Vinylidene cyclopropanes

The effect of the type of unsaturated moiety exocyclic to a cyclopropane ring on the optical rotation is demonstrated by the methylenecyclopropane 91 and the vinylidene-cyclopropane 92 Both compounds have the same absolute configuration, (R)-( —). 

In vinylidene cyclopropanes such as 3, the three-membered rings will also be hydrogenated ... 

In the presence of fluoride anion, electronically poor silyl enol ethers will undergo an anti-Baldwin 3-exo-dig cyclization to provide vinylidene cyclopropanes. The use of TBAT at room temperature provided product rapidly and in good yield (eq 25). ... 

Halocarbenes undergo this reaction very well and usually in high yields. For example, the cyclopropanation of the diaryl vinylidene cyclopropanes 94 with dihalocarbenes (X = Cl, Br) affords the 1-dihalomethylenespiropentanes 95. ... 

A [3+2] cycloaddition reaction of the vinylidene cyclopropanes 334 with MeCN does not proceed across either one of the C=C bonds, but occured via ring opening to form the cycloadduct 335 °. ... 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

Ohe, Uemura et al. further developed this electrocyclization into the [3,3]-sigmatropy of a cyclopropane system substituted with ethynyl and an acyl or an alkenyl group [33]. Thus, treatment of cis-l-acyl-2-ethynylcyclopropanes 125 with 5 mol% Cr CO)5(thf) in the presence of EtgN at rt induced isomerization to phenol derivatives through the [3,3]-sigmatropy of the vinylidene intermediate 126 to give... 

Unusual bridging (//-cyclopropyIidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis248. The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(/(-Co)(//-C,H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and mww-vinylidene complexes [CpCoFe](//-CO)(//-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent /z-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

The most important discoveries in ruthenium catalysis are highlighted and innovative activation processes, some of which are still controversial, are presented in this volume. They illustrate the usefulness in organic synthesis of specific reactions including carbocyclization, cyclopropanation, olefin metathesis, carbonylation, oxidation, transformation of silicon containing substrates, and show novel reactions operating via vinylidene intermediates, radical processes, inert bonds activation as well as catalysis in water. 

This reaction was extended to other alkenes such as vinylidene carbonate and 1,3-diacetyl-1 f-imidazol-2(3//)-one which gave rise to the formation of m-spiro[2.3]hexane-4,5-dicarb-oxylic acid anhydride (60%) and 2, 4 -diacetylspiro cyclopropane-l,6 -2, 4 -diazabicyclo-(3.2.0]heptan -3 -one (74%) respectively. 

Ethylidyne (8) has been recognised on Pt/SiOa at 300 K using the SEDOR NMR technique applied to heavily C-labelled ethene the C—C bond length was 149 pm. This seemed to occur on large platinum particles, where areas of (111) face are most likely it was also seen by SIMS on platinum black, but on small particles vinylidene (17) predominated. Similar SEDOR experiments with ethyne showed 75% vinylidene and 25% ethyne as 12A or 15. " Adsorbed benzene was shown to rotate freely at 300 K, and cyclopropane was adsorbed, but not strongly, i.e. without loss of hydrogen. ... 

C8H,2F2Si]- (CH3CH2)3SiCH2CF2 Addition of TEST to vinylidene-fluoridine/ Viny lidenefluoride or cyclopropane EPR/ 203 2H(iS) 1.085 2F 9.811 74Che6... 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

In 2002, Ohe et al. used electrocyclization in the [3,3]-sigmatropic rearrangement of cyclopropanes bearing both an ethynyl group and an acyl group 18, where chromium-vinylidene complexes were supposed to work as key reactive intermediates (Scheme 21.8) [14]. Formation of the corresponding oxepins 20 and their conversion to phenols 19 via arene oxides 21 were proposed. In 2004, Sangu et al. 

Two research groups independently extended the electrocyclization system into the stoichiometric formation of pyranylidene complexes (25 and 27), which also proceeds via vinylidene complexes as key intermediates (Scheme 21.10) [16,17], Further reactions of the pyranylidene complexes 25 with a variety of alkenes gave the corresponding naphthalenes 28 in good to high yields. In addition, Miki and others found the catalytic formation of cyclopropanes bearing a furan ring 30 from reactions of ene-yne ketones 29 with a variety of alkenes, where furyl carbene complexes 31 were proposed as key intermediates (Scheme 21.11) [18],... 

Nishibayashi and coworkers [10] have used a variant of the same reaction that does not require the cobalt carbonyl group. Here, a diruthenium catalyst is employed, which reacts with a terminal alkyne, reportedly going through a vinylidene complex and opening the cyclopropane ring in situ. Reaction with the aldehyde followed by ring closure provides the product, regenerating the catalyst (Scheme 10.8). 

Cyclobutene derivatives (66) have been synthesized from a diyne and an alkene via a novel Au(I)-catalysed reaction. A highly active vinylidene intermediate (67), formed by a dual Au(I)-mediated activation of the diyne precursor, is believed to act as an alkylidene Au(I)-carbenoid to effect stereospecific cyclopropanation of the alkene the resulting methylenecyclopropane (68) converts to (66) via an Au(I)-catalysed ring-expansion cascade. 

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