Methylene, carbene

In contrast to methylene, carbenes which contain an oxygen atom bonded to the carbenic center are known to exist as singlet ground states. The rational for this is that the lone pair electrons on the adjacent oxygen atom have a resonance stabilizing effect on the electron deficient carbenic center ... 

The product of a elimination is a neutral species that resembles a carbocation in having only six carbon valence electrons. The simplest carbene is CH2, methylene. Carbenes are highly reactive, so much so that they cannot be isolated. Their involvement in reactions usually has to be inferred from the nature of the products or the reaction kinetics. The characteristic carbene reactions involve forming an electron-pair bond to the carbene carbon by reacting with cr bonds, it bonds, or unshared pairs ( ), Some of these reactions are illustrated here for methylene ( CH2). ... 

The cyclopropanation requires a methylene carbene that can add to the double bond. Under which of the following sets of conditions can methylene be prepared in situ from diiodomethane a) CH.I2, Cu/Zn b) CH2I2, KMn04 c) CH2I2, ZnEt2 ... 

Structures of type XVIII or XIX as proposed for the diazodicyanomethane complex (Table 12) may also apply to the complexes of diphenyldiazomethane and 9-diazo-fluorene. The formation of the ketenimine complex (45) from the reaction of (CN)2CN2 and Ni(t-BuNC)4 probably occurs via attack of the complexed dicyano-methylene carbene on the isocyanide ligand113 The observation that these complexes112 113 catalyze the formation of ketenimines from isocyanides and diazo compounds, a reaction which does not proceed under same conditions without the transition metal, may be of preparative value 113 ... 

Carbonylation of a mixture of ethylene, ethanol, and diphenylacetylene in the presence of rhodium carbonyl gives the butenolide (10). 2 Another formation of a butenolide, that of compound (12), is by flash vacuum pyrolysis of diphenylmethyl propiolate, Ph2CH02C H. It has been suggested that the ester isomerizes to the methylene-carbene (11), which yields the product by intramolecular insertion. Ozonization of tetraphenyl-... 

Terminal propynes and olefins have also been found to inhibit CYPs via an irreversible mechanism. Furans and thiophenes are frequently found to be irreversible CYP inhibitors. Epoxidation of either system may be involved, and for thiophenes oxidation to the S-oxide can enhance electrophilicity and may be involved in binding to CYPs. Benzodioxoles often display quasi-irreversible CYP inhibition via conversion to the corresponding methylene carbenes which form MI complexes at the CYP heme unit. Compounds containing the hydrazine group, like isoniazid, can also be irreversible CYP inhibitors. 

Proton addition to the anionic methylene carbene complex [Tp W( = CH2) (CO)2] (prepared in situ via addition of Na[HBEt3] to a tetrahydrofuran solution... 

Analogous situations exist with allyl hydrides and propene complexes, agostic propene, propene hydrides, and isopropyl complexes. Exchange of hydride with methylene carbene is relevant to the Fischer-Tropsch reaction,... 

Interestingly, Z- and E-non-3-ene and Z- and -non-2-ene could not be identified as reaction products of Z- and -l-methyl-2-n-pentylcyclopropane. These products would correspond to the fission of the most substituted central cr-bond of the cyclopropane. Furthermore, methylene-carbene elimination leading to the lower homologous alkenes seems to be an unfavourable process with those catalysts. This is in contrast to the behaviour of bicyclo[2.1.0]pentane, which reacts in the presence of PhWCl3/EtAlCl2 to cyclobutene in 70% yield . 

Cleavage of a rutheniiun dimer such as [(TTP)Ru]2 [124] upon treatment with diazoalkanes and diazoesters affords the corresponding carbene complexes, (TTP)Ru(CHCH3) and (TTP)Ru(CHC02CH2CH3), respectively (Scheme 13) [125,126]. These carbene complexes were the first such met-alloporphyrin species to contain a proton on the carbene carbon atom. Unfortunately, the methylene carbene complex was not detected when diazomethane was used as the reagent. Instead, the ethylene complex was formed. 

The isolation of a diamagnetic bridging methylene complex [(OEP-N-yx-CH2) Ru(CH3)](BF4) from decomposition of [(OEP - N - CH3)Ru(CH3)](BE4) was also possible. This complex has been characterized by H NMR and partially by an X-ray structure [145]. Unfortunately, reduction of this complex did not result in formation of an axial methylene carbene complex as was postulated by James and Dolphin [146]. Although M = CH2 species have been prepared [147,148], similar metalloporphyrin complexes are not yet known. Ruthenium carbene complexes which are involved in catalytic reactions will be discussed below. 

The ring forming reactions described in this section, with the exception of the reaction of triplet methylene (carbene, CH2), the second example in Table 6.5, follow the symmetry rules outlined in Chapter 4. Thus, the simplest example of an allowed (2 -H 2) addition is shown the first item in Table 6.5. 

Chapter 7. On the other hand, methylene (rCH ) and alkyl substituted methylenes (carbenes, CHR CR2 R H) can be readily prepared either by the thermal (or photochemical) decomposition of the corresponding diazo compounds (Equation 6.51) themselves prepared from an amine (Chapter 10) or ketone (or aldehyde) (Chapter 9) precursor. Alternatively, for a species that behaves as if it were singlet methylene (rCH ), the treatment of methylene iodide (diiodomethane, I2CH2) with copper (Cu)-activated zinc (Zn) metal (Equation 6.52) can be used (the Simmons-Smith reaction ). 

Unsaturated carbenes are classified (Figure 10) into methylene carbenes (e.g., vinylidene, H2C=C ), vinylidene carbenes (e.g., propadienylidene, H2C=C=C ), vinylcarbenes and related a, p unsaturated carbenes (e.g., aryl carbenes and cyclopentadienylidenes), and propargylenes (e.g., propynyli-dene). ... 

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