Diazo Transfer to Alkenes

A similar example with a relatively electrophilic diazoalkane is the diazo transfer (2-66) from ethyl diazonitroacetate (2.164) to 5,5-dimethylcyclohexane-l,3-dione (2.120) described by Schdllkopf et al. (1969). 

As mentioned already in Section 2.6, it is somewhat arbitrary to discuss diazo transfer reactions to alkenes in isolation from those to activated methylene compounds. The most important activation in methylene compounds is that of a neighboring carbonyl group and, as a consequence, the active methylene compound is in equilibrium with the corresponding enol, i.e., with an alkene as established by the systematic work of Huisgen (review Huisgen, 1984), typical diazo transfers involve 1,3-dipolar cycloaddition of a 1,3-dipole (azides) to a multiple-bond system, the dienophile (see Chapt. 6). In diazo transfer, this dienophile is an alkene or an alkyne, and the primary product is a A -l,2,3-triazoline or a A -l,2,3-triazole, 

2 Methods for the Preparation of Alkane, Alkene, and Alkyne Diazo Compounds 

In the subsequent part of the diazo transfer one or two bonds of these five-membered heterocycles is (are) cleaved, leading to diazo compounds. 

Some substituents may be charged (e.g., 0 ), but, for convenience, charge is not indicated in the scheme. Competitive and consecutive reactions, which may be dominant in many cases (e. g., dediazoniations), are also not included. The scheme is not intended for mechanistic considerations. In particular, the formation of the triazoline may be a one- or a two-step reaction (see Sect. 6.3). 

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Conjugated dienes, styrenes and electron-rich alkenes are cyclopropanated with ethyl diazoacetate using a triarylamminium salt of appropriate oxidation potential as a cata-lyst/initiator (equation 96)185. These reactions are initiated by electron transfer from the unsaturated substrate to the amminium ion and the double additions of the diazo esters to the conjugated dienes are effectively suppressed. Cyclopropanes geminally bearing two... 

Another cyclopropanation procedure that is quite general involves the use of Rh-carbene complexes, which can act catalytically to effect ring formation. Scheme 10.7 shows some of the details of this method. Ccaibene is derived from corresponding diazo compounds, which were traditionally used directly as sources of free carbenes. The scheme includes a catalytic cycle for conversion of the diazo compound to the Rh-carbene complex, which then delivers Ccarbene to the alkene. Transfer of Ccaibene regenerates an active catalyst that can react with another mole of diazo compound. The detailed mechanism of step c in the cycle resembles path b from Scheme 10.6. 

Transfer of a formylcarbene unit from an a-diazo aldehyde to an alkene has never become popular. The copper-catalyzed cyclopropanation with diazoacetaldehyde to give 5 occurred in low yield, since ketene formation is competitive (for experimental procedure, see Houben-Weyl Vol.E19b, pi224). It appears that with other catalysts and at a lower temperature the cyclopropanation could be more effective. 

Photochemical decomposition of diazo(trimethylsilyl)methane (1) in the presence of alkenes has not been thoroughly investigated (see Houben-Weyl Vol. E19b, p 1415). The available experimental data [trimethylsilylcyclopropane (17% yield) and la,2a,3j8-2,3-dimethyl-l-trimethylsilylcyclopropane (23% yield)] indicate that cyclopropanation occurs only in low yield with ethene and ( )-but-2-ene. In both cases the formal carbene dimer is the main product. In reactions with other alkenes, such as 2,3-dimethylbut-2-ene, tetrafluoroethene or hexafluoro-propene, no cyclopropanes could be detected.The transition-metal-catalyzed decomposition of diazo(trimethylsilyl)methane (1) has been applied to the synthesis of many different silicon-substituted cyclopropanes (see Table 3 and Houben-Weyl Vol.E19b, p 1415) 3.20a,b,2i.25 ( iQp. per(I) chloride has been most commonly used for carbene transfer to ethyl-substituted alkenes, cycloalkenes, styrene, and related arylalkenes. For the cyclopropanation of acyl-substituted alkenes, palladium(II) chloride is the catalyst of choice, while palladium(II) acetate was less efficient, and copper(I) chloride, copper(II) sulfate and rhodium(II) acetate dimer were totally unproductive. The cyclopropanation of ( )-but-2-ene represents a unique... 

