Hydrocarbonation methylenecyclopropanes

The chemistry of the closest relative of hydrocarbon 1- methylenecyclopropane (2) - has recently been reviewed extensively [2]. The presence of the second three-membered ring in 1 strongly increases the total strain of this molecule, this enhances its chemical reactivity and leads to specific physical properties. The meanwhile large variety of experimental material obtained for compound 1, some of its derivatives and analogs prompted us to summarize the results in order to help set the stage for future developments. 

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. 

Homofulvenes 8 (4-alkylidenebicyclo[3.1.0]hex-2-enes <25%), in addition to several hydrocarbon products (10-35%), are formed in the thermal decomposition of sodium salts of bicyclo[3.2.0]hept-2-en-6-one tosylhydrazones 5 at 130-150°C in both protic and aprotic solvents.8 The formation of homofulvenes 8 may be explained by the initial conventional ring contraction of the cyclobutylidenes 6 to the methylenecyclopropanes 7, the strain of which is released in a thermal rebonding to give homofulvenes 8.8... 

In the hydrocarbonation of methylenecyclopropanes 8 with nitriles, the hydro-palladation of 8 with 16 gives the alkylpalladium complexes 23 and/or 24 (Scheme 5). The complex 23 would undergo rearrangement by distal bond cleavage to give the Jt-allylpalladium 25 (route A). The reductive elimination of Pd(0) from 25 produces 9. The palladium complex 24 would isomerize to the Jt-allylpalladium complex 27 via proximal bond cleaved ring-opened intermediate 26 (route B). The reductive elimination of Pd(0) from 27 gives 10. 

In the co-oligomerization of methylenecyclopropane (26) with allene (63 equation 25) on a palla-dium(O) catalyst, prepared in situ from [Pd(acac)2], triisopropylphosphine and diethylaluminum ethoxide, or from [Pd(DBA)2] and triisopropylphosphine, cyclic hydrocarbons such as 2-methylenespiro[2.4]hep-tane (27), 1,3-dimethylenecyclopentane (64) and 1,3,5-trimethylenecycloheptane (65) are formed. ... 

The product ethylene is formed in a stoichiometric yield with the nitrile and isonitrile, and for methylallenimine, (C2H4) = 0.25 and deactivating mechanism is also operative. The overall mode of decomposition bears a close resemblance to that of the hydrocarbon analog, the methylenecyclopropane ring system. 

Direct photolysis of cis- and rraH5-bicyclo[5.2.0]non-8-ene (47) in hydrocarbon solution with monochromatic far-ultraviolet (185-214 nm) light sources afforded (Z,Z)- and (Z, )-cy-clonona-1,3-dienes via formal electrocyclic ring-opening, cycloheptene via formal a2s + a2s) cycloreversion and only a small amount of methylenecyclopropane derivative 44. The formation of this three-membered ring has been proposed to arise from a cyclobutylidene, formed as a result of [1,2]-hydrogen migration in the 71,R(3a) state. ... 

The intermediates generated from transition metal induced cleavage of methylenecyclopropanes are, in contrast to other trimethylenemethane equivalents, capable of undergoing [3 -I- 2] cycloaddition not only with activated, i.e. electron-deficient alkenes, but also with unsaturated, even nonstrained, hydrocarbons. Some of the reactions summarized in this section are also briefly discussed in Sections 2.2.2.2. and 2.2.2.3.I. because homodi- and homooligomerization of the methylenecyclopropane, as well as [2-1-2] cycloadditions, may efficiently proceed as side reactions of the [3 -f 2] cycloadditions. 

As indicated for the hydrocarbon reactants vide supra), a high selectivity for distal ring cleavage of disubstituted methylenecyclopropanes, and formation Of the methylenecyclopentanes with a doubly-substituted alkene moiety (e.g. 2), is usually observed under palladium(O) cataly-... 

In addition to strained and unstrained hydrocarbons, as well as unsaturated esters, a variety of cyclic substrate types undergo [3-1-2] cycloaddition with methylenecyclopropanes. Some representative examples of such cyclopentaannulation reactions are discussed in this section. 

