Exo-methylenecyclopentane

The catalytic intramolecular coupling of two C=C bonds at a ruthenium site leads to cyclization reactions. For example, although generally less reactive than a,co-diynes or enynes, 1,6-dienes react with [RuC12(COD)] in 2-propanol, leading to exo-methylenecyclopentanes in excellent yields [13] (Eq. 8). The mechanism suggests the formation of the ruthenacyclopentane(hydrido) intermediate 19. 

Functionalized exo-methylenecyclopentanes can also be obtained by ruthenium-catalyzed intramolecular C-H bond activation [15]. l-(2-Pyridyl)-, l-(2-imidazolyl)-, and l-(2-oxazolyl)-l,5-dienes proceeded in a regiospecific manner to give five-membered ring products (Eq. 10). The proposed mechanism initially involves the activation of the vinylic C-H bond of the exocyclic C=C bond assisted by preliminary coordination of the nitrogen atom, followed by intramolecular insertion of the other C=C bond (see Eq. 6). 

The exo-methylenecyclopentane (197) is also functionalized on the less hindered side upon rearrangement (Scheme 24). The isomeric sulfoxide epi-(197) gave the same product (198) but more slowly. The retarding effect is due to the enforced exo orientation of the phenyl group in the transition state while endo is preferred (c/. Scheme 23). 

Using the catalyst system based on [Ni(allyl)(cod)l [BARF] and Wilke s aza-phospholene ligand, several 1,6-dienes including diene 48 were converted to exo-methylenecyclopentanes 49 through the asymmetric cycloisomerization with enantiomeric excess of up to 91% (Scheme 28) (66). 

Laurene is a sesquiterpene isolated from marine red algae Laurencia elate. It possesses an exo-methylenecyclopentane core with two contiguous stereocenters, one of which is quaternary (Scheme 8.8). Fukumoto et al. reported the enantioselective synthesis of (-l-)-laurene by ring-expansion of 1-vinylcyclobutanol induced by an electrophilic palladium(II) catalyst [28]. 

The SiCaC reaction of 5-hexyn-l-al 23 gives the corresponding 2-(exo-silymethylene)-l-cyclopentanol 24 in high yield (Scheme 7.11) [20]. The reaction is accelerated by gem-disubstitution for example, the reaction of 3,3-gem-disubstituted 5-hexyn-l-al 23 (X = C(C02Et)2, C(CH20Me)2) is substantially faster and cleaner than the unsubstituted derivative (X=CH2), which is accompanied by a small amount of silylformylation product [21]. It should also be noted that the formation of l-siloxy-2-methylenecyclopentane is not observed, in sharp contrast with the nickel-catalyzed version of this reaction [20]. 

An efficient way of introducing selenium in a radical precursor is the use of selenium containing building blocks. The selenides are particularly appropriate when the reaction sequence involves reaction steps that are incompatible with halides or when the corresponding halides are not stable. In Eq. (7), preparation of the selenenylated alkenyl sulfoxide 35 by alkylation of malonate 33 with the bromide 34 is described [18]. This procedure is not feasible with the corresponding halide due to cyclopropane formation via intramolecular alkylation. Radical cyclization of 35 in a 5-exo mode affords, after elimination of the phenylsulfinyl radical, the methylenecyclopentane 36 in good yield and excellent enantioselec-tivity. 

A variety of alkenes can participate in the cycloaddition. Simple alkenes such as ethylene and allene will react to form methylenecyclopentane adducts. Facile cycloadditions with strained alkenes are also observed. For example, norbomene reacts smoothly with (79) to give only the exo adduct (80) in good yield (equation 64). Electron-deficient alkenes, having an ester, ketone or sulfone activating group, are also substrates. However, the methylenecyclopropane cycloaddition does not appear to be very chemoselective. This is demonstrated by the Pd-catalyzed reaction of (79) with 2,3-dimethoxycar-bonylnorbomadiene, where both double bonds of the norbomadiene react to an almost equal extent (equation 65). ... 

A thia-Wittig rearrangement in the methylenecyclopentane (154) proceeded to (155) without formation of the epimer (equation 45). Obviously, the reaction proceeds on the less hindered (convex) face of the bicyclic framework. Equally high diastereoselectivity — unassigned, but presumably also on the less hindered (exo) face — is recorded for the 2,3-rearrangement of a methylenecyclopentane-derived sulfur ylide. ... 

Starting contemporaneously with Skinner, Turner s group (Turner et al., 1957) published a much larger body of work ending in 1973. Turner s work produced a substantial amount of data on the relative stability of isomers, for example, exo-endo isomers like methyl-cyclopentene and methylenecyclopentane. 

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