Fischer carbene complexes, cyclic

The reactions of Fischer carbene complexes with 1,3-dienes (carbodienes or heterodienes) lead to the formation of cyclic products with different ring sizes depending upon both the nature of the reaction partners and the reaction conditions. Between these synthetically useful transformations are found [2c+2s], [3C+2S], [4S+1C], [3S+3C], [4S+2C], [4S+3C] and [2S+1C+1C0] cycloaddition reactions which will be summarised further on, in addition to the [2S+1C] cycloaddition processes here described. 

The formation of cyclic Fischer carbene complexes by ruthenium-mediated activation of 3-butyn-l-ol and 4-pent5m-l-ol has been reported before (147,155,158-164). Correspondingly, a reaction of [Ru(bdmpza)Cl(PPh3)2] (24) with these terminal alkynols results... 

Scheme 5.7 Formation of a cyclic Fischer-carbene complex via vinylidene by Dotz.

The reactions of both alkoxy and amino complexes are highly stereoselective and give the unnatural epimer at the C-6 position in the penicillin analogs, but methods are known for inversion at this center, llie mechanism of these reactions is thought to involve the intermediacy of a metal-4cetene complex whose formation is photo-induced.Early indications that this was the case came when it was found these reactions fail to give cyclic products of any kind under thermal conditions. More recently, vi-nylketene complexes have been trapped from these reactions.With the recent realization that metal-ketene intermediates are likely to be involved in these reactions further development of the photo-induced reactions of Fischer carbene complexes can be anticipated. 

Cycloaddition. Cyclic Fischer carbene complexes are generated from o-ethynylaryl carbonyl compounds with the title reagent at room temperature. When enol ethers or enamines are present, a Diels-Alder reaction-retro-Diels-Alder sequence occurs to furnish substituted naphthalenes (7 examples, 54—84%). 

A very rapid growth of novel and synthetically useful reactions of Fischer-type carbene complexes see Fischer type Carbene Complexes has occurred over the last decade. Reactions leading to cyclic and polycyclic compounds of high complexity and reactions wherein chirality has been transferred from the starting material to the product are of particular importance. A number of recent reviews on the chemistry of Fischer carbene complexes and their application in organic synthesis are available. 

The intramolecular Diels-Alder reactions of the Fischer carbene complexes, (L)(0C)4W=C(0R)CH=CH(CH2) CH=CHCH=CH2, (L = CO, PPh3 R = Me or Pri n = 3 or 4) are compared with known reactions of the analogous methyl esters. The stereoselectivites are comparable to those of the Lewis acid catalysed reactions but are more tolerant of functionalised diene units." The reaction between Mo(CO)6 and All(H)N(CH2)2NHAll/CH(NMe2)(OMe)2 (All = allyl) in methylcyclohexane at 1(X) C affords the chelated cyclic carbene complex 122, while polymeric carbene materials 123 are available from electron rich alkene precursors containing pendant phosphine donor groups such as 124. ... 

Electrochemical measurements permit also a quantitative comparison of two types of Fischer carbene complexes with different stabilizing heteroatom the aminocarbene complexes of Cr and W (data from Fig. 48.1 Table 48.1) and some analogous alkoxycarbene complexes of Cr and W (data published in Reference 26), whose reduction potentials were measured by cyclic voltammetry under similar conditions (in nonaqueous acetonitrile vs SCE). 

The reaction of a,(3-unsaturated Fischer carbene complexes 42 with cyclic enamines 43 gives cyclopentannulation products 46 with complete regioselectivity by a formal [3-1-2] cycloaddition (Scheme 5.11) [20]. The formal [3-1-2] cycloaddition is assumed to occur in a stepwise manner via the 1,4-addition of cyclic enamines 43 to the alkynyl carbene complexes 42 (formation of iminium intermediate 44), the intramolecular cyclization to the formed iminium moiety from the carbene carbon (formation of intermediate 45), and the hydrolysis reaction. When (—)-8-phenylmenthyl derivatives of Fischer carbene complexes 42 are reacted with indoles 43, the corresponding indolinones 46 are obtained with >97% ee. 

