Ammonium ylide decomposition

Another Rhn-catalyzed decomposition of a a-diazoester as described by Sabe and coworkers [198] was used for the synthesis of indolizidine alkaloids (Scheme 6/2.8). It can be assumed that, first, an ammonium ylide is formed which then undergoes a 1,2-shift with ring-expansion. Thus, reaction of 6/2-40 with Rh2(OAc)4 led to a 72 28 mixture of 6/2-41 and 6/2-42 in 85 % yield. Cu(acac)2 can also be used with even better yields, but lower selectivity (65 35). 

A ring enlargement process was used effectively to access the enantiopure pyrrolo [ 1,4]oxazepine-9a(7//)-carboxylate derivatives 142 and 143. The sequence involved copper (Il)-catalysed decomposition of an a-diazocarbonyl derivative attached to a chiral morpholinone, and a carbenoid, spiro-[5,6]-ammonium ylide, Stevens [1,2] rearrangement sequence. The Stevens and related rearrangements have considerable further potential for novel heterocyclic syntheses <00TA3449>. 

Similar to those of oxygen and sulfur ylide, ammonium ylide or azomethine ylide can be generated by the interaction of metal carbene and amine or imine, respectively. As is the case of sulfur, nitrogen also has a strong coordinating ability to a metal complex. Consequently, metal complex-catalyzed diazo decomposition in the presence of an amine or imine usually requires high reaction temperatures (Figure 6). 

A new synthesis of cr-substituted and a,a-disubstituted a-amino acid derivatives based on the ammonium ylide formation/[2,3]-sigmatropic rearrangement has been recently reported by Clark s group.Decomposition of a-diazo -keto ester 153 was studied in detail with Rh2(OAc)4, Cu(acac)2, and Cu(hfacac)2 as the catalyst. Cu(acac)2 and Cu(hfacac)2 gave similar results, but Rh2(OAc)4 turned out less effective (Equation (23)). 

The cyclic ammonium ylide/[l,2]-shift approach has been successfully applied by West and Naidu to a key step in the total synthesis of (—)-epilupinine, one of the biologically active lupin alkaloids. Cu(acac)2-catalyzed diazo decomposition of enantiomeric pure diazoketone 160 in refluxing toluene generates a spiro ammonium ylide 161 and 162, which then undergoes [l,2]-shift to give rise to a quinolizidine skeleton as a mixture of diastereomers (95 5) (Scheme Major diastereomer 164 has enantiomeric purity of 75% ee. The partial retention of stereo-... 

The Cu(acac)2-catalyzed decomposition of the diazo ketone 55 afforded the ring-expanded 5,7-fused benzazepine 57 via the initial formation of ammonium ylide 56 followed by Stevens rearrangement of the benzylic... 

Sulfonium compounds undergo elimination similar to that of their ammonium counterparts (7-6 and 7-7) in scope and mechanism. The decomposition by heat of sulfonium hydroxides has been known for many years.218 The ylide reaction was discovered more recently.219 Neither is important synthetically. 

The addition compounds (I) are insoluble in diethyl ether, and the slurries obtained are quite stable. In more strongly solvating media, such as tetrahydrofuran or dimethoxyethane, the compounds are soluble but show rapid decomposition, with trimethylamine and polymethylene as the main products. These experiments indicate (9, 40) that when the lithium salt is trapped by donor solvent molecules, the free ylide quickly undergoes decomposition (40). No free trialkylammonium ylide has yet been prepared, even under very mild conditions (35). On the other hand, it has been shown, that the tetramethylammonium cation can even be metalated twice by organolithium reagents (102) to afford dimethyl-ammonium bismethylides ... 

Quaternary ammonium and phosphonium, as well as dialkylimidazolium ions, are typical ions used for preparing moderately hydrophobic ILs. It is known that these ions decompose through Hofmann elimination or ylide formation in strongly alkaline environment, e.g., solid alkyltrimethylammonium hydroxide hydrate, even at room temperature and the decomposition rate increases with elevating temperature [58, 59]. However, these cations are stable in an aqueous alkaline solution [58, 60], and, hence, ELSBs made of these irnis are stable when they are in contact with an aqueous alkaline solutimi, e.g., 0.1 mol dm NaOH. 

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