Radicals ketene

The AMI-calculated structure and charge distribution of radical the trication of [1-carotene have been reported and its UV absorption spectrum estimated from INDO/S methods.158 The decomposition of the ftiran radical cation proceeds by two separate pathways according to a recent theoretical study, one via formation of propene radical cation and CO, the other a lower energy process via acetylene and a ketene radical cation.159 As a result of a reflection mass spectrometric study, a likely mechanism is... 

Harrington-Frost and Pattenden have reported a new synthesis of pental-ene, a natural angular triqiunane, using a novel tandem cychzation involving a ketene radical intermediate [26]. 

There are many synthetic examples that use radical cyclization as a key step, and the radical precursor is not limited to iodides or bromides. In Pattenden s synthesis of pentalenene, conjugated selenyl ester 156 was treated with Bu3SnH and AIBN to give a 45% yield of tricyclic ketone 159. Loss of PhSe generated the acyl radical 157, which exists in equilibrium with the ketene radical 156. Radical cyclization via the latter intermediate leads to 159. Cyclization via aryl radicals is also possible. In Schultz s synthesis of hexahydro-phenanthren-2-one derivatives, " aryl bromide 160 was cyclized to 161 in 78% yield under standard conditions. Radical cascade reactions have become quite popular for the synthesis of polycyclic ring systems. In these reaction, polyenes are subjected to radical cyclization, generating tricyclic or even tetracyclic ring systems. 5 Chiral auxiliaries have been used effectively in radical cyclization reactions. ... 

Ionized hydroxyacetylene (HC=C—9 ), the enol isomer of the ketene radical cation (H2C=C=0 , cannot be formed directly because 9 does not exist in... 

The product-ion scan (the old, now-unaccepted term, still used by some, is daughter-ion scan) is the most common mode of MS/MS operation. That spectrum is useful in the structure elucidation of a specified analyte. Information obtained in this scan is similar to that derived from a normal mass spectrum, except that the spectrum contains only those product ions that are formed exclusively from a mass-selected precursor ion. To acquire this spectrum, the first mass analyzer is set to transmit only the precursor ion chosen, and the second mass spectrometer is scanned over a required miz range. As an example, the ketene radical cation reacts with neutral ketene to form a cycloadduct, the structure of which was delineated by mass-selecting the mIz of the adduct (at 84) and acquiring the product-ion scan (see Figure 4.3). 

C. Dass, Dimerization of ketene radical cation in the gas phase. Rapid Commun. Mass Spectrom. 7, 95-98 (1993). 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

During this early period, a very ingenious free-radical route to polyesters was used to introduce weak linkages into the backbones of hydrocarbon polymers and render them susceptible to bio degradabihty (128—131). Copolymerization of ketene acetals with vinyl monomers incorporates an ester linkage into the polymer backbone by rearrangement of the ketene acetal radical as illustrated in equation 13. The ester is a potential site for biological attack. The chemistry has been demonstrated with ethylene (128—131), acryhc acid (132), and styrene (133). 

Although these reactions are thus closely related to the acyl-alkyl diradical disproportionation to ketenes, the stereospecificity of (55) -> (56) and (57) -> (58) shows that these hydroxyketones cannot proceed through free radicals capable of rotating about single bonds prior to the intramolecular hydrogen... 

PenCafluorosulfur chloride adds under free-radical conditions to fluorinated olefins, to acetylene, to ketene, to methyl tnfluorovinyl ether [/] and to perfluoroal lene [8]... 

The N,0- and N,S-heterocyclic fused ring products 47 were also synthesized under radical chain conditions (Reaction 53). Ketene acetals 46 readily underwent stereocontrolled aryl radical cyclizations on treatment with (TMSlsSiH under standard conditions to afford the central six-membered rings.The tertiary N,0- and N,S-radicals formed on aryl radical reaction at the ketene-N,X(X = O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from (TMSlsSiH was investigated and found to provide higher selectivities than BusSnH. 

Appearance energies for CH2 have been measnred from a large nnmber of precursors, including methane, ketene, and even methyl radical. 

Fig. 3.17. Kinetics of conductivity of a zinc oxide film in the process of adsorption of CH2-biradicals at 100 C. The radicals have been produced by means of photol)rsis ketene vapour at pressure P = 0,5 Torr. a - after adsorption of Zn-atoms b - prior (/) and after adsorption of (2) Zn-atoms.

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

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