Carbonyl elimination

In contrast to the 4-hydroxy isomers, the thermally stable 5-hydroxy-THISs add to the C=C bond of cyclopropenylidenes (4. 18, 27. 28). The adducts eliminate carbonyl sulfide, and the strained bond breaks resulting in ring-expansion with formation of pyridin-4-ones. -thiones, or -imines. or 4-alkylidenedihydropvridines (20, X = 0. S.NR. or CRR ) (Scheme 19). 

Hydroxy-THISs add regioselectively to the C=N bonds of isocyanates or isothiocyanates. The initially formed cycloadducts eliminate carbonyl sulfide with formation of 4-hydroxy- or 4-mercaptoimidazolium hydroxide inner salts (21) (Scheme 21). 4-Hydroxyimidazolium hydroxide... 

Like alkynes, a variety of mechanistic motifs are available for the transition metal-mediated etherification of alkenes. These reactions are typically initiated by the attack of an oxygen nucleophile onto an 72-metalloalkene that leads to the formation of a metal species. As described in the preceding section, the G-O bond formation event can be accompanied by a wide range of termination processes, such as fl-H elimination, carbonylation, insertion into another 7r-bond, protonolysis, or reductive elimination, thus giving rise to various ether linkages. 

The meso-ionic l,3-dithiol-4-ones (134) participate - in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes including tetramethylethylene, cyclopentene, norbomene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, A -phenylmaleimide, and acenaphthylene. Alkynes such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly. - ... 

Carbonyl reduction is a metabolic pathway widely distributed in nature. Many endogenous substances, such as prostaglandins, biogenic amines, and steroids, together with xenobiotic chemicals of several varieties, are transformed to the corresponding alcohols before further metabolism and elimination. Carbonyl reduction in several continuous cell lines was investigated using metyrapone as a substrate ketone. Quercitrin was reported to inhibit carbonyl reductase. 

Zeolite-trapped clusters are stable toward oxidation-reduction cycles. A sample of [Rh6(CO),e]-NaY (A) was subjected to mild oxidation with dry O2 followed by heating from 293 to 473 K (to eliminate carbonyl ligands), and this treatment was followed by reduction with hydrogen at 473 and... 

Perfluorocarbamoyl fluorides likewise eliminate carbonyl fluoride to afford the corresponding carbonimidoyl difluorides ( 5,46,155 or example, heating of bis(trifluoromethyl)carbamoyl fluoride (XLII) at 490-575°C produced an almost quantitative yield of trifluorcarbonimidoyl difluoride (XLIII) and carbonyl fluoride (XLIV)... 

In a step-growth polymerization, the monomers polymerize following different reaction mechanisms, such as carbonyl addition-elimination, carbonyl addition-substitution, nucleophilic substitution, double bond addition or free radical... 

By treatment of an amide with sodium hypobromite or sodium hypochlorite solution (or with the halogen and alkali), the amine of one less carbon atom is produced, the net result being the elimination of the carbonyl group. An example is ... 

AJdoJ Condensation -Aldol condensation Initially give p-hydroxy ketones which under certain conditions readily eliminated to give a,p-unsaturated carbonyls. 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The Julia-Lythgoc olefination operates by addition of alkyl sulfone anions to carbonyl compounds and subsequent reductive deoxysulfonation (P. Kocienski, 1985). In comparison with the Wittig reaction, it has several advantages sulfones are often more readily available than phosphorus ylides, and it was often successful when the Wittig olefination failed. The elimination step yields exclusively or predominantly the more stable trans olefin stereoisomer. 

The elimination of 3-hydrogen of Pd alkoxide (17) to afford a carbonyl compound is a similar reaction. 

As a unique reaction of Pd(II), the oxidative carbonylation of alkenes is possible with Pd(ll) salts. Oxidative carbonylation is mechanistically different from the hydrocarboxylation of alkenes catalyzed by Pd(0), which is treated in Chapter 4, Section 7.1. The oxidative carbonylation in alcohol can be understood in the following way. The reaction starts by the formation of the alkoxy-carbonylpalladium 218. Carbopalladation of alkene (alkene insertion) with 218 gives 219. Then elimination of /3-hydrogen of this intermediate 219 proceeds to... 

Formation of ketones. Ketones can be prepared by the carbonylation of halides and pseudo-halides in the presence of various organometallic compounds of Zn, B, Al, Sn, Si, and Hg, and other carbon nucleophiles, which attack acylpalladium intermediates (transmetallation and reductive elimination). 

The carbonylation of aryl iodides in the presence of alkyl iodides and Zn Cu couple affords aryl alkyl ketones via the formation of alkylzinc species from alkyl iodides followed by transmetallation and reductive elimination[380]. The Pd-catalyzed carbonylation of the diaryliodonium salts 516 under mild conditions in the presence of Zn affords ketones 517 via phenylzinc. The a-diketone 518 is formed as a byproduct[381],... 

Organotin compounds such as aryl-, alkenyl-, and alkynylstannanes are useful for the ketone synthesis by transmetallation of acylpalladium 529 and reductive elimination of 530 as shown[389-393]. Acetophenone (531) is obtained by the carbonylation of iodobenzene with Me4Sn. Diaryl ketones... 

The aryl- and heteroarylfluorosilanes 541 can be used for the preparation of the unsymmetrical ketones 542[400], Carbonylation of aryl triflate with the siloxycyclopropane 543 affords the 7-keto ester 545. In this reaction, transme-tallation of the siloxycyclopropane 543 with acylpalladium and ring opening generate Pd homoenolate as an intermediate 544 without undergoing elimination of/3-hydrogen[401],... 

The 2-substituted 3-acylindoles 579 are prepared by carbonylative cycliza-tion of the 2-alkynyltrifluoroacetanilides 576 with aryl halides or alkenyl tri-flates. The reaction can be understood by the aminopalladation of the alkyne with the acylpalladium intermediate as shown by 577 to generate 578, followed by reductive elimination to give 579[425]. 

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