Oxygenated structures, ring expansion

However, studies on the scope of this sequence revealed that the substrate has to be an N-tosyl sulfonamide and that certain boronic acids are not trans-metallated but rather give rise to the formation of the pyrrole 21 or a pyridine derivative 22 (Scheme 7). The peculiar outcome as a carbopalladation-Suzuki sequence is rationalized by co or dinative stabilization of the insertion intermediate 18 by the sulfonyl oxygen atom, as represented in structure 19, now suppressing the usual /3-hydride elimination. If the transmetallation is rapid the Suzuki pathway is entered leading to product 17. However, if the transmetallation is slow, as for furyl or ferrocenyl boronic acid, either /i-hydride elimination or a subsequent cyclic carbopalladation occurs. The former leads to the formation of the diene 20 that is isomerized to the pyrrole 21. The latter furnishes the cyclopropylmethyl Pd species 23, which rearranges with concomitant ring expansion to furnish piperidyl-Pd intermediate 24 that suffers a -hydride elimination to give the methylene tetrahydro pyridine 22. 

Dibenz[/>, ]oxocin (385) has been synthesized by successive expansions of six- and seven-membered oxygen-containing rings. It is a stronger acid than xanthene, demonstrating the aromatic stabilization of anion (386). 2-Thiabicyclo[6,l,0]non-6-en-3-one (387) is reported as the product of irradiation of 9-thiabicyclo[3,3,l]nonenones in pentane, although the reaction takes a different course in methanol. Its structure has been established by A"-ray analysis. The first preparation of a phosphocane, (388), has arisen as a... 

Figure 13 Representation of the Sn-beta structure as derived from EXAFS data, viewed along the b-axis (for clarity the oxygen atoms are not shown). The only Sn distribution consistent with the experimental EXAFS data is one where pairs of Sn atoms occupy opposite vertices of the six-member rings. A pair of T5 sites (red), with the required 5.1 A separation is shown, representing one possible tin pair within the beta structure. This tin pair distorts two of the 12-membered ring channels as viewed from the [100] direction and all four 12-membered ring channels as viewed from the [010] direction. This distortion is either direct by the replacement of silicon by tin or by the expansion of the neighboring Si04 tetrahedra. Reproduced with permission from Ref (152) Copyright 2005, The American Chemicai Society.

As we shall see in the following Sections, phosphorus also forms a number of cage structures based on expansion of the P4 tetrahedron, containing three- or four-coordinate phosphorus. It has also been found to form ring or chain structures from linked tetrahedra, with oxygen or nitrogen substituents on phosphorus. 

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