Reactions of Alkyne Complexes

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

Table 3. Alkyne scission by reaction of alkyne complexes with cluster-building reagents.

Few reports on the reactions of alkyne complexes with electrophiles other than protic acids have appeared. Bromine has been found to react with a few systems to afford vicinal dibromoalkenes [e.g., Eqs. (41) and (42)] (Kruerke and Hubei, 1961 Hiibel and Merenyi, 19M). These reactions may, in fact, involve bromine addition subsequent to oxidative release of the ligand from the metal. The platinum dicyanoacetylene complex behaves differently in forming a product in which addition has occurred but the complex has remained intact (McClure and Baddley, 1970). 

Initial step is the formation of a dicobalthexacarbonyl-alkyne complex 5 by reaction of alkyne 1 with dicobaltoctacarbonyl 4 with concomitant loss of two molecules of CO. Complex 5 has been shown to be an intermediate by independent synthesis. It is likely that complex 5 coordinates to the alkene 2. Insertion of carbon monoxide then leads to formation of a cyclopentenone complex 6, which decomposes into dicobalthexacarbonyl and cyclopentenone 3 ... 

Scheme 8 Various modes of reaction of ethenylcarbene complexes 43 with alkynes 44 [ 11 ]...

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

The reaction of alkenylcarbene complexes and alkynes in the presence of Ni(0) leads to cycloheptatriene derivatives in a process which can be considered as a [3C+2S+2S] cycloaddition reaction [125]. As shown in Scheme 77, two molecules of the alkyne and one molecule of the carbene complex are involved in the formation of the cycloheptatriene. This reaction is supposed to proceed through the initial formation of a nickel alkenylcarbene complex. A subsequent double regioselective alkyne insertion produces a new nickel carbene complex, which evolves by an intramolecular cycloprop anation reaction to form a nor-caradiene intermediate. These species easily isomerise to the observed cycloheptatriene derivatives (Scheme 77). 

An iron-catalyzed carbonylation reaction of alkynes 120 forming succinimides 121 by the aid of Fe(CO)5 78 or [Fe3(CO)i2] 119 has been reported by Beller et al. (Scheme 31) [94]. This reaction seems interesting as iron-carbonyl complexes are kinetically relatively inert. As a model system 3-hexyne was reacted with excess ammonia under 20 bar CO pressure. Employing a higher pressure leads to... 

Sugihara et al. in 1997.106 They utilized aqueous ammonium hydroxide as a reaction medium, which provided ammonia as a hard ligand to labilize the CO ligands and therefore enhance the rate of the PKR. The reaction of dicobalthexacarbonyl complexes of enynes and alkynes provided expected cyclopentenones via intramolecular and intermolecular modes respectively (Scheme 4.10). 

Among the most exciting frontiers in boratabenzene chemistry is the development of transition metal-boratabenzene complexes as catalysts. As early as 1984, it had been demonstrated that these adducts can accelerate useful reactions— specifically, Bonnemann established that (C5H5B-Ph)Co(cod) serves as a catalyst for pyridine-forming cyclotrimerization reactions of alkynes and nitriles.39... 

Addition Reactions of Alkynes Catalyzed by Gold Complexes 266... 

Professor Stone s paper points out that the reactivity of [ (ti-C5H5) (OC) 2WsCR] towards transition metal complexes is similar to that of an alkyne. It would be of interest to examine this compound and several of its derivatives which contain OW double bonds with respect to their reactivity patterns towards the BH3 group to determine if reactions analogous to the hydroboration reaction of alkynes and olefins would occur (1) or reactions similar to the attempted hydroboration described below would take place. 

NbBrs, and NbCls-Pl Sn evidently proceeds via cyclotrimerization of diynes, which most probably involve cyclic carbometallation, details are not very clear.246 2463 Related reactions of Ta and Mo complexes were also investigated in this study. Formation of tantallacyclopropenes by complexation of alkynes with Ta complexes has also been reported247 (Scheme 51). In addition to the Ta-catalyzed polymerization of diynes mentioned above, Ta-catalyzed or -promoted cyclotrimerization reactions of alkynes to produce benzene derivatives, a Ta-promoted ethylene... 

Similarly, 73-allylpalladium complexes were also employed as promoters for the cyclization.39,39a,39b The palla-dium(0)-catalyzed reaction of alkyne 168 with allylic carbonate in THF at 60 °G gave the A-allyl product 170 instead of the desired product 169. However, without isolation of 170, the reaction mixture was heated at 80 °G with 5 equiv. of K2CO3, providing 169 in 77% yield (Equation (7)). The nitrogen atom intervenes in the process as a nucleophile in the A-allylation step and as a leaving group in the cyclization step. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

Reaction of acetylenic complexes with triosmium dodecacarbonyl leads to a variety of products involving one, two, or three acetylenic units. As with ruthenium, for the monosubstituted alkynes, hydrogen transfer can occur to the metal cluster. Thus, Os3(CO)12 and phenyl-acetylene (L) yield, in refluxing benzene, the derivatives Os3(CO)10L, Os3(CO)10L2, Os3(CO)9L, and HOs3(CO)9(L-H). The general chemistry is summarized in Scheme 2 (131). 

Diboration provides another means of obtaining organoboranes. Studies [30-36] have been focused on the diboration reactions of alkynes and olefins with pinacol ester derivatives catalyzed by Pd(0) and Pt(0) metal complexes. Interestingly, it has been shown that Pt (0) complexes catalyze cis addition of the B-B bond in pinacol ester derivatives to alkynes but not to olefins. On the other hand, Pd (0) complexes do not catalyze diboration reactions, neither for alkynes nor for olefins. 

For the diboration reactions of alkynes catalyzed by Pt(0) complexes, the reaction mechanism involves the oxidative addition of diborane to the Pt(0) center, followed by the insertion of alkyne into the Pt-B bond and reductive... 

---