Carbonyl groups, bridging

So far, only reactions in which the internal nucleophile is tethered to the nitrogen atom of the A -acyliminium ion have been discussed, however, cyclizations with nucleophiles attached to other positions are also possible. If the nucleophile is connected to the carbon atom adjacent to the carbonyl group, bridged azabicycloalkane derivatives are obtained in high yield by using the more reactive allyl- or propargylsilanes. 

In the case of [Fe3(CO)n]2-, a triply bridging carbonyl group is asymmetrically positioned (as deduced by the bond distances) above the Fe3 triangle, which is not found for [Ru3(CO)i i]2-,18 and [Os3(CO)i i]2-.15 In all the three compounds there is a carbonyl group bridging two metal atoms. 

Four of the eight triangular faces in octahedral Rh6(CO)16 (Rhftfri3-CO)4(CO),. J (Fig. 15.10) contain carbonyl groups bridging three metal atoms. The corresponding cobalt compound is thought to have a similar structure.17... 

In dimethyltin diformate, however, the two carbonyl groups bridge intermolecularly, to form infinite sheets similar to those in dimethyltin difluoride or dimethyltin di(fluorosulfonate).29 If the linearly polymeric diorganotin carboxylates are dissolved in chloroform, the 119Sn NMR chemical shift moves downfield by 50 to 100 ppm, and is then almost independent of concentration, showing that the compounds are monomeric in solution.23-30 Dibutyltin glutarate is a linear polymer as shown in 13-9.31... 

This complex contains six terminal carbonyl groups and three carbonyl groups bridging the two atoms of iron. When the complex is treated with acetylene at 20-25° and 20-24 atm., acetylene displaces the bridging carbonyls with formation of tropone iron tricarbonyl (2), which is an orange solid of double melting point (63.5-64° and 83-84°). This complex is obtained in only trace amounts with Fe(CO)s and Fe(CO)i. Treatment of the complex (2) with triphenylphosphine affords free tropone (3,69% yield) and a new complex (4), tropone iron dicarbonyltriphenylphosphine. Tropone can also be obtained, if in low yield, by oxidation of (2) with ferric chloride ... 

Recently a new oxo-derivative of kopsine, kopsifine (189) was obtained from K. dasyrachis from Malaysian Borneo (133). The C NMR spectral data indicated the presence of a lactam carbonyl in addition to the ketonic carbonyl group bridging C(6) and C(16). The presence of the characteristic H(3) and H(21) resonances of kopsine-type compounds ruled out oxygenation at these positions. Oxygenation at position 5 was supported by the observation of H(6) as a singlet which had been shifted downfield as a result of anisotropy exerted by the proximate carbonyl function, as well as from the observed correlations in the HMBC spectrum... 

C. The structure, which involves two bridging carbonyl groups as shown in Fig. 26.8a, can perhaps be most easily rationalized on the basis of a bent Co-Co bond arising from overlap of angled metal orbitals (d sp hybrids). However, in solution this structure is in equilibrium with a second form (Fig. 26.8b) which has no bridging carbonyls and is held together solely by a Co-Co bond. 

Oxidation of diethyl 3,6-hexanooxepin-4,5-dicarboxylate with ruthenium(VIII) oxide, generated from ruthenium(III) chloride and sodium periodate in situ, gives the corresponding hexa-no-bridged furan 1 a with loss of two carbon atoms (see Houben-Weyl, Vol. E6a, p 77).200 201 One of the central methylene groups can be replaced by a carbonyl group to give lb.200... 

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