Bridged systems ethylene

An even broader structural variety has been evidenced for ethylene-bridged PBs. As for methylene-bridged systems, the presence of n-donat-ing substituents at boron (such as catechol or pinacol) prevents intramolecular P-B interaction and leads to linear monomeric structures (compounds 40g and 40h).56 This is clearly apparent in solution from the 3IP and 11B NMR data, and was further confirmed crystallographi-cally for 40h (Figure 5). The environment around boron remains trigonal planar and the anti arrangement of the PCH2CH2B skeleton maintains the distal phosphorus and boron atoms far away from each other (4.231(2) A). 

Alkyl spacers thus offer the opportunity to finely tune the interplay between the antagonist sites of ambiphilic compounds. The proximity of the phosphine and borane moieties, and thus the possible formation of intramolecular P-B interaction depend on the number of carbon atoms present in the backbone and on the Lewis acidity of the borane. Conformational considerations may also play an important role as for the ethylene-bridged systems. 

Note Figure 3.23 shows different orientations for the oxazoline ring in the ethylene bridged system compared with the other two. Consequently, the chiral centre is now in the bridge rather than on the wing. 

Ethylene-bridged bis-indenyl zirconocene dichloride-methylalu-moxane system. 

Similarly, efficient tetracyclization (MeAlCl2-promoted) of the bis-allylic silane/ bis-epoxide 97 constitutes the key step in the synthesis of (+)-a-onocerin. In this case, because of the presence of the bis-allylic silane group, a double bis-annula-tion occurs, with the formation of the ethylene-bridge linked bis-decalin system present in the target compound (Scheme 8.26) [46],... 

One feature common to both designs in Figures 12.21 and 12.22 is that single-feed columns were used with the entrainer being mixed with the feed. In the case of the ethanol-water-ethylene glycol, the operation leaves for the top and bottom sections of the column do not meet, and there is a gap. In some systems, it is possible to bridge the gap between the operation leaves between the top and bottom sections by creating a middle section in the... 

The ability to selectively direct chemistry at indole positions C4 to C7 remains a synthetic challenge. Takayama and co-workers have developed a procedure for masking the 2,3-Jt bond of indole as a bridged ethylene glycol system that allowed for functionalization of the... 

For noncrowded complexes, the influence of the ethylene bridge conformation (and hence of the chirality of the tertiary carbon atom of the bridge) on the stereospecific behavior is poor (compare calculations for ligands 38 and 39, in Table 1.1). This is in qualitative agreement with measured stereospecificities of corresponding catalytic systems (39% vs. 34% of mmmm pentads, respectively).63... 

The activity of CMe2-bridged Zr(II) and Hf(II) boratabenzene cationic complexes toward ethylene trimerization has been explored computationally using a gradient-corrected DFT approach the Zr species was calculated to be a highly efficient catalyst, exceeding the efficiency of the corresponding Cp system.108... 

The bridge effect was scrutinized in the range of diferrocenyl derivatives, especially of those that are applicable in catalysis and material science (Atkinson et al. 2004). One-electron oxidation of these derivatives also proceeds easily, reversibly, and gives rise to cation-radicals (ferrocenium ions). In contrast to the cation-radical of ferrocenylacrylonitrile, the hole transfers through conjugated systems were proven for the bis(ferrocenyl)acetylene cation-radical (Masuda and Shimizu 2006), the bis(ferrocenyl) ethylene cation-radical (Delgado-Pena et al. 1983), and the cation-radical of bis(fulvaleneiron) (LeVanda et al. 1976). These structures are presented in Scheme 1.30. 

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