Diazoalkanes metallation

In this chapter we discuss host-guest complexes of arenediazonium salts with crown ethers and related compounds. Transition metal complexes of arenediazonium ions are treated together with those of dinitrogen and of diazoalkanes in our second book (Zollinger, 1995, Sec. 10.1). 

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

In the presence of metal catalysts such as Yb(OTf)3, diazoalkanes add to imines to... 

Reaction of diazo compounds with a variety of transition metal compounds leads to evolution of nitrogen and formation of products of the same general type as those formed by thermal and photochemical decomposition of diazoalkanes. These transition... 

It appears to be a stable covalent highly crystalline compound (unlike other metal methanediazoates). Alkanediazoates are easily converted to diazoalkanes, so should be regarded as capable of detonation. (Though named by the author as a methanedia-zoate, it is indexed and registered in CA as a nitrosomethylamide salt) [1], A sample of the freshly synthesised compound was dissolved in dichlorodideuteromethane and sealed into an NMR tube. Four days later, when the tube was being opened for recovery of the sample, the tube exploded. This was attributed to diazomethane formation, possibly from reaction with traces of moisture sealed into the tube [2], See other heavy metal derivatives, n-o compounds... 

The reaction of low-valent Ru, Os, and Ir substrates with diazoalkanes provides the most general route to ds alkylidene complexes of these metals. Some features of this reaction are outlined below. 

A Mechanism for Alkylidene Formation. There is no unambiguous example of free-carbene capture by a metal substrate, and the mild reaction conditions used in the generation of these carbene complexes from diazoalkanes suggests that such a mechanism is highly unlikely here. Transition metal diazoalkane complexes, then, are almost certainly implicated as intermediates in these reactions. 

Formal 1,3-dipolar addition of a diazoalkane to an unsaturated metal fragment ML would produce 53. Such a species would be expected to be unstable with respect to nitrogen extrusion, and would seem a plausible intermediate in the diazo reaction ... 

Similar metallacyclic species have been proposed as intermediates in other reactions of metal compounds with diazoalkanes (57,55). The presence of unfavorable steric interactions in an intermediate such as 53 could well explain the failure to observe any reaction of diaryldiazoalkane with 45 (55). 

The EfZ ratio of stilbenes obtained in the Rh2(OAc)4-catalyzed reaction was independent of catalyst concentration in the range given in Table 22 357). This fact differs from the copper-catalyzed decomposition of ethyl diazoacetate, where the ratio diethyl fumarate diethyl maleate was found to depend on the concentration of the catalyst, requiring two competing mechanistic pathways to be taken into account 365), The preference for the Z-stilbene upon C ClO -or rhodium-catalyzed decomposition of aryldiazomethanes may be explained by the mechanism given in Scheme 39. Nucleophilic attack of the diazoalkane at the presumed metal carbene leads to two epimeric diazonium intermediates 385, the sterically less encumbered of which yields the Z-stilbene after C/C rotation 357,358). Thus, steric effects, favoring 385a over 385 b, ultimately cause the preferred formation of the thermodynamically less stable cis-stilbene. 

Some examples of carbene dimer formation resulting from diazoalkane decomposition on transition-metal surfaces have been reported. Diazomethane is decomposed to give ethylene and N2 upon passage over a C0O/M0O3 catalyst as well as on Ni, Pd, Fe, Co, Ru and Cu surfaces 367). Similarly, 2-diazopropane is readily decomposed on Raney nickel 368). At room temperature, propene and N2 were the only detectable products, but above 50 °C, the carbene dimer 2,3-dimethyl-2-butene started to appear which reached its maximum yield at 100 °C, where approximately 40 % of the carbene fragments dimerized. It is assumed 367,368), that surface carbenes are formed as intermediates from both diazomethane and 2-diazopropane which either dimerize or desorb by migration of a P-hydrogren, if available (Scheme 40). 

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). 

Taking together the results of reactivity and stereoselectivity comparisons, one may conclude that the cyclopropanation mechanism as such is quite similar in all cases and involves a metal carbene, but that the stereoselectivity is determined by the nature of the diazoalkane substituent. Doyle has developed a mechanistic scheme which accounts for these observations (Scheme 44). 

Metallic groups as in case (c) lead to electrophilic or even carbocation-like carbene complexes. Typical examples are Fischer-type carbene complexes [e.g. (CO)5Cr=C(Ph)OMe] and the highly reactive carbene complexes resulting from the reaction of rhodium(II) and palladium(II) carboxylates with diazoalkanes. Also platinum ylides [1,2], resulting from the reaction of diazoalkanes with platinum(Il) complexes, have a strong Pt-C o bond but only a weak Pt-C 7t bond. In situation (d) the interaction between the metal and the carbene is very weak, and highly reactive complexes showing carbene-like behavior result. Similar to uncomplexed carbenes. 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

It has been known for a long time that the decomposition of diazoalkanes can be catalyzed by transition metal complexes [493-496]. Carbene complexes were proposed as possible intermediates by Yates in 1952 [497]. However, because reactions of diazoalkanes with metal complexes tend to be difficult to control, it was not until 1975 [498] that stable carbene complexes could be directly obtained from diazoalkanes (Figure 3.19). 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

Because of the high nucleophilicity and reactivity of diazoalkanes, catalytic decomposition occurs readily, not only with a wide range of transition metal complexes but also with Brpnsted or Lewis acids. Well-established catalysts for diazodecomposition include zinc halides [638,639], palladium(II) acetate [640-642], rhodium(II) carboxylates [626,643] and copper(I) triflate [636]. Copper(II)... 

The transition metal-catalyzed reaction of diazoalkanes with acceptor-substituted alkenes is far more intricate than reaction with simple alkenes. With acceptor-substituted alkenes the diazoalkane can undergo (transition metal-catalyzed) 1,3-dipolar cycloaddition to the olefin [651-654]. The resulting 3//-pyrazolines can either be stable or can isomerize to l//-pyrazolines. 3//-Pyrazolines can also eliminate nitrogen and collapse to cyclopropanes, even at low temperatures. Despite these potential side-reactions, several examples of catalyzed cyclopropanations of acceptor-substituted alkenes with diazoalkanes have been reported [648,655]. Substituted 2-cyclohexenones or cinnamates [642,656] have been cyclopropanated in excellent yields by treatment with diazomethane/palladium(II) acetate. Maleates, fumarates, or acrylates [642,657], on the other hand, cannot, however, be cyclopropanated under these conditions. 

Most electrophilic carbene complexes with hydrogen at Cjj will undergo fast 1,2-proton migration with subsequent elimination of the metal and formation of an alkene. For this reason, transition metal-catalyzed cyclopropanations with non-acceptor-substituted diazoalkanes have mainly been limited to the use of diazomethane, aryl-, and diaryldiazomethanes (Tables 3.4 and 3.5). 

Sulfonium ylides R2S=CR 2 [672,673] and metallated sulfones [674-676] can cyclopropanate simple alkenes upon catalysis with copper and nickel complexes (Table 3.6). Because of the increased nucleophilicity and basicity of these ylides, compared with diazoalkanes, these reagents are prone to numerous side-reactions,... 

Table 3.5. Transition-metal-catalyzed cyclopropanations with diazoalkanes.

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. 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

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