Chromium vinylcarbenes

The facile route for introduction of the phosphole ring into the coordination sphere of the chromium vinylcarbene complex is via [4 - - 2] intramolecular... 

Experimental Procedure 2.2.8. [4 + 3] Cycloaddition of a Chromium Vinylcarbene Complex to a 1-Azadiene rranj-i-(2-Furyl)-2-inethoxy-5-methyl-4,5-dihydro-3H-azepine [390]... 

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. 

Even the distorted boat-like deck in [2.2]metacyclophanes can be constructed by an intramolecular version of the benzannulation. A suitable precursor bears a chromium vinylcarbene and an allcyne moiety linked to a meta-phenylene core by two-atom bridges, as shown for complexes 80. Benzannulation under the typical conditions affords hydro-quinonophanes 81 in fair yields (Scheme 31) [73]. Interestingly, the intramolecular benzannulation approach even tolerates heteroatom bridges, which impose both additional strain and helicity on the cyclophane skeleton [73b]. 

Barluenga and co-workers extended the scope of [3+4] annulations of chromium vinylcarbenes by using 2-amino-1,3-butadienes as the diene component. In most cases the reaction with this diene is highly diastereoselective and favors the formation of cw-divinylcyclo-propanes that rapidly rearrange to the cycloheptadienes. Furthermore, the amino functionality can be a chiral auxiliary such that an asymmetric [3+4] annulation is achieved. Examples of this approach are shown in Scheme 17. Reaction of the chromium vinylcarbene complex... 

Many of the syntheses we have seen within this review depend on the carbonylation of a vinylcarbene complex for the generation of the vinylketene species. The ease of this carbonylation process is controlled, to some degree, by the identity of the metal. The electronic characteristics of the metal will clearly have a great effect on the strength of the metal-carbon double bond, and as such this could be a regulating factor in the carbene-ketene transformation. It is interesting to note the comparative reactivity of a (vinylcarbene)chromium species with its iron analogue The former is a fairly stable species, whereas the latter has been shown to carbonylate readily to form the appropriate (vinylketene)iron complex. 

In addition to the reaction of vinylcarbene complexes with alkynes, further synthetic procedures have been developed in which Fischer-type carbene complexes are used for the preparation of benzenes. Most of these transformations are likely to be mechanistically related to the Dbtz benzannulation reaction, and can be rationalized as sequences of alkyne-insertions, CO-insertions, and electrocycli-zations. A selection of examples is given in Table 2.18. Entry 4 in Table 2.18 is an example of the Diels-Alder reaction (with inverse electron demand) of an enamine with a pyran-2-ylidene complex (see also Section 2.2.7 and Figure 2.36). In this example the adduct initially formed eliminates both chromium hexacarbonyl ([4 -I- 2] cycloreversion) and pyrrolidine to yield a substituted benzene. 

Cyclobutanones3 The reaction of pentacarbonyl(methoxymethylmethylene)-chromium (1) with the enyne 2 in acetonitrile results in two isomeric bicyclohep-tanones [(E)-3 and (Z)-3] as the major products. These probably arise by initial reaction with the triple bond to give a vinylcarbene complex (a), which undergoes insertion of CO to give a vinylketene complex (b). An intramolecular [2 + 2] cycloaddition results in the cyclobutanones (3). 

Treatment of the vinylcarbene chromium complex 97 with /-BuC=P affords the dihydrophosphetylketene complex 98 (Scheme 26). This transformation is believed to proceed via r -phosphaalkyne carbene-, phosphaalkenylcar-bene-, and phosphaalkenylketene complexes as intermediates. An intramolecular [2+2] cycloaddition completes the reaction sequence.53 Different carbene/carbon monoxide/phosphaalkyne cycloaddition products (e.g., 1,3-oxaphospholes, phosphaphenanthrenes) are obtained depending on substitution at the carbene ligand (vide infra). 

Inter- and intramolecular trapping of vinylcarbene complexes generated in situ, including those of metals such as chromium, molybdenum and tungsten, have also been described. ... 

However, yields in the intermolecular cycloaddition reactions of vinylcarbene complexes, formed by intramolecular insertion of an alkynyl tethered metal carbene complex, are higher when molybdenum rather than chromium or tungsten carbene complexes are employed. Mild thermolysis (THF, 65 °C, 1 h) in the presence of ten equivalents of an electronically undemanding alkene directly leads to the 2-alkyl-2-(2-methoxycyclopentenyl)cyclopropanes 31. ... 

Fischer-type chromium carbene complexes with 1-ethoxycyclopropylalkynyl substituents at the carbene carbon, e.g. 25, on reaction with dimethylamine and subsequent conversion of the resulting vinylcarbene with alkynes surprisingly did not give phenol derivatives, as would be expected from the known Dotz reaction, but gave cyclopenta[ )]pyrans, e.g. 26. The reaction is interpreted as a double alkyne-insertion/CO-insertion sequence with formation of a trienylketene intermediate, which undergoes intramolecular hetero-Diels-Alder cycloaddition and dimethylamine elimination. ... 

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 reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

Cyclopropanation. These carbenes are particularly useful for cyclopropanation of electron-poor olefins. The reaction occurs under milder conditions and at a faster rate with molybdenum carbenes than with chromium- or tungsten-derived complexes. This cyclopropanation has been used to trap a molybdenum vinylcarbene generated by intra-... 

The vinylcarbene complexes Cr(CX))5 =C(OEt)C=CXR) (X = NMe2, OEt R = alkyl, aryl) react with alkynes R OCH to form various cyclopenta[b]pyrans.30S In a two step process, addition of the alkynes NEt20Chfe u> the l-metalla-l,3-diene W(CX))s =C(OEt)CHsCHHi) results in metallatriene species that cyclise U) fwm cyclopentadiene complexes 40. Chromium Fischer type alkoxyalkenyl carbene complexes react with ketoalkynes to fcxm bicyclic lactones in a number of different processes involving 8 and 10-e cyclizations. Various dienyl carbene complexes whose parent in Crortho-substituted aromatic alcohols through a variation upon the more usual benzannulation reaction involving alkyne cycloaddition with carbene complexes. ... 

Another route to vinylcarbene C-glycosides is based on a TiCl4-assisted aldol condensation of pentacarbonyl[(methoxy)methylcarbene]chromium (229) and formyl glycosides 230-232. This reaction is trans-selective and affords sugar carbene complexes 233-235 in good yields (Scheme 11.53) [111]. 

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