Alkyne insertion metal carbene complexes

The photochemistry of metal carbyne complexes is in many ways similar to the photochemistry of metal carbene complexes, but the reactions have not been developed or become as synthetically useful as the photochemistry of metal carbene complexes. Among reported reactions are couplings with ancillary CO ligands to form ketenyl complexes, protonation of the carbyne carbon, insertions into C-H bonds, addition of the carbyne carbon to an alkyne to produce a cyclopropenyl complex, and electron-transfer reactions. ... 

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. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

In most of the reactions of heteroatom-substituted carbene complexes with alkynes the first event is insertion of the alkyne into the carbon-metal double bond. If vinylcarbene complexes undergo insertion reactions with alkynes, (1,3-butadien-l-yl)carbene complexes result (Figure 2.27). 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Carbometallation is a term coined for describing chemical processes involving net addition of carbon-metal bonds to carbon-carbon Jt-bonds [1] (Scheme 4.1). It represents a class of insertion reactions. Whereas the term insertion per se does not imply anything chemical, the term carbometallation itself not only explicitly and clearly indicates carbon-metal bond addition but also is readily modifiable to generate many additional, more specific terms such as carboalumination, arylpalladation, and so on. In principle, carbometallation may involve addition of carbon-metal double and triple bonds, that is, carbene- and carbyne-metal bonds, as well as those of metallacycles. Inasmuch as alkene- and alkyne-metal Jt-complexes can also be represented as three-membered metallacycles, their ring expansion reactions via addition to alkenes and alkynes may also be viewed as carbometallation processes (Scheme 4.1). 

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. 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

A further approach to the synthesis of pyrroles other than that shown in Scheme 47 involves formation of l-metalla-5-aza-l,3,5-trienes 123 (M = W) by insertion of an alkyne into the M=C bond of an (imino)carbene complex 121, leading to chain extension by two carbon atoms ( alkyne route to pyrroles). Compounds 123 cyclize spontaneously to 2//-pyrrole complexes 124, from which ///-pyrroles 122 are obtained by disengagement of the metal unit (Scheme 48).l2d 2H-Pyrrole complexes of type 124 have been characterized by X-ray analysis, and 2/Z-pyrroles could be eliminated from such complexes if substituents other than hydrogen are attached to C2 of the compound.136125 137 l2al2b... 

Knox and co-workers (190) reported that irradiation of the Fe-and Ru-carbene complexes 157 in the presence of alkynes results in loss of CO and insertion of the alkyne into a metal-carbene bond to give 158 [Eq. (150)]. A variety of alkynes (R"=H, CH3, or C02Me) and... 

Reactions with other carbon triple bonded functional groups. The substitution of nitrile for alkynes does not lead to pyridines or quinolines in the benzannulation reaction.Instead noncyclic products are obtained that are the result of insertion of the carbon-nitrogen triple bond into the metal-carbene bond. On the other hand, in a very recent report it was found that X -phosphaalkynes will undergo the benzannulation reaction to produce phosphaarene chromium tricafbonyl complexes. 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

C(OMe)C6H4-o-C=CPh (CO)j leads directly to the formation of a chrysene derivative via the formal dimerization of the carbene ligand. A plausible explanation for the formation of the final product involves a doubly alkyne-bridged dinuclear complex, alkyne insertions into metal-carbene bonds, and coupling of the carbene carbons. 

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). ... 

Alkyne polymerization in organic media has been reviewed [131]. A large variety of catalysts has been reported to polymerize alkynes in organic media. Similar to the polymerization of olefins, early transition metal as well as late transition metal catalysts are effective for this polymerization. Depending on the nature of the metal, two different mechanisms of polymerization have been suggested polymerization via a metal alkyl intermediate, or via a metal carbene (Scheme 7.9). With metal alkyl complexes, polymerization proceeds via migratory insertion of the alkyne into the metal-carbon bond [path (a) in Scheme 7.9] whereas with metal carbenes the mechanism is equivalent to that of metathesis [path (b)]. 

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],... 

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. 

Schrock suggested that metallabenzenes might be involved as intermediates in the reactions of metal-lacyclobutadienes with alkynes. For example, treatment of metallabutadiene A (Scheme 26) with 2-bu-tyne leads to the formation of the cyclopentadienyl— metal complex E. perhaps through the intermediacy of metallabenzene C. In this case, alkyne insertion into a metallacyclobutadiene M—C bond is proposed, followed by carbene migratory insertion. An alternative mechanism involves the formation of a Dewar metallabenzene intermediate (D, Scheme 26). followed by reductive elimination. 

Diazo compounds, with or without metal catalysis, are well-known sources of carbenes. For synthetic purposes a metal catalyst is used. The diazo compounds employed are usually a- to an electron-withdrawing group, such as an ester or a ketone, for stability. In the early days, copper powder was the catalyst of choice, but now salts of rhodium are favoured. The chemistry that results looks very like the chemistry of free carbenes, involving cyclopropanation of alkenes, cyclopropenation of alkynes, C-H insertion reactions and nucleophilic trapping. As with other reactions in this chapter, free carbenes are not involved. Rhodium-carbene complexes are responsible for the chemistry. This has enormous consequences for the synthetic applications of the carbenes - not only does the metal tame the ferocity of the carbene, but it also allows control of the chemo-, regio- and stereoselectivity of the reaction by the choice of ligands. 

We know that double oxidative addition of an alkene to a metal to produce a metal-bis-carbene or triple oxidative addition of an alkyne to produce a metal-bis-carbyne complex is not possible. Olefin insertion into a metal-hydride giving a metal-alkyl, however, can be followed by a-elimination to yield a hydrido-metal-carbene or even further double a-elimination can give a hydrido-metal-carbyne. Such a case is known the reaction of styrene or phenylacetylene with [Os(H)Cl2L2] (L = Pi-Pr3) yields the carbyne complex [OsHCl2(CCH2Ph)L2] via the metal-carbene intermediate. PhEt is also formed as a result of hydride consumption (K.G. Caulton, J. Organomet. Chem. 2001,617-618,56). 

---