Catalysts with acceptor-substituted diazomethanes

Fig. 4.20. Complexes for asymmetric cyclopropanation with acceptor-substituted diazomethanes. 1 [1372], 2 [1373], 3 [1033], Rh2(55-MEPY>4, Rh2(55-MPPIM)4 [1001,1074], For related rhodium-based catalysts, see, e.g., [997,1000,1002].

The most frequently used ylides for carbene-complex generation are acceptor-substituted diazomethanes. As already mentioned in Section 3.1.3.1, non-acceptor-substituted diazoalkanes are strong C-nucleophiles, easy to convert into carbene complexes with a broad variety of transition metal complexes. Acceptor-substituted diazomethanes are, however, less nucleophilic (and more stable) than non-acceptor-substituted diazoalkanes, and require catalysts of higher electrophilicity to be efficiently decomposed. Not surprisingly, the very stable bis-acceptor-substituted diazomethanes can be converted into carbene complexes only with strongly electrophilic catalysts. This order of reactivity towards electrophilic transition metal complexes correlates with the reactivity of diazoalkanes towards other electrophiles, such as Brpnsted acids or acyl halides. 

The conversion of acceptor-substituted diazomethanes into carbene complexes often does not proceed smoothly in the presence of sulfides. This might be because of poisoning of the catalyst by the sulfide [975]. Higher yields of sulfonium ylides can, however, be obtained by HBF4-mediated reaction of acceptor-substituted diazomethanes with sulfides [1011,1328,1330,1331]. 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

According to Scheme 1 methyl 2-siloxycyclopropanecarboxylates should also be available from donor-acceptor-substituted olefins like 100, which are easily synthesized by silylation of the corresponding 1,3-dicarbonyl compounds. Cyclopropanation of 100 with methyl diazoacetate or diazomethane could be realized in the presence of Cu(II)-catalysts, but due to the relatively low reactivity of the olefins a large excess of diazoalkanes had to be employed. This makes the isolation of 101 troublesome and therefore direct hydrolysis with acid to give 1,4-dicarbonyl compounds 102 is advantageous (Eq. 32) 66). 

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See also in sourсe #XX -- [ , , , ]

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