Diazoalkanes reactions

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

C=C double bonds which are neither strained nor activated by an EWG or EDG react slowly with azides. The reaction may take weeks or even months. The use of high temperatures is restricted as 1,2,3-triazolines are thermally unstable. In such cases, the imine-diazoalkane reaction is a suitable alternative. This method is a versatile route for the preparation of 1,5-substituted triazolines, especially 1,5-diaryltriazolines (848), from the reaction of a SchifTs base and diazomethane under mild conditions <93JHCl 191>. A wide range of substituents can be incorporated from readily available imines. 

Isatin and N-substituted isatins react with diazoalkanes to give a variety of products. Simple adducts of the type 181 have been obtained when N-substituted isatins react with methanol-free diazomethane and are reported to lose nitrogen and react further in methanol.505 Such adducts have also been reported in other cases.196,506-508 Ring enlargement to quinolones (182 R is the R in RCHN2 and R is usually OH but sometimes OMe from diazomethane) frequently occurs.123,196,506-509 Finally, epoxides of the type 183 have sometimes been obtained from the diazoalkane reaction.123,249,507,510 With diphenyldiazomethane, ring expansion does not take place and 183 (R = Ph) is the only... 

There are two main approaches to the synthesis of A2-l,2,3-triazolines these involve the union of the C—C and N—N—N fragments as in the olefin-azide additions or alternately building from the C—N and C—N—N moieties as achieved through the imine-diazoalkane reactions (Scheme 2). The choice is determined by the type of substitution desired on the triazoline ring. Imines in general can be more easily prepared than the corresponding olefins. 

Although several early reviews provide brief discussions on olefin-azide additions,6-9 the greater part of the work on imine-diazoalkane reactions is more recent and not covered previously.10,11 The major methods of triazoline synthesis are outlined in a recent treatise on 1,2,3-triazoles.13... 

Although the olefin-azide and the imine-diazoalkane reactions constitute the two major general routes to triazoline synthesis, there are several other interesting pathways that have not been explored sufficiently and hence have only limited application. 

Finally, epoxides of the type 183 have sometimes been obtained from the diazoalkane reaction.With diphenyldiazomethane, ring expansion does not take place and 183 (R = Ph) is the only... 

The first step in the decomposition of nitrosoamides 123) is formation of the diazo ester 125) which fragments to a diazonium ion pair (128)129 The ion pairs thus produced differ from those obtained in the reaction of diazoalkanes with acids. The ratio of ester to ether formed in the decomposition of rV-nitroso-fV-benzhydrylbenz-amides in alcohol is lower than that found in the reaction of diphenyldiazomethane 132) with acids, and in the solvolysis of benzhydryl benzoate (I35)135,136 This effect has been attributed to the intervention of trans-diazo ester in the decomposition of 125) which leads to a greater distance between carbocation and carbox-ylate anion. In the diazoalkane reaction attack of the acid occurs at the electron-rich carbon atom to generate the carboxylate in the immediate vicinity of the incipient carbocation. 

The vast majority of diazoalkane reactions are based on the ambident nucleophilic character of diazoalkanes, as shown by the mesomeric structures of diazomethane 4.32 a and 4.32 b. Bronsted and Lewis acids can be added at the C- and the N()8)-atoms. The reaction with Bronsted acids is particularly important. Alkanediazonium ions are obtained by proton addition at the C-atom giving rise to dediazoniation and various reactions of carbocations (see Chapt. 7). 

Two recent investigations of the groups of Olah and Adam led to impressive examples of diazoalkane reactions with unconventional electrophilic reagents. 

The sites of alkylation are the oxygen atoms of the hydroxyl, carbonyl, and carboxyl groups in lignin. Reactions a and b are selective with diazoalkanes (a) reacting under anhydrous conditions to alkylate, chiefly, the slightly acidic hydroxyl groups (98) of the phenolic, enolic(99-102), and carboxylic (103-107) units to form ethers. The diazoalkane reaction with carboxylic acids only occurs in solvents in which the acid is deprotonated to an enolate anion. The RN reactions are shown in Equation 2. 

Irradiation of diazoketones (reactions 8.7 and 8.8) [17-19], diazoesters (reaction 8.9) [20], and various other diazoalkanes (reactions 8.10 and 8.11) [21,22] as well as diazirines (reactions 8.12 and 8.13) [23,24] under matrix isolation conditions also led to the detection of the corresponding carbene intermediate, which could be trapped with carbon monoxide in the form of ketenes. For example. 

The dediazotation reaction of various diazoalkanes (reaction 8.34) was successfully applied in the preparation of stable dinuclear cobalt carbene complexes (16) containing Tj -cydopentadienyl ligands [66-71]. 

A number of thermal cycloadditions have been reported. The thiocarbonyl group may behave as a dienophile with butadiene or as a 1,3-dipolarophiIe with benzonitrile oxide and, especially, with diazoalkanes. Reaction of... 

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