Bridging symmetric

Diphosphinoamines (X2P-NR-PX2) are examples of a more unusual potentially chelating diphosphine. The dimer Co2(F2PN(CH3)PF2)3(CO)2 has three of these ligands bridging symmetrically between the two tetrahedral Co centers, which also carry a terminal CO group this molecule can undergo reduction with LiEt3BH in THF to form a stable mixed valence (d9—d10) radical anion.163... 

As for LiC2H2, the authors have shown spectroscopically by using partial asymmetrical deuteriation [Li (CH2CD2)m] that lithium bridged symmetrically the carbon-carbon bond . Possible structures for the Li(C2H4) , n = 1, 2, and 3 complexes are depicted in Scheme 4. 

V atom, which are bridged symmetrically by the carboxylate groups. At the center of the cluster is a trigonal-planar O atom. These compounds undergo Nernstian reductions in the range —1.32 to —1.61... 

Methylzinc A,A-diethyldithiocarbamate (232) (Figure 106) exists as a centrosymmetric dimer in the solid state . One sulfur atom of each of the dithiocarbamate groups bridges symmetrically [Zn—S 2.512(4) and 2.501(4) A] between the two zinc atoms to form a fiat four-membered ring. The other two sulfur atoms each coordinate to a zinc atom with a considerably shorter bond distance [Zn—S 2.370(3) A]. As a consequence of... 

Coronene (9) is known to form a 7r-complex with a chain structure in which the iodine molecule bridges symmetrically between double bonds of adjacent coronene molecules26 the double bonds are perpendicular to the I-I axis. Such a structure explains the large value of the asymmetry parameter. 

In order to install a benzophenone at the bicyclic scaffold we relied on the previously used oxazole linkage. To this end, the known amino-hydroxybenzophenone 37 (Aichaoui et al. 1990) was coupled to the free acid rac-38, which is available from Kemp s triacid in five synthetic steps. Remarkably, an 0-aeylation instead of the expected /V-acylalion was observed resulting in ester rac-39. As a consequence, oxazole formation was less straightforward but could eventually be achieved under more forceful conditions. The reaction sequence led to the racemic benzophenone rac-40, i.e. to a 1/1 mixture of the enantiomers (+)-40 and (-)-40, which was separated by chiral HPLC (Daicel Chiralpak AD). It is important to mention that a separation of enantiomers at an earlier stage is not sensible. While carboxylic acid 38 can be obtained in enantiomerically pure form, racemisation occurs upon activation, presumably due to a bridged symmetrical intermediate (Kirby et al. 1998) (Scheme 16). 

The early methylenic bridged symmetrical dicopper knots... 

The energy difference between open and bridged 2-norbomyl cations is probably small57. Both intermediates may occur in succession, particularly if generated by strongly exothermal reactions. Capture of unsymmetrical 2-norbornyl cations in such reactions does not disprove the existence of the bridged, symmetrical species. 

Hexakis(tert-butylisocyanide)triplatinum (Pt3[CN-/-Bu],5) reacted with 2,3-diphenylcycloprop-enone to give /t-2-oxo-l,3-diphenylpropanediylidenebis[bis(t< -Tt-butylisocyano)platinum] 4. Synthesis of the complex involves C-C double-bond cleavage in cyclopropenone the resulting C3 fragment bridges symmetrically the two platinum atoms. ... 

Another group of cytostatic, antiviral and antibacterial yet highly toxic peptides are the quinoxalines [64] with two quinoxaline-carboxylic acids attached to eight membered, disulfide bridged symmetric peptide-lactone rings. Further inhibitors of protein biosynthesis isolated from Streptomyces species here mentioned only by name are streptogramin B and the tuberactinomycins (e.g. Viomycin) isolated, elucidated and synthesized mainly by T. Shiba s group in Osaka. 

Only symmetrical alkenes react by means of true bromonium ions (i.e., strongly bridged, symmetrical structures). If the alkene is unsymmetrically substituted with alkyl groups, the bromonium ion takes on the character of a /7-bromocarbocation with restricted rotation about the formerly olefinic carbon-carbon bond. If an aryl group is present on one or both of the olefinic carbon atoms, the evidence points to an intermediate that is even more nearly like a benzylic carbocation. 

Ogoshi T, Kanai S, Fujinami S, Yamagishi T-a, Nakamoto Y. para-Bridged symmetrical pillar[5]arenes Their lewis add catalyzed synthesis and host—guest property. J Am Chem Soc 2008 130 5022-3. 

If an appropriately positioned olefin can interact with a cationic center to make a carbo-nium ion, we might expect that the edge of a cyclopropane ring could do the same. An example of this is the ion derived from pentacyclo[4.3.0.0 .0 .0 - 0]nonane, which is known as Coates ion (shown in the margin). Even the most ardent opponents of the non-classical ion concept accept that this is a bridged, symmetrical carbonium ion. The value of fully sup-... 

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