Chromium pentacarbonyl carbenes

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... 

This synthesis of (6), like the synthesis of (5) in Section 3.16.4.1.5 uses a phosphaalkyne. Its reaction with a chromium pentacarbonyl carbene complex provides a carbon monoxide and a carbene unit as the other ring constituents. 

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). 

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

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... 

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. 

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

Fischer and co-workers have prepared several novel carbene derivatives of chromium pentacarbonyl and have confirmed the molecular formulas by mass spectrometry (5, 6, 73, 74, 77, 128, 147). The spectra show the expected stepwise loss of the five carbonyl groups giving the ions [Ligand Cr(CO) ]+ (w = 0-5). The compounds for which spectral data have been reported are given in Table III. The selenium derivative (XXIX) shows... 

Universitat in Miinchen (for his studies on arene chromium tricarbonyls see Chap. 5) prepared the diaminocarbene chromium pentacarbonyl 9 from the imidazolium salt 8 by heating the starting material at 120°C [13]. Although Wanzlick s and Ofele s results received considerable attention, the opportunity to isolate stable carbenes was missed. 

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

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. ... 

A recent study by Frenking [84] investigated in great detail the influence of the carbene substitutents X and R at a pentacarbonyl-chromium Fischer-type complex. The electronic characteristics of these substituents control the reac-... 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

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). 

The reaction of methyl acrylate and acrylonitrile with pentacarbonyl[(iV,iV -di-methylamino)methylene] chromium generates trisubstituted cyclopentanes through a formal [2S+2S+1C] cycloaddition reaction, where two molecules of the olefin and one molecule of the carbene complex have been incorporated into the structure of the cyclopentane [17b] (Scheme 73). The mechanism of this reaction implies a double insertion of two molecules of the olefin into the carbene complex followed by a reductive elimination. 

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