Vinylcarbene from metallates

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. 

Weiss studied68a the reactivity of both new complexes, and found that a variety of phosphines and phosphites would also convert the vinylcarbene complex 139 into the corresponding vinylketene complex (140), capturing one of the carbonyl ligands from the coordination sphere of the metal to become the ketene carbonyl. Only in the case of triphenylphosphine was the dicarbonyl(phosphine)vinylcarbene complex (141) isolated, which then required addition of carbon monoxide to convert it to the dicarbonyl(triphe-nylphosphine)vinylketene complex 140.a. This interconversion was reversible and proceeded quantitatively. 

Further evidence for the intermediacy of an -vinylcarbene has been provided by Garcia-Mellado and Alvarez-Toledano.139 They have shown that lithium dimethylcuprate may be used in place of an alkyllithium reagent in the absence of carbon monoxide, necessary CO being scavenged from other metal-carbonyl species hence the low yields observed from these reactions. 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

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. 

Methoxyfuran also reacted with vinylcarbenes that were derived from sec-O-propargyl thiocarbamates in a transition metal-catalyzed ring opening to give a mixture of geometrical isomers, as depicted below <07TL6651>. 

The mechanism of the Dotz benzannulation reaction has not been fully elucidated. The first step is the ratedetermining dissociation of one carbonyl ligand from the Fischer carbene complex, which is cis to the carbene moiety. Subsequently, the alkyne component coordinates to the coordinatively unsaturated carbene complex, and then it inserts into the metal-carbon bond. After the alkyne insertion, a vinylcarbene is formed that can lead to the product by two different pathways (Path A or Path b). ... 

Among the methods at hand to synthesize cyclopropane derivatives, carbene addition to alkenes plays a prominent role 63). As a source of vinylcarbenes, cyclopropenes might be useful in this kind of approach. In 1963, Stechl was the first to observe a transition metal catalyzed cyclopropene-vinylcarbene rearrangement64). When treating 1,3,3-trimethylcyclopropene with copper salts, dimerization occurred to give 2,3,6,7-tetramethyl-octa-2,4,6-triene (9), the product from a formal recombination of the corresponding vinylcarbene (Eq. 8). 

In the meantime thermal65 and metal catalyzed66,67> rearrangements of cyclopropenes have been detected as convenient methods for the preparation of vinylcyclo-propanes via formal [2+ l]-cycloadditions of vinylcarbenes to alkenes (Eq. 9) (for an alternative entrance starting from allylidene dichloride or 1,3-dichloropropene, see Ref. 68)). 

Indenes, like cyclobutenones and furans, are common side-products in the reaction of chromium arylalkoxycarbene complexes with alkynes, especially internal alkynes [9]. The in-dene structure comes about by a process that is very similar to naphthol formation annula-tion to the aryl ring still occurs, but without carbon monoxide insertion, and, instead, bond formation takes place directly between an alkyne carbon and the aryl carbon ortho to the metal carbene substituent [Eq. (18)] [4]. Scheme 5-1 shows two pathways that have been suggested for this transformation beginning from the vinylcarbene intermediate 3, naphthol formation can be diverted to intermediate 8, either by direct cyclization (3 -+ 8) or through the chromacyclohexadiene (3->6- 8). Aromatization and decomplexation yield the indene [7 b, d, 43], More detailed mechanistic analyses consider the roles of the stereochemistry of 3, as an ( )- or (Z)-vinylcarbene, as well as the coordination of external ligands, in the production of indenes, naphthols, furans, cyclobutenones, and other common side-products [8 a, 9, 13, 44],... 

In the two separate, initial reports on the reactivity of Fischer carbenes with enynes, one study found cyclobutanone and furan products [59], while the other found products due to olefin metathesis [60]. These products have turned out to be the exceptions rather than the rule, as enynes have since been found to react with Fischer carbenes to produce bicyclic cyclopropanes quite generally. The proposed mechanistic pathway is included as part of Bq. (28), in which vinylcarbene 10, produced by insertion of the alkyne into the metal carbene, may then cyclize with the pendant olefin to metallacyclobutane 11, leading to product. The first reported version of this reaction suffered from extreme sensitivity to olefin substitution [Eq. (28) compare R=H, Me] often producing side-products due to metathesis (through 11 to yield dienes) and CO insertion (into 10 to yield cyclobutanones and furans) [61]. Since then, several important modifications have been developed which improve yield, provide greater tolerance for alkene substitution, and increase chemoselectivity for the bicyclic cyclopropane... 

The subsequent steps of the reaction are too fast in order to allow further kinetic investigation. A previously proposed 16-electron chromacyclobutene intermediate arising from a formal [2-i-2]-cycloaddition of the alkyne ligand aross the metal-carbene bond was later discarded as a result of theoretical studies which support a direct insertion of the alkyne into the metal-carbene bond to generate an 18-electron valence-isomer, the rj -vinylcarbene complex D. [48] A related species III has been isolated from the reaction of an aminocarbene complex. [46] Subsequent insertion of a carbonyl ligand leads to an rj -yinylketene complex E, of which structural analogues as enaminoketene complex IV have been synthesized. 

Addition of diazo compounds to metallic complexes allows the formation of metal carbenoid species which can react with unsaturated molecules to form new carbon-carbon bonds. The Cp RuCl(cod)-catalyzed addition of diazo compoimds to alkynes led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoaUcane and one molecule of alkyne [115,116] [Eqs. (53) and (54)]. The ruthenium carbene, generated from diazo compound, reacts with the C=C bond to produce vinylcarbene intermediate able to add a second molecule of diazo compotmd to generate dienes. The stereoselective formation of these conjugated dienes results from the selective creation of two C=C bonds, probably due to the possibility for (C5Me5)RuCl moiety to accomodate two cis carbene ligands. This reaction occurred with terminal or internal alkynes as well as 1,3-diynes [115] and was applied successfully to alkynylboronates [116]. 

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