Boron-containing alternating

Borasiloxane 92 with boron and silicon atoms at alternate corners and the general formula B4Si40io for the central cage is composed of two dib-oratetrasiloxane rings, that are joined by two Si-O-Si bridges [129]. Unlike the related and extensively studied silasesquioxanes 93 (Fig. 25), in which the silicon atoms can be substituted by various combinations of group 13 and 15 elements [137], this molecule contains alternately three-coordinate boron and four-coordinate silicon corner sites. 

Hydride donor strengths can be summarized as follows 24 > 19 > 12 > 22. This type of reaction provides a useful alternative to formyl synthesis by borohydride reductants neutral metal carbonyl by-products are produced instead of boron-containing species. 

Although this reaction represents only an alternative synthesis for the chromium species 24 it was conducted as an initial experiment since the expected product was known to be stable under the chosen conditions. Monitoring the progress of the transfer by nB-NMR spectroscopy proved that the borylene transfer was quantitative after 12 h at —30 °C in toluene and no boron containing byproducts were detected. A corresponding... 

The boronic esters (Chart 9) are easily hydrolyzed to the corresponding homoallylic alcohols using triethanolamine 98). Consequently, the allylboration sequence provides a synthesically useful alternative to the familiar Grignard synthesis of homoallylic alcohols. However, the protonolysis by triethanolamine causes a problem in the isolation of homoallylic alcohol from the thick, sticky, air sensitive boron-containing mixture. Fortunately, treatment of a pentane solution of borinate esters of 9-BBN with 1-equivalent of ethanolamine results in the rapid formation of a fluffy white... 

Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCl4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts. 

The lUPAC method 2.302 (Paquot, 1979a) is identical to the method described above. lUPAC has a separate method 2.301 (Paquot, 1979a) for the preparation of the fatty acid methyl esters, which is also identical to the procedures described above. lUPAC refers to the procedure, using methanolic potassium hydroxide as the special method . lUPAC method 2.301 also contains alternative methods not involving the use of boron trifluoride. 

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