Aziridine catalysts, rhodium complexes

Numerous studies have been directed toward expanding the chemistry of the donor/ac-ceptor-substituted carbenoids to reactions that form new carbon-heteroatom bonds. It is well established that traditional carbenoids will react with heteroatoms to form ylide intermediates [5]. Similar reactions are possible in the rhodium-catalyzed reactions of methyl phenyldiazoacetate (Scheme 14.20). Several examples of O-H insertions to form ethers 158 [109, 110] and S-H insertions to form thioethers 159 [111] have been reported, while reactions with aldehydes and imines lead to the stereoselective formation of epoxides 160 [112, 113] and aziridines 161 [113]. The use of chiral catalysts and pantolactone as a chiral auxiliary has been explored in many of these reactions but overall the results have been rather moderate. Presumably after ylide formation, the rhodium complex disengages before product formation, causing degradation of any initial asymmetric induction. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

In a similar vein, a resin-supported rhodium-complexed dendrimer 340 has been shown to promote the carbonylative ring expansion of aziridines to /3-lactams <1988CC710, 2006JHC11>, as illustrated by the conversion of the A-7-butyl aziridine 341 to the corresponding lactam 342 in almost quantitative yield. The supported catalyst, which shows reactivity comparable to the solution-phase variety, is easily recovered by filtration and exhibits no significant loss of activity upon recycling (Scheme 88). 

The aziridination of olefins has also been studied, but fewer complexes catalyze this reaction as efficiently as iron and manganese complexes catalyze the epoxida-tion of olefins. Nevertheless, the aziridinations of olefins catalyzed by copper, ruthenium, and rhodium complexes have been reported. The source of nitrogen is usually [N-(p-toluenesulfonyl)imino]phenyliodinane (PhI=NTs) or a precursor to a related iodoarylimine. The aziridine is likely generated from these copper- and rhodium-catalyzed reactions by an outer-sphere process in which the olefin interacts with the LUMO of the complex, which is located at the nitrogen. This mechanism is more likely to be followed by these catalysts than a [2-t-2] process, followed by reductive elimination. 

Che has reported that both achiral and chiral rhodium catalysts function competently for intramolecular aziridination reactions of alkyl- and arylsulfonamides (Scheme 17.29) [59, 97]. Cyclized products 87 are isolated in 90% yield using 2 mol% catalyst, PhI(OAc)2, and AI2O3. Notably, reactions of this type can be performed with catalyst loadings as low as 0.02 mol% and display turnover numbers in excess of 1300. In addition, a number of chiral dimeric rhodium systems have been examined for this process, with some encouraging results. To date, the best data are obtained using Doyle s Rh2(MEOX)4 complex. At 10 mol% catalyst and with a slight excess of Phl=0, the iso-... 

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