Methyl methoxy carbene

Methyl)(methoxy)carbene]pentacarbonyl chromium(O) Chromium,... 

Methyl) (methoxy)carbene] jentacarbonyl chromium(O) Chromium, pentacarbonyl(1-methoxyethylidene)-, (8) Chromium, pentacarbonyl(1-methoxyethylidene)-, (OC-6-21)-, (9) (20540-69-6)... 

Pentacarbonyl[methyl(methoxy)carbene]tungsten and other carbene complexes containing a hydrogen atom a to the carbene carbon atom react with butyllithium (but also with OMe" ) at low T to generate carbene anions. The moderate reactivity of these carbene anions toward carbon nucleophiles, including epoxides, aldehydes, a-bromoesters and a, -unsaturated carbonyl compounds can be used to prepare carbene complexes inaccessible via other synthetic routes (see refs. 7-9). The anion generated by treatment of (CO)5Cr[C(OMe)Me] with BuLi in THF at — 78°C is isolated as the air-stable bis(triphenylphosphane)iminium salt . 

Other examples of [2C+2S+1C0] cycloaddition reactions have been described by Herndon et al. by the use of chromium cyclopropyl(methoxy)carbenes. These complexes react with alkynes releasing ethene and forming cyclopenta-dienone derivatives, which evolve to cyclopentenone derivatives in the presence of chromium(O) and water [122] (Scheme 76). This reaction has been extended to intramolecular processes and also to the synthesis of some natural products [123]. These authors have also described another process involving a formal [2C+2S+1C0] cycloaddition reaction. Thus, the reaction of methyl and cyclo-propylcarbene complexes with phenylacetylene derivatives does not afford the expected benzannulated products, and several regioisomers of cyclopentenone derivatives are the only products isolated [124] (Scheme 76). 

The rearrangement exhibits some stereochemical preference for c/s-vinyl carbene complex (with respect to the metal) compared to the //ww-isomer. Thus, 2-methyl-2-m-vinyl cyclopropyl (methoxy) carbene chromium pentacarbonyl rearranges to 5-methyl-5-vinyl-2-methoxycyclopentenone approximately 4 times faster (THF, 52 °Q than the trans-isomer, which in turn rearranges faster than phenyl derivatives. This suggests that vinyl complexes undergo initial Cope-type rearrangement to form metallacycloheptadienes, which then rearrange to jt-allyl complexes. Subsequent CO insertion and reductive elimination leads to the vinylcyclopentenones (equation 89)150. 

Transient carbene (9) is expected to be a reaction intermediate in the transformation of butadiyne-linked triazene (8) to alkyne-linked bis-2//-indazolcs (10).18 Experimental data and DFT calculations support carbene (9) as an intermediate in this stepwise, non-synchronous coarctate reaction. Methoxy(methylthio)carbene (12) has been observed by UV photoelectron spectroscopy after gas-phase decomposition of oxadiazoline (11).19 The oxadiazoline (13) allows the generation of the acetoxy(methoxy)carbene (14) 20 -pjjg reaC jvj y 0f (he latter with isocyanates has been explored. Such reactions generally yield methyl (acetylamino)oxoacetates (15). DFT calculations permitted interpretation of the experimental data. 

Thermolysis of 2-acetoxy-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazoline affords acetoxy(methoxy) carbene.60 The thermal rearrangement of acetoxy(methoxy) car-bene to methyl pyruvate was studied by DFT at the B3PW91/6-31G(d,p) level. The conformation of the carbene was considered, as were competing fragmentations to radical pairs. The authors concluded that the reaction is a concerted 1,2-migration rather than a fragmentation-recombination process. 

A. De Renzi, and E. O. Fischer, l,l-Dihalo-2-phenylvinyl Methyl Ethers from [Phenyl-(methoxy)carbene]pentacarbonylchromium(0) and Phenyl(trihalomethyl)mercurials, Inorg. Chim. Acta 8, 185-189 (1974). 

The anions generated from alkylamino carbene complexes can be alkylated in high yields with simple alkyl halides without any detectable amount of dialkylation. This is illustrated for the methyl pyiro-lidine complex (109), which can be alkylated cleanly with ethyl bromide to give the monoalkylated product (110) in 87% yield. The methyl pyrrolidine complex (109) can be prepared in nearly quantitative yield quite simply by treating an ether solution of the methyl methoxy complex (88a) with pyrrolidine at room temperature for a few minutes. A few examples of diastereoselective alkylations are known. The 0-alkylimidate carbene complex (112) can be alkylated with methyl triflate to give a 93 7 mixture of (113) and (114), which are diastereomers as a result of the chiral axis about the aza-allenyl linkage. Other examples of diastereoselective alkylations will be presented in Section 9.2.2.7. 

The reaction of methyl trichloromethyl ether with methyllithium, lithium iodide and cyclo-pentadiene gives anisole. It is formed by the rearrangement of the chloro(methoxy)carbene (or carbenoid) adduct... 

The reactions are stereospecific. The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), mo-lybdenum(O) or tungsten(O) with methyl ( )-but-2-enoate, diethyl (Z)-but-2-enedioate and ethoxyethene depends on the type of transition metal, which is consistent with a carbenoid transition state. 

A carbene carrying both a donor and an electron-withdrawing substituent presents a new pattern of reactivity, often called ambiphilic, since such species can show both nucleophilic and electrophilic properties. Thus chloro(methoxy)carbene 4.222 has a low enough energy LUMO, making it electrophilic towards simple alkenes, and yet a high enough HOMO to make it able to react with electrophilic alkenes like methyl acrylate.443 None of the carbenes discussed above is capable of both of these reactions. 

The a-protons of the alkyl substituent are acidic, just as the protons a- to an ester carbonyl group are acidic. Measurement of pK values for carbene complexes is not straightforward as such numbers are often determined in an aqueous environment in which the conjugate base of the carbene is unstable. A value of 12.5 for the methoxy substituted methyl chromium carbene in 1 1 H2O/ CH3CN appears reasonable. Values for... 

Cyclopent-2-en-l-one, 2-hydroxy-3-methyl-synthesis, 3, 693 Cyclopentenone, 4-methoxy-formation, 1, 423 Cyclopenthiazide as diuretic, 1, 174 Cyclopent[2,3-d]isoxazol-4-one structure, 6, 975 Cyclophane conformation, 2, 115 photoelectron spectroscopy, 2, 140 [2,2]Cyclophane conformation, 2, 115 Cyclophanes nomenclature, 1, 27 Cyclophosphamide as pharmaceutical, 1, 157 reviews, 1, 496 Cyclopiloselloidin synthesis, 3, 743 Cyclopolymerization heterocycle-forming, 1, 292-293 6H-Cyclopropa[5a,6a]pyrazolo[l,5-a]pyrimidine pyrazoles from, 5, 285 Cydopropabenzopyran synthesis, 3, 700 Cyclopropachromenes synthesis, 3, 671 Cyclopropa[c]dnnolines synthesis, 7, 597 Cyclopropanation by carbenes... 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

---