Metallocarbenoid

Muroni et al. have reported a tandem metallocarbenoid/ylide/[l,2]-Stevens rearrangement <2004TA2609>. The azoniaspirocycle ylide 5 is formed by trapping of the metallocarbene precursor (Equation 14). Heating the ylide in toluene then gave indolizidinone 55 as a single diastereoisomer. 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

These reactions are covered in other chapters of Volume 11 (Chapters 11.06 and 11.07). This part deals only with examples which are in connection with other sections of this chapter. Additions of metallocarbenoids to unsaturated partners have been extensively studied. Most of the initial studies have involved the transition metal-catalyzed decomposition of cr-carbonyl diazo compounds.163,164 Three main reaction modes of metallocarbenoids derived from a-carbonyl diazo precursor are (i) addition to an unsaturated C-C bond (olefin or alkyne), (ii) C-H insertion, and (iii) formation of an ylid (carbonyl or onium).1 5 These reactions have been applied to the total synthesis of natural... 

Over the past decade, a reliable, high-yielding method has been developed for the preparation of carbonyl ylide intermediates, namely, the addition of stabilized metallocarbenoid species to carbonyl compounds. 

Carbonyl ylides can be viewed as an adduct between a carbonyl group and a carbene and, in fact, some ylides have been prepared this way (see above). The application of carbonyl ylides to the synthesis of complex natural products has been greatly advanced by the finding that stabilized carbenoids can be generated by the decomposition of ot-diazocarbonyl compounds with copper and rhodium complexes. The metallocarbenoids formed by this method are highly electrophilic on carbon and readily add nucleophiles such as the oxygen of many carbonyl derivatives to form carbonyl ylides. This type of reaction is in fact quite old with the first report being the addition of diazomalonate and benzaldehyde (33,34). 

Landgrebe and co-workers (35) studied the reaction of ketones with metallocarbenoids and observed the generation of enol ethers via an intramolecular rearrangement (Scheme 4.24). 

Decomposition of diazoketone 110 with rhodium acetate produced the highly electrophilic rhodium stabilized metallocarbenoid that suffers attack by the Lewis basic oxygen of the pendant ketone, producing cyclic carbonyl ylide 111. This ylide was trapped by the addition of an activated acetylene such as DMAD to furnish... 

Structurally complex substrates can be realized when an alkyne is inserted between the metallocarbenoid and the carbonyl trap (Scheme 4.27). 

Padwa et al. (42,43) investigated the cyclization of diazoalkynyl ketones such as 116 and found that upon exposure to rhodium acetate, the transient metallocarbenoid... 

SYNTHESIS USING NONMETALLOCARBENOID PRECURSORS AND NOVEL METALLOCARBENOID PROCESSES... 

---