Silyloxy ligand

Basset and his group have observed that propane and propene metathesis give similar Cn+i/Cn+2 ratios of cross-metathesis products on silica-supported tantalum-neopentylidene catalyst at 150°C. The olefin-metathesis activity of these Schrock-type supported complexes results from the presence of the silyloxy ligand (vide infra) - Organometallic complexes are bound to silica or alumina by reaction of soluble complexes and involve die formation of one or several bonds between the central metal and the oxygen atom of the oxide support. 

Metal-alkylidene and alkylidyne complexes bonded to silica via silyloxy ligands are also well-defined active alkene and alkyne metathesis catalysts. Recall that Schrock had turned metathesis-inactive alkylidene complexes into active ones by the introduction of alkoxy ligands. In Basset s catalysts, this beneficial role is played by a silyloxy ligand from silica. 15.2,15.31a the catalysts [(SiO)M(=CH-r-Bu)... 

The common point that makes the Schrock and Basset catalysts active in alkene metathesis is the alkoxy or aryloxy ligand in Schrock s catalysts and the silyloxy ligand in Basset s catalysts. Also note that Basset s catalysts are in fact Schrock s complexes or catalysts that are grafted on silica in order to link the metal by a Si-O-M bond and make catalysts heterogeneous. Schrock s high oxidation state compounds also give o-bond metathesis upon a-H elimination, but they do not catalyze alkane disproportionation. 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Dankwardt s vinylidene-mediated reaction apparently operates concurrently with cationic cycloisomerization to give 3-silyl-l-silyloxy naphthalenes (e.g., 13). From his data, a direct comparison can be made of the effect of different metal complexes and silyl groups on selectivity for a vinylidene-mediated reaction pathway (Table 9.2). At least in this instance, Rh(I) is more vinylidene friendly than Pt(II). Iwasawa and coworkers [7], in an isolated related report, also obtained high selectivity for silyl-shifted products in the presence of a Rh(I)-catalyst, albeit one with a substantially different ligand set from that employed by Dankwardt. 

In the case of 0-silyl ethers, thermolysis of (triphenylsilyloxy)tetraphenylantimony (13) did not afford the ligand coupling product, phenyl(triphenylsilyl)ether. Instead, a disproportionation reaction took place to give the bis(silyloxy) ether (14) and pentaphenylantimony (3), which decomposed into triphenylantimony (4) and biphenyl (5) under the conditions of the reaction. 1... 

The same conditions used above (i.e., with ligand 152) apply to acyclic enones bearing a silyloxy group a to the enone carbonyl, as in 155 (Scheme 1-15). Asymmetric addition of R2Zn, where R = Me, Et, /-Pr, and n-Bu,... 

Catalysts derived from organo-chromium compounds supported on silica gel are veiy different to the original Phillips chromium oxide catalyst. In the original catalysts one molecule of chromium oxide is bound to two silica units, to form the silyloxy chromate sites. In contrast, organo-chromium compounds are usually only bound to a single site on the sihca support and the other coordination sites are still occupied by the organic ligands. For example, tris-7t-allyl chromium reacts with a hydroxylated support, with the evolution of propylene and the formation of a bis-a-allyl chromium siloxy derivative. 

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