Fragmentation reactions, loss

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Aliphatic onium ions such as immonium, oxonium, and sulfonium ions have been introduced as even-electron ionic products of the a-cleavage occurring from molecular ions of amines, alcohols, and ethers or thiols and thioethers, respectively (Chap. 6.2.5). All these and analogous onium ions are capable of further fragmentation reactions, the majority of which are alkene losses [141] yielding fragments of high relevance for structure elucidation. 

Bukovits, G.J. Budzikiewicz, H. Mass Spectroscopic Fragmentation Reactions. XXVIII. The Loss of Water From n-Alkan-l-ols. Org. Mass Spectrom. 1983, 75,219-220. 

Griitzmacher, H.-F. Mechanisms of Mass Spectrometric Fragmentation Reactions. XXXII. The Loss of Ortho Halo Substituents From Substituted Thiobenzamide Ions. Org. Mass Spectrom. 1981, 16, 448-450. 

The mechanism of the reaction presumably involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

As part of a mechanistic and synthetic study of nucleophihc carbenes the spirocyclic 4(5/l)-oxazolone 18 has been obtained from benzoyl isocyanate (Scheme 6.1) Thermal extrusion of nitrogen from the 1,3,4-oxadiazoline 14 produced the carbonyl ylide 15 that fragmented via loss of acetone to the aminooxycarbene 16. Spectroscopic data [gas chromatography-mass spectrometry (GC-MS), infrared (IR), proton and C-13 nuclear magnetic resonance ( H and NMR)] of the crude thermolysate was consistent with 18. The formation of 18 was rationalized to result from nucleophihc addition of 16 to benzoyl isocyanate followed by cyclization of the dipolar intermediate 17. Thermolysis of 19 and 21 under similar reaction conditions afforded 20 and 22 respectively, also identified spectroscopically as the major products in the thermolysate. 

Wu and Brodbelt have studied the gas-phase fragmentation reactions of HOMg(L) complexes of crown ethers and glymes . A common loss involves units of C2H4O, which can either directly occur from the precursor ion, or can be triggered by an initial interligand reaction between HO and L. This latter reaction is illustrated in Scheme 11 for the complex of 12-Crown-4. Thus loss of H2O from the initial adduct 56 yields the ring-opened complex 57, which contains a coordinated alkoxide moiety, which can then lose an epoxide to form the related complex 58. 

High-throughput analysis requires speed of analysis, hence no chromatographic separation of analytes and detection by making use of a common fragmentation reaction (neutral loss). 

Decarboxylation of carboxylic acids.1 Reaction of methyllithium with non-enolizable a-phenyl- or a-phenythio carboxylic acids such as 1 in HMPT results in a Haller-Bauer type fragmentation with loss of C02. The intermediate carbanion can be trapped by various electrophiles. 

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