Metallated cyclopropanes

To determine the stereochemical relationship between the allyl group and ZnBr in intermediate 81, the reaction was performed with the more simple substrate 83, and the intermediate metallated cyclopropane 85 was quenched with allyl bromide after transmet-allation to an organocopper (equation 37). 

Treatment of functionalized vinyllithium, 28, with allylmaghesium bromide in the presence of zinc bromide leads to a stereoselectively substituted, metallated cyclopropane, 29, via a tandem metalla-Claisen-cyclization reaction (equation 48 )M. The produced cyclo-... 

The allylmetallation of vinyl metals, y-heterosubstituted with a methoxy-methyl ether as the chelating group, leads to the corresponding gembismetal-lic derivatives,20 but now, warming the reaction mixture to room temperature promotes an internal nucleophilic substitution, leading to a metallated cyclopropane which can react with different electrophiles21 (Equation 7.5 and Protocol 9). 

Aminocyclopropanes have also been prepared from metalated cyclopropanes 123 and... 

The formation of metalated cyclopropanes is primarily the reason for the inefficiency of de-hydrohalogenation of cyclopropyl monohalides with strong base. This reaction yields for the most part alkylated cyclopropanes, e. g. formation of 24 with allene 25 as the minor product. 

During the past several decades, with the aid of transitiOTi metals, cyclopropanes have been incorporated into cycloadditions via C-C bmid cleavage and have led to many desirable types of transformations, mainly involving vinylcyclopropanes (VCPs) and methylenecyclopropanes (MCPs) [140]. These methods are complementary to traditional cycloaddition reactions. In particular, the VCP- or MCP-involved cycloadditions can provide efficient accesses to different sized... 

The same disconnection is also effective for cyclopropanes but the reagent for the earbene synthon is a diazocompound RCHN2 or a dihalo compound treated with a metal e.g. 

Alkali metal and other iodides are effective catalysts in reactions involving aUphatic chloro and bromo compounds, such as the preparation of cyclopropane from 1,3-dichloropropane andmetaUic 2inc (141). 

FREUNO-GUSTAVSON Cyclopropane Synthesis Cyclopropane synthesis from 1,3-dihaloalkenes with metals... 

Catalytic, enantioselective cyclopropanation enjoys the unique distinction of being the first example of asymmetric catalysis with a transition metal complex. The landmark 1966 report by Nozaki et al. [1] of decomposition of ethyl diazoacetate 3 with a chiral copper (II) salicylamine complex 1 (Scheme 3.1) in the presence of styrene gave birth to a field of endeavor which still today represents one of the major enterprises in chemistry. In view of the enormous growth in the field of asymmetric catalysis over the past four decades, it is somewhat ironic that significant advances in cyclopropanation have only emerged in the past ten years. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

Similar results are observed in the conjugative addition of CH-acidic methylene compounds with the metal derivatives of 2-nitro-5,10,15.20-tetraphenylporphyrin (6). The nickel porphyrin 6 (M = Ni) yields with an excess of dimethyl malonate the cyclopropane derivative 7 whereas the copper porphyrin 6 (M — Cu) forms with two equivalents of malononitrile the bisadduct 8.111... 

Cyclopentene-l-dithiocarboxylic acid, 2-amino-meta complexes, 2, 800 Cyclophane chlorophylls, 3, 58 Cyclophane hemes iron complexes, 4,1269 Cyclophosphazenes metal complexes, 2, 81 Cyclopropane carbonylation... 

TABLE 9. Preparation of cyclopropanes 111 from a,/ -unsaturated sulfones 109 and a-metallated nitriles 11071... 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Cyclopropanes can be cleaved by catalytic hydrogenolysis. Among the catalysts used have been Ni, Pd, and Pt. The reaction can often be run under mild condi-tions." ° Certain cyclopropane rings, especially cyclopropyl ketones and aryl-substituted cyclopropanes," can be reductively cleaved by an alkali metal (generally Na or Li) in liquid anunonia." Similar reduction has been accomplished photo-chemically in the presence of LiC104." ... 

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