Diazomethane metal complexes

Diazirine is a cyclic isomer of diazomethane. According to the organometallic literature, scission of both C-N and N-N bonds can occur when diazirines interact with metal complexes. The formation of carbene ligands arises from selective cleavage of the C-N bond, whereas selective N-N bond scission results in the formation... 

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. 

Cyclopropanations with diazomethane can proceed with surprisingly high diastereo-selectivities (Table 3.4) [643,662-664]. However, enantioselective cyclopropanations with diazomethane and enantiomerically pure, catalytically active transition metal complexes have so far furnished only low enantiomeric excesses [650,665] or racemic products [666]. These disappointing results are consistent with the results obtained in stoichiometric cyclopropanations with enantiomerically pure Cp(CO)(Ph3P)Fe=CH2 X , which also does not lead to high asymmetric induction (see Section 3.2.2.1). 

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. 

Table 4.1. Transition metal complexes suitable for the conversion of acceptor-substituted diazomethanes into carbene complexes.

Ylides other than acceptor-substituted diazomethanes have only occasionally been used as carbene-complex precursors. lodonium ylides (PhI=CZ Z ) [1017,1050-1056], sulfonium ylides [673], sulfoxonium ylides [1057] and thiophenium ylides [1058,1059] react with electrophilic transition metal complexes to yield intermediates capable of undergoing C-H or N-H insertions and olefin cyclopropanations. 

The first bix(hydroxymeihyl) transition metal complex has been prepared. The synthesis was achieved by an interesting sequence of reactions. Diazomethane converted (1,5-... 

There is a reasonably extensive reported chemistry for diazomethane complexes483 obtained by oxidative addition of a diazomethane to relatively low oxidation state metal complexes. A representative example is given in equation (176).483... 

Diazomethane adds not only to monomeric metal complexes but also to compounds containing metal metal double bonds (equation 38), a reaction that has analogy with the addition of a free carbene to a C=C double bond to give a cyclopropene. ... 

In an improved process for the synthesis of tropilidene (5) by E. Muller, a solution of diazomethane in benzene is added gradually to refluxing benzene containing cuprous bromide as catalyst. Benzene is used in large excess, and the product is isolated most easily by filtering the solution from the catalyst and adding it to a solution of phosphorus pentachloride in carbon tetrachloride. The tropylium chloride which separates is dissolved in water and treated with perchloric acid to afford tropylium perchlorate in 85% yield. The success of the method is attributed to formation of the intermediate (3), a deactivated electrophilic carbon metal complex. Tropilidene... 

The first examples of the tervalent tautomer of phosphinous acids have been prepared as transition-metal complexes (134). The structures were assigned on the basis of distinctive spectral evidence and reactions with diazomethane and triethylamine to give (135) and (136) respectively. ... 

All the P-As and P=As bonded compounds are sensitive to hydrolysis and to oxidation by air. The reaction of Cp As=PMes with diazomethane adds a CH2 group across the double bond and yields the three-membered heterocyclic phosphaarsirane Cp AsCH2PMes. Sulfur and selenium add across the double bond to form three-membered heterocycles.Dimerization of Cp As=PCp produces two isomeric diphosphadiarsetanes, one containing the P-P-As-As and the other the P-As-P-As core. Photolysis of Cp As=PMes, Cp P=AsMes, and (2,4,6-r-Bu3C6H2)P=AsCp yields the diarsadiphosphacyclo-butanes. Metal complexes of several of the P=As compounds have been prepared and characterized. ... 

Dzhemilev et al. (1991) conducted an interesting investigation on the yield of cycloheptatriene formed in the reaction of benzene with diazomethane with various catalysts. The yield decreases in the presence of transition metal complexes in the series Rh-C (100<7o), Rh2(CF3COO)4 (57q/o), CuCl (39< o), CuBr (37<7o), Rh2(CH3COO)4 (17< o), activated charcoal (15 /o). Toluene, biphenyl, and dimethyl-benzenes yield mixtures of the corresponding regioisomeric cycloheptatriene derivatives in 82-98 yield. With naphthalene, cyclopropanation took place in the 1,2-position only (98< o). The benzonorcaradiene formed resisted isomerization to benzocycloheptatriene. 

The syntheses of fcp differ from those of other transition metal complexes of cyclophanes and starts with 70 itself [8, 65]. Only ethano bridged fcp [66] have to be prepared from the free ligand and iron-II-ions. A typical reaction sequence is illustrated in Fig. 16 [67] 72 is reduced to 73 using LAH/AICI3. Ring extension [67, 68] with diazomethane followed by reduction yields 75 (Fig. 16)... 

Attempts to carry out carbene transfer reactions with chiral palladium catalysts were unsuccessful so far. Demnark et al. conducted a detailed study in which cyclopropana-tions of a,/3-unsaturated carbonyl compounds with diazomethane catalyzed by bis-(oxazoline)palladium(n) complexes were investigated. Virtual no asymmetric induction was obtained in these reactions which led to the conclusion—especially in light of the excellent asymmetric enviromnent bis(oxazolines) metal complexes offer in general—-that partial or complete ligand dissociation must have been occurred during the course of the reaction. 

Reactions of diazomethane with transition metal complexes also furnish derivatives possessing M —C [Pg.223]

In organorhodium compounds, reaction with diazomethane [50-52], and rearrangement with acetylene compounds, etc. [53-62], afford a variety of carbene complexes. The carbenes are a general term for a chargeless divalent carbon (X(Y)C ) and the triplet state bond angle is ca. 150°) is more stable by ca. lOkcal/moI than the singlet state (bond angle ca. 100°). The metal complexes of a carbene are represented as M=CRR and when R and R are C or H, it is called an alky-lidene complex. The carbene is labile, but the carbene complexes are relatively stable. 

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