Higher diazoalkanes

The use of excess diazoalkane in its reaction with sulfur dioxide will necessarily lead to symmetrically substituted thiirane dioxides. When monoalkyl or monoaryl diazoalkanes are used, mixtures of cis- and trans-isomers are formed The cis/trans ratio of the products varies significantly with the polarity of the reaction medium the higher the polarity of the solvent, the lower is the yield of the cis-product. 

The higher diazoalkanes can be made by Pb(02CCH3)2 oxidation of hydrazones.129... 

For the formation of stilbenes from aryldiazomethanes, Rh2(OAc)4 was shown to be superior to other catalysts such as CufClO or CuBr2 357), LiBr363 or Ce(NH4)2(N03)6 364) in terms of efficiency, Z-selectivity and compatibility with substituents on the aromatic ring of the diazoalkane 358 . Even higher Z-selectivity was provided by the bulky catalyst iodorhodium(III) mew-tetraphenylporphyrin, but reduced yields had to be acknowledged358 . Contrary to copper catalysts, RhjfOAc failed to induce the formation of carbene dimers from secondary aryldiazoalkanes azines were produced instead 358). 

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. 

Esterification with higher diazoalkanes [205] has also been suggested. A 2-ml volume of 50% potassium hydroxide solution was added to 5 ml of diethyl ether in a small flask. N-n-Butyl(or propyl)-N-nitrosoguanidine (1 g) was suspended in diethyl ether and added to the flask through a separating funnel. The reaction was carried out in a water-bath at 45°C. The diazo compound have been passed through a cooler, was trapped in diethyl ether. Hydrochlorides of amino acids (about 5 mg) were dissolved in 4 ml of methanol... 

The reaction is usually carried out by reacting both components (diazoalkane and olefin) in an etheral solution for several days. The reaction time must be prolonged when highly substituted olefins are used and, in these cases, the yields are not higher than 75% in general . 

The synthetic methods outlined above for diazomethane woik well, in general, for the higher diazoalkanes. Again, these are usually generated and handled as solutions in diethyl ether. In the special case of the conversion of the amine (98) to diazoalkane (100), the urethane approach proved most effective (equation 41). ... 

Reaction conditions for the acylation of higher diazoalkanes by acid chlorides are more critical. Since azo coupling of the diazo ketone with the diazoalkane can occur to give azines (equation 5), it is essential to avoid an excess of the diazoalkane and to carry out the acylation at lower temperatures. Typically, the acid chloride (1 equiv.) in ether is added to a stirred solution of the diazoalkane (1 equiv.) and triethylamine (1 equiv.) in ether at -40 C. Yields of the diazoketones are then ca. 74-91 %, and equation (6) is fairly representative. ... 

Allyl sulfides and allyl amines. Rhodium-catalyzed decomposition of ethyl diazoacetate in the presence of these allyl compounds generates products 136 and 137, respectively, derived from [2,3] rearrangement of an S- or N-ylide intermediate, besides small amounts of carbene dimers No cyclopropane and no product resulting from the ylide by [1,2] rearrangement were detected. Besides RhjfOAc) and Rhg(CO)i6, the rhodium(I) catalysts [(cod)RhCl]2 and [(CO)2RhCl]2 were found to behave similarly, but yields with the only allyl amine tested, CH =CH—CH NMe, were distinctly lower with the latter two catalysts. Reaction temperatures are higher than usually needed in rhodium-promoted diazoalkane decomposition, which is certainly due to competition between the diazo compound and the allylic hetero-... 

Methods have been developed for obtaining other non-methyl esters. In addition to simpler techniques these methods were aimed at increasing the sensitivity and volatility and, sometimes, decreasing the volatility (for lower acids with high volatihty) and improving the separation and selectivity of determination. Other workers [174—176] have described the application of diazoethane, diazopropane, diazobutane and diazotoluene to obtain esters. Solutions of higher diazoalkanes are more stable and less explosive than diazomethane solutions. 

Aliphatic diazo compounds are all poisonous and colored — colors range from yellow or purplish-red. The lower diazoalkanes are gaseous and explosive, higher members are liquid to solid and much more stable. Comparative studies of the rate of decomposition by acid... 

The Bamford-Stevens reaction is frequently used not for the isolation of aliphatic diazo compounds, but for the synthesis of the products obtained by dediazoniation either by the carbene or the carbocation mechanisms in (2-38). As 4-toluenesulfonyl-hydrazones are fairly stable under Bamford-Stevens conditions, the reactions must be carried out at higher temperature, which is, of course, a disadvantage when it is desired to stop the reaction at the diazoalkane stage. Yields above 90% can, however, be obtained in some cases, particularly for the synthesis of 9-diazofluorene (Dudman and Reese, 1982). 

The greater weight of the mesomeric structure 5.13 c in a-diazo ketones causes a shift of the NN stretching frequency to higher values by ca. 30-40 cm relative to diazoalkanes. 2-Diazo-l,3-diones show even larger shifts (see examples given by Regitz and Maas, 1986, in Table 1.8). The partial double-bond character of the C(0) — C(N) bond, as visualized by the mesomeric structure 5.13 c, is the reason why it is feasible that stable (Z)- and ( )-isomers 5.21 and 5.22 exist. 

If the dipolarophile is assymmetric (d e in d=e. Sect. 6.2, Scheme 6-5), there are two alternatives for the diazoalkane and all other unsymmetrical 1,3-dipoles in their cycloadditions with that dipolarophile. For example, diazomethane and an ethene derivative with an alkyl substituent R may yield the 3- or the 4-alkyl-l-pyrazolines (6-19), or a mixture of both. These primary products rearrange at higher temperature or in the presence of base to give the corresponding alkyl-2-pyrazolines. 

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