Diazoalkenes, reaction with

Thiete sulfones show an irregular behavior pattern when involved in cycloaddition reactions. With 1,3-dienes, dienamines, enamines, ynamines, diazoalkenes, cyclopropadiene, and its substitution products, furan, and anthracene, the addition proceeds in the normal fashion. With certain Diels-Alder reagents such as tetraphenylcyclopentadienone (tetracycloneX however, the cyclic sulfones react anomalously. The Diels-Alder adducts undergo decomposition with SO 2 and CO extrusion to a seven-membered ring, the tetraphenylcycloheptatriene 223. Bicyclic octadienone is produced as well (Eq. 62). The mechanism of this unusual reaction is proposed by... 

A general approach to the problem of distinguishing free carbenes from carbenoids is to compare product distribution and stereochemistry in reactions involving several carbene sources, including unequivocal methods such as diazoalkene decomposition. By such comparisons, it has been concluded, for example, that the intermediate generated from a,a-dibromotoluene and butyl-lithium is not the free carbene but, instead, a carbenoid reagent in which the incipient carbene remains attached to the elements of LiBr. LiBr is eliminated only when reaction with the substrate occurs. 

Carbenes from Diazo Compounds. Decomposition of diazo compounds to form carbenes is a quite general reaction that is applicable to diazomethane and other diazoalkanes, diazoalkenes, and diazo compounds with aryl and acyl substituents. The main restrictions on this method are the limitations on synthesis and limited stability of the diazo compounds. The smaller diazoalkanes are toxic and potentially explosive, and they are usually prepared immediately before use. The most general synthetic routes involve base-catalyzed decomposition of V-nitroso derivatives of amides, ureas, or sulfonamides, as illustrated by several reactions used for the preparation of diazomethane. 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

CHEC(1984) <1984CHEC(2)1> and CHEC-II(1996) <1996CHEC-II(6)1> covered examples of the cycloaddition reaction of diazoalkenes with alkenes yielding 2,3,4,5-tetrahydropyridazines. The diazadiene is normally generated in situ from an a-halogenated hydrazone with the assistance of base. 

Kerth and Maas have reported reactions of 2-acyl-l,2,3-diazaphospholes 73 with diazo ketones 71 to form bicyclic compounds 74, the products of a 1,3-dipolar cycloaddition reaction of diazoalkenes 72, which are in equilibrium with diazo ketones 71 (Scheme 3) <1999EJ02633>. 

The formal addition of carbenoids to the thiocarbonyl -bond [C + C=S] represents another major route used for the preparation of thiiranes. The reaction of thiocarbonyl compounds with diazoalkenes generally gives good yields of thiiranes (Scheme 11). The mechanism may involve an addition of a carbene across the thiocarbonyl group, especially in the presence of Rh(II) acetate, CuCl, CuS04, or other metal catalysts, but involvement of a zwitterion is also possible . 

The reaction is somewhat selective for the cis-diastereomer. The use of chiral additives in this reaction leads to aziridines enantioselectively. " Imines can be formed by the reaction of an aldehyde and an amine, and subsequent treatment with Me3SiI and butyllithium gives an aziridine. " A-Tosyl imines react with diazoalkenes to form A-tosyl aziridines, with good cis-selectivity " and modest enantioselectivity in the presence of a chiral copper catalyst, " but excellent enantioselectivity with a chiral rhodium catalyst. . It is noted that A-tosyl aziridines are formed by the... 

The Wittig-Homer type reaction of aliphatic ketones with dimethyl diazomethylphosphonate gave the diazoalkenes (19), which were trapped by amines thus affording enamines (18) of the next higher aldehydes (Scheme 11). For the preparation of allylic amines, the reader is referred to a comprehensive re-... 

Alternatively, Homer-Wittig reaction of diazomethylphosphonate (11) with carbonyl compounds gives, presumably, the diazoalkenes (12). Replacement of the diazo group in situ by an alkoxy group gives the enol ethers (13 Scheme 3). ... 

This chapter has shown the great control and selectivity of modern radical reactions, especially intramolecular radical coupling reactions. Carbenes, especially those derived from diazoalkenes, have also played a prominent role in organic synthesis. The chemical reactions illustrated in this chapter are excellent additions to the list of nucleophilic, pericyclic, and electrophilic reactions presented previously. This chapter concludes the methodology for generating carbon-carbon bonds. With the functional group reactions in the first part of the book, all the tools for pursuing a synthesis are at hand. 

Cycloaddition of 1,3-dipolarophiles to alkynes for the synthesis of diazo compounds can also be applied to reaction of diazoalkanes with alkynes (2-91). 2-Diazopropane and 1,2-diarylethynes readily form 3//-pyrazoles (2.229). These pyrazoles isomerize photochemically to the 4-diazo-2-methyl-3,4-diarylbutenes (2.230), i.e., to a vinyldiazo compound (Pincock et al., 1973 Arnold et al., 1976 Leigh and Arnold, 1979). Some cyclopropene (2.231) is formed in a consecutive dediazoniation, i. e., by cyclization of the carbene formed. The method is not useful for unsymmetrically substituted alkynes because these cycloadditions are not regiospecific. It is, however, applicable to the synthesis of diazoalkenes with alicyclic... 

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