Carbon eliminators, cyclic

Takata and Endo, 1988]. A cyclic carbonate and cyclic ether are eliminated as by-products in reaction pathways a and b, respectively. Polymer LXXIV contains both ether and carbonate functional groups in the polymer chain. 

Metathesis of alkenes has been reviewed in terms of cross-metathesis, ring opening and closing, disproportionation, transmutation, and self-metathesis.34 A review on catalytic processes involving ft -carbon elimination has summarized recent progress on palladium-catalysed C-C bond cleavage in various cyclic and acyclic systems.35... 

For examples of P-carbon elimination in late transition metal systems, Bergman et al. identified P-methyl transfer with four-membered ruthenacycles, which is driven by the formation of Ji-allyl and Ji-oxallyl complexes. Warming the solution of oxaruthenacycle 58 to 45°C led to formation of methane and cyclic enolate complex 60 [76]. ji-Oxallyl complex 59 initially arises from P-methyl... 

Aldehydes show an elimination reaction (loss of carbon monoxide, CO), that is not possible with ketones. Butanal, for example, photodissociates to propane and carbon monoxide. Cyclic ketones dissociate to a diradical (41 from cyclopentanone), which then reacts in any of several ways including elimination to ethene or 42 and coupling to cyclobutane. Formation of cyclobutane and ethene is accompanied by expulsion of CO prior... 

Abstract Palladium-catalyzed C-C bond cleavage via p-carbon elimination occurs in various cyclic and acyclic systems. Thus, the reaction can be utilized as one of fundamental and effective tools in organic synthesis. The recent progress in this field is summarized herein. 

Figure 9. Structure of the cyclic carbonate elimination product (9) isolated from the BTFE cationic polymerization of SOCM. Symmetrical bis(chloromethyl) spiro model monomer (10) used in comparative cationic polymerizations.

The nickel-catalyzed reductive coupling reaction of methyl vinyl ketone with enyne 43 bearing a P-dicarbonyl moiety proceeds with liberation of the P-dicarbonyl enolate (Scheme 5.56) [39]. Reductive coupling of 43 with enone would afford nickelaoxacyclooctadiene 44. Transmetalation is accompanied by carbonickelation to yield 45. Intermediate 45 engages in P-carbon elimination through a six-membered cyclic transition state to liberate nickel enolate 46, which should be transformed to the corresponding zinc enolate to complete the catalytic cycle, and zinc enolate 47, which is eventually protonated upon hydrolysis. 

Cyclic 2-azidoalcohol 41 was converted to pyridine 46 upon treatment with a palladium(II) catalyst (Scheme 7.13) [16]. Mechanistically, palladium(II) reacts with the azidoalcohol 41 to form the palladacycle 42, which undergoes retro-oxidative cyclization (p-carbon elimination) to form a-azidoalkylpalladium 43. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Cycloalkene (Section 5 1) A cyclic hydrocarbon characterized by a double bond between two of the nng carbons Cycloalkyne (Section 9 4) A cyclic hydrocarbon characterized by a tnple bond between two of the nng carbons Cyclohexadienyl anion (Section 23 6) The key intermediate in nucleophilic aromatic substitution by the addition-elimination mechanism It is represented by the general structure shown where Y is the nucleophile and X is the leaving group... 

The addition of nucleophiles to cyclic fluoroolefins has been reviewed by Park et al. [2 ]. The reaction with alcohols proceeds by addition-elimination to yield the cyclic vinylic ether, as illustrated by tlie reaction of l,2-dichloro-3,3-di-fluorocyclopropene Further reaction results in cyclopropane ring opening at the bond opposite the difluoromethylene carbon to give preferentially the methyl and ortho esters of (Z)-3-chloro-2-fluoroacrylic acid and a small amount of dimethyl malonate [29] (equation 8). 

In 1970, it was disclosed that it is possible to achieve the conversion of dimethylformamide cyclic acetals, prepared in one step from vicinal diols, into alkenes through thermolysis in the presence of acetic anhydride." In the context of 31, this two-step process performs admirably and furnishes the desired trans alkene 33 in an overall yield of 40 % from 29. In the event, when diol 31 is heated in the presence of V, V-dimethylforrnamide dimethyl acetal, cyclic dimethylformamide acetal 32 forms. When this substance is heated further in the presence of acetic anhydride, an elimination reaction takes place to give trans olefin 33. Although the mechanism for the elimination step was not established, it was demonstrated in the original report that acetic acid, yV, V-dimethylacetamide, and carbon dioxide are produced in addition to the alkene product."... 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

Reactions of the hydrido(hydroxo) complex 2 with several substrates were examined (Scheme 6-14) [6]. The reactions are fairly complicated and several different types of reachons are observed depending on the substrate. Methyl acrylate and small Lewis bases such as CO, P(OMe)3, BuNC coordinate to the five-coordinated complex 2 affording the corresponding six-coordinate complexes. In reactions with the unsaturated bonds in dimethylacetylenedicarboxylate, carbon dioxide, phenylisocyanate indications for the addition across the O-H bond but not across the Os-OH bond were obtained. In reactions with olefins such as methyl vinyl ketone or allyl alcohol, elimination of a water molecule was observed to afford a hydrido metalla-cyclic compound or a hydrido (ethyl) complex. No OH insertion product was obtained. 

The elimination of carbon monoxide can occur by a concerted process in some cyclic ketones. The elimination of carbon monoxide from bicyclo[2.2.1]heptadien-7-ones is very facile. In fact, generation of bicyclo[2.2.1]heptadien-7-ones is usually accompanied by spontaneous decarbonylation. 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

Hydroxy-( , )-a,y-dienylsulfones are prepared by an analogous one-pot dehydration procedure via an elimination reaction of the corresponding cyclic carbonate [21]... 

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