C-O bond fragmentation

One of the possible synthetic ways to obtain heterocyclic phosphines is the insertion of carbonyl compounds into the P—E (E = Si, Ge) bond of sila- and germa-phospholanes. Thus, the enlargement of the ring takes place and the P—C—O—E fragment is formed (9) [Eq. (7)] (74MI1 75JOMC35 77JOM35). The heterocyclic phosphepanes are obtained as a mixture of stereoisomers. 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

The reaction gave only the rearrangement products 333 and 334, and the side product 335, as expected from the reactivity of alkylidenecyclopropane derivatives (Scheme 49). Compound 333 might arise from the 0-0 bond cleavage followed by the rearrangement of a cyclopropyloxy cation to an oxoethyl cation (Scheme 49, path a). Spiro-hexanone 334 could arise from a different fragmentation of ozonide C-O bond and further cyclopropyloxy-cyclobutanone rearrangement (Scheme 49, path b). Oxirane 335 can eventually derive from the same path b or from other side processes [13b]. 

The gaseous dichlorocarbene radical cation reacted with alkyl halides via a fast electrophilic addition to form a covalently bonded intermediate (CI2C—X—R)+ in a Fourier transform ion cyclotron resonance mass spectrometer. This intermediate fragments either homolytically or heterolytically to produce net halogen atom or halogen ion transfer product. Addition of carbonyls to the carbene ion is followed by homolytic cleavage of the C-O bond to yield a new carbene radical cation. 

Anion-radicals of benzyl benzenesulfenate and tcrt-butyl benzenesulfenate prepared electro-chemically undergo fragmentation at the expense of the sulfenate esterial group, but in a different mode. In the benzyl benzenesulfenate anion-radical, the S—O bond cleaves, whereas in the tert-butyl benzene sulfenate, the C—O bond splits (Stringle and Workentin 2005). 

One of the first kinetic studies of the fragmentation of a C—O bond in an ether radical anion was reported by Maslak and Guthrie. " In this study substituted benzyl phenyl ethers (as well as some other benzyl-type phenyl ethers) were treated with 2,4,6-tri-tert-butylnitrobenzene radical anion to produce ArCH20Ph or PhCH20Ar and the unimolecular decay of the anion radical was monitored using EPR. Despite some discrepancies between the values of the reported rate constants, ... 

Fremy s salt has been used to degrade a />-hydroxybenzylamine [263], Aminoxy-lation at the para position of the phenol triggers fragmentation the C-O bond formation gives rise to a conjoint 1,3-disubstituted system, despite contrapolarization at the intervening oxygen atom is required. 

C—O bond cleavage with the charge remaining on the alkyl fragment. 

Addition reactions of carbon radicals to C—O and C—N multiple bonds are much less-favored than additions to C—C bonds because of the higher ir-bond strengths of the carbon-heteroatom multiple bonds. This reduction in exothermicity (additions to carbonyls can even be endothermic) often reduces the rate below the useful level for bimolecular additions. Thus, acetonitrile and acetone are useful solvents because they are not subject to rapid radical additions. However, entropically favored cyclizations to C—N and C—O bonds are very useful, as are fragmentations (see Chapter 4.2, this volume). 

Adsorption of CO on NifOOll. The response of a surface to ion bombardment covered with a molecularly adsorbed species is mechanistically distinct from the atomic absorbate case. For CO on Ni 001, for example, the strong C-O bond of 11.1 eV and the weak Ni-CO bond of 1.3 eV allows the CO molecule to leave the surface without fragmentation. In the experimental Btudies, the main peaks in the SIMS spectra for a Ni 001 surface exposed to a saturation coverage of CO are Ni, Nit, Nit, NiCO, NitCO, and NitCO. All... 

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