Carbenes chromium complexes

Hydroquinone synthesis (regiospecific) from alkynes and carbonyl carbene chromium complexes... 

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... 

An unexpected varying regiochemistry in intramolecular benzannulation has also been observed in the synthesis of cyclophanes. As mentioned above, there are only two possible regiochemical outcomes in the benzannulation reaction, which differ in the direction of alkyne incorporation. / -Tethered vinyl-carbene chromium complexes undergo an intramolecular benzannulation reaction with incorporation of the tethered alkyne with normal regioselectivity to give meta-cyclophanes [28]. 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

Cycloaddition of the carbene chromium complexes 97 with CO incorporation provides a versatile method for naphthol synthesis, in which the metallacy-clic intermediates 99 are involved [47]. An alternative entry to 101 is achieved by metal carbonyl-catalyzed rearrangement of the cyclopropenes 98 via the same metalla-cyclobutenes 99 and vinylketene complexes 100 [52], Mo(CO)6 shows a higher activity than Cr(CO)6 and W(CO)6. The vinylketene complex 103 is formed by the regioselective ring cleavage of 1,3,3-trimethylcyelopropene 102 with an excess of Fe2(CO)9 [53]. (Scheme 35 and 36)... 

The thermal reaction of vinyl- or aryl(alkoxy)carbene chromium complexes with alkynes can give high yields of 4-alkoxyphenols (Figure 2.24). This elegant synthetic procedure was reported for the first time by Dotz in 1975 [326] and has proven robust and of broad scope. 

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]. 

Aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols upon treatment with alkynes, but form indene derivatives. Vinyl(dialkylamino)car-bene complexes, however, react with alkynes to yield aminophenols as the main products if solvents of low nucleophilicity are used [335]. (2-Amino-1-vinyl)carbene complexes do not undergo benzannulation when treated with alkynes, but form cyclopentadienes or heterocycles instead [251]. 

The intermediate vinylketene complexes can undergo several other types or reaction, depending primarily on the substitution pattern, the metal and the solvent used (Figure 2.27). More than 15 different types of product have been obtained from the reaction of aryl(alkoxy)carbene chromium complexes with alkynes [333,334]. In addition to the formation of indenes [337], some arylcarbene complexes yield cyclobutenones [338], lactones, or furans [91] (e.g. Entry 4, Table 2.19) upon reaction with alkynes. Cyclobutenones can also be obtained by reaction of alkoxy(alkyl)carbene complexes with alkynes [339]. 

The reaction of alkoxy(alkyl)carbene chromium complexes with alkynes has been reported to give modest yields of cyclopentenones [368] and a few examples of intramolecular carbene C-H insertions of Fischer-type carbene complexes, leading to five-membered heterocycles, have been reported [369,370] (Table 2.22). 

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

The photochemical generation of metal-bound ketenes from carbene-chromium complexes and the subsequent coupling with imines to give azetidin-2-ones is treated separately (Section 2.01.3.10.5). 

M. F. Semmelhack, J. J. Bozell, L. Keller, T. Sato, E. Spiess, W. Wulff, and A. Zask, Synthesis of Naphthoquinone Antibiotics by Intramolecular Alkyne Cycloaddition to Carbene-Chromium Complexes, Tetrahedron 41, 5803-5812 (1985). 

Hydrostannation of (l-alkynyl)carbene chromium complexes involves addition to the M = C bond as well as to the C=C bond and is strongly influenced by the substituent at the alkynyl moiety. By the bulky trimethyl-silyl group of compound lg, a 1,1-addition to the carbene carbon atom is promoted with formation of a densely functionalized 1,3-heterobimetal lie propargyl reagent 93 containing both a stannyl and a silyl group. A 3,1-addition is observed with phenyl derivative la and also the vinyl compound lo, which leads to production of an allene 94 and 95, respectively (Scheme 31).111... 

