By Fragmentation Reactions

S-alkyl-N,N,N -trialkylisothioureas 10 react with tosyl isothiocyanate to give ionic adducts 11, which sometimes undergo a [2+2] cycloreversion reaction to give N-sulfonylcarbodiimides 12 and dimethylthiourea.  

Heating of the cycloadducts derived from arenesulfonyl isocyanates with dialkyl-carbodiimides in refluxing o-dichlorobenzene gives rise to the formation of N-sulfonylcarbodiimides and the lower boiling alkyl isocyanate. For example, reaction of p-toluenesulfonyl isocyanate 13 with di-n-butylcarbodiimide affords N-p-toluenesulfonyl-N -n-butylcarbodiimide 14 in 42 % yield. Reactions with DCC and diisopropylcarbodiimide proceed similarly. 

Addition of N-sulflnyl-p-toluenesulfonamide 15 to methyl-t-butylcarbodiimide causes an exothermic reaction with formation of the exchange products 16 and 17.  

The thermolysis of perfluoroalkyl(N-pentafluorosulfanyl)azidomethines 18 affords N-fluoroalkyl-N -sulfanylcarbodiimides 19 (Rf=CF3, C2F5).  

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SCHEME 50. Preparation and uses of allylic zinc reagents by fragmentation reaction... 

Conducting radical cyclizations by fragmentation reactions offers a powerful alternative to the tin hydride method. Instead of. obtaining reduced products, one obtains products of substitution an alkene is regenerated in the fragmentation step. 

The second route of getting free radical center near is mediated by low-molecular free radicals or compounds which are either present in the polymer or are gradually formed there by fragmentation reactions. While tertiary peroxy radicals propagate only by abstraction of hydrogen atom from surrounding C —H bonds, secondary peroxy radicals may easily cleave to hydroxy radicals and ketones as follows ... 

VIII. 1. Cleavage of the Zero Bridge in Bicycles by Fragmentation Reactions... 

Synthesis of unsaturated open chain sugars by fragmentation reactions... 

The beam produces M in a variety of energy states. Some are produced with rather large amounts of internal energy (rotational, vibrational, and electronic) that is dissipated by fragmentation reactions for instance. 

As an alternative to 5-exo cyclization the peroxyl radical may also undergo 4-exo cyclization to form 1,2-dioxetanyl carbinyl radicals. Such reactions appear to only be favored if there are no double bonds appropriately positioned for 5-exo cyclization. Dioxetanes have been postulated to serve as precursors to lipid aldehydes by fragmentation reactions. 

Many peaks in mass spectra can be explained by fragmentation reactions that involve the breaking of two covalent bonds. When a radical cation undergoes this type of fragmentation, the products are a new radical cation and a neutral molecule. Some important examples, starting from the initial radical cation, are the following ... 

The mass spectra of materials containing many chemical components generally show a large number of peaks, rendering unambiguous identifications difficult. Further complications are added by fragmentation reactions, or in-source recombination of ions and neutrals to form higher mass species. Typical examples include the formation of sodium and potassium adducts. 

The carbohydrate dienophiles in the above examples are all derived from cyclic structures. Acyclic sugar dienophiles have also been studied. These substrates arc accessible through the reaction of a carbohydrate with a stabilized Witdg reagent or by fragmentation reactions. Two examples of the use of acyclic carbohydrate dienophiles in Diels-Alder reactions are presented below. 

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