Oxygen conjugation

Yoshida reported the oxygenative conjugate addition of perfluoroalkyl radicals to styrene derivatives (Scheme 4) [1 Ij. The reaction is photochemically initiated and used hexabutyldistannane as radical chain mediator and as reducing agent for the intermediate peroxide. 

Typical auxochromes are hydroxyl, alkoxyl and aroxyl, amino, alkyl-amino and arylamino, all of which promote conjugation with lone pairs on oxygen or nitrogen atoms. 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

FIGURE 18 6 Acrolein w (H2C=CHCH=0) is a planar molecule Oxygen and each carbon IS sp hybridized and each contributes one elec tron to a conjugated tt elec tron system analogous to that of 1 3 butadiene... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

A meta nitro group is not directly conjugated to the phenoxide oxygen and thus stabi hzes a phenoxide ion to a smaller extent m Nitrophenol is more acidic than phenol but less acidic than either o or p nitrophenol... 

Conjugated with a carbon-carbon double bond —C=C—C=C— Conjugated with a carbon-oxygen double bond —C=C—C=0 Cumulative —C=C=C— or —C=C=0 Triple bond... 

As the temperature approaches the NTC zone, the reversibility of reaction 2 comes into play and the steady-state concentration of alkyl radicals rises. There is a competing irreversible reaction of oxygen with radicals containing an alpha hydrogen which produces a conjugate olefin (eq. 23). 

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