Pyrazolin-decomposition

Diazoalkane Method of pyrazoline decomposition Products and yields... 

The stereochemical outcome of the pyrazoline decomposition can be varied according to the substrate and the reaction conditions. Although retention predominates, inversion... 

From Pyrazolines.—Investigations of the mechanisms of pyrazoline decomposition continue. A mixture of cis- and trans-3,5-divinyl-l-pyrazoline was synthesized by cycloaddition of vinyldiazomethane to butadiene. Kinetics of the thermolysis in diphenyl ether at 35—65 °C, studied by measuring the rate of nitrogen evolution, gave data which, when compared with those for 1-pyrazoline and 3-vinyl-l-pyrazoline, led to the conclusion that both C—N bonds are broken in the rate-determining step. Thus, substituted 1-pyrazolines decompose concertedly, whereas the parent cyclic azoalkane does not. ... 

The full details of oxa-bis- and oxa-tris-a/ic-homobenzene syntheses, cw-aza-bis-and cis-aza-tris-a-homobenzene and cis-aza-tris-a-homotropilidene synthesis, and the decomposition of spiro[fluorene-9,3 -indazole] have become available, and there have been a number of routine applications of pyrazoline decomposition for cyclopropane synthesis. ... 

Owing to their particular interest two individual reactions will now be discussed separately. The reaction of methoxycarbonylhydrazine and 3-bromo-2,4-pentanedione affords, in addition to the expected pyrazole (608), a pyrazolium salt (609), the structure of which was established by X-ray crystallography (74TL1987). Aryldiazonium salts have been used instead of arylhydrazines in the synthesis of pyrazolines (610) and pyrazoles (611) (82JOC81). These compounds are formed by free radical decomposition of diazonium salts by titanium(n) chloride in the presence of a,/3-ethylenic ketones. 

Pyrolysis at 190° of the resulting diastereomeric A -pyrazolines (8) and (11) leads to elimination of nitrogen and formation of the cis- and tmns-cydo-propanecarboxylates (9) and (12), respectively. Thermal decomposition of the A -pyrazoline (13) affords methyl tiglate (14) in addition to the cyclopropane derivative (15) in a ratio 2 1, while A -pyrazolines such as (3) give only 0L,[i- or, y-unsaturated esters, and no cyclopropane derivatives. 

The preparation of cyclopropane derivatives has been greatly facilitated by the development of carbene-type intermediates (see Chapter 13) and their ready reaction with olefins. The preparation of phenylcyclopropane from styrene and the methylene iodide-zinc reagent proceeds in only modest yield, however, and the classical preparation of cyclopropane derivatives by the decomposition of pyrazolines (first employed by Buchner in 1890) is therefore presented in the procedure as a convenient alternative. 

As it is known from experience that the metal carbenes operating in most catalyzed reactions of diazo compounds are electrophilic species, it comes as no surprise that only a few examples of efficient catalyzed cyclopropanation of electron-poor alkeiies exist. One of those examples is the copper-catalyzed cyclopropanation of methyl vinyl ketone with ethyl diazoacetate 140), contrasting with the 2-pyrazoline formation in the purely thermal reaction (for failures to obtain cyclopropanes by copper-catalyzed decomposition of diazoesters, see Table VIII in Ref. 6). 

Based on a detailed investigation, it was concluded that the exceptional ability of the molybdenum compounds to promote cyclopropanation of electron-poor alkenes is not caused by intermediate nucleophilic metal carbenes, as one might assume at first glance. Rather, they seem to interfere with the reaction sequence of the uncatalyzed formation of 2-pyrazolines (Scheme 18) by preventing the 1-pyrazoline - 2-pyrazoline tautomerization from occurring. Thereby, the 1-pyrazoline has the opportunity to decompose purely thermally to cyclopropanes and formal vinylic C—H insertion products. This assumption is supported by the following facts a) Neither Mo(CO)6 nor Mo2(OAc)4 influence the rate of [3 + 2] cycloaddition of the diazocarbonyl compound to the alkene. b) Decomposition of ethyl diazoacetate is only weakly accelerated by the molybdenum compounds, c) The latter do not affect the decomposition rate of and product distribution from independently synthesized, representative 1-pyrazolines, and 2-pyrazolines are not at all decomposed in their presence at the given reaction temperature. 