In 1985, Dbtz et al. reported during a study on the reaction of Fischer-type carbene complexes with alkynes [10] that 2-oxacyclopentylidene chromium complex 24 was obtained as a side product. Thus, treatment ofmethyl(methoxy)carbene complex with 3-butynol at 70 °C in dibutyl ether gave the cyclic carbene complex 24 in 23% yield along with the desired metathesis product 23. The authors briefly commented that the cyclic carbene complex 24 might be obtained through the vinylidene complex 25, generated by the reaction of the alkyne with the liberated pentacarbonylchromium species (Scheme 5.7). 

Then, in 1987, Dotz reported an improved procedure for this transformation, for which the use of Et20 as solvent improved the yield of the cyclic carbene complexes considerably [12]. For example, the five-membered Fischer-type carbene complex 29 (n=l) was prepared in 58 66% yield by the reaction of preformed M(CO)5(L) (M = Cr, W, L = Et20) and 3-butynol in Et20 at room temperature. The six-membered cyclic carbene complex could also be prepared by this method. This method has been applied to the preparation of functionalized cyclic Fischer-type carbene complexes from the corresponding alkynols. For example, Dotz et al. reported the preparation of various carbohydrate-functionalized cyclic Fischer-type carbene complexes, one of which is shovm in Scheme 5.9. 

In 1994, Quayle et al. reported the application of this cyclic Fischer-carbene synthesis from 3-butynols to spirolactone synthesis, although the process was stepwise and a stoichiometric amount of the complex was employed [17]. The key transformation was the chromium or tungsten carbene complex formation followed by the CAN oxidation of the complex to give y-lactone. The reaction was further applied to the synthesis of andirolactone and muricatacin, the former being shown in Scheme 5.14. 

The reaction of deprotonated 2-aminophenol 115 with an alkynyl-substituted Fischer carbene 277 gave a cyclic complex 279 in 40% yield, most probably via an intermediate adduct 278 (X = 0). On the other hand, complex 280, obtained from 115 and 277 without base, was heated in tetrahydrofuran to afford benzoxazepine derivative 281 (38%) and benzoxacinone 282 (36%) with reverse regiochemistry (Scheme 49) <2003CEJ4943>. 

Complexes of type [LyM (CH2)IIX ], where n > 1, have been shown to be useful precursors for hetero- and homobimetallic n(a.,a>) alkanediyl complexes, [LjcM(CH2)bM L, ] (where ML, is not necessarily the same as M Lj,). Such hydrocarbon-bridged binuclear compounds have been proposed as models for intermediates in the Fischer-Tropsch reaction (18,19) and other significant catalytic processes (20-23). Some [LyM (CH2)BX ] complexes are precursors to cyclic carbene complexes (Section III), whereas others have been shown to have synthetic utility in organic chemistry (24). 

The cyclic carbene complex (23) formed in equation 10.722 is analogous to the original Fischer-type carbene complex. The ring, however, offers an added complexity that has utility in the realm of organic synthesis. 

In the case of using lactones as electrophiles in the presence of cerium trichloride, spiroketals are obtained after acidic work-up (90JA6389). Cyclic Fischer-type chromium carbene complexes (21) have also been prepared when hexacarbonyl chromium was added to intermediates (18), followed by treatment with trimethyloxonium tetrafluoroborate (Scheme 7) (92JCS(CC)1623). 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

Both 211 and 297 (see also Section II-C.2) have been foxmd to effect double C—H activation of cyclic ethers to afford Fischer-type carbene complexes (Section II-D.2), leading in some instances to C-C coupling reactions." Comparable double C-H activations have also been effected using 194," and the /" -isoprene complex Tp Ir(f/" -2-MeC4H5) (238) with several substrates (MeOPh, Me2NPh,... 

The double a-C-H activation of cyclic and acyclic ethers and amines, affording Fischer-type, i.e. hetero-atom substituted carbene complexes, has also been well... 

SCHEME 5.11 Cyclopentannulation of Fischer alkynyl carbene complexes 42 with cyclic enam-ines 43. 

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