Pentacarbonyl carbene chromium

In spite of the fact that silver(i) X-heterocyclic carbene complexes were widely employed as carbene-transfer reagents for the synthesis of other transition metal carbene complexes, their synthesis could also be achieved by the reaction of silver salts with relatively more labile carbene metal complexes, albeit rare. Complexes 71a-71c were reported to be synthesized from the reaction of the corresponding pentacarbonyl(carbene)chromium(i) complexes with silver(i) hexafluorophosphate in CDC13 under inert atmosphere (Scheme 17).117... 

Pentacarbonyl(carbene)chromium complexes with a primary amino group at the alkenyl terminus 32, readily obtained by addition of ammonia to alkynyl-substituted complexes, initially rearranged upon heating to pentacarbonylchromium-coordinated 1-azabuta-l,3-dienes 33, which subsequently underwent [4 + 2] cycloaddition of an alkyne to give coordinated 4-ethoxy-1,4-dihydropyridines the corresponding pyridines, e.g. 34, were formed by 1,4-elimination of ethanol. ... 

Although most of the examples of [3S+2C] cycloaddition reactions with carbene complexes are referred to as 1,3-dipolar processes, we should include in this section another kind of non-dipolar transformation dealing with the reaction of pentacarbonyl(methoxymethylcarbene)chromium with a base followed by treatment with an epoxide in the presence of boron trifluoride. This reaction gives cyclic carbene complexes in a process that can be considered a [3S+2C] cycloaddition [44] (Scheme 14). 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

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. 

The fact that pentacarbonyl carbene complexes react with enynes in a chemo-selective and regiospecific way at the alkyne functionality was successfully applied in the total synthesis of vitamins of the Kj and K2 series [58]. Oxidation of the intermediate tricarbonyl(dihydrovitamin K) chromium complexes with silver oxide afforded the desired naphthoquinone-based vitamin K compounds 65. Compared to customary strategies, the benzannulation reaction proved to be superior as it avoids conditions favouring (E)/(Z)-isomerisation within the allylic side chain. The basic representative vitamin K3 (menadione) 66 was synthesised in a straightforward manner from pentacarbonyl carbene complex 1 and propyne (Scheme 38). 

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. 

Triethylamine (8) Ethanamine, N,N-diethyl- (9) (121-44-8) Pentacarbonyl[phenyl(methoxy)chromium]carbene Chromium, pentacarbonyl(a-methoxybenzylidene)- (8) ... 

Arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal, or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-1-naphthols, with the sterically more demanding substituent of the alkyne (Rl Figure 2.24) ortho to the hydroxy group. Acceptor-substituted alkynes can also be used in this reaction (Entry 4, Table 2.17) [331]. Donor-substituted alkynes can however lead to the formation of other products [191,192]. Also (diarylcarbene)pentacarbonyl chromium complexes can react with alkynes to yield phenols [332]. 

Historically, vinylidene complexes of zero-valent pentacarbonyl Group 6 metals appeared as a fleeting intermediate for the preparation of Fischer-type carbene complexes. Probably the first example of the formation of such a pentacarbonyl vinylidene complex of a Group 6 metal was proposed in 1974 by Fischer et al, who examined the reaction of pentacarbonyl[hydroxy(methyl)carbene] chromium 1 with dicyclohexylcarbodiimide(DCC) [3]. Thus, treatment of 1 with DCC in CH2CI2 at —20°C rt gave a novel azetidinylidene complex 2 in 47% yield. As a possible... 

Scheme 5.1 The reaction of pentacarbonyl[hydroxy(methyl)carbene] chromium with DCC.

Pentacarbonyl[ethoxy(diethylamino)methylene ] chromium(O). Pentacarbonyl [ ethoxy (diethylamino)carbene ] chromium(0) ... 

This is a rapidly developing area of Cr chemistry, and the two main classes discussed here are the chromium(0) pentacarbonyl carbenes (56) and the chromium(III) alkyls (57). 

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). 

When cyclopropylidenemethy](ethoxy)carbene chromium pentacarbonyl is treated with diphenylacetylene in THF, a mixture of products is obtained, from which only the tricarbonyl(5-ethoxy-6,7-diphenylbenzofuran)chromium complex could be isolated, in... 

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. 

Carbene precursors Chromium carbene complexes, 82 Pentacarbonyl(ethylmethoxymethylene)-chromium, 82... 

LACTAMS Benzenesulfenyl chloride. Pentacarbonyl(methoxyalkyl)chromium carbenes. Triphenylphosphine-Diethyl diazodicarboxylate. 

Pentacarbonylchromium, oxygen difluoride reaction, 5, 231 Pentacarbonyl[ethoxy(2-amnioethenyl)carbene]chromium complexes, preparation, 5, 270 Pentacarbonyliron... 

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.

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... 

We could show further that not only methoxy(organo)carbene complexes react with boron trihalides in the above-mentioned sense. For instance, it was found that b aws-(bromo)tctracarbonyl(phenylcarbyne)chromium(0) and -tungsten(0) are also accessible from pentacarbonyl[hydroxy (phenyl)-carbene [[chromium (0) (02) and from pentacarbonyl[methoxycarbonyl-... 

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). 

Certain pentacarbonyl carbene complexes of chromium rearrange spontaneously by loss of a CO ligand to give trans-substituted aminocarbyne complexes ... 

The carbanion derived from pentacarbonyl(methoxymethyl-carbene)chromium(0) reacts with () )-( ) in the presence of BF3-OEt2 to give a lactone in low yield after oxidation of the ring-opened intermediate. ... 

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