Alkynylcarbene cycloadditions

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...

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. 

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). 

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). 

In a similar study, Park has shown125 that cyclic dienes other than cyclo-pentadiene will also undergo a [4 + 2] cycloaddition reaction with the (trimethylsilyl)alkynylcarbene complex (184.b). When 184.b was reacted with 1,3-cyclohexadiene in THF, the diene complex 191 was isolated along with the vinylketene complex 192, which was prone to hydrolysis during column chromatography as expected.123 When 192 was stirred with silica gel and water in hexane, an almost quantitative conversion to the aldehyde complex 193 was observed. 

However, a crucial difference was seen in analogous reactions with the (tert-butyl)alkynylcarbene complex 184.a, carried out under a pressure of carbon monoxide. A [4 + 2] cycloaddition takes place, but in this case the ene fragment is not the alkyne functionality, but the carbene-alkyne bond. The mechanism presented by Park implies that this is due to the preliminary... 

Also alkynylcarbene complexes can react as Michael acceptors with nucleophiles, forming 1,3-dien-l-ylcarbene complexes (Figure 2.17). Both carbon nucleophiles, such as, e.g., enamines [246-249], and non-carbon nucleophiles, such as imidates [250], amines [64,131,251], aliphatic alcohols [48,79,252], phenols [252], and thiols [252] can add to the C-C triple bond of alkynylcarbene complexes. Further reactions of the C-C triple bond of alkynylcarbene complexes include 1,3-dipolar [253,254], Diels-Alder [64,234,238,255-258] and [2 -i- 2] cycloadditions [259 -261], intramolecular Pauson-Khand reactions [43,262], and C-metallation of ethynylcarbene complexes [263]. 

Benzoindole 114 has been synthesized by a remarkable sequence, in which the benzannulation precursor 113 is pre-assembled starting from two different chromium carbenes. It is formed in a [3+2] cycloaddition, in which the acylamino carbene complex 111 acts as the dipolar component and the alkynylcarbene complex 112 serves as the dipolarophile. The resulting 3-pyrrolylcarbene complex 113 undergoes a photoinduced intramolecular benzannu-lation to give the benzoindole 114 [84a]. This strategy complements an approach towards carbazoles [84b], Isoindolines and 1,2,3,4-tetrahydroisoquinolines are accessible from the reaction of pentacarbonyl (a-methoxyethylidene) chromium with 7r,co-dialkynes bearing a nitrogen atom in the carbon ether [84c]. 

Almost exclusively reported for 1-alkenylcarbenes and 1-alkynylcarbenes are examples dealing with mechanism-based and simple diastereoselectivity. The diastereofacial selectivity has been occasionally examined with bicyclo[2.2.1]heptene (norbornene) derivatives which provide the expected exo-adducts, while enantioselective [2 + 1] cycloadditions of these carbenes or their equivalents have apparently not yet been studied. 

Cycloadditions. Alkynylcarbene complexes furnish three-carbon units for -2]-cycloadditions. The component can be an imine or an enamine. ... 

ALKYNYLCARBENE COMPLEXES OF TRANSITION METALS AS SUITABLE SUBSTRATES FOR STEREOSELECTIVE CYCLOADDITIONS,... 

ABSTRACT. Strong electronic effects of the carbene-metal unit on the adjacent fimctionalities in alkynylcarbene metal (Cr,W) complexes may be regarded as the main cause for easy and stereocontrolled cycloadditions with a variety of substrates. 

These systems are known to behave as vinyl cations displaying, as a consequence, a potentially interesting chemistry towards a variety of olefmic substrates[2]. Provided that this similarity applies, it will confer a promising outlook to alkynylcarbene metal complexes in cycloaddition chemistry concerning versatility and, what is even of the utmost importance, a strict control of the stereochemistry of the products may be expected[3]. 

At the present, our studies with alkynylcarbene complexes of Cr and W suggest that their cycloadditions with electron-rich olefins follow, mainly, a concerted [2+2] pathway by activation of the triple bond[6], whereas, the intramolecular Co-induced carbonylative cycloaddition of the corresponding allylamino complexes seems to be facilitated by the alternative influence of the metal on the heteroatom side. A strict control of the stereochemistry in these reactions has been observed making these complexes valuable auxiliaries in organic synthesis. Efforts to broaden the scope of application are under way. 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocycUc (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes ". ... 

Metal carbonyl carbene fragments are potent electron acceptors and as such effectively promote [n+2]cycloaddition reactions of vinyl- and alkynylcarbene complexes. Typical examples of regioselective [2-h2] and [4+2]cycloaddition reactions are depicted in Schemes 11.14[36a, b] and 11.15 [36c],... 

Scheme 11.14 Regioselective [2+2]cycloaddition reaction of alkynylcarbene complexes.

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