M. 1-Aza-t,3-butadiene Complexes via 1,4- and N-Atkenyl(amino)carbene Chromium Complexes via 1,5 Hydrogen Shifts... 

The reaction of an azadiene RIN = CH—CH = CHR2 (R = Ph, Me R2 = rc-Pr, Bn, i-Pr, n-Pr) with a (l-alkynyl)carbene chromium complex la (R = Ph, SiMe3, f-Bu) quite unexpectedly affords [4+3] cycloaddition products 133 in 57-93% yield, from which azepinones 132 are obtained on contact with wet silica (Scheme 52).141 Reaction of azadienes R N=CH — CH = CHR2 (R = Ph, Me R2 = n-Pr, Bn, i-Pr, n-Pr) with a (l-alkynyl)car-bene tungsten complex lb (R = Ph) was shown to produce 1,4-dihydropyri-dine complexes (of the type shown in Scheme 50) as the major products.142... 

Pentacarbonyi[cyclopropyl(methoxy)carbene]chromium (Complex 2/ ... 

Pentacarbonyl[cyclopropylethynyl(ethoxy)carbene]chromium complex 19 reacted with cyclo-penta-1,3-diene to give a cyclopropyl-substituted carbene complex of norbornadiene 22, which was converted to the corresponding carboxylic acid 23 upon oxidation or to a 1,4-quinone-type derivative 24 after Dotz-type cyclization with phenylethyne. ... 

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. ... 

Phosphorylglycines are easily prepared, but in limited yield, by alkoxycarbonylation of the a-lithiated Schiff base of diethyl aminomethylphosphonate in THF at low temperature. The acylated Schiff base is converted in high yield (80%) to the corresponding p-lactam by photochemical reaction with the [(methoxy)(methyl)carbene]chromium complexes in CHjCF. ... 

Merrifield resin, polystyrene matrix, Argogel matrix Figure 31.7 Polymer-supported Fischer carbene chromium complexes. 

Straightforward access to pyrazole carbene chromium complex LXI results from the 1,3-dipolar addition of diazo compounds to acetylenic chromium carbene LX the reagent thus obtained is a useful synthon for the preparation of novel heterocyclic systems such as pyrazole [1, 5-a]pyridine quinone LXIII derivatives [100] in low to moderate yield (10-51 o) according to the substituents (Figure 5). 

Yamashita and colleagues reported an indole synthesis absence of Ac O. The pyrrole carbene complex was from pyrrole-carbene chromium complexes reacting with prepared from 2-Uthio-l-methylpyrrole and chromium... 

Cyclopropanation proceeds through the formation of an electrophilic (2-furyl)carbene-chromium complex which in turn forms through 5-exo dig cyclization. As shown in Scheme 10.13, cyclization of ene-yne-ketone 15 begins with the nucleophilic attack of carbonyl oxygen to an internal carbon of an alkyne in q -alkyne complex A. A might be the most plausible pathway for generation of (2-furyl)carbene-chromium complex 18. A slipped, polarized q -complex B would be an alternatively possible intermediate. It is known that A is prone to isomerize to B, which has been... 

Another example of the application of the chromium carbene complex for the synthesis of benzannulated compounds was described by Herndon and Wang." The coupling of substituted carbene chromium complex 27 with conjugated enediyne 28 results in the formation of intermediate enyne-ketene 29, which undergoes the Moore cyclization to produce the intermediate chromium-complexed diradical 30. The... 

Herndon et al. have investigated the reactions of Fischer carbene chromium complexes with conjugated enediynes that feature a pendant alkene group such as 32. The experimental results confirm that arene... 

Miscellaneous Triple Bond Additions. An example has been published in which a Fischer carbene chromium complex adds to 3-trimethylsilyl-1-propyne, to give the Dotz benzannulation product in 62% yield (eq 44). ... 

A number of miscellaneous methods for forming carbon-carbon bonds are of interest. Carbene-chromium complexes were involved in the synthesis of deoxyfrenolicin, the important step being conversion into the naphthoquinone (Scheme 32). Silylated tricarbonylarenechromium(0) complexes of indole may... 

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