Metals, activated alkynes

The metal-oxo molecular models outlined above have a quite remarkable potential for studying the metal activity in a quite unusual environment. Some of the possibilities could be (1) the generation and the chemistry of M—C, M=C, M=C functionalities (2) the interaction with alkenes, alkynes, hydrocarbons, and hydrogen (3) the activation of small molecules like N26 and CO (4) the support of metal-metal bonded functionalities and (5) the generation of highly reactive low-valent metals. 

Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

Heteroatom Nucleophiles with Metal-activated Alkenes and Alkynes... 

ADDITION OF HETEROATOM NUCLEOPHILES TO METAL-ACTIVATED ALKYNES 567... 

Metal activation, and sonochemistry, 1, 314 Metal alkoxides, synthesis, 12, 51 Metal-to-alkyne ligand charge-transfer transitions, rigid-rod transition metal—acetylide polymers,... 

Asymmetric alkynyl additions to aldehydes by prior, separate generation of the alkynylides (e.g. dialkylzinc reagents) have recently been reviewed and are a topic of current research [10], They will not be covered in the context of this chapter. Instead, in line with the theme of this book, this chapter will focus on the metala-tion of terminal alkynes by activation of the terminal C-H and the use of the corresponding metal acetylides in aldehyde and ketone addition reactions. 

This chapter is divided into two main parts. The first part focuses on reactions where the dithiolene ligand is generated independently of the metal center. For the most part, these preparations give alkenedithiolate dianions, which ordinarily are treated with metal electrophiles to form dithiolene complexes. In the second part, transition metals actively participate in the assembly of the dithiolenes, usually via the reaction of a metal sulfido species with an alkyne or hydrocarbon in an equivalent oxidation state. 

Hegedus, L. S. Heteroatom Nucleophiles with Metal-Activated Alkenes and Alkynes. in Comp. Org. Synth, (eds. Trost, B. M.,Fleming, I.), 4,... 

As outlined in 5.8.2.3.4, coordination of an alkene to a neutral or cationic transition metal activates the alkene toward nucleophilic attack, leading to an alkylmetal product. Development of the analogous synthesis of alkenylmetal complexes starting with /y -alkyne-metal complexes is more recent. An early reaction of this type is ... 

Perhaps the most remarkable illustration of the ability of metals to activate alkynes comes from reactions in which complete scission of the carbon-carbon triple bond occurs. On the stoichiometric level these include examples in which carbyne complexes are produced from alkyne completes as in the melt-thermolysis of CpCo(PPh3)(RCsCR) [112] or from reactions of alkynes with unsaturated metal species (Scheme 4-34) [113]. The remarkable alkyne metathesis reaction (Scheme 4-35), which involves overall cleavage and regeneration of two o-and four rt-bonds, is conceptually related. A variety of functionalized alkynes can be tolerated as metathesis substrates [114] and especially effective catalysts for these reactions are Mo(VI)-and W(VI)-carbyne complexes. Metallacyclobutadienes 64, formed by the reaction of the alkyne with a metal-carbyne complex, appear to be central intermediates in these reactions and the equilibrium between metallacycle and alkyne/metal-carbyne is observable in some cases [115]. 

ABSTRACT. Alkynes, when activated by an electron-rich d Re, Mo or W phosphinic centre, undergo hydrogen shift reactions (to give, e.g., vinylidene species) or oxidatively add to the metal (forming alkynyl--hydrido or alkynyl complexes). These alkyne-derived products undergo 3-protonation to afford a variety of carbyne-fluoro or -chloro complexes, whereas aminocarbynes are obtained upon 3-electrophilic attack (e.g., by a protic or a Lewis acid) at isocyanides when ligating such metal sites. Mechanistic studies, by stopped-flow spectrophotometry, are also indicated. 

An extension of Hashmi s Au(III)-catalyzed phenol synthesis [81] to furan substrates 9 bearing an additional alkyne moiety allowed the preparation of C6-C7-heterofused benzofuran 11 (Scheme 9.3) [82]. According to the proposed mechanism, the Au(III)-catalyzed arene formation reaction generates o-alkynylphenol 10. A subsequent Au(III)-catalyzed cycloisomerization of the latter, following the general mechanism for an intramolecular nucleophilic addition of heteroatom to transition metal-activated carbon-carbon multiple bonds, gives 11 (Scheme 9.3). 

The same group demonstrated that a variety of mono- and disubstituted 1-oxyindolizine derivatives 3S2 could be readily synthesized via a facile Ag-catalyzed cydoisomerization of skipped propargylpyridines 3S1 (Scheme 9.121) [300, 301]. It was suggested that this Au-catalyzed reaction involved a 5 -endo-d cyclization of the alkyne 351 activated by a jt-philic metal. Formation of the indolizine product 352 was accomplished via a subsequent proton transfer in cyclic vinylmetal zwitterion 354 (Scheme 9.122). It should be noted that a variety of transition metals, such as Au(I), Au(III), Cu(I), Pt(II), and Pd(II), were shown to catalyze this transformation with variable degrees of efficiency. 

Almost all the electrophilic groups or electrophilic intermediates can be employed as the hydride acceptors, e.g., alkylidene malonates, carbophilic transition metals activated alkynes or allenes, enal/enones, aldehyde/ketones, imines, ketenimine/car-bodiimides, metal carbenoids, alkynes carrying electron-withdrawing groups, as well as in situ-generated carbocations (Fig. 3). 

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