Alkyllithium has been reported to add to cyclopropene [54]. In the presence of alkali amides this hydrocarbon can form oligomers. The chance of these additions, however, will be less great if the double bond bears an alkyl group. The kineti-cally favoured reaction between 1-methylcyclopropene and potassium amide undoubtedly will be formation of the cyclopropenyl anion. Although the deprotonation equilibrium is expected to be strongly on the side of the anion, (compare Ref. [8]) in the protic solvent ammonia it may easily shift to the other direction to give via the allylic anion) methylenecyclopropane ... 

The modest yields may be ascribed to a number of side reactions. Allylic halides are very prone to nucleophilic attack, which in this case will give rise to the formation of an allylic amine. One could also imagine a nucleophilic attack of the intermediary carbenoid on the allylic halide, or metallation of the cyclopropene by alkali amide and subsequent coupling of the cyclopropenylide with the allylic halide. The relatively high temperatures in the preparations of the cyclopropenes will certainly favour oligomerization, which may be a merely thermal one or an ionic process (similar to that observed by Dutch investigators [16] in the case of the parent hydrocarbon). Such reactions afford products which are considerably less volatile than the cyclopropenes and therefore will remain in the reaction mixture. One obvious subsequent reaction is the irreversible base-induced isomerization to methylenecyclopropane, which cannot be separated from the desired product [79] ... 

In the Pd-catalyzed hydrocarbonation of methylenecyclopropanes 68 with pronucleophiles of type 69 (Scheme 18), the direction of the initial hydropalladation depends craciaUy on the electron density distribution in the double bond of 68. Thus, a competition of the reactions along both pathways A and B can be observed. When R = alkyl or substituted alkyl, the reaction proceeds mainly via intermediates I and II furnishing the hydrocarbonation products 70 in good to very good yields. This type of reaction was performed both as an inter- as well as an intramolecular version. The reaction proceeded... 

In contrast to boranes 17 and 18, no dynamic effects were found in the NMR spectra of the compounds 33-35 (Scheme 2.14). The borane with cyclopropane moiety 33 exists exclusively as 2-(di-H-butylboryl)-l-methylenecyclopropane. On the other hand, only endo isomers can be detected in the NMR spectra of the five- and six-membered boranes 34 and 35 (Scheme 2.14). The order of relative thermodynamic stability of triorganoboranes 17, 18, 33-35 nicely corresponds to the known order for the corresponding hydrocarbons [37]. 

In the presence of transition metal complexes, certain strained hydrocarbon systems are activated under mild thermal conditions and undergo characteristic transformations. Methylenecyclopropane (XXVI) (Noyori et al., 1970, 1972b), bicyclo[2.1.0]pentane (XXVII) (Noyori et al., 1971c, 1974a), and quadricyclane (XXVIII) (Noyori et al., 1975a) add to electron-deficient olefins with the aid of a nickel(0) catalyst such as bis(l,5-cyclooctadiene)-nickel(O) or bis(acrylonitrile)nickel(0). These cycloaddition reactions proceed... 

Next to isobutene, other 1,1-dialkyl substituted ethenes can also be polymerized cationically. Suitable monomers are 2-methyl-1-butene, 2-methyl-1-pentene, and 2,3-dimethyl-1-butene [613] Polymers with very high molecular weights of Mn>300000 are obtained by catalysis with aluminum alkyl halogenides. Also, cyclic hydrocarbons with a methylene group (methylenecyclopropane, methylene cyclobutane, methylene cyclohexane, a-pinene) are suitable monomers [614-619]. 1,1-Disubstituted ethenes with stronger steric hindrance as camphene or 2-methylene-bicyclo-[2.2.1] heptane, however, could not be polymerized cationically [574]. 

Diboration of Unsaturated Hydrocarbons. B2pin2 adds to unsaturated hydrocarbons in the presence of a catalytic amount of a Pt complex to afford diborated products in high yield with excellent regio- and stereoselectivity. The reaction is recognized to proceed through a catalytic cycle, which involves (a) oxidative addition of the B-B bond to Pt , (b) insertion of the unsaturated hydrocarbon into the B-Pt bond, and (c) reductive elimination of the product to regenerate Pt (eq 1). The diboration of alkynes, allenes, conjugated dienes, methylenecyclopropanes, anda, -unsaturated carbonyl compounds is efficiently catal) ed by phosphine-based Pt complexes (eqs 2-6), whereas phosphine-free Pt complexes are favorable for the reaction of simple alkenes because of the low coordination ability of the alkene over phos-... 

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