Methylenecyclopropanes isomerization

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

These reactions of 1-Me resemble that of (dichloromethylene)cyclopropane [31] and radicophilic alkenes with a capto-dative substitution pattern [32]. Thus, it is not surprising that 1-Me reacts with a-ferf-butylthioacrylonitrile (18), yielding the two isomeric cyclobutane derivatives 19a, b (ratio 2.2 1) as a mixture of two diastereomers each [29] (Scheme 5), and this reaction occurs under milder conditions than the [2-1-2] cycloaddition of 18 onto methylenecyclopropane. 

Other isomerization products of excited methylenecyclopropane were butadiene, methyl allene, dimethyl acetylene, and ethylacetylene. The rate of isomerization of excited methylallene was slow compared to that of excited methylenecyclopropane. 

Ruthenium complexes mediate the hydroamination of ethylene with pyridine.589 The reaction, however, is not catalytic, because of strong complexation of the amine to metal sites. Iridium complexes with chiral diphosphine ligands and a small amount of fluoride cocatalyst are effective in inducing asymmetric alkene hydroamination reaction of norbomene with aniline [the best enantiomeric excess (ee) values exceed 90%].590 Strained methylenecyclopropanes react with ring opening to yield isomeric allylic enamines 591... 

Use of excess of reagents and prolonged reaction times lead to dialkylation, dimerization, isomerizations and cross-coupling reactions (equation 187). In the presence of methoxy groups in the side chain, alkylation is followed by migration of the double bond to form the thermodynamically more stable methylenecyclopropanes (equation 188). 

Deprotonation of (alkylcyclopropylidenemethyl)cyclopropanes (alkyl = methyl, cyclopropyl) with BuLi and subsequent reactions with various electrophiles afforded the corresponding ring-substituted methylenecyclopropanes (equation 295)365. When the lithiated compounds are treated with C02, carboxylic acids are obtained, together with isomeric lactones. These can be regarded as formal 3+2 adducts of the methylenecyclopropanes with C02 (equation 295)366. 

Silylation of lithiated 2-methyl-l-methylenecyclopropane is reported to give selectively tra s-2-methyl-3-trimethylsilyl-1 -methylenecyclopropane361. However, when 2-trimethyl-silyl-l-methylenecyclopropane is again lithiated in situ, and subsequently reacted with Me3SiCl, silylation occurs mainly at the already silylated carbon, giving 2,2-bis-(trimethylsilyl)-l -methylenecyclopropane, along with minor amounts of the exo-silylated isomeric cyclopropene and syn and anti trisilyl derivatives (equation 296)368. 

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). 

From a mechanistic point of view (see below) it is important to note that the chloropalladation of labeled 2,2-diphenyl-l-methylenecyclopropane-3,3-d2 gave only two isomeric complexes, with absence of the isomer in which both the phenyl and deuterium reside on the allylic moiety (equation 323). This allows the exclusion of a symmetrically bound >/4-trimethylenemethane (TMM) intermediate or rapidly equilibrating >/3-TMM species,... 

Unlike the alkyl and aryl-substituted methylenecyclopropanes discussed above, both cis- and franj-Feist s esters undergo chloropalladation with proximal 1,2-ring opening, to give isomeric n3-[3-chloro-l,2-bis(methoxycarbonyl)but-3-enyl]palladium complexes (equation 331)397. Formation of the but-3-enyl complexes is rationalized by sequential... 

The highly strained double bond in methylenecyclopropane displays enhanced reactivity in cycloaddition reactions. In addition to normal [4+2] cycloaddition to 1,3-dienes (e.g. equation 13)32, methylenecyclopropane and its derivatives have a pronounced tendency to undergo thermal [2+2] cycloaddition reactions. For example, thermal dimerization of methylenecyclopropane in the gas phase results in formation of isomeric dispirooctanes 16 and 17 (equation 14)33. This unusual cyclization is considered to proceed via a stepwise radical mechanism involving the intermediacy of biradical 18 (equation 15)34. Equation 15 demonstrates that methylenecyclopropanes possessing substituents capable of stabilizing intermediate radicals undergo efficient [2+2] dimerization even... 

In this section we compare the enthalpy of formation of isomeric pairs of cyclic olefins, one with an exocyclic double bond and the other with the double bond endocyclic. We start with 1-methylcycloalkenes and related methylenecycloalkanes, species genetically 18a and 18b, respectively. From prior experience with cyclopropanes and the thermochemical consequences of replacement of sp3 carbons by sp2 carbons in three-membered rings52, we expect 1-methylcyclopropene to be considerably less stable than methylenecyclopropane because the former compound has two trigonal carbons within the ring while the latter has but one. And so it is found56 the former has a gas-phase enthalpy of formation 43.1 2.2 kJ mol-1 more positive than the latter. 

The palladium-catalyzed Heck-type reaction of methylenecyclopropane 66 in the presence of soft nucleophiles such as sodium diethylmalonate gives a mixture of isomeric alkenes 70 and 71 (Scheme 8.32) [77]. In this process, there is first a carbopalladation of the double bond of 66 giving the cyclopropylcarbinylpalladium in-... 

Pd(PPh3)4-catalysed isomerization of methylenecyclopropanes in acetic acid proceeds smoothly at 1-substituted or 1,1-disubstituted dienes (Scheme 90). A plausible mechanism is hydropalladation and /3-carbon-Pd elimination followed by /3-hydride elimination, established from a deuterium labelling experiment.133... 

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. 

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