Dinitrogen reaction with carbenes

In a similar way, 1,3-dinitrogen systems such as diamines, amidines, guanidines, aminothiazoles, aminopyridines, ureas and thioureas react with alkynyl-carbene complexes generating the corresponding heterocycles. Of particular interest is the reaction with ureas, as the process can be applied to the easy synthesis of pyrimidine derivatives [88] (Scheme 41). 

Finally, a racemic cyclopropanation process has also been developed that utilizes an iron Lewis acid catalyst (72) that presumably proceeds through an iron carbene intermediate (73) (Scheme 28). The catalyst is activated by reaction with diazo compounds to produce an intermediate (74) that loses dinitrogen see Dinitrogen Dinitrogen Complexes) to afford the cyclopropane. This chemistry has been extended to the production of epoxides and aziridines and has recently been reviewed. ... 

We proposed two possible reaction pathways. Under the heat, 6-31 decomposes and releases dinitrogen, acetone, and carbene 6-32. In Pathway A, nucleophilic addition of in situ generates 6-32 to C=0 bond generated zwitterionic intermediate 6-36. Migration of more substituted and electron-rich a-carbon leads to ring expansion product 6-33. In Pathway B, [2-1- 1] cycloaddition of 6-32 with C=0 bond affords spiro compounds 6-37. Homolysis of C-O bond gives diradical... 

In the photolysis of difiuorodiazirine (218) a singlet carbene was also observed (65JA758). Reactions of the difiuorocarbene were especially studied with partners which are too reactive to be used in the presence of conventional carbene precursors, such as molecular chlorine and iodine, dinitrogen tetroxide, nitryl chloride, carboxylic acids and sulfonic acids. Thus chlorine, trifiuoroacetic acid and trifiuoromethanesulfonic acid reacted with difiuorodiazirine under the conditions of its photolysis to form compounds (237)-(239) (64JHC233). 

Working with diazo compounds, known since the early 1900s to undergo loss of dinitrogen when treated with copper or copper salts, Yates described in 1952 the possibility that transition metals could form an intermediate that combined units of the diazo compound and the metal (Eq. 1, L = ligand) and acted like a carbene in addition and insertion reactions. Somewhat later, but independently, E. O. Fischer isolated and characterized stable metal carbenes that could also undergo cyclopro-panation reactions." They were derived from transition metals on the left side of the... 

By 1960, there was recognition that copper salts could cause the loss of dinitrogen from diazocarbonyl compounds with addition of the resulting carbene intermediate to a carbon-carbon double bond to form a cyclopropane product. That this reaction, first reported by G. Stork in 1961 (Eq. 6), could occur in an intramolecular fashion and thus avoid the formation of isomers, ushered in the first significant synthetic... 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

Chiral Rh(II) oxazolidinones Rh2(BNOX)4 and Rh2(IPOX)4 (25a,b) were not as effective as Rh2(MEPY)4 for enantioselective intramolecular cyclopropanation, even though the steric bulk of their chiral ligand attachments (COOMe versus i-Pr or CH2Ph) are similar. Significantly lower yields and lower enantioselectivides resulted from dinitrogen extrusion from prenyl diazoacetate catalyzed by either Rh2(4.S -lPOX)4 or Rh2(4S-BNOX)4. This difference, and those associated with butenolide formation [91], can be attributed to the ability of the carboxylate substituents to stabilize the carbocation form of the intermediate metal carbene (3b), thus limiting the Rh2(MEPY)4-catalyzed reaction to concerted carbene addition onto both carbon atoms of the C-C double bond. 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

At an early date, Moore and Pimentel (1964 d) carried out photolyses of diazirine in a solid matrix of N2. They detected diazomethane, but concluded from results using a matrix that diazomethane was derived from the reaction of primarily formed methylene with dinitrogen (for later work on reactions of other carbenes with N2 see Sect. 8.1). 

The metallocarbene intermediates are most often formed from thermal, photolytic, or metal-catalyzed deconposition of diazocarbonyl compounds, with concomitant loss of dinitrogen. Under transition metal catalysis, the initially formed species is a metallocarbene rather than a free carbene, and this is usually desirable due to the moderated reactivity (and, hence, fewer undesired side reactions) of the metal-complexed carbene. The two most common methods for introduction of the diazo group are acylation of diazoalkanes with suitably activated carboxylic acid derivatives and diazo transfer reactions in the case of more acidic active methylene substrates fScheme 16.12T... 

Notably, the diaryl diazomethane here showed the reactivity of a nitrene, rather than a carbene, forming two C-N bonds in one cycloaddition reaction [45]. This is probably because the a-C in diaryl diazomethane is less nucleophilic and steric-hindered, while the y-N behaves as nucleophile and initiates the ring opening of NSBV 6-la [46 8]. Although [3 + 2] cycloaddition of diazo compounds with other unsaturated compounds have been reported with the preservation of the dinitrogen moiety, few reports show nitrene reactivity with formation of two C-N bonds [49-51] (Scheme 6.7). 

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