Vinyl chromium carbene complex

P. Quayle and co-workers utilized the Dotz benzannulation reaction for the synthesis of diterpenoid quinones." The authors developed a novel synthetic approach to 12-O-methyl royleanone using a simple vinyl chromium carbene complex along with a disubstituted oxygenated acetylene. The bicyclic hydrazone was converted to the corresponding vinyllithium derivative by the Shapiro reaction and then functionalized to give the desired crude Fischer chromium carbene complex. The benzannulation took place in refluxing THF with excellent regioselectivity, and the natural product was obtained in 37% overall yield from the hydrazone. 

Hegedus and co-workers8 reported the synthesis of (4S,5R)-4,5-diphenyl-3-vinyl-2-oxazolidinone (the enantiomer of the compound prepared here) via the chromium carbene complex in a fair yield. This is an interesting method, but the procedure is complicated (e.g., low temperature, argon atomsphere) and the chromium waste must be disposed of in an appropriate way. On the other hand, this procedure, consisting of transacetalization9 and pyrolysis,10 is simple and safe. Optically active 3-vinyl-2-oxazolidinone is also used for the synthesis of (1 R,2S)-2-fluorocyclopropylamine11a b that is the key intermediate for novel antibacterial quinolonecarboxylic acids. 

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

DIELS-ALDER DIENOPHILES Bis(2,2,2-trichloroethyl) azodicarboxylate. 1-Bromo-2-chlorocyclopropene. Chromium carbene complexes. (R)-Ethyl p-tolysulfinyl-methylenepropionate.l (S)-a-Hydroxy-p,p-dimethylpropyl vinyl iketone. (IR)-m-3-Hydroxy isobomyl ndopentyl ether. Oxa-zolidones, chiral. Phdnylselenenyl ben-zenesulfonate. 

Generally, arene(alkoxy)carbene chromium complexes react with aryl-, alkyl-, terminal or internal alkynes in ethers or acetonitrile to yield 4-alkoxy-l-naphthols, with the more hindered substituent ortho to the hydroxyl group . Upon treatment with alkynes, aryl(dialkylamino)carbene chromium complexes do not yield aminonaphthols, but they form indene derivatives . Vinyl(dialkylamino)carbene complexes, however, react with alkynes to yield aminophenols as the main products The solvent is one of the many factors that affects this type of reaction, for which the most important is the polarity and/or coordinating ability of the solvent. The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. 

But-2-enedinitrile, 2-nitropropene, phenyl vinyl sulfoxide, styrene and but-2-enal do not form 1-methoxy-l -phenylcyclopropanes with the corresponding chromium-carbene complex. " On the other hand, methyl vinyl ketone gave a formal 1,4-adduct, which, due to its instability, was further transformed under acidic conditions. ... 

This method has been applied to the synthesis of taxodione and related diterpenes through cyclohexadienone annulations of a chromium-carbene complex which was prepared from a hindered vinyl triflate12lb (equation 99a). 

Aryl LXIV and vinyl LXV chromium carbene complexes are respectively implied in these synthesis as key intermediates ... 

Three component cyclization of an aromatic or vinylic alkoxy pentacarbonyl chromium carbene complex, an alkyne, and carbon monoxide, generating a Cr(CO)3 coordinated phenol ... 

Carbonylation of the chromium carbene complex (CO)5Cr=C(OCH3)Ph at 150bar pressure in the presence of l-vinyl-2-pyrrolidone resulted in Cr(CO)6 and organic products that were rationalized by the intermediate formation of methoxyphenylk-etene [33]. The formation of a free vinylketene and vinylketene chromium complexes were found in the reaction of (CO)5Cr=C(OCH3)Ph and bis(trimethylsilyl)acetylene at 50 °C (reaction 8.19) [34-36]. 

Yamashita, A. and Scahill, T.A. (1982) Reaction of aryl chromium carbene complexes with ethyl propiolate. A versatile vinyl ether formation. Tetrahedron Letters, 23, 3765-3768. 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

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]. 

Experimental Procedure 2.2.8. [4 + 3] Cycloaddition of a Chromium Vinyl-carbene Complex to a 1-Azadiene rra/w-4-(2-Furyl)-2-methoxy-5-methyl-4,5-dihydro-3H-azepine... 

Cr(CO)3-coordinated hydroquinone from vinylic alkoxy pentacarbonyl chromium carbene (Fischer carbene) complex and alkynes. 

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. 

Insertion of the alkyne into the chromium carbene bond in intermediate B affords vinyl carbene complex D, in which the C=C double bond may be either (Z) or (E). A putative chromacydobutene intermediate resulting from a [2+2] cydoaddition of the alkyne across the metal-carbene bond on the way to chromium vinylcarbene D, as was sometimes suggested in early mechanistic discussions, has been characterized as a high energy spedes on the basis of theoretical calculations [9c]. Its formation and ring-opening cannot compete with the direct insertion path of the alkyne into the chromium-carbene bond. An example of an (E)-D alkyne insertion product has been isolated as the decarbonylation product of a tetracarbonyl chromahexatriene (4, Scheme 4) [14], and has been characterized by NMR spectroscopy and X-ray analysis. 

The stepwise coupling of two cis ligands as depicted in Scheme 3 has been verified as involving a sequence of three discrete steps at low temperatures, allowing the isolation of the relevant intermediates as individual compounds [18]. When a chelated tetracarbonyl amino-vinyl carbene complex (chelated analogue of intermediate B in Scheme 3) was reacted with an electron-deficient alkyne under controlled conditions, a l,4,5- 3-dienylcarbene tetracarbonyl chromium complex (corresponding to intermediate D in Scheme 3) was formed. It underwent thermal decomposition to give phenol derivatives as the final products. 

Finally, the a,/ -unsaturated carbene complex may be generated in situ by alkyne insertion into a chromium-carbene bond of a saturated chromium carbene leading to a chromium vinyl carbene (equivalent to intermediate (f )-D in the mechanism of the benzannulation reaction, see Section 8.2.1, Scheme 3), which may undergo subsequent benzannulation with a second equivalent of the alkyne [43a]. This strategy was subsequently applied to the synthesis of (Z)-enediynes and related compounds [43b], and to that of substituted benzofurans (see also Section 8.5) [43c, 43d]. 

Vinyl-substituted benzo[c]furans can be prepared by reaction of n-alkynylbenzaldehydes with chromium Fischer carbene complexes. Initially a benzo[c]furan chromiumtricarbonyl complex is believed to be formed which is converted into an alkylidenephthalan derivative or can be trapped with electron-deficient dienophiles with excellent ( 3 ti-selectivity (Equation 128) <20000L1267>. More elaborate vinylidene Fischer carbene complexes yield dienyl benzo[c]furans that undergo [8-1-2] cycloaddition with DMAD to furnish furanophane derivatives <2003JA12720>. An equilibrium between 7] -(o-ethynylbenzoyl)rhenium complexes and rhenium benzo[f]furyl carbene complexes has been observed. These species behave like other benzo[< ]furans in the reaction with DMAD <20040M4121>. 

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