In a study aimed at elucidating the mechanism of the thermal decomposition of spiropentane 229, the two regioisomeric pyrazolines 227 and 228 were obtained in high yield by allowing a solution of MCP (1) and diazomethane (226) (or diazomethane-d2) in diethylether to stand at 3 °C for three weeks (Scheme 37) [59]. 

Reduction of unsaturated aromatic aldehydes to unsaturated hydrocarbons poses a serious problem, especially if the double bond is conjugated with the benzene ring or the carbonyl or both. In Clemmensen reduction the a,)8-unsaturated double bond is usually reduced [160], and in Wolff-Kizhner reduction a cyclopropane derivative may be formed as a result of decomposition of pyrazolines formed by intramolecular addition of the intermediate hydrazones across the double bonds [280]. The only way of converting unsaturated aromatic aldehydes to unsaturated hydrocarbons is the reaction of... 

Thermal or photochemical extrusion of nitrogen from frawi-pyrazolines, for example, 482 gives frani-cyclopropylamino acid derivatives 483. " Photochemical decomposition of c/i-pyrazolines 476 leads to c -cyclopropylamino acid derivatives 477 642-644 thermal decomposition of 476 affords the corresponding... 

Pyrazoline 68 is converted into the V-acetyl derivative 69 by treatment with acetic anhydride and triethylamine at —5 °C (Scheme 5). Treatment of 68 with acetic acid at 40 °C caused decomposition of the dihydrotriazole ring to give the enamine 71 <1997TL5891>. Treatment with trifluoroacetic acid in dichloromethane at room temperature, however, caused decomposition of both the dihydrotriazole and the oxazolidine rings yielding the pyroglutaminol 70 <2001J(P1)2997>. 

Dihydrobenzoxocinone (149) is produced by decomposition of the pyrazoline (148), from the reaction of 3-acetylcoumarin with excess diazoethane (74JCS(P1)66). The boat-like conformation (150) is assigned to the eight-membered ring. 

Lack of stereospecificity, extensive formation of olefinic products, and extensive tar formation limit the thermal decomposition of pyrazolines as a route to cyclopropanes.182 263 Light-induced decomposition of stereoisomeric pyrazolines establishes a method for the formation of cyclopropanes stereospecifically.222 Photolysis of 3-carbomethoxy-cis-3,4-dimethyl-l-pyrazoline (CCLI) produced cis-l,2-dimethylcycIopropane-l-carboxylate (CCLII) and without olefinic formation. Furthermore, irradiation of 3-carbomethoxy-trans-3,4-dimethyl-l-pyrazoline (CCLIII) gave [Pg.123]

Pyrazolines, in general, undergo a photochemically induced ring contraction in solution to form a cyclopropane derivative and nitrogen. This process, unlike some equivalent thermal decompositions, is stereospecific, and methyI-as-3,4-dimethyl-l-pyrazoline-3-carboxyl-ate (88) is converted in high yield into methyl-as-1,2-dimethylcyclo-propane carboxylate (89).69 This route is of considerable preparative... 

Diazomethane and its simple analogs undergo cycloaddition to unsaturated compounds both directly and after conversion to carbenes. The direct cycloadditions are 1,3-dipolar for the most part and provide access to pyrazolines and pyrazoles. Intramolecular cyclizations were recognized as early as 1965 95 The two main methods used in generation of diazo compounds for subsequent intramolecular cycloaddition include thermolysis of tosylhydrazone salts and thermolysis of iminoaziridines. Decomposition of nitros-amines has also been employed. 

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

In the case of alkyl substituted pyrazolines such as (307), thermal decomposition at 40-70 °C or photolysis leads to complex products which could be explained either in terms of a diradical intermediate or of rearrangement to a diazo-compound followed by loss of nitrogen to produce a carbene. However, on brief heating to 80 °C or on... 

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