Zinc, carbene complexes

More useful for synthetic purposes, however, is the combination of the zinc-copper couple with methylene iodide to generate carbene-zinc iodide complex, which undergoes addition to double bonds exclusively to form cyclopropanes (7). The base-catalyzed generation of halocarbenes from haloforms (2) also provides a general route to 1,1-dihalocyclopropanes via carbene addition, as does the nonbasic generation of dihalocarbenes from phenyl(trihalomethyl)mercury compounds. Details of these reactions are given below. 

Chromium carbene complexes having electron-rich arenes tethered to the car-bene oxygen or carbon underwent photodriven intramolecular Friedel-Crafts acylation in the presence of zinc chloride (Eqs. 32 and 33) [118]. The process was highly regioselective, undergoing acylation exclusively para to the activating group. 

In contrast to the Simmons-Smith reagent and similar carbenoids, which are reactive and therefore difficult to characterize, adducts of the fV-heterocyclic l,3-diorganylimidazol-2-ylidenes are remarkably stable. The first iV-heterocyclic carbene complex of zinc, namely the l,3-di(l-adamantyl)imidazol-2-ylidene diethylzinc complex 48 (Figure 22), was reported by Arduengo et al. in 1993.98 Because of the general utility of these iV-heterocyclic... 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

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. 

Related to metal complexes are metal carbenoids such as those formed when zinc reacts with di-iodomethane. In early examples, such as the efficient cyclopropanation of cyclohexenone 54, the zinc was activated by some copper.13 The active reagent is the zinc a-complex 56. One might suppose an a-elimination 57 would occur to give the carbene, but this is apparently not so. The active reagent 56, nearly but not quite a carbene, is known as a carbenoid. 

However, an encouraging result was obtained very recently for the 1,4-conjugate addition of dialkyl zinc to a variety of Michael acceptors catalyzed by copper. Alexakis, Roland and coworkers have investigated the addition of diethyl zinc to cycloheptenone and observed an enantiomeric excess of 93% (95% yield) in the presence of Cu(OAc)2 and the silver carbene derivative of imidazolium 1 (Scheme 3) [10]. Silver carbene complexes are efficient transfer agents to copper(II) and therefore the potentially harmful use of a base to generate the catalytic species is avoided. 

The synthesis and characterization of Et2Zn adducts with nucleophilic carbenes see Carbene Complexes) have been described. They are directly available and in good yields from a smooth reaction between Et2Zn and stable carbenes see Carbene Complexes) such as l,3-dimesityhmidazol-2-yhdene or l,3-di(l-adamantyl)imidazol-2-ylidene (equation 13). The solid-state structure of [Et2Zn l,3-di(l-adamantyl)imidazol-2-ylidene ] shows a zinc atom in a rather unusual trigonal planar arrangement, certainly due to a steric... 

The ionic liquids, [BMIMjBr, [BMIM][BF4], [BMIMjlPFe], [BDMIM][BF4], and [BPY][BF4], were examined as the solvent media for dialkylzinc addition to aldehydes giving the corresponding alcohols. The ionic liquid [BPY][BF4] was found to be the solvent of choice, giving the best yields, and was found to be easily recovered and reused [226] (Scheme 5.2-95). It was found that the imidazoUum salts react with diethyl zinc to form a carbene complex of zinc, but the 2-methylimidazolium or pyridinium salts did not react and hence could be recycled. 

Philip Kocienski published an elegant synthesis of racemic olean. The starting material is the THP ether of 4,4-dibromobutanol. In spite of the acid-sensitivity of the acetal, the formation of a carbene complex with titanium tetrachloride and zinc can be achieved. Its reaction with a corresponding ester leads to an enol ether, which cyclises to olean under acidic conditions. [217]... 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

In the presence of a large excess of tetracarbonyl nickel and ethanol diphenyldiazo-methane, diazofiuorene, bis-(4-methoxyphenyl)diazomethane, and ethyl diazoacetate give carbonylation products trapped by ethanol and processed into the corresponding carboxylic acid in 74,38,26, and 8.5% isolated yields, respectively, which presumably arise from a nickel carbene carbonyl intermediate that releases a substituted ketene upon decomposition at 50-66 °C. In the absence of ethanol, by refluxing a solution of 1 mol diphenylketene with 6.4 mol tetracarbonyl nickel in diethylether 35% isolated yield of diphenylketene was observed [86]. The r -(C,C)-ketene complex of nickel (31) was isolated in 17% yield from the reaction of nickelacyclobutane (29) with carbon monoxide (3 bar) at 50 °C (reaction 8.54) [87]. Complex 29 is believed to be in equilibrium with the nickel-carbene-olefrn complex 30 [88]. The nickel-ketene complex 31 was also obtained either by direct reaction of Ni(PPh3)4 with ketene or by carbonylation of the nickel-carbene complexes presumably formed from the reaction of Ni(PPh3)4 and CH2Br2 in the presence of metallic zinc [89]. 

Takai reported a simple, general, and stereoselective method for the alkylidenation of ester to give Z-enol ethers The titanium carbene complex is easily prepared in situ by the reaction of RCHBr2 with a low-valent titanium species generated by treatment of TiCU with zinc and tetramethylenediamine (TMEDA) in THE. Without isolation, the complex is used for carbonyl alkenation. It was reported that the presence of a small amount of lead in the zinc was crucial to the reaction ... 

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

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