Bridging moieties alkene

Tab. 2 Racemization barriers in overcrowded alkenes 23 with different bridging moieties X and Y.

Table 2 summarizes the racemization barriers in unsubstituted chiral alkenes 23 with different bridging moieties in their upper and lower halves. As is evident from these data, the tetrahydrophenanthrene-type upper part is large enough to prevent fast racemization by movement of the aromatic moieties of upper and lower halves through the mean plane of the molecule. On the other hand, there is enough conformational flexibility in the molecules to prevent excessive distortion of the central olefmic bond (leading to ground state destabilization), which would lower the racemization barrier. 

The reaction of the iron-carbene complex [[Bu C CC(OEt)=)Fe(CO)4] with 1,3-cyclohexadiene afforded223 (32), which was characterised by X-ray diffraction. The addition of the anionic derivatives of i-(diphenylmethane)-il-fluorene- and ii-(9,10-dihydroanthracene)-bis tricarbonylchromium to the hydrocarbon moiety (alkene, benzene, cyclohexadienyl, cycloheptadienyl and cycloheptatrienyl) in various cationic compounds of manganese, rhenium, iron, chromium, molybdenum, tungsten and cobalt was reported224 to provide a new route to hydrocarbon-bridged heteronuclear species such as (33). 

Ogawa et al. (12) used an intramolecular azide-alkene cycloaddition strategy to synthesize the oxygen-bridged aza[15]annulene 52 and the aza[15]annulene dicar-boxylate 55 (Scheme 9.12). 1,3-Dipolar cycloaddition of vinyl azide to the acrylate moiety followed by extrusion of nitrogen gave the aziridine 51. Rearrangement of 51 afforded the aza[15]annulene 52. The same approach was used to synthesize the aza[15]annulene 55. 

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (H ), strong positive hfcs for the non-allylic bridgehead protons (Hy), and negligible hfcs for the... 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

This unique reactivity was subsequently utilized in the functionalization of unsaturated hydrocarbon substrates with CO (Scheme 39) [285, 286]. In the course of these studies the stereospecific synthesis of a diolato unit was observed via a samarium-mediated CO and CH (alkene) activation. A bridging trans-5,10-dihydroindeno[2,l-a]-5,10-diolate ligand forms terminal Sm-O bonds in [Cp Sm]2(O2C16H10) and its THF adduct. Two molecules of CO are formally inserted into the carbon carbon double bond of l,2-di-2-pyridylethene to form a terminal bisenolate moiety [286] (Table 18). 

The oxidation of p-/ert-butylthiacalix[4]arene or its tetra-O-benzyl ethers generated all four stereoisomers of j-tert-butylsulfinylcalix[4]arenes based on the orientations of the sulfonyl moieties relative the mean plane of the four bridging sulfur centers <03JOC2324>. Alkenes, when treated with W-bromosuccinimide and dimercaptoethane, gave the corresponding... 

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