The seminal report of an asymmetric homogeneous metal-catalyzed reaction described the copper-catalyzed group-transfer reaction from a diazoester to an alkene, Eq. 3 (2). This article provided experimental verification of the intervention of copper carbenoid olefin complexes in the catalytic decomposition of diazo com-... 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

In a long-term research project, Hossain and coworkers investigated the usefulness of the CpFe(CO)2+ fragment [35-38] in the cydopropanation reaction of alkenes by a carbene transfer utilizing diazo esters as the carbene source (Scheme 9.17). The cydopropanation products of styrene derivatives could be obtained in good yields of up to 80% and excellent cis selectivity by using an excess of the alkene, whereas the cydopropanation of aliphatic alkenes was less effective, yielding the desired cyclopropane derivative in up to 51% yield. 

Reactions of rhodium porphyrins with diazo esters - According to Callot et al., iodorhodium(III) porphyrins are efficient catalysts for the cyclopropanation of alkenes by diazo esters [320,321], The transfer of ethoxycarbonylcarbene to a variety of olefins was found to proceed with a large syn-selectivity as compared with other catalysts. In their study to further develop this reaction to a shape-selective and asymmetric process [322], Kodadek et al. [323] have delineated the reaction sequences (29, 30) and identified as the active catalyst the iodoalkyl-rhodium(III) complex resulting from attack of a metal carbene moiety Rh(CHCOOEt) by iodide. 

A cyclic iminocarbene was transferred from 2-diazo-5-isopropyl-3,6-dimethoxy-2,5-dihy-dropyrazine to an alkene under unusually mild conditions. This diazo compound decomposes under the conditions of its synthesis from 2,5-dihydropyrazine 8, and in the presence of cy-cloalkenes, 2,5-dihydrospiro[pyrazine]bicyclo[n.l.O]alkanes 9 (n = 3-5) were obtained (an experimental procedure is given in Houben-Weyl Vol. E19b, pi 194). 

The catalytic activity of low-valent ruthenium species in carbene-transfer reactions is only beginning to emerge. The ruthenium(O) cluster RujCCO), catalyzed formation of ethyl 2-butyloxycyclopropane-l-carboxylate from ethyl diazoacetate and butyl vinyl ether (65 °C, excess of alkene, 0.5 mol% of catalyst yield 65%), but seems not to have been further utilized. The ruthenacarborane clusters 6 and 7 as well as the polymeric diacetatotetracarbonyl-diruthenium (8) have catalytic activity comparable to that of rhodium(II) carboxylates for the cyclopropanation of simple alkenes, cycloalkenes, 1,3-dienes, enol ethers, and styrene with diazoacetic esters. Catalyst 8 also proved exceptionally suitable for the cyclopropanation using a-diazo-a-trialkylsilylacetic esters. ... 

For the carbene transfer from 2-diazo-3-butenoates to terminal alkenes (e.g. styrene, hex-l-ene, vinyl acetate) und 1,3-dienes, the chiral catalyst tetrakis[A-(4-rer -butylphenyl-sulfonyl)prolinato]dirhodium(Il) produced enantioselectivities of up to more than 90% (see also Table 13). 

The ability of chiral bis(camphorquinone-a-dioximato)cobalt(Il) complexes (Section 1.2.1.2.4.2.6.3.1.) to catalyze carbene transfer from diazocarbonyl compounds (diazoacetic esters, 2-diazo-l-phenylethan-l-one) to terminal alkenes conjugated with vinyl, aryl, carbonyl, and cyano groups, has already been mentioned. The ee-values are 75-88 /o at best, often lower. The highest values are again obtained with bulky diazoacetic esters. The significance of these catalysts, however, is their ability to promote cyclopropanation of electron-deficient alkenes, such as acrylates and acrylonitriles, in contrast to the rhodium and copper catalysts discussed above. 

A number of 1-azetines have been obtained from thermolysis of cyclopropyl azides, nitriles and alkenes being side products. The cyclopropyl azides were obtained by transfer of a diazo-group from tosyl azide to cyclopropylamine anions or from carbene/carbenoid additions to vinyl azides. The azetine (1) was oxidized... 

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