Siloxyl groups

Enantioselectivity is generally reduced by the presence of two functional groups, probably because of competitive ligation generally, the effect of an ester group overrides that of an alkoxyl or siloxyl group. 

Adsorption of pyridine on goethite and on hematite involved both coordination to the Lewis acid sites and hydrogen bonding to surface OH groups. Trimethylchlorosil-ane reacted with the surface OH groups on these oxides to form surface trimethyl siloxyl groups and HCI (Rochester and Topham, 1979c). 

I ILPHENYLENE-SILOXANE POLYMERS (I) are, in a sense, modified polysi-loxanes in which every third siloxyl group in the main chain backbone is replaced by a suitable silphenylene group see structure on page 742). 

Hunter and Hutchings [33] approached this issue by examining the reaction of TM0+SbClg" with lithium tetraisopropoxide, which they regarded as a reasonable model of the zeolite basic site, with isopropoxyl replacing siloxyl groups. No MeOEt was detected, but only products of isopropoxide decomposition, or nucleophilic chloride and isopropoxide attack. 

The corresponding reaction with DPE was completed in a matter of hours [109]. The reduced rate of addition to the siloxyl-substituted DPE is consistent with the Hammett p value of +1.8 for these reactions [95] as discussed in Sect. 3.2.1. The phenol-end-functionalized polystyrenes were obtained in >99% yield after hydrolysis with 1% HCl in tetrahydrofuran under reflux for 3 h. The efficiency of these functionalization reactions was evaluated by end-group titration, elemental analyses, H NMR and CNMR analyses, as well as by thin-layer chromatography. All of the available evidence suggested that this is an essentially quantitative functionalization reaction <1% unfunctionalized polystyrene was formed. It is noteworthy that Heitz and Hocker [148] have carried out similar functionalizations using l,l-(4,4 -dimethoxyphenyl)ethy-lene. 

The catalyst preparation appeared to be a very simple procedure and consisted of impregnation a silica support, made by precipitation of silica gel, with a soluble chromium salt. It was found that siloxyl chromium complexes were formed by the reaction of chromic oxide with the hydroxyl groups on the silica surface, as the catalyst was activated prior to operation. The stracture of the... 

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