Carbene complexes with vinyl ethers

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

The reaction of these transition metal carbene complexes with some nucleophiles such as isocyanante, thiophenol, hydrazine, orhydroxylamine,have been studied. For example, the carbene part of the complex is converted into vinyl ether by pyridine 100>. 

Subsequent experiments probed the reaction of these species with vinyl ether compounds, which irreversibly yield catalyticaUy inactive Fischer car-bene complexes (the Fischer carbene is far more thermodynamically favor-... 

Similarly, reaction with vinyl ethers generates less active Fischer carbene species. Grubbs and coworkers have prepared and characterized a series of these complexes, via metathesis of the corresponding alkene with G1 or G2 (e.g. Scheme 2.34). While these species can still perform metathesis, they are poorly active, and require high temperatures at which there are competing decomposition reactions to yield hydridocarbonyl complexes. 

Although no cyclopropane formation was observed in the reactions of heteroatom-stabilized carbene complexes with simple alkenes such as cyclohexene or tetramethylethylene, cyclopropane formation has been observed both with electron-deficient a,) -unsaturated esters and with electron-rich vinyl ethers. The mechanisms involved in cyclopropane formation from these very different classes of alkenes may be substantially different. 

The reaction of metal-carbene complexes with electron-rich vinyl ethers occurs under milder conditions than the reaction with electron-poor unsaturated esters. The conditions are also milder than those required for ligand substitution of carbene complexes. The reaction products depend strongly on the external CO pressure with no added CO, alkene scission products predominate under 100 atm CO pressure, cyclopropanes are formed in 60% yield (Dotz and Fischer, 1972b). The ratio of isomeric cyclopropanes formed... 

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. 

Anionic alkoxy Fischer-type carbene complexes were shown to react with (Ph3P)AuGl to give a unique vinyl ether complex, which is bound in a quasi-771 fashion to the Cr/Mo centers. Upon treatment with triphenylphosphine, the gold-vinyl ether ligand can be liberated and isolated. This reaction thus gives access for the first time to aurated vinyl ethers (Scheme 57).38... 

Z,Z)-l,4-Dialkoxy-l,3-dienes can be readily prepared from propargyl ethers and molybdenum carbene complexes (equation 185)307. High stereoselectivity in this reaction may be due to the formation of stable vinyl hydride complex with the enol ether. 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

In 2002, Grubbs and co-workers reported the first CM reactions of allyl phosphines.In an initial reaction, subjecting allyl diphenylphosphine to catalyst 5 (5 mol%) failed to produce any of the desired cross-product. However, by protecting the phosphine as its borane complex, CM reactions could be achieved in good yield with high E-selectivity (Equation (5)). Notably, catalyst 5 failed to dimerize borane-protected vinyl diphenylphosphine. This result was attributed to substrate trapping of the catalyst as an unreactive Fischer carbene, a situation analogous to that observed in the CM reactions of alkyl vinyl ethers. 

Casey and coworicers have shown that ketone etiolates add efficiently to a,3-unsaturated vinyl carbene complexes (164), irrespective of 3,3-disubstitution on the complex or high substitution on the enol-ate 133 thus, contiguous 3 and y quaternary centers are easily assembled. When coupled with the ease of release of the carbene ligand from the complexes by either oxidation to the ester functionality1331 or elimination to the corresponding enol ether,133 the vinyl carbene complexes are synthetic equivalents for a,3-unsaturated esters or a,3-unsaturated aldehydes, respectively (Scheme 63). 

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

Three different strategies have been envisaged. The chiral information can either be incorporated into the alkyne or linked to the heteroatom or to the a,/ -unsaturated substituent at the carbene complex carbene carbon. High diastereoselectivities (57a 57b >96 4) have been observed in reactions of vinyl carbene complex 55 with the chiral propargylic ether 56 bearing the bulky trityloxy substituent [57a]. A more general approach is based on chiral alcohols incorporated into the alkoxycarbene complex. Upon benzannulation with tert-butylethyne, the menthyloxy carbene complex 58 gave a diastereoselectivity of 10 1 in favor of the naphthalene tricarbonylchromium complex 59a [57c, 57d]. Finally, the tandem benzannulation-Mitsunobu reaction of optically active carbene complex 60 with 5-hexyn-l-ol afforded the anti-benzoxepine complex 61 as the only diastereomer (Scheme 23) [57b]. 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

It has been reported that treatment of 70 with silyl enol ether generates active species only toward olefin isomerization (Eq. 12.41) [47]. When vinyl acetate was added to the reaction instead of silyl enol ether, neither metathesis nor isomerization took place. Although details of the active species remain unclear, Fischer-type carbene complexes would be formed in the reaction of 70 with silyl enol ether. It has also been recognized that hydride-carbonyl complexes were formed by the thermolysis of... 

Vinyl ethers.2 This stable metal-carbene complex (1) reacts with Wittig reagents at room temperature to give vinyl ethers (2) in high yield and pentacarbonyltriphenyl-phosphinetungsten(O) (3). 

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. 

These ruthenium complexes react rapidly and quantitatively with ethyl vinyl ether to form a Fischer carbene that is only weakly metathesis active at elevated temperatures [86, 87]. This property can be employed to end-cap ROMP and ADMET polymers and to ensure that there are no polymeric ruthenium alkyhdenes present. Since ruthenium alkylidenes are relatively robust complexes they could survive workup procedures, although experimental evidence has yet to confirm this notion. Treatment of an ADMET polymer with ethyl vinyl ether gives the polymer well-defined terminal olefinic endgroups and should prevent backbiting metathesis upon dilution of the polymer (Scheme 6.22). 

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