Formation of Conjugated Carbonyls

Equation 2.9. Trimer formation from butanal 2.6.2.4 Formation of Conjugated Carbonyls... 

Although definitive evidence for the intermediacy of oxabicyclobutanes in the peracid oxidation of cyclopropenes is lacking, the production of conjugated carbonyl compounds from such reactions is most readily rationalized from the formation and spontaneous rearrangement of such species (equation Calculations suggest... 

Baylis-Flillman reaction Formation of a C-C single bond between the a-position of conjugated carbonyl compounds or conjugated carboxylic acid derivatives and aldehydes or ketones. 48... 

Silane hydrides can be used for the reduction of carbonyls and alkenes. Reaction of methylcyclohexene with a mixture of triethylsilane (EtsSiH) and trifluoroacetic acid (CF3CO2H) reduced the alkene moiety to give methylcyclohexane in 72% yield. 2 Under the same conditions, however, 1-pentene was not reduced. More commonly, this reagent is used for reduction of conjugated carbonyls, probably via formation of a silyl enolate (secs. 9.2, 9.3.B) as in the reduction of cyclohexenone to cyclohexanone in 85% yield with Ph2SiH2. Addition of transition metals such as ZnCl2, or copper salts to the silane facilitates the reduc-tion,594 as in the conversion of 576 to 577 in 96% yield. ... 

Benzoxepine formation by the 1,7-electrocyclisation of conjugated carbonyl ylides studied by Sharp was reported in last year s review. This work has now been extended in a study of the relative rates of cyclisation via intramolecular competition reactions <97JCS3025>. It has been found, for example and not unexpectedly, that the rate of cyclisation for alkenyl groups and the thiophene ring are ca. 10-20 times that for phenyl. The selectivity is unaffected by the nature of the substituent on the terminal atom of the ylide. 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

In a well-known reaction of conjugated carbonyl compounds, a nucleophile adds to the C=C unit. The formation of 38 previously used the C=C unit of the conjugated system as a Brpnsted-Lowry base, donating electrons to the acidic proton. It is known from Chapter 16 that the acyl carbon will accept electrons in a reaction with a nucleophile. Imagine that the 7i-system effectively extends the reactivity of the carbonyl to the C=C unit then, the C-C should also react with a nucleophile. The extension of points of reactivity by conjugating n-bonds is called vinylogy. 

Figure 8.17 Formation of conjugated acids from carbonyl compounds in acidic media.

Indirect but mechanistically interesting use of the Tebbe reagent for the olefination of carbonyl compounds is the formation of conjugated dienes via titanacydo-... 

A. Hydrogenation of Alkenes (and Aryls) B. Formation of Aryls C. Alkylations and Arylations of Alkenes D. Conjugate Reduction of Conjugated Carbonyl Compounds and Niuiles E. Conjugate Alkylations F. Cyclopropanations, including balocyclopropaiiations... 

Compounds containing a double or triple bond, usually activated by additional unsaturation (carbonyl, cyano, nitro, phenyl, etc.) In the ap position, add to the I 4-positions of a conjugated (buta-1 3-diene) system with the formation of a ax-membered ring. The ethylenic or acetylenic compound is known as the dieTwphile and the second reactant as the diene the product is the adduct. The addition is generally termed the Diels-Alder reaction or the diene synthesis. The product in the case of an ethylenic dienophile is a cyctohexene and in that of an acetylenic dienophile is a cyctohexa-1 4-diene. The active unsaturated portion of the dienophile, or that of the diene, or those in both, may be involved in rings the adduct is then polycyclic. 

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