Complexes, alkyne-metal aldehydes

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

We see from these examples that many of the carbon nucleophiles we encountered in Chapter 10 are also nucleophiles toward aldehydes and ketones (cf. Reactions 10-104-10-108 and 10-110). As we saw in Chapter 10, the initial products in many of these cases can be converted by relatively simple procedures (hydrolysis, reduction, decarboxylation, etc.) to various other products. In the reaction with terminal acetylenes, sodium acetylides are the most common reagents (when they are used, the reaction is often called the Nef reaction), but lithium, magnesium, and