Bridges, oxygen

Cr03] , dichromale, containing oxygen-bridges, in acid solution. The Cr04 ion is yellow and the ion red. Free HjCrO ... 

Figure C2.12.3. Secondary building units in zeolites. Each comer represents a T-atom (Si, Al) while tire connecting lines represent oxygen bridges witli tire oxygen atom in tire middle.

The 1-alkoxytitatranes can be synthesized by the reaction of equimolar amounts of tetraalkyl titanates and triethanolamine (105). X-ray crystallographic analysis of the soHd isolated from the reaction of one mole of triethanolamine and one mole of TYZOR TPT confirms the stmcture as a centrosymmetric dimer having a Ti isopropoxy nitrilotriethoxy ratio of 1 1 1. The titanium atoms have achieved a coordination number of six via a rather unsymmetrical titanium—oxygen bridge (106). 

Thermal stability is enhanced in chelates thus dimethyl-2-methy1pentane-2,4-dio1titanium [23916-35-0] (22) is much more stable than (CH2)3Ti(OCH(CH2)2)2 (68)- The stmcture of the former has been shown by x-ray diffraction to be dimeric and five-coordinate through oxygen bridges. The more highly substituted the six-membered ring, the mote thermally stable the compound. 

A single-crystal x-ray diffraction study has shown that the borate anion in anhydrous borax is polymeric in nature and is formed via oxygen bridging of triborate and pentaborate groups (83). The chemistry of anhydrous borax has been reviewed (73,84). 

Pure silica contains no metal ions and every oxygen becomes a bridge between two silicon atoms giving a three-dimensional network. The high-temperature form, shown in Fig. 16.3(c), is cubic the tetrahedra are stacked in the same way as the carbon atoms in the diamond-cubic structure. At room temperature the stable crystalline form of silica is more complicated but, as before, it is a three-dimensional network in which all the oxygens bridge silicons. 

Shortly after their first report of all-oxygen bridged cryptands, Dietrich, Lehn and Sauvage reported incorporation of sulfur in the strands. The experimental methods used were essentially similar to those applied in the syntheses of the parent cryptands. As in previous cases, a diacyl chloride was condensed with a diamine under high dilution conditions. In this case, however, the diamine contained sulfur atoms rather than oxygen. The synthesis of compound 5 was accomplished in two stages as illustrated below in Eq. (8.3). The first cyclization step affords the macrocyclic amine in 55% yield. The macrobicyclic product (5) is formed in 25% yield from the monocyclic diamine and the acid chloride. 

Small and Browning agree with Schbpf and Winterhalder that thebainone methyl enolate (formed by 1 6 addition in which the oxygen bridge is ruptured and ethylenic linkages produced at C —C and C —C ) may be the primary intermediate in tWs reaction since it is hydrogenated... 

Since dihydrothebainone is also formed by hydrogenation of thebainone (Schopf and Hirsch i), and as Schopf and Pfeifer have shown that 1 5-dibromodihydrothebainone, on treatment with alkali, is converted into. 1-bromodihydrocodeinone by formation-of the C -C oxygen bridge, and this, on catalytic hydrogenation, yields dihydrocodeinone (LIII, p. 246), the constitution of which has been demonstrated by Schopf (p. 244), there can be little doubt that dihydrothebainone is represented by (LI). Schopf and Winterhalder have also isolated as an oxime (m.p. 228°, [a] °° — 115-8°) an e.pidihydrothebainone, which is regarded as the epimeride of dihydrothebainone. 

The oxygen-bridging mode (v) is less common but occurs together with modes (iii) and (iv) in the following centrosymmetrieal trimeric Ni complex and related compounds. 

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