Miscellaneous Unsaturated Carbons

In addition to their characteristic downfield positions, carbonyl carbons can also be recognized by their relatively low intensity. Except in the case of a formyl carbonyl [one attached directly to a hydrogen, -C(=0)H], there are no hydrogens attached to carbonyl carbons, so their signal cannot benefit from NOE enhancement. Carbonyl carbons also tend to have longer T) relaxation times than many other carbons, another cause of lower intensities in a signal-averaged pulsed-mode spectrum (Section 5.4.1). 

There is a host of less common functional groups that involve unsaturated (i.e., multiply bonded) carbon and appear in the same downfield region of the C spectrum as carbonyls, for example, the central (sp hybridized) carbon of an allene linkage (C=C=C). Such carbons give very weak signals 

TABLE 7.6 C Chemical Shifts of Other Functional Groups 

armed with only the H spectrum of a molecule, one can estimate the chemical shift of the attached carbons (or vice versa) with a fair level of confidence. 

Solution It is easy to pick out the carbonyl signal it is the one at 5 190.6. From Table 7.5 we can see that this signal is within the normal range for aldehydic carbons but too far downfield for an acid halide carbonyl. So, we pick the structure on the right  

TABLE 7.5 13C Chemical Shift Ranges of Carbonyl Compounds 

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MISCELLANEOUS UNSATURATED CARBONS 97 TABLE 7.5 C Chemical Shift Ranges of Carbonyl Compounds... 

Miscellaneous Unsaturated Substrates. Exposure of l,l -bis(trans-2-cyanovinyl)ferrocene to a mixture of two equivalents of triethylsilane and 320 equivalents of trifluoroacetic acid at 50° for three hours gives a product with the carbon-carbon double bonds reduced in 83% yield, but leaving the nitrile groups intact (Eq. 122).179... 

Miscellaneous Reactions. Epoxy compounds yield chlorosubstituted carbonates (45). The reaction of chloroformates with hydrogen peroxide or metal peroxides results in the formation of peroxydicarbonates that are used as free-radical initiators of polymerization of vinyl chloride, ethylene, and other unsaturated monomers (46,47). 

Miscellaneous. Aside from the oxidation chemistry described, only a few catalytic applications are reported, including hydrogenation of olefins (114,115), a, [3-unsaturated carbonyl compounds (116), and carbon monoxide (117) and the water gas shift reaction (118). This is so owing to the kinetic inertness of osmium complexes. A 1% by weight osmium tetroxide solution is used as a biological stain, particulady for preparation of samples for electron microscopy. In the presence of pyridine or other heterocyclic amines it is used as a selective reagent for single-stranded or open-form B-DNA (119) (see Nucleic acids). Osmium tetroxide has also been used as an indicator for unsaturated fats in animal tissue. Osmium tetroxide has seen limited if controversial use in the treatment of arthritis (120,121). 

The reaction of thienyllithium with organic halides to form C—C bonds has been discussed in Section 2.10.4.7.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable thienyl metal derivative. The metal is usually zinc, magnesium, boron or tin occassionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated thiophenes with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. Besides these, a few other miscellaneous Pd-catalyzed substitution reactions on thiophene will also be referred to. 

Among miscellaneous routes to fluoro-olefins may be mentioned the so-called halogen metathesis in which a mixture of saturated and unsaturated chlorofluorocarbon, when passed over an appropriate heated catalyst e.g. activated carbon), imdergoes disproportionation, e.g. Scheme 9 the... 

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