Metal‐catalyzed carbenoid reactions with iodonium and sulfonium ylides

Abstract

Transition metal-catalyzed decomposition of phenyliodonium and diphenylsulfonium ylides was investigated with regard to application in asymmetric carbenoid reactions. Phenyliodonium ylides react in the presence of Rh(II) catalysts with the same selectivity in inter-and intramolecular cyclopropanations as the corresponding diazo compounds, and intramolecular CH insertions proceed with identical enantioselectivities. With diphenylsulfonium ethoxycarbonylmethylide the Cu(I)-catalyzed cyclopropanation of olefins affords trans/cis ratios and asymmetric inductions identical with those of diazo compounds, but with Rh(II) catalysts some small, although significant, selectivity variations occur, which are ascribed to coordination of diphenyl sulfide to one of the coordination sites of the catalyst. CARBENOID REACTIONS WITH PHENYLIODO-NIUM YLIDES Synthesis and thermal, and photochemical decomposition of phenyliodonium ylidesThe chemistry of phenyliodonium ylides has been pioneered by Neilands. 15 The ylides (4) are synthesized by heating activated methylene compounds (1) with iodosylbenzene (2) in chloroform. Other procedures use iodosylbenzene and acetic anhydride 16 or diacetoxyiodosylbenzene (3) in MeOH in the presence of KOH at À5 to 0°C. 17 This latter method of Schank 17 is the method of choice. It is, however, limited to ylides such as 4 carrying two electron-withdrawing substituents, such as carbonyl,

Advances in Synthetic Applications of Hypervalent Iodine Compounds

“…Iodonium ylides are applied as useful carbene or carbenoid sources for the reactions under thermal, catalytic, or photochemical conditions. A discussion of the reaction mechanisms and an overview of synthetic uses of iodonium ylides as carbene precursors can be found in earlier reviews. ,, The generated carbenes can be trapped with various substances to give the corresponding products of inter- or intramolecular cyclization, products of transylidation, or products of C–H insertion. Numerous recent examples of inter- or intramolecular cyclopropanation of alkenes under metal-catalyzed or metal-free conditions have been published by several groups. ,,, Moriarty and co-workers have developed an intramolecular cyclopropanation of alkenes using the sterically rigid iodonium ylides 491 under metal-free condition (Scheme ).…”

Asymmetric Synthesis through C–H Activation

“…Mn(III) -, Fe(III) -, or Rh(II) -catalyzed intra -and intermolecular C -H insertions has been achieved with tosylimidophenyliodinane 184 [251] . Effective intermolecular C -H amination was later demonstrated by Muller and others using p -nitrophenylsulfonyl imidoiodinane 185 [71,75,77] . This class remains the most widely used among nitrene precursors.…”

Carbenoid Pathways in Copper-Catalyzed Intramolecular Cyclopropanations of Phenyliodonium Ylides

“…Both precursors afforded the same selectivities in the intermolecular cyclopropanation of substituted styrenes and in the intramolecular competition for cyclopropanation vs. CH insertion. In addition, identical enantioselectivities resulted when the intramolecular CH insertion of a diazoacetoacetate and the corresponding phenyliodonium ylide was carried out in the presence of Ikegamis chiral [Rh II (carboxylato)] catalyst [6]. These results are consistent with a carbenoid mechanism.…”

“…Unfortunately, the lack of reactivity of diazo compound 14a does not allow comparison with the enantioselectivities achieved with the ylides, and it is not clear whether the less than 100% enantioselectivity must be attributed to an inadequate choice of catalysts or to competing, unselective cyclopropanation pathways. Although we have not observed spontaneous cyclopropanations of ylides in the absence of catalysts in the present investigation, such reactions have been reported in the past not only by Moriarty et al [7], but also by ourselves [6], and for these reactions the mechanism outlined in Scheme 1, or a radical version thereof, is plausible. The present results lend some support for a metal carbenoid intermediate in the main pathway of the Cu-catalyzed ylide decomposition.…”