Diazoalkane complexes, transition metal

Occasionally, the term 1,3-dipolar cycloaddition is also used for the reaction of diazoalkanes with transition metal complexes, e. g., (6-9), investigated by McCrindle and McAlees (1993). This is not, however, a 1,3-dipolar cycloaddition as coined by Huisgen. This term should not, therefore, be used for reactions that involve a dipolar reagent, but only in a cycloaddition with a dipolarophile, as shown in Huisgen s mechanism (Schemes 6-5 and 6-6). They may be called [3 + 1] dipolar cycloadditions, however, in order to underline the difference to the [3 + 2] reactions. 

Until today, it is a general rule that 1 1 adducts of diazoalkanes with transition metals are unstable in most cases. The stability can be significantly increased if dibenzoyldiazomethane is used as ligand. This was demonstrated by Cowie et al. (1986 a) in the synthesis of iridium complexes. As shown in (10-20), dibenzoyldiazomethane replaces the dinitrogen ligand in the starting material. The reaction is run... 

One could imagine generating carbene complexes from typical precursors to car-benes in organic chemistry, such as diazoalkanes. Fewer carbene complexes have been isolated by addition of diazoalkanes to transition metal complexes than by the more indirect methods presented previously, but many carbene or carbenoid complexes have been generated by this method as reactive intermediates. Two examples of carbene complexes that have been isolated from the reaction of a diazoalkane are shown in Equations 13.4a and 13.4b. 

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). 

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. 

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)... 

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). 

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],... 

The normal byproducts formed during the transition metal-catalyzed decomposition of diazoalkanes are carbene dimers and azines [496,1023,1329], These products result from the reaction of carbene complexes with the carbene precursor. Their formation can be suppressed by slow addition (e.g. with a syringe motor) of a dilute solution of the diazo compound to the mixture of substrate and catalyst. Carbene dimerization can, however, also be a synthetically useful process. If, e.g., diazoacetone is treated with 0.1% RuClCpIPPhjij at 65 °C in toluene, cw-3-hexene-2,5-dione is obtained in 81% yield with high stereoselectivity [1038]. 

The other example to be discussed in this context comes from Pettit s group. Simultaneous treatment of the iron complex (/u.-CH2)[Fe(CO)4]2 (35) with hydrogen and ethylene gives both methane (66%) and propylene (6%), the expected products from the two separate reactions. In addition, ethane (—600%) is formed, with the actual hydrogenation catalyst still to be determined (72). Because simple diazoalkanes provide the cleanest method to metal-attached alkylidenes, and with the expectation that dissociative chemisorption of diazomethane to absorbed CH2 and free N2 would occur, the reactions of CH2N2 with and without H2 over various transition metals were examined in a careful study with regard to the product ratio (73). It was found, that gas-phase decomposition of the parent diazoalkane upon passage over active Ni, Pd, Fe, Co, Ru, or Cu-... 

The complications that occasionally arise in the use of diazoalkanes reflect the possible further reactions of carbene ligands, which will be dealt with subsequently, e.g. insertion into adjacent M-H or M-halide bonds and the formation of bimetallic complexes supported by bridging carbene ligands. In some cases, transition metals may catalyse reactions of diazoalkanes, leading to products which are suggestive of the reactions of free carbenes, i.e. dimerization, addition to alkenes (cyclo-propanation) and insertion into C-H bonds (Figure 5.9). In such cases, however, the actual mechanism does not involve free carbenes but rather transient diazoalkane/carbene complexes. This is supported by the obser-... 

Polymer-supported benzenesulfonyl azides have been developed as a safe diazotransfer reagent. ° These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, paUadium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Diazoketones and diazoesters with alkenes to give the cyclopropane derivative, usually with a transition-metal catalyst, such as a copper complex. The ruthenium catalyst reaction of diazoesters with an alkyne give a cyclopropene. An X-ray structure of an osmium catalyst intermediate has been determined. Electron-rich alkenes react faster than simple alkenes. ... 

Of special interest is the behavior of 1-aza-zirconacyclopentene complexes which allows electrophilic sp, sp2, and sp3 C-H bond activations as well as M-H bond activations (M=N, O, P, S) and the preparation of the first C-metallated diazoalkanes LnMC(N2)R with an early transition metal. 

Although transition metal complexes do not usually react directly with free carbenes (with the exception of the formation of NHC-metal complexes, Section 10-2-3), low-valent Group 7 to 9 metal complexes in particular react with diazoalkanes to produce alkylidenes. Equation 10.17 shows a general example of this reaction. The complex must either be unsaturated or possess a labile L-type ligand so that the reaction can occur. The intermediate in this reaction is unlikely to be a free carbene. 

One of the most general approaches has already been mentioned and exemplified by equation 10.16. In this approach, diazoalkanes are used as free carbene precursors. These are photochemically or thermally decomposed to the free carbene, which reacts with a low-valent, mid- to late transition metal complex to give the alkylidene. Although general, this method suffers from the difficulty in handling often unstable diazo compounds. 

Basically, there are three types of process by which diazoalkanes dissociate into dinitrogen and a carbene, namely thermolysis, photolysis, and catalysis with a transition metal or a metal complex. 

Metal catalysis in the decomposition of diazoalkanes has been known for almost a century (Silberrad and Roy, 1906), but it was only in 1952, i.e., shortly after carbene chemistry had started (see Sect. 8.1) that Yates realized that transition metal catalysts generate transient electrophilic metal carbenes (8.8, Scheme 8-10). As these complexes are not carbenes in the proper sense, but react in most cases like carbenes. 

There are three leading reviews on metal carbenoid transformation, written by Doyle (1986), by Brookhart and Studabaker (1987), and by Maas (1987). They include aspects of synthesis and of mechanism. The more recent reviews in Trost and Fleming s Comprehensive Organic Synthesis were written by Helquist (1991, use of diazoalkanes) and by Davies (1991, diazo carbonyl compounds). The review of Ye and McKervey (1994) on a-diazocarbonyl compounds also contains examples of metal carbenoid transformations. The book of Hegedus (1994) contains representative syntheses of complex organic molecules obtained by transition metal-catalyzed reactions of diazo compounds. 

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