Alkynylcarbene

Scheme 4 Access to various a,/ -unsaturated carbene complexes from alkynylcarbene complexes 23. A 1,3-Dipolar cycloaddition. B Diels-Alder reaction. C Ene reaction. D [2+2] Cycloaddition. E Michael-type addition followed by cyclization. F Michael-type additions...

This reaction mode of alkynylcarbene complexes of type 23 undoubtedly provides the most convenient access to /J-amino-substituted a,/J-unsaturated Fischer carbene complexes 27 (X=NH2, NHR2, NR2). Fischer et al. reported the very first such addition of an amine to an alkynylcarbene complex of type 23 and observed a temperature-dependent competition between 1,4- and 1,2-addition [12]. In a later systematic study, de Meijere et al. found that in addition to the 1,4-addition products 30,1,2-addition-elimination (formal substitution)... 

Scheme 5 Access to / -amino-substituted a,/ -unsaturated Fischer carbene complexes 30 by Michael-type addition of amines to alkynylcarbene complexes 23 (R=Et) [30,33]...

Diamino-substituted complexes of type 37 were first obtained by Fischer et al. [12] in two steps via the 1,2-addition-elimination product 34 from di-methylamine and 35 (Scheme 6). The (3-aminoallenylidene)chromium complexes 36, which can be prepared either from 33 [47,48] or directly from 35 [33], can also be transformed to l,3-bis(dialkylamino)-substituted complexes of type 37 (e.g., R2=z Pr) by treatment with dimethylamine in excellent yields [33]. Although the complex 37 is accessible by further reaction of the complex 34 with dimethylamine, and 34 itself stems from the reaction of 35 with dimethylamine, the direct transformation of 33 to 37 could not be achieved [12]. In spite of this, heterocyclic carbene complexes with two nitrogens were obtained by reactions of alkynylcarbene complexes 35 with hydrazine [49] and 1,3-diamines [50]. 

In contrast to other terminal alkynes, the lithiated dimethylaminoethyne 40 does not give the corresponding alkynylcarbene but the cyclopropenylidene complex 41 (Scheme 7) [51]. Further addition of dimethylamine to 41 affords the substitution product 42 in excellent yield. This 2,3-bis(dimethylamino-cyclopropenylidene)pentacarbonylchromium (42) is extremely stable, and it cannot be transformed to the corresponding carbonyl compound, 2,3-bis... 

An analogous cyclization to eventually form five-membered rings has also been observed for l-metalla-l,3,5-hexatrienes with an additional heteroatom within the chain, such as in the complexes 157. These are obtained by Michael additions of imines to alkynylcarbene complexes in good to excellent yields (reaction type F in Scheme 4), and their configurations were determined to be Z (>91%) in all cases. Upon warming in THF solution, complexes 157 underwent cyclization with reductive elimination to furnish 2Ff-pyrroles 158 in up to 97% yield (Scheme 34). With two cyclopropyl substituents at the terminus in... 

Fig 1 Heteroatom-stabilised (X=OR, NR2) and non-heteroatom-stabilised (X=alkyl, aryl) alkyl- (1), alkenyl- (2), aryl- (3) and alkynylcarbene (4) complexes of group 6 metals... 

The first examples of alkene cyclopropanation reactions with alkynylcarbene complexes were reported by Barluenga et al. in 2002 [15]. These intermolecular... 

Alkoxy)alkynylcarbene complexes have been shown to react with nitrones to give dihydroisoxazole derivatives [47]. Masked 1,3-dipoles such as 1,3-thia-zolium-4-olates also react with alkynylcarbene complexes to yield thiophene derivatives. The initial cycloadducts formed in this reaction are not isolated and they evolve by elimination of isocyanate to give the final products [48]. The analogous reaction with munchnones or sydnones as synthetic equivalents of... 

Non-enolizable imines such as 9-fluorene imines react with alkynylcarbene complexes to afford mixtures of mesoionic pyrrolium carbonyltungstates and dihydropyrrole derivatives [68] (Scheme 23). Although both compounds can be considered as [3C+2S] cycloadducts, formation of each of them follows a very different pathway. However, the first intermediate of the reaction is common for both compounds and supposes the conjugated addition of the imine to the alkynylcarbene complex to form a zwitterionic intermediate. A cyclisation... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

All around this chapter, we have seen that a,/J-unsaturated Fischer carbene complexes may act as efficient C3-synthons. As has been previously mentioned, these complexes contain two electrophilic positions, the carbene carbon and the /J-carbon (Fig. 3), so they can react via these two positions with molecules which include two nucleophilic positions in their structure. On the other hand, alkenyl- and alkynylcarbene complexes are capable of undergoing [1,2]-migration of the metalpentacarbonyl allowing an electrophilic-to-nucleophilic polarity change of the carbene ligand /J-carbon (Fig. 3). These two modes of reaction along with other processes initiated by [2+2] cycloaddition reactions have been applied to [3+3] cyclisation processes and will be briefly discussed in the next few sections. 

Alkynylcarbene complexes react with /J-dicarbonyl compounds and catalytic amounts of a base to generate formal [3+3] pyranylidene derivatives [87]. The... 

In a similar process, tertiary enaminones react with alkynylcarbene complexes to give the corresponding pyranylidene complexes following a reaction pathway analogous to that described above. First, a [2+2] cycloaddition reaction between the alkynyl moiety of the carbene complex and the C=C double bond of the enamine generates a cyclobutene intermediate, which evolves by a conrotatory cyclobutene ring opening followed by a cyclisation process [94] (Scheme 49). 

Alkoxy alkynylcarbene complexes undergo Diels-Alder reactions with neutral and electron-rich dienes [36f, 104] and also with 1-aza- and 2-aza-l,3-butadi-ene derivatives [84a, 105] (Scheme 57). 

Following the same tendency as alkenylcarbene complexes, the substitution of the alkoxy group for an amino group in alkynylcarbene derivatives greatly decreases the rate of Diels-Alder reactions [102, 104b]. In fact, substituted... 

The cycloprop anation of fulvenes has been effected with alkynylcarbene complexes (see Sect. 2.1.1). However, this reaction is inhibited in the presence of CO and under these conditions a formal [4S+3C] cycloadduct is formed [15a]... 

Aumann et al. have observed an unusual formal [6S+2C] cycloaddition reaction when they performed the reaction between an alkynylcarbene complex and 1-aminobenzocyclohexenes. The solvent used in this reaction exerts a crucial influence on the reaction course and products of different nature are obtained depending on the solvent chosen. However, in pentane this process leads to cyclooctadienylcarbene complexes in a reaction which can be formally seen as a [6S+2C] cycloaddition [117] (Scheme 71). The formation of these compounds is explained by an initial [2+2] cycloaddition reaction which leads to a cy-clobutenylcarbene derivative which, under the reaction conditions, undergoes a cyclobutene ring opening to furnish the final products. 

Aumann et al. have described the synthesis of biscarbene complexes by the reaction of 1-alkylimidates with two equivalents of a tungsten alkynylcarbene complex [131]. An initial [4S+2C] cycloaddition generates an intermediate which further reacts with a second molecule of the alkynylcarbene complex... 

Alkynylcarbene complexes react with strained and hindered olefins yielding products that incorporate up to four different components by the formation of five new carbon-carbon bonds [15b]. This remarkable transformation is explained by an initial [2+2] cycloaddition followed by CO insertion. The resulting intermediate suffers a well precedented [1,3]-migration of the metal fragment to generate a non-heteroatom-stabilised carbene complex intermediate which reacts with a new molecule of the olefin through a cyclopropana-tion reaction (Scheme 85). 

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