Fragmentation reactions, according to the definition of Grob,1,2 are processes (equation 1) where the reacting molecule breaks into three fragments.

5.4 – Fragmentation Reactions

Silica-supported phosphorus chloride has been proved to be an efficient and recyclable catalyst for Beckmann rearrangement of a variety of ketoximes and dehydration of various aldoximes in anhydrous THF under microwave irradiation. This protocol has advantages of high conversion, high selectivity, short reaction time, no environmental pollution, and simple work-up procedure.

Silica-supported Phosphorus Chloride: An Efficient and Recyclable Catalyst for Beckmann Rearrangement of Ketoximes and Dehydration of Aldoximes Under Microwave Irradiation

A structural, spectroscopic and computational study of a series of oximes was undertaken to investigate how geometric and structural changes relevant to the reaction coordinate for the Beckmann reaction (normal Beckmann) and Beckmann fragmentation (abnormal Beckmann) manifest in the ground state. X-ray structures of a range of oximes derived from dihydrolevoglucosan (Cyrene™; which undergoes the abnormal Beckmann reaction), in which the oxygen substituent was systematically varied were determined. As the electron demand of the OR group increased, the major structural changes included lengthening of the N-OR bond distance, and a decrease in the magnitude of the C2[double bond, length as m-dash]N-O bond angle, consistent with the changes seen for cyclohexanone oximes, which undergo the normal Beckmann reaction. However, unique to the Cyrene oximes, an increase in the length of the fissile C1-C2 bond was observed, which correlated with a decrease in the 13C-13C 1-bond coupling constants as the electron demand of the OR substituent increased. Computational studies of Cyrene and cyclohexanone oximes using Natural Bond Orbital analysis support an electronic structure involving n(O) → σ*C1-C2 and σC1-C2 → σ*N-O localized orbital interactions.

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Structure-reactivity correlations of the abnormal Beckmann reaction of dihydrolevoglucosenone oxime.

5,6,7,8,9,10-Hexahydro-6,9-methanobenzocyclo-octen-11-one (3) was prepared through condensation of o-xylene-α,α′-diyl dibromide with N-cyclopentylidenepyrrolidine and converted by Beckmann rearrangement of its oxime (4) into 2,3,5,6-tetrahydro-2,5-ethano-3-benzazocin-4(1H)-one (6). 2,3-Bis(bromomethyl)benzo[b]thiophene was converted similarly into a mixture of 1,4,5,6-tetrahydro-2,5-ethano[1]benzothieno[2,3-d]azocin-3(2H)-one (15) and the isomeric lactam (19). Lactam (6) was N-methylated and both the parent lactam (6) and the N-methyl derivative (23) were reduced with lithium aluminium hydride to the saturated products (24) and (25), respectively. Conversion of lactam (6) into the corresponding thiolactam (26) with phosphorus pentasulphide followed by alkylation of the latter compound gave an N-(29) or S-alkylated derivative, (27) or (28), depending on the reagent and reaction conditions. Similar reactions are reported also for lactams (15) and (19). The 4(1H)-one structures of compounds (15)–(22) are based on an X-ray analysis of 3-methyl-2,3,5,6-tetrahydro-2,5-ethano[1]benzothieno[3,2-d]azocin-4(1H)-one (20). We also report syntheses of 2-azido-5,6,7,8,9,10-hexahydro-6,9-methanobenzocyclo-octen-11 -one (33) and 3-nitro(and 1,3-dinitro)-6,7,8,9,10,11 -hexahydro-7,10-methanocyclo-octa[b][1]benzo-thiophen-12-one, (36) and (37), respectively.

Synthesis and reactions of 2,3,5,6-tetrahydro-2,5-ethano-3-benzazocin-4(1H)-one and a thieno-extended analogue: X-ray structure of 3-methyl-2,3,5,6-tetrahydro-2,5-ethano[1]benzothieno[3,2-d]azocin-4(1H)-one

Aryl alkyl ketoximes react with the TiCl4/Et3N reagent to give 2,3,4,5-tetrasubstituted pyrroles in moderate to good yields (55–81%).

B. Gary^ McKinnie

Papers

Novel products from Beckmann fragmentation of camphor oxime in polyphosphoric acid