Vinylcarbene complexes

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

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

Barluenga et al. have described novel vinylcarbene complexes containing a cyclic BF2 chelated structure which temporarily fixes the s-cis conformation of the exocyclic C=C and Cr=C double bonds. These boroxycarbene complexes behave as dienophiles with 2-amino-l,3-butadienes in a remarkably regio- and exo-selective way. Moreover, high degrees of enantioselectivity are reached by the use of chiral 2-aminodienes derived from (S)-methoxymethylpyrrolidine [101] (Scheme 54). 

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 benzannulation reaction with small alkynes such as 1-pentyne may generate a two-alkyne annulation product. In this case the original [3+2+l]-benz-annulation is changed to a [2+2+1+1]-benzannulation. After CO dissociation and insertion of the first alkyne, the coordinated a,/J-unsaturated moiety in the vinylcarbene complex is supposed to be replaced by the second alkyne. The subsequent reaction steps give the phenol 112 (Scheme 50). 

The starting point of the central part of the reaction is assumed to be the ri -vinylcarbene complex 13a... 

Figure 6. Optimized geometries of V-vinylcarbene complex 13a, T)3-vinylcarbene complexe 13b, and the transition state connecting them. Selected bond distances are given in A and energies are given with respect to 13a in kJ mol 1.

A particularly interesting piece of evidence concerning the nature of this sequence has been presented by Barluenga et al.21 When the vinylcarbene complex 14 was heated, decarbonylation afforded the alkene-stabilized complex 15. Upon treatment with dimethyl acetylenedicarboxylate, the alkyne-insertion product 16 was isolated. This complex proved to be unstable in solution at room temperature and decomposed readily to 17, the expected product of a Dotz reaction with an aminovinylcarbene. 

Complex exo-60 is then protonated to give the 773-vinylcarbene complex exo-64, which subsequently inserts carbon monoxide in the well-established manner (see Sections II,B, V,B, VI,B, VI,C, VI,E, VI,J, and VII), affording the 16-electron species endo-65. Anion trapping of the unsaturated species finally yields the vinylketene complex endo-62. 

Following the precedent set by Green,77 the reaction is assumed to proceed through the i73-vinylcarbene complex 70, which is formed by the protonation of the CHPh carbon of 68. This then undergoes carbonyl insertion to afford the 16-electron complex 71, whose coordination sphere is subsequently saturated by iodide, affording the i74-vinylketene product (67). 

In a later paper by Weiss,68 the methodology was extended to a more complex cyclopropene, and an intermediate cobaltacyclobutene (103) was proposed. In an analogous insertion reaction with nonacarbonyldiiron, a vinylcarbene complex was isolated along with the expected vinylketene complex (see Section VI,B). However, no such vinylcarbene cobalt complex was isolated, even when cyclopentadienyl bis(ethene) cobalt was used in place of dicarbonylcyclopentadienyl cobalt, and the only product isolated was the vinylketene complex 104, represented here in the rf -allylacyl structure. 

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. 

Treatment of complexes 167.c and 167.d with a further equivalent of a coordinating ligand was found to yield substituted ferracyclopentenones (168), in which the L and L groups adopt a cis geometry. The product 168.a could also be synthesized directly from the precursor vinylcarbene complex by treatment with 2 equiv of the appropriate ligand. Note that when 167.a or 167.b was treated with PPh3 or CO(g) (80 atm), only starting... 

When the initial vinylcarbene complex is substituted with a second me-thoxycarbonyl group (complex 169), a different reactivity pattern is observed. Addition of methyldiphenylphosphine or dimethylphenylphosphine to 169 results in formation of the expected vinylketene complex 170. However, the analogous reaction with triphenylphosphine yielded a complex mixture at room temperature, and upon heating the simple ligand-substituted product 171 is formed. When 169 is reacted with carbon monoxide, the pyrone complex 172 is formed. Finally, reaction of the vinylketene... 

As part of ongoing research into the behavior of (vinylcarbene)iron complexes,119120 Mitsudo and Watanabe found that the trifluoromethyl-substituted vinylcarbene 174 exhibited a reactivity different from that of both 166 and 169.107 Upon treatment of the complex 174 with triphenylphos-phine the vinylketene complex 175 is formed, a reaction identical to that seen in the series of vinylcarbene complexes 166 (R = H). However, when the vinylcarbene 174 is exposed to a high pressure of carbon monoxide, it is converted cleanly to the ferracyclopentenone 176. Remember that when the vinylcarbene complex 166 (R = H) was treated in the same manner, conversion stopped at the vinylketene complex 167 Even when exposed to a pressure of 80 atmospheres of CO(g), no further reaction was seen to occur. An electron donating ligand (L = PR3) is required for conversion to cyclopentenone structure 168. Conversely, when the more electron-rich vinylcarbene 169 is exposed to carbon monoxide in the same manner, the pyrone complex 172 is formed. 

J- Synthesis from rf-Vinylketone Complexes (via if-Vinylcarbene Complexes)... 

They have also shown that the dimeric bis-77-allyl complex 227 is formed, under different conditions, from 221.g, 228, and 229, suggesting that all three reactions proceed through a common intermediate, the most likely candidate being the 773-vinylcarbene complex 230. 

The vinylketene complex 221.a was shown3 to undergo ligand replacement when treated with triphenylphosphine to yield the dicarbonyl(triphe-nylphosphine)vinylketene complex 234. This complex was also isolated from the reaction of vinylketone 222.a with methyllithium in the presence of triphenylphosphine, indicating that a phosphine, as well as carbon monoxide, could induce the migration of a coordinated carbonyl ligand to form a ketene carbonyl. Mitsudo, Watanabe, and Weiss have all shown that this process occurs in the reaction between phosphines and isolated rf-vinylcarbene complexes (see Sections VI,B and VI,C).48,68,106... 

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. 

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

Heteroatom-substituted vinylcarbene complexes are easily prepared by aldol condensation of aldehydes with alkylcarbene complexes [228]. The latter also react readily with imidates to yield either (2-aminovinyl)- or (2-alkoxyvinyl)carbene complexes [229]. 

The chemical behavior of heteroatom-substituted vinylcarbene complexes is similar to that of a,(3-unsaturated carbonyl compounds (Figure 2.17) [206]. It is possible to perform Michael additions [217,230], 1,4-addition of cuprates [151], additions of nucleophilic radicals [231], 1,3-dipolar cycloadditions [232,233], inter-[234-241] or intramolecular [220,242] Diels-Alder reactions, as well as Simmons-Smith- [243], sulfur ylide- [244] or diazomethane-mediated [151] cyclopropanati-ons of the vinylcarbene C-C double bond. The treatment of arylcarbene complexes with organolithium reagents ean lead via conjugate addition to substituted 1,4-cyclohexadien-6-ylidene complexes [245]. 

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