides in just 2 1 molar ratio with Pt, to dramatically suppress hydrosilation of vinylsilane (with EtsSiH) or vinyl-terminated siloxane with methylhydrogensiloxane copolymer until 60-70 °C. Above this temperature, rapid hydrosilation occurs. This is in contrast with traditional cure inhibitors where a large excess of inhibitor (relative to Pt), typically alkynols or maleates, is needed to prevent premature cure. 

A series of simple alkene Pt silyl complexes that are the first true catalyst species ever to be observed in Pt-catalyzed hydrosiiation was recently identified by Roy and Taylor. Of these, 19a was isolated and characterized by multinuclear NMR spectroscopy and X-ray crystal structure determination. Although the focused aim of the project was to elucidate the catalytic cycle with Pt eatalysts (as discussed in more detail in Section X), 19a as well as other members of the series were proven highly active, stable and recyclable catalysts in the hydrosiiation of 1-hexene and styrene with chlorohydridosilanes. 

Although their work does not strictly fall within the scope of this review, Hartwig and Tsukuda s report on dehydrogenative silation via arene-H and alkane-H 

---

Among the characterized metal homoenolates, only zinc homoenolate of alkyl propionate undergoes substitution reactions with electrophiles under suitable conditions. Two types of metal catalysts, copper(I) and metals of the nickel triad (e.g. Pd), have successfully been used to effect allylation, arylation, and vinylation reactions. 

Addition of active methylene compounds to butadiene has been catalyzed by complexes of the nickel triad. A catalyst mixture of Ni(acac)2, PPr(OR )2, and borohydride was effective for the addition of R1R2CH2 (Ri = Ph, R2 = COCH3 Ri = Ph, R2 = CN Ri = Ra = COgEt Ri = COCH3, Rg = COgEt) to butadiene 26). Both Pd(0) and Pd(II) complexes are effective as catalysts for the addition of active methylene compounds such as 2,4-pentanedione and ethyl acetoacetate to butadiene 177, 269). Several products were obtained from these reactions since both the 1 1 and 1 2 addition occurred. 

Heating [Ir(acac)3] with 2-phenylpyridine (Hppy) affords the tris-cyclometallated complex [Ir(ppy)3],392 and the synthesis of other complexes of the ppy ligand have also been reported.393,394 cyclometallated benzoquinoline Ir complex was shown to be an active catalyst for the H/D exchange of alcohols with D2.395 2.3 The Nickel Triad... 

The polymers generated by Ni(acac)2/MAO and other naked nickel-type catalysts are soluble in toluene and differ significantly from those derived from metallocene catalysts that are insoluble. Chemical shifts for the bridge carbon (C7) in the soluble polynorbornene were found between 33.8 and 34.3 ppm.Based on the assignment made by Al-Samak et for norbornene hydrotrimers (Figure 12), the chemical shift of 33.8 ppm belongs to mm and that of 34.2 ppm to mr and rm triads and an atactic polymer. Metallocene-based polynorbornenes show more mm triads and are more or less isotactic. Polynorbornenes synthesized by neutral nickel catalysts show in the C-NMR spectrum a peak at 35.0 ppm from C7 that indicates no substantial content of mm triads. 

The Nickel Triad. - The square-planar nickel(II) complex [Ni(Me)(PMe3)(2-phosphanylphenolato)] and the five-coordinate species [Ni(Me)(PMe3)2(2-phosphanylphenolato)] have been prepared and found to be effective one-component catalysts for the oligomerisation of ethene. The aryl-nickel(II) complexes 38 have been found to be remarkably efficient catalysts for... 

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

This monotonic decrease in the reactivity in the rates of substitution is the usual behaviour for triads of transition metal complexes, e.g. cobaIt(III) > rhodium(III) > iridium(III) and nickel(II) > palladium(II). However, some years later it was observed that in low oxidation state metal complexes the second row transition metal is the fastest to react, e.g. rhodium > cobalt > iridium and palladium > nickel > platinum. The most important commercial homogeneous catalysts use organometallic complexes of the second row transition metals. ... 

The PNBs from all of these Ni-based catalysts are soluble in common organic solvents such as toluene or hexane, and resemble PNBs produced using Ni(acac)2 (acac = acetylacetonate) or di(2-ethylhexanoate)nickel in combination with MAO. Goodall and coworkers describe the structure of PNB formed using [(ti -crotyl)Ni(l,4-COD)]+[PF6] as containing almost equal amounts of mm and mr triads. 

---