Homoallylic systems

Rh+ catalyst is more selective than Ir+ for acyclic stereoselection, Acyclic homoallylic systems ... 

Homoallylic systems may isomerize under hydrogenation conditions to allylic systems, causing hydrogenolysis to occur when it would not have been expected (39b,45a-45c). In these cases, if hydrogenolysis is unwanted, it is best to avoid those catalysts and conditions that favor isomerization. Double-bond migration to an allylic position may occur even if the double bond is required to leave a tetrasubstituted position (26a). 

Structural rearrangements are not encountered with saturated Grignard reagents, but allylic and homoallylic systems can give products resulting from isomerization. NMR studies indicate that allylmagnesium bromide exists as a CT-bonded structure in which there is rapid equilibration of the two terminal carbons.101 Similarly,... 

Winstein, S. Shatavsky, M. Neighboring carbon and hydrogen. XXI. Anti-7 derivatives of norbomene as homoallylic systems. /. Am. Chem. Soc. 1956, 78, 592-597. 

It is only when one gets to the j5-homoallyl system (88) of Julia and coworkers that one observes interaction of the cyclopropyl radical with the 7c-system resulting in a cyclization. 

The anti diastereoselectivity can be achieved by coordination of the nucleophile to the opposite face of the a-oxygen atom. This was the case with the homoallylic system (Scheme 82). 

Such a homoallylic carbocation may rearrange to either a cyclopropylmethyl or a cyclobutyl carbocation, which leads to the formation of cyclopropane or cyclobutane derivatives. In addition, elimination or direct substitution of the homoallylic system leads to considerable amounts of acyclic alkenes. Therefore, this rearrangement has found little synthetic application as mixtures of products are generally obtained. ... 

When a cyclohexyl cation is produced in the solvolysis of a homoallylic system, formation of a three-membered ring can occur. 

Various types of [3 + 0] cyclizations via y-functionalized metal alkyls (either as stable compounds or reactive intermediates) are known. The y-carbon can act as an acceptor, a donor or radical center. Homoallyllic systems 1 can cyclize via alkene insertion to form (metallamethyl)cyclo-propanes 2. 

As a reverse of fi-C elimination in methylcyclopropane complexes to form homoallyl systems, homoallylmetal complexes can undergo intramolecular alkene insertion to yield metallated methylcyclopropanes. 

The homoallylmetal complex 27, obtained via addition of methanol to dichloro-/ -norbor-nadienepalladium(II) (26), can also be assigned as a nonclassical homoallylic system. Upon treatment with methanol/CO, complex 27 gives 3-e rfo-carbomethoxy-5-exo-methoxynortricyc-lene... 

Careful conformational analysis of acyclic systems is needed. Homoallylic Systems... 

Due to our engagement in carbanion rearrangements we were especially interested in the homoallylic system 4,4-dilithio-l-butene 33. In the case of homoallyllithium 29 ring-chain equilibrium strongly favors the open-chain lithiumorganic compound and the introduction of four methyl groups into the a- and -positions is necessary... 

Cationic cyclization and its reverse have particular relevance, since ring closure and ring opening have been invoked to account for a large number of biosynthetic pathways.The gaseous cyclopropylcarbinyl-cyclobutyl-homoallyl system represents an extreme instance of a rapid, reversible isomerization of this sort, which has been studied over a period of 20 years both by /1-decay of tritiated cyclobutane and by radiolytic methods. This interconversion is so fast that the three structures... 

This example is related to the rearrangement of cyclopropylmethyl radicals 32 to give homoallylic systems 33 (Scheme 8). The parent reaction is very fast =... 

The cases we have considered involve aryl rings as the governing structural feature for enantioselectivity. The AD systems also show excellent enantioselectivity toward functionalized alkenes, especially allylic and homoallylic systems with oxygen substituents. In these systems, another important structural variable comes into play, that is, the conformation of the allylic substituent and its possible interaction with the reaction site. ... 

Two research groups report the formation of appreciable quantities of cyclopropane compounds by reaction of appropriate derivatives of methyl ricinoleate, which contains a homoallylic system. One group employed the crystallized tosyl ester of the /ru/ij -isomer, the other used methyl 12-mesyloxy-oleate. Hydroxy-, methoxy-, acetoxy-, and chloro-cyclopropane esters (30) were formed (30—60% yield) under controlled pH conditions in acidic media the more stable homoallylic system is regenerated. 

A very similar behavior was noticed when the investigation was extended to homoallylic systems of type 9. 

Structural rearrangements are not encountered with saturated Grignard reagents. Allylic and homoallylic systems can give products resulting from isomeriz-... 

Hanack, M., and H.-J. Schneider Neighboring-Group Effects and Rearrangements in Reactions of Cyclopropylmethyl, Cyclobutyl, and Homoallyl Systems. Angew. Chem. Intern. Ed. 6, 666 (1967). 

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