Intramolecular Halogen Shift

Subsequently to rhodium coordination with the enyne to form X, oxidative addition with the allyl chloride affords a rhodium-7r-allyl complex. Then isomerization 

Moreover, both the as- and trons-aUcenes react at the same rate and yield the same product (Tab. 8.9, entries 1 and 2). As demonstrated above, this was not the case with the N-tethered enynes (Tab. 8.2, entry 6) which gave a mixture of products. Nitrogen-and oxygen-tethered enynes were subjected to the rhodium(I) conditions affording high yields of the halogen shift product (Tab. 8.9, entries 1-10). 

The rhodium catalyzed Alder-ene reaction represents just one exciting cycloisomerization process that is still in its infancy. The synthetic potential of this reaction is currently being investigated, and new findings will undoubtedly be made. 

1 For a review on the Alder-ene reaction, see Snider, B. B. Ene Reactions with Alkenes as Enophiles, in Comprehensive Organic Synthesis Trost, B.M., Ed. Pergamon Press Oxford, UK, 1991 Vol. 5, pp. 1-27. 

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The cyclization of 1,6-enynes by RhCl(PPh3)3 can generate functionalized 1,3- or 1,4-diene cyclic compounds. For example, treatment of l,6-en)mes containing a haloalkenyl group with Wilkinson s catalyst in dichloromethane at reflux produces cyclization product which incorporates an intramolecular halogen shift a wide spectmm of enynes can be applied in this transformation (eq 75). ... 

Thus, halocarbenes undergo smooth addition to the P/C triple bonds of compounds 9 to furnish 1-phosphirenes (for intramolecular ring closures phosphavinylcarbenes to give the same products, see [16]), which rearrange to the thermodynamically more stable 2-isomers 11 by [l,3]-halogen shifts [17]. 

Among the halogens, fluorine does not undergo rearrangement, and what evidence there is suggests that the rearrangement of chlorine is an intramolecular process (1,2 shift) whereas that of bromine appears to take place by both inter-and intramolecular routes. Less is known about iodine migration. 

An interesting aspect of the radiolysis of longer-chain alkyl bromides and chlorides is the high yield of isomerization (G(isomer) > 6 pmol e.g., 1-chloroalkane isomerizes to 2-chloroalkane). These reactions are attributed to free-radical chain reactions, which involve intramolecular shift of a halogen atom. 

Similarly, the introduction of heteroatoms like oxygen, nitrogen, or halogens results in lower chemical shifts when compared to alkylboranes. The reason for this resides in the inter- or intramolecular donation of the lone pairs of the heteroatom into the p orbital of the central boron. This has the same effect as unsaturated systems, leading to upheld shifts of B signals. 

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