Methylenecyclopropanes metal derivatives

Albeit the transition metal catalysed reactions of methylenecyclopropane derivatives have already been thoroughly reviewed [2], it should be noted here that the cyclodimerization of these compounds can also be achieved by catalysis with Ni or Co complexes. The regioselectivity of the process is surprising and opposed to that of the thermal reaction, giving dispiro[2.1.2.1]octane derivatives (Scheme 69) [2],... 

Lithiation of 2-bromo-3,3-disubstituted-methylenecyclopropanes by metal halogen exchange reaction with EtLi in ether, followed by alkylation with epoxides, gave selectively ring-alkylated /8-alcohols, derived from attack at the epoxide primary carbon (equation 299). When R1 R2 a mixture of isomers is obtained369. 

Complexes of di- and tri-methylenecyclopropanes are unknown. However, the alkali metal salts of substituted trimethylenecyclopropane dianions and their corresponding radical anions are stable. The dianions are prepared by base-induced condensation of tetra-chlorocyclopropene with three equivalents of malonic derivatives, or alternatively in two steps, via a zwitterionic aminotriafulvene (equation 368). Further oxidation with potassium persulfate gave the corresponding radical anions431. 

Methylenecyclopropane (79), and its substituted derivatives, have been shown to function as TMM equivalents in the presence of a transition metal catalyst. These cyclic materials are useful synthetic reagents because they are quite stable at ambient temperature and are also readily available. For example, parent compound (79) can be. prepared from methallyl chloride on a kilogram scale with high yield (equation 62). Other efficient syntheses of various substituted derivative systems are outlined in Scheme 2. ... 

In the field of olefin carboxylation, stoichiometric reactions have been described to occur between non-activated alkenes, CO2 and an electron-rich transition-metal complexes, such as Ni(0) [3], Ti(II) [4] or Fe(0) [5]. A Pd-catalyzed CO2 fixation occurs into methylenecyclopropane derivatives affording lactones [6]. The reaction of carbon dioxide with ethylene is difficult and its carboxylation to propionic acid, catalyzed by Rh derivatives [7], needs drastic experimental conditions. 

FSg.. 1. Synthons derived from cyclopropenes and methylenecyclopropanes via transition metal catalysis (dipolar structures not considered)... 

Whereas the transition metal catalyzed cyclotrimerization and cyclotetramerization of alkynes leading to benzene or cyclooctatetraene and their derivatives is a rather common reaction, there exist only a few examples of cooligomerizations between alkynes and alkenes or 1,3-butadienes leading to 1,3- or 1,4-cyclohexadiene derivatives20S). It is therefore surprising that the [3+2]-cycloaddition between methylenecyclopropanes and alkynes, catalyzed by triarylphosphite modified Ni(0) compound, is a rather convenient method to synthesize 4-methylenecyclopentenes 206). A wide range of methylenecyclopropanes and alkynes, in the latter case mainly 1,2-disubstituted ones, can be used for these reactions (Eqs. 98-100, see p. 127-128). 

In the thermally induced [2+2]-eycloaddition of methylenecyclopropanes with ketenes only the C=C-double bond of the ketenes react to give spiro[3.2]hexanones in good yields 21S). The reaction proceeds stereospecificly with methylenecyclopropanes substituted at the double bond. With diphenylketene, tetrahydronaphthalene derivatives are also formed216>. Transition metal compounds do not catalyze these or other reactions between methylenecyclopropanes and ketenes. 

Cyclopropene, methylenecyclopropane and their derivatives have proved to be valuable reagents in transition metal-catalyzed cycloaddition reactions. Small and medium carbocycles can be prepared by this method. The chemoselectivity observed in some of these reactions is quite remarkable. In addition, high degree of regio- and stereoselectivity is obtained in most cases. In particular the new [3+2] cycloaddition described here and which involves methylenecyclopropane and its derivatives as trimethylenemethane synthones, shows great synthetic promise as a method for constructing fivemembered rings. 

In contrast, cyclodimers and cyclooligomers are obtained from strained monoolefins by oligomerizations with Ni(0) and other Group 8-10 metal catalysts . Methylenecy-clopropane is dimerized by [Ni(cod>2] to a mixture of the cyclobutane derivative 2 and the cyclopentane derivative 3 (Scheme 1). By modifying the catalyst with phosphines, the cyclodimerization is suppressed and a mixture of trimers forms in which, depending on the nature of the phosphine, the linear trimer 4 or the cyclotrimer 5 prevails . A metallacycle mechanism has been developed for these reactions. The intermediate A may react further with methylenecyclopropane to form the metallacyclic precursors to the... 

All these Ni(0) catalyzed [2- -2]-cycloadditions proceed through nickelacyclopentane derivatives as intermediates. This has been supported by the isolation of model complexes and the study of their reactivity The results of these model studies have shown that transition metal catalyzed [2 -)- 2]-cycloadditions of methylenecyclopropanes are — at least at the moment — not very useful for the controlled formation of four-membered carbocycles. 

The two double bonds in 2,3-dimethoxycarbonyInorbomadiene are almost equally active. Furthermore, the reactions with methylenecyclopropane are stereoselective leading exclusively to the exo-isomers. Both observations are in striking contrast to the results obtained in the Pd(0) catalyzed cycloadditions of 2-[(tri-methylsilyl)methyl]allyl acetate with norbornadiene derivatives The last observations automatically lead to the conclusion that non-activated alkenes also could undergo these reactions. Indeed it was found that ethylene, norbornene, norbornadiene and allene react with methylenecyclopropane to give cycloadducts (Scheme 7). The reason for the limitation to these alkenes lies in the ability of methylenecyclopropane to compete successfully with alkenes in n-complexation to the metal. Thus cyclodimerization of methylenecyclopropane is much faster than codimerization with other alkenes, which give less stable rt-com-plexes with Pd(0). 

Abstract Transition metal-catalyzed cycloadditions of cyclopropanes have been well developed over the past several decades, leading to numerous new types of cycloadditions which are complementary to the traditional cycloadditions for the synthesis of carbocycles. Cycloadditions of vinylcyclopropanes (VCPs) and methylenecyclopropanes (MCPs) constitute two main aspects of this field. VCPs can act either as five-carbon synthons or three-carbon synthons, depending on whether the vinyl substituent is acting as an additional two-carbon synthon or not. As five-carbon synthons, VCPs are involved in [5-1-1], [5-1-2], [5-I-2-1-1], and [5+1+2-I-1] cycloadditions. As three-carbon synthons, VCPs are mainly involved in [3-1-2] and [3-1-2-t-l] cycloadditions. MCPs mostly act as three-carbon synthons and can have [3-1-2] cycloadditions with different jt systems. Other types of cycloadditions involving MCPs are also reviewed, such as [3-rl], [3+2+2], and [4+3+2] cycloadditions. CycloadditirMis of some other unusual cyclopropane derivatives are also introduced briefly. The cycloadditions of VCPs and MCPs have found applications in total synthesis and some representative molecules are tabulated as selected examples. 

Reactions of arenes carrying a coordinating substituent with alkenes may give alkylated derivatives when catalysed by ruthenium biscarboxylate complexes. Experiments with deuterium-labelled compounds indicate that carbon-hydrogen metallation is reversible, so that reductive elimination from intermediates such as (90) is rate determining. Carboxylate-assisted ruthenium catalysis also allows the reaction of 2-arylpyridines with methylenecyclopropane to give derivatives, (91), in which the cyclopropane ring is conserved. ... 

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