Pyrroles reactions with ketocarbenoids

Reaction of ketocarbenoids with pyrrole and N-alkylpyrroles yields the product of formal insertion into the a-C—H bond (256) in many cases the -insertion product 257 is formed concomitantly, but generally in lower yield 238-241 >. The regioselectivity varies according to the catalyst, the diazo compound and the N-alkyl substituent. Some examples concerning the former two variables are given in Table 18 239 240). 

The reaction of ketocarbenoids with pyrroles leads to either substitution or cyclopropanation products, depending on the functionality on nitrogen. With N-acylated pyrrole (209) reaction of ethyl diazoacetate in the presence of copper(I) bromide generated the 2-azabicyclo[3.1.0]hex-3-ene system (210) and some of the diadduct (211 Scheme 44).162163 On attempted distillation of (210) in the presence of copper(I) bromide rearrangement to the 2-pyrrolylacetate (212) occurred, which was considered to proceed through the dipolar intermediate (213). In contrast, on flash vacuum pyrolysis (210) was transformed to the dihydropyridine (214). A plausible mechanism for the formation of (214) involved rearrangement of (210) to the acyclic imine (215), which then underwent a 6ir-electrocyclization. 

In an intramolecular version of ketocarbenoid a-C/H insertion, copper-promoted decomposition of l-diazo-3-(pyrrol-l-yl)-2-propanone (258a) or l-diazo-4-(pyrrol-l-yl)-2-butanone (258b) resulted in quantitative formation of the respective cycli-zation product 259 242 >. The cyclization 260 -> 261, on the other hand, is a low-yield reaction which is accompanied by olefin formation. The product ratio was found to vary with the copper catalyst used, but the total yield never exceeded 35 % 243>. 

Whereas pyrrole was reported not to give N/H insertion by ketocarbenoids, such a reaction mode does occur with imidazole Copper-catalyzed decomposition of ethyl diazoacetate at 80 °C in the presence of imidazole gives ethyl imidazol- 1-ylacetate exclusively (93 %) small amounts of a C-alkylated imidazole were obtained additionally under purely thermal conditions 244). N/H insertion also takes place at benzimidazole 245 a). The reaction is thought to begin with formation of an N3-ylide, followed by N1 - C proton transfer leading to the formal N/H insertion product. Diazomalonic raters behave analogously however, they suffer complete or partial dealkoxycarbonylation under the reaction conditions 244) (Scheme 34). N-alkylation of imidazole and benzimidazole by the carbenoids derived from co-diazoacetophenone and 2-(diazoacetyl)naphthalene has also been reported 245 b>. 

---

See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

---