Diazoalkene

JA173) illustrates this possibility (Scheme 34). Thus 3,3,5-trimethyl-3//-pyrazole (371 R = H) on irradiation in pentane solution gives 1,3,3-trimethylcyclopropene (372 R = H) the intermediate diazoalkene (373) has been characterized. The tetramethyl derivative (371 R" = Me) when irradiated at -50 °C in methylene chloride leads to a species believed to be a l,2-dlazablcyclo[2.1.0]pent-2-ene (374). This isomerization is thermally reversible, the 3H- pyrazole being regenerated at room temperature. 

Electrocyclization of certain a,/ y,<5-unsaturated diazo compounds, in which one of the double bonds is embedded in an aromatic ring, affords benzodiazepines. The diazoalkenes 1, produced by heating the sodium salts of the corresponding tosylhydrazoncs, can undergo two kinds of cyclization, [1,5] to yield 3//-pyrazoles, and [1,7] to give diazepines. 

Thermolysis of the l-(2-thienyl)-3-diazoalkenes 1 gives 3//-thieno[3,2-f]-l,2-diazepines 2 by sequential 1,7-dipolar electrocyclization and [l,5]-hydrogen shifts, together with 37/-pyrazoles 3, which are produced by a competing 1,5-cyclization.140... 

Heating the l-(3-thienyl)-3-diazoalkenes 5 gives 3//-thieno[2,3-f]-l,2-diazepines 4. In the case of 5b, the pyrazole 6b is also formed.140... 

In this section we first discuss photolytic reactions of arenediazonium salts and report on quinone diazides at the end of the section in the context of imaging technology. Diazoalkenes, non-quinonoid diazo ketones, and the Wolff rearrangement are treated in the book on aliphatic diazo compounds (Zollinger, 1995, Chap. 8). 

Cycloadditions of the 1,3-dipolar nitrile oxides and diazoalkenes to acyclic vinyl sulfones are in general highly selective, the particular regioisomer formed depending on the substituents of both reactants213,214. Nitrones, on the other hand, tend to yield mixtures of the two possible isomers (see equation 78). 

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. 

Vinylcarbenes. The use of diazoalkenes as vinylcarbene precursors is often precluded by rapid cyclization, with formation of pyrazoles. However, on photochemical generation of the diazoalkenes in situ, cyclization and nitrogen extrusion can proceed competitively. Photolysis of 1,3-diphenylpropenone to-sylhydrazone sodium salt (5) in MeOD afforded 3,5-diphenylpyrazole (9) and l,3-diphenyl-3-methoxypropene (10) in similar amounts.17 If 10 is formed by way of the 1,3-diphenylallyl cation (8), the deuterium should be distributed between C-l and C-3 of 10 (Scheme 6). The observed ratio of 10a to 10b was 66 34 the methoxy group is bound preferentially to the deuterated site, which originates from the divalent carbon of 7 (for a discussion of this effect, see below). 

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. 

As shown by the scheme on p. 118, the triplet diazoalkene 44 can lose N2 to give the triplet carbene 45, which can now add to the olefin (b) in a non-stereo-specific manner. Since the triplet state of the diazoalkane 44 has a longer lifetime, it can also add to the olefin directly, producing the biradical 47 which after ex-... 

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... 

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. 

As shown in Scheme 8.67, the cyclization of diazoalkenes 273 requires thermal activation and not only affords 3/7-pyrazoles 274, but also cyclopropenes 275 that are formed from carbene intermediates (319). The activation parameters for cyclopropene formation (i.e., N2 elimination from 273) have been determined (320). A novel example involves the cyclization of the 3-nitro-l-diazoprop-2-ene derivative 276 into pyrazolopyridine derivative 277 (45). 

Another drawback of the TEA (both in the GC and HPLC modes) is that A-nitrosamides give extremely poor yields ( 1%) of NO upon pyrolysis. When heated, these compounds, instead of splitting into NO and the parent amides, rearrange to yield diazoalkenes and nitrogen. Recently reported modifications of the TEA have somewhat improved this deficiency in the GC mode, but the corresponding improvements in the HPLC mode are yet to be worked out (78). 

The extension of this reaction to include the conversion of 3H-pyrazoles into cyclopropenes is established.86 3,3,5-Trimethyl-3/i-pyrazole (100 R = H), for example, on photolysis in pentane solution gives 1,3,3-trimethylcyclopropene (101), and an intermediate diazo-alkene (102) has been characterized. The proposed conversion of the diazoalkene into the cyclopropene (101) via a vinylcarbene has a... 

Diazoalkenes always have the intervening chain attached on carbon. The dipole HOMO-dipolarophile LUMO interaction is usually dominant in intermolecular cycloadditions. Consequently, nonactivated al-kenes which have a relatively high-energy LUMO react poorly. Nevertheless, intramolecular cycloaddi-... 

Diazoalkenes have been known to cyclize to pyrazoles since 1935. This reaction has been rationalized as another variation of intramolecular 1,3-dipolar cycloaddition.103 The 3-diazoalkenes, derived from nitrosamines (190a-b), gave typical results cyclization afforded pyrazoles in 89% and 87% yield, respectively (Scheme 60). 

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