1,4-Rhodium 2 migration

Remote C-H Activation via Through-Space Palladium and Rhodium Migrations... 

In addition to palladium, rhodium is also known to migrate from one carbon to another under suitable conditions. However, compared with palladium migration processes, rhodium migration is scarce. To date, only a few examples are reported, all involving 1,4-alkyl/vinylic to aryl migrations. 

There are two reports of an alkyl to aryl rhodium migration process. The first example was reported in 2000 by Miura [75], It was discovered that, upon reaction of phenyl boronic acid with norbomene under rhodium-catalyzed conditions, a merry-go-around type sequential alkylation occurred up to four times on the aromatic ring, resulting in a 1,2,3,4-tetranorbomylated benzene as the final product (Scheme 16). Mechanistically, this reaction involves an alkyl to aryl migration of rhodium. Thus, aryl rhodium intermediate 29, generated via an initial transmetalation step,... 

Scheme 16 Sequential alkylation of an aromatic boronic acid by norbomene under rhodium migration conditions...

The other example of a rhodium migration was discovered when 3-substituted-3-(2-hydroxyphenyl)cyclobutanones were subjected to rhodium-catalyzed conditions (Scheme 17) [76], The reaction generates dihydrocoumarin derivatives in high... 

Scheme 18 Isotope labeling experiment and the proposed mechanism for the rhodium migration...

Table 11 Rhodium migration process trapped by Heck coupling in the synthesis of dihydrocoumarins3...

A few examples of vinylic to aryl rhodium migration have also been observed. One such example was discovered independently by two groups almost at the same time [77,78], In this reaction, aryl propargyl alcohols undergo a rhodium-catalyzed isomerization to furnish indanone products (Table 12). The reaction proceeds by a fairly complicated mechanism shown in Scheme 19. Thus, after base-promoted ligand... 

Rhodium migration has also been observed in the rhodium-catalyzed reaction of aryl boronic acids (or equivalents) with alkynes or enynes. Three reports have been disclosed in the literature [79-81], The first is an alkyne hydroarylation method reported by Hayashi and coworkers (Scheme 20) [79], It was observed that under the standard rhodium-catalyzed hydroarylation conditions, protonolysis does not occur on the rhodium-bearing vinylic site, but rather on the aromatic ring after an apparent rhodium migration. 

Scheme 20 Rhodium-catalyzed hydroarylation of alkynes with vinylic to aryl rhodium migration...

Murakami and coworkers reported a further use for this rhodium migration [80, 81]. Instead of protonolysis, they noticed that the aryl rhodium species after the vinylic to aryl migration is nucleophilic enough to attack an ester moiety in an intramolecular fashion to afford a cyclic ketone. Thus, an internal alkyne equipped with ester groups at a specific place was subjected to the rhodium-catalyzed hydroarylation conditions (Scheme 21). Indeed, the desired ketone was obtained in an 89% yield. Not only methyl esters can serve as acylation agents ethyl esters and isopropyl esters are also suitable substrates. 

Fewer examples of rhodium migration chemistry have been reported. Other than palladium and rhodium, platinum [82] is also known to migrate from one position to another under suitable reaction conditions. A better understanding of the migration in these metal-catalyzed reactions in terms of substrate scope and mechanism is needed. Extensions of this migration chemistry to other metals, such as perhaps ruthenium and iridium, can be expected. 

Figure 3 compares the light-off temperatures determined on Rh/A and Rh/CeA catalysts. Ce02 is known to stabilize noble metals [18] and probably limits here the rhodium migration into the support. Nevertheless, contrary to what was observed on Rh/A, a reducing treatment increases the deactivation of Rh/CeA. The most probable explanation is that a RhCexOy mixed oxide could be... 

The hydrosilylation of methylenecyclopropanes is proposed to proceed via oxidative addition to the olefin, followed by rhodium migration across the strained cyclopropane ring, and eventual reductive elimination to give the silyl-substituted olefins. The process is compatible with aromatic and aliphatic substitution on the olefin and often requires heating. Additionally, cyclopropyl-substituted methylenecyclopropanes may be selectively silylated to give alkenes containing one, two, or three /3-silylated olefin chains. 

Asymmetric ring-opening reorganization of cyclobutanol 42 proceeded via 1,4-rhodium migration to give 1-indanone 43 in a diastereo- and enantioselective manner (Scheme 3.24) [35]. 

It has been reported that aryl-to-aryl 1,5-rhodium migration is involved in decyanative silylation of aryl cyanides bearing a tethered arene. The 1,5-migration proceeds through remote C-H bond activation (Scheme 148). " ... 

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