Direct cleavage

Salicylonitrile is believed to arise by direct cleavage with subsequent hydrogen transfer, while the benzoxazoles were produced by an isocyanide intermediate (73JA919, 74HCA376). Photolysis in D2O tends to confirm this possibility and rule out an azirine intermediate (39), due to deuterium corporation into the molecule (Scheme 10) (74HCA376). 

The alkaloids cotamine (259), hydrastinine (261), and berberinal (260), each possessing a grouping formed by interaction of an aldehyde with a secondary amino group in their molecule, are unusual. The Grignard reaction of free base 166 does not occur as readily as that of the corresponding salt 167. Both reactions lead to the alkylated product 168. For example, only 50% of hydrastinine reacts and 50% is regenerated, whereas hydrastinine hydrochloride reacts almost quantitatively (261). The salt undoubtedly contains a C=N double bond. In the case of the free base, the presence of a C=N double bond was not proven, and the reaction probably occurs by direct cleavage of the C—OH bond. 

A related hydroxyl-directed cleavage has been observed using LiBp4/BH3 or UBH4/BF3. ... 

Intermediate 8, the projected electrophile in a coupling reaction with intermediate 7, could conceivably be derived from iodolactone 16. In the synthetic direction, cleavage of the acetonide protecting group in 16 with concomitant intramolecular etherification could result in the formation of the functionalized tetrahydrofuran ring of... 

Essentially different from the reactions described in (2) are radical eliminations in which the radical X is already-present in the ionized molecule Rt — X)+ but where the actual dissociation of the R, — X bond is preceded or accompanied by an isomerization of the charge carrying part R. This situation (3) may occur in cases, in which the direct cleavage 11- 12 is energetically less likely than the two-step (or generally multi-step) pathway 11-+13-+14. 

There is another mechanistic variation to circumvent the direct cleavage process 11- 12. The isomerization 11- 13 does not necessarily have to involve a skeletal reorganization. For example, it can be envisaged that a specific hydrogen transfer occurs onto a suitable acceptor function Y, 24- 25 (5). In this way a reactive radical site is created which induces the dissociation step (elimination of X ) by... 

In an aprotic solution, the mechanism of oxidation of diaryl disulfides was shown to be more complex than a direct cleavage of the S—S linkage [116,117,123], The occurrence of two consecutive reactions being of second kinetic order if potential-determined and of first order if current-determined, was established for the two-electron transfer steps. Dimerization of the cation radicals occurs on the ArS fragment, whose contribution to the HOMO is more important, and produces an intermediate disulfonium dication. The subsequent cleavage of the latter results in two ArS+ cations and a molecule of a disulfide (the same as a starting disulfide in the case of symmetrical compounds). This mechanism, EC2C1E (E = electrochemical, C = Chemical), has... 

Direct Cleavage of Transition Metal-Silicon of Germanium Bonds. . . 93... 

M. J. Damha, Efficient RNase H-directed cleavage of RNA promoted by antisense DNA or 2 F-ANA constructs containing acyclic nucleotide inserts, J. Am. Chem. Soc., 125 (2003) 654-661. 

While the direct cleavage of a carbon-carbon double bond to two carbonyl groups by unzipping of polyperoxides (10,11) or by ozonolysis is well known, the cleavage of a-methylstyrene to acetophenone and formaldehyde (11), the cleavage of polyisoprene in solution (2,12), and the... 

A direct cleavage of the ester to radicals similar to that observed in the vapor state94 and an internal hydrogen abstraction followed by radical formation98 have been proposed to explain the reaction in benzene. The internal hydrogen abstraction mechanism appears to be the more reasonable. Direct cleavage would yield ethoxy radicals, but no products derived from them are observed. The low quantum yield of ester disappearance observed for ethyl... 

Hydrozirconation of alkyne 14, followed by cuprate addition, was then used to install the Cl3 20 side chain of 15. Reduction of the ketone (L-selectride) followed by hydrogen fluoride cleavage provides the F-series derivatives 16, while direct cleavage of 15 gave the E-series compounds 17. 

N-Carboxylic esters are readily cleaved under basic conditions. It is not known whether direct cleavage of the ester group is initiated by nucleophilic attack on the carbonyl group (c/. cleavage of acyl groups, Section 3.05.2.6) or whether the ester group is initially hydrolyzed. 

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