Apical allyl

The weakening of the ligand bonded trans- to the 0x0 group results in a large number of square-pyramidal VO(IV) complexes where the oxo-ligand occupies the apical site. Such complexes can serve as useful oxidation catalysts. For example, VO(acac)2 is a good catalyst for the oxidative (by O2) polymerization of diphenylsulfide [81] and is used for the epoxidation of allylic alcohols with MC3COOH [82]. Its electrochemistry has been examined in detail elsewhere [83]. 

The structure of the cationic part of 7 consists of a [Mg3C2] trigonal bipyramidal arrangement (Figure 8) with the magnesium atoms in the equatorial plane and the carbon atoms at the apical positions. The two allyl groups are /r3-bonded (one above and one... 

A similar transition state 22 is calculated for the reaction of allylsilanes with aldehydes (equation 10)43. In this case, however, the calculations show that the oxygen attacks at an apical site of the silicon centre, while the allyl group departs directly from an equatorial position without causing a pseudorotation, in contrast to the mechanisms previously discussed. 

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

The introduction of lipophilic substituents is of interest for producing surface-active compounds (surfactants) and liquid-crystal systems. The complexes with allyl substituents at the apical boron atoms are precursors for the synthesis of linear and netlike polymeric clathrochelates. 

The IR spectra of the apically functionalized compounds, together with characteristic C=N, N-0, and B-0 stretching vibrations, have been found to contain some additional intense bands of functionalizing substituents (for the hexadecyl fragment at ca 2850-t2950 (vc-h) and at 1440-1470 cm-i ([Pg.214]

Reaction of [Co(R)(PhTt Bu)] with CO yielded the acyl adducts [Co(CO)C(0)(R)(PhTt Bu)] (R=Me, Et, Ph). These five-coordinate, low-spin complexes possess square pyramidal stereochemistry with a thioether occupying the apical position. [Co(R)(PhTtBu)] (R = Bn or allyl), upon reaction with CO, yields [Co(CO)2(PhTt Bu)]. [Ni(r]3-allyl)(K2-PhTt Bu)] shows no reactivity with CO under similar conditions. [Co(R)(PhTtBu)] also reacts with NO, giving [Co(NO)2(K2-PhTt Bu)].13... 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

Structural characterization of these species has revealed that complex 69 adopts a trigonal-bipyramidal geometry in which the methylallyl moiety occupies the apical position (Ni-GN 189 pm), whereas complex 71 adopts a square-pyramidal structure with the cyanide ligand at the apical position at a relatively long distance from the Ni center (Ni-CN 199 pm). The latter structure is similar to the bromo analog NiBr(allyl)(dippe), prepared from the reaction of the nickel(i) hydrido dimer [NiH(dippe)]2 and allyl bromide. The involvement of allyl cyano species such as 69 and 71 in the catalytic hydrocyanation of butadiene is supported by the following observations (i) complex 69 catalyzes the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile (ca. 100 turnovers at 100 °G), (ii) complex 71 decomposes slowly to give Ni(0) complexes of cis- and // 77i--crotonitrile (Scheme 20). 

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