Pentacarbonyl methoxy carbene

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

Pentacarbonyl[phenyl(methoxy)carbene]chromium was prepared by the checkers in 75% yield according to the literature Hegedus, L. S. McGuire, M. A. Schultze, L. M. Org. Synth. 1987, 65,140. This material was stored under nitrogen at -30°C and purified immediately prior to use by filtration through a plug of Celite (hexane solvent). If the chromium carbene is purchased from Aldrich Chemical Company, Inc., the submitters found it was only 65% pure based on capillary GLC analysis. 

In 1979, Rudler ef al. reported another example of the presence of a vinylidene complex during the reaction of pentacarbonyl[methoxy(methyl)carbene] tungsten 5 with MeLi followed by acidification with TFA [4]. It was proposed that the vinylidene complex 7 was generated by deprotonation of the a-proton of the carbene complex followed by elimination of methoxide and reaction with the dimethylcarbene complex 8, the addition-elimination product of MeLi with the starting carbene complex, to give the dinuclear complex 6 (Scheme 5.2). 

Pentacarbonyl(methoxyphenylmethylene)tungsten(0), [Pentacarbonyl[methoxy(phenyl)carbene] tungsten(O)]... 

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

Simple alkenes, norbomene and styrene do not undergo cyclopropanation or insertion reactions with cyclopropyl(methoxy)carbene chromium pentacarbonyl complex. However, the conjugated 1-vinylcyclopentene is cyclopropanated under the reaction conditions at the terminal double bond, affording an isomeric mixture (trans cis = 40 60), in 66% yield (equation 72). 

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. 

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

Preparation of pentacarbonyl[phenyl(methoxy)carbene]chromium ... 

Charge the first flame dried and argon flushed two-necked flask with pentacarbonyl-(methoxy(4-pentenyl)carbene)chromium(0). 

The first X-ray crystal structure determination, carried out by Mills in cooperation with us 31) on pentacarbonyl[methoxy (phenyl) carbene ]-chromium(0), confirmed our originally postulated bonding concept. According to this concept, the carbene carbon atom is sp1 hybridized. It should therefore possess an empty p-orbital and be electron-deficient. 

Fig. 2. The structure of pentacarbonyl [methoxy(phenyl)carbene]chromium (0) bond lengths in angstroms.

Fig. 4. Reactions of pentacarbonyl[methoxy(methyl)carbene]chromium(0) with tertiary phosphines.

Naturally, reactions are especially interesting to us if the products obtained are not readily accessible by the methods of classic organic chemistry but are easily preparable with our complexes. We found one such example upon treatment of pentacarbonyl[methoxy(aryl)carbene]chrom-ium(O) complexes with oxygen, sulfur, or selenium (76). In this way one obtains conveniently the corresponding methyl esters of arylcarboxylic acids and the O-methyl esters of arylthio- as well as arylselenocarboxylic acids this seems to us to be synthetically useful in the last two cases ... 

Fig. 6. Hypothesis concerning the course of the reaction during the treatment of pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with JV-vinyl-2-pyrrolidones under a CO pressure of 150 atm.

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

It was also of interest to us how changes in the organic residue of the carbyne ligand influence the stability and the behavior of carbyne complexes. Hence, we treated with boron tribromide a series of pentacarbonyl-[methoxy (aryl) carbene tungsten (0) complexes which were substituted at... 

K. H. Dotz, Synthesis of the Naphthol Skeleton from Pentacarbonyl[methoxy(phenyl)-carbene]chromium(0) and Tolane, Angew. Chem. Int. Ed. Engl. 14, 644-645 (1975). 

Methylation of(CO)5W[C(OH)Me] with CH Nj [MejOjCBFJ or MeOSO F and of [(C0),WC(0)Me]- with H20/CH2N2 HjO/CMejOJCBFJ MeOSOjF or CHjI finally gives pentacarbonyl[methoxy(methyl)carbene]-tungsten ... 

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 . 

Dotz, K. H. Synthesis of the naphthol skeleton from pentacarbonyl[methoxy(phenyl)carbene]chromium(0) and tolan. Angew. Chem. 1975, 87, 672-673. 

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. 

The reaction of pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with vinyl ethers gives 1-methoxy-l-phenylethene as the main product. If these substrates are allowed to react in the presence of carbon monoxide under pressure, the corresponding cyclopropanes are formed in good yield. 

A solution of pentacarbonyl[phenyl(methoxy)carbene]chromium(0) (3.12 g, 10 mmol) in ethoxyethene (9.66 mL, 100 mmol) was placed in a rotary autoclave (250 mL volume) and heated at 100 atm pressure of carbon monoxide at 50 "C for 65 h. Hexacarbonylchromium formed was filtered and the residue, after the evaporation of excess of vinyl ether, was distilled at 50 °C under high vacuum yield 1.18 g (61%) (mixture of isomers). The isomers were separated by preparative GC the tranj-isomer has mp 29.5 °C, the cii-isomer is a colorless liquid. 

The chromium complexes of alkoxy(styryl)carbenes frequently react with alkenes to give a mixture of products. Thus, pentacarbonyl[(4-methoxyphenyl)vinyl(methoxy)carbene] or [(2-... 

Procedure A A solution of pentacarbonyl[methoxy(cyclopropyl)carbene]chromium(0) (0.276 g, 100 mmol) and an alkene (2.00 mmol) in THE (20 mL) was added dropwise via a syringe pump over a period of 5 h to refluxing THE (tOOmL) under N2. 

Pentacarbonyl[butyl(methoxy)carbene]moIybdenum(0) reacts with electrophilic alkenes under mild conditions (65 C, 1 h, THF) to give a mixture of isomeric cyclopropanes 5 in satisfactory yields." The isomers with methoxy and electron-withdrawing substituent predominate. 

Pentacarbonyl[methoxy(phenyl)carbene]chromium(0) reacts regioselectively across the 4,5-double bond of electrophilic conjugated dienes. The reactions run at a much slower rate than those with electrophilic alkenes, e.g. formation of 6, 7 and 8-10. The diastereomer with the methoxy group cis to the olefinic moiety is largely favored (85 15). Double adducts are only formed as minor side products, but they become the major product in the case of (E,E)-7V,iV-dimethylhexadienamide. ... 

Cyclopropanation of Alkenes with pentacarbonyl[cyclopropyl(methoxy)carbene chroinium (Complex 2) ... 

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