Methylenecyclopropanes MCPs

The parent methylenecyclopropane (MCP) (1) [1 ] is a highly volatile compound (bp 11 °C) which can be prepared in multigram quantities and is commercially available. Numerous efficient and straightforward syntheses of the different types of methylene- and alkylidenecyclopropanes have appeared in the literature and the matter has been reviewed by Binger and Buch [2]. In the last decade other selective syntheses have been developed which gave easy access to compounds containing specific substitution patterns [3], to optically active derivatives [4], or more sophisticated derivatives like dicyclopropylideneethane (2) [5], bicyclopropylidene (BCP) (3) [6] and chloromethoxy carbonyl-methylenecyclopropane (4) [7],... 

An example is trimethylenemethane (TMM), one of the first hydrocarbon biradicals whose triplet ESR spectrum was recorded (cf. Chapter 5 in this volume), but which eluded 25 years of efforts (among others by the present author) to obtain its UV-vis or IR spectrum, because the predominant product of photolysis or pyrolysis of different precursors was invariably methylenecyclopropane (MCP). Eventually, Maier et al. successfully generated a sufficient quantity of TMM, by irradiation of MCP in halogen-atom doped Xe matrices, to record the IR spectrum of this elusive compound. 

Spiro- and 4,5-bis(spiro)-cyclopropane isoxazolidines 85a and 85b prepared by 1,3-DC of acyclic nitrones with methylenecyclopropane (MCP), MCP derivatives or bicyclopropylidene (BCP) smoothly underwent fragmentation upon heating in the presence of a protic acid to yield monobactams 86a and spirocyclopropanated p-lactams 86b in moderate to good yields (56-96%) <04EJ02205 04EJO4158>. 

Like other pyrazolines, 4-methylene-1-pyrazoline (MP) undergoes thermal extrusion of dinitrogen to form methylenecyclopropane (MCP) [35], but it does so much more rapidly the energy of activation is about 9 kcal/mol lower than that of the parent 1-pyrazoline, more than enough to offset a hundredfold reduction in the pre-exponential factor [40, 41]. This kinetic behaviour is prima facie evidence that the reaction proceeds stepwise via a triplet intermediate. The obvious choice was trimethylenemethane (TMM), that had been shown to have a triplet ground-state [36, 37, 38], in confirmation of numerous theoretical predictions [39], [18, pp. 141ff.j. 

The methylenecyclopropanes (MCPs) reacted with various imines to give the aza-Diels-Alder adducts in good to excellent yields (Scheme 12.17) [33]. In this reaction, cyclopropyl rings of MCPs remained unopened, and a lot of quinoline derivatives were synthesized by using this method. A possible mechanism might... 

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. 

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... 

Similar to VCPs, transition-metal-catalyzed reactions of methylenecyclopropanes (MCPs) have been the subject of intensive investigation since the 1970s. Both oxidative addition and P-carbon elimination achieved cleavage of the C-C bond in MCPs. Two C-C bond cleavage patterns can operate with MCPs either the proximal or the distal bond is cleaved (Figure 2.1). This makes the chemistry of MCPs a fertile area of research. Since there are excellent reviews on transition-metal-catalyzed reactions of MCPs [88], only the advances of the last decade are summarized here. 

Whereas the parent MCP (1) is not reported to give [4 + 2] cycloadditions, bicyclopropylidene (3) has been shown to give Diels-Alder adducts. Bicyclo-propylidene (3) is a unique olefin, combining the structural features of a tetra-substituted ethylene and two methylenecyclopropane units. The central... 

Apparently, the dimerization of 4 is considerably more facile than that of MCP or BCP, resembling those of (dichloromethylene)cyclopropane (455) and radicophilic olefins with a capto-dative substitution pattern [125], some of which are known to cyclodimerize even at room temperature. Indeed, the capto-datively substituted methylenecyclopropane 85 undergoes the homodimerization at 60 °C (Scheme 65) [126]. 

CpCo(mcp)2, which in turn can be further transformed to the mono-complex CpCo(PPh3)(mcp) by exchange of one methylenecyclopropane ligand with PPhj (equation 312). Although both complexes are isolable crystals, they are thermally less stable than the analogous Feist s ester complexes. CpCo(mcp)2 readily undergoes thermal isomerization at 110 °C, to give cyclopropyl-substituted -butadiene complexes (see below). 

The reactivity pattern of methylenecyclopropanes is highly dependent on the transition-metal catalyst employed for the activation. The different reactivity of MCP towards transition metals is obvious from the variety of topologically different metal complexes that are stoichiometrically formed upon reaction with... 

Additional aspects of regioselectivity which arise for substituted methylenecyclopropanes are closely related to both the thermodynamic stability or kinetic availability of competing intermediates within the cycloaddition sequences. A variety of products can, in principle, be expected from a [3 + 2] cycloaddition between a monosubstituted MCP and a nonsymmetrical, disubstituted alkene (XHC = CHY). This can be attributed to variability arising in several different steps of the overall reaction. From a topological point of view, these structural features of the product methylenecyclopentanes can be classified as shown in Table 1. Only one selected example for the specific type of isomerism is given in each case. 

Di- and oligomerization reactions of methylenecyclopropanes are also the most important competing side reactions in many of the codimerization reactions employing methylenecyclo-propane (MCP), which are summarized in Section 2.2.2.3. They are specially favored if the cosubstrate, i.e. the alkene in a [3-I-2]-cycloaddition reaction, is only weakly bound to the metal center thus allowing it to be replaced by a second molecule of MCP. 

Methyl-2-methylenecyclopropane (1) and 1,1 -dimethyl-2-methylenecyclopropane (4) give rise to the formation of monospiro and dispiro dimers, 3, 6 and 2, 5, respectively. In the case of 1,1,2,2-tetramethyl-3-methylenecyclopropane (7), the dispiro compound 9 is formed along with an open-chain monomer 8. With 1 as substrate, several other dimers and trimers are additionally obtained. On the other hand, with 7, phenylated ring-opened products, such as 10 and 11 arising from transfer of the ring-cleaved MCP to the solvent benzene, are formed in 4% (L = EtsP) to 34% (L = i-Pr P) yield. ... 

An interesting case of an intramolecular dimerization reaction occurs on treatment of the ethylene-bridged MCP 12 with a nickel/phosphane catalyst system. The tricyclic dimer 13 is formed in 33% yield.The only obvious rationale for this reaction is a proximal ring cleavage occurring at one of the methylenecyclopropanes and subsequent addition to the alkene of the other. 

Due to its pronounced tendency to undergo [3-1-2] cycloaddition (see Section 2.2.2.3.2.), only a few examples are known for predominant [2-1-2] cyclocodimerizations involving the parent compound methylenecyclopropane. Preparatively most useful, a [2-1-2] cycloadduct endo-3 is formed as the major product (86% yield) from MCP (2) and norbornadiene (1) in the presence of a combination of bis(t/" -cycloocta-l,5-diene)nickel(0) and triphenylphosphane as catalyst at 20... 

---