Fragmentation process alkanes

We will now look at some of the most common fragmentation processes, starting with the fragmentation of alkanes, e.g. hexane, CoHh (Scheme 5.4). 

The reactions proposed for the formation of the labeled products in the triton transfer to ethane, propane, butane and isobutane are illustrated in Fig. 6. The formation of the parent tritiated alkane was ascribed, without invoking the intervention of long-lived protonated ions, to a mechanism involving the protonation reaction (48), followed by the fragmentation process (50b) and by a thermoneutral hydride-ion transfer from the unlabeled alkane to the tritiated alkyl ion ... 

A further interesting example is the use of l,l-di(phenylthio)med)yllithium to open an epoxide followed by a Grob fragmentation process as shown in Scheme 56 (entry b). The reaction of 1,1-di-(thio)methyllithium with epoxides has also been used - in the one-pot synthesis of l,l-di(thio)cyclopropanes involving an intramolecular cyclization of the 7-tosyloxydi(phenylthio)alkylli-thium. This intramolecular alkylation reaction proceeds even more efficiently than its intermolecular version, and allows the synthesis of a large variety of l,l-di(thio)cyclopTDpanes from 3-chloro- and 3-phenylthio-l,l-di(thio)alkanes and n-butyllithium in THF (Scheme 57 and Scheme 58). 

Another approach to carrying out tin-free radical fragmentation processes, developed by Fuchs, utilizes trifluoromethyl sulfone, or triflone, derivatives. Fuchs first reported examples of free radical alkynylation reactions using acetylenic triflone 102 [62]. What is most remarkable about these reactions is that the radicals being alkynylated are formed from the cleavage of C-H bonds standard radical precursors are not required. For example, when tetrahydrofuran is mixed with triflone 102 at room temperature, alkynylation occurs a to the ether oxygen in 92% yield (Scheme 21). In this case, the radical chain process is most likely initiated by traces of peroxides in the THF. Similarly, unactivated alkanes such as cyclohexane will react with triflone 102 in good yield (83% for cyclohexane) when heated with a catalytic amount of AIBN. 

Alkyl triflates can also be used as efficient leaving groups for intramolecular cyclization in the process of generating heterocycles, such as thioindenes, cyclic enones, iV-heterocyclic carbene precursors, or tetrahydropyridines via a Grob fragmentation process (93). They can also be reduced to the corresponding alkane in the presence of a copper catalyst. ... 

Five-membered ring systems are also formed on transfer of single-skeletona-tom fragments, usually in a stepwise process CR2 from diazo alkanes [28], NH from azourude (hydrazoic acid) [134], O from peroxy acids [/ii], S from phos-... 

Cracking (Chapter 3 Focus On) A process used in petroleum refining in which large alkanes are thermally cracked into smaller fragments. 

Duffield and coworkers65 studied the El- induced mass spectra of five arene- (215-219) and four alkane sulfonylthioureas (220-223) and observed two rearrangement processes, namely loss of S02 from 215-219 and the elimination of ArS02 and RS02 with the thione sulfur atom from 215-223. The other fragmentations involved simple bond cleavages with and without hydrogen transfer (equation 48). The loss of H2S was evident for all the compounds studied except 221 and 222. It was, however, found to be a thermal and not an ionization process. 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

As pointed out, carbenium ions make up the largest portion of alkane mass spectra and dissociate further by alkene loss. This type of fragmentation is rather specific, nevertheless isomerizations of the carbenium ion can happen before. Generally, such processes will yield a more stable isomer, e.g., the tcrt-butyl ion instead of other [C4H9] isomers (Fig. 6.19). 

Consequently, this reaction could be identified with a process where an alkyl fragment is transferred from one alkane molecule to another. The methyl group is the smallest alkyl fragment and the easiest to be transferred. However, crossmetathesis shows that ethyl and propyl groups can also be transferred. Indeed, heavier homolog alkanes can result from successive reactions or from heavier alkyl group transfer. Consequently, because of the successive reactions, the real product distribution corresponds to the following equation ... 

In isomerizations, zeolites have special merit in their ability to admit straight-chain but not branched-chain molecules into the pores. Thus, normal alkanes up to n-Ci4H30 can penetrate the pores of zeolite 5A to reach the cavities where the C—C or C—H bonds may be catalytically broken the fragments, on reemerging from the pores, can recombine as isomerized molecules. The reverse process is not possible, since the isomers, having... 

For an alkane, the ionization process will involve the loss of an electron from a o-bond, as shown in Scheme 5.4 for the loss of an electron from the central C—C bond of hexane (the loss of an electron from all of the a-bonds in this molecule is almost equally probable, so a large number of fragments can be produced Scheme 5.5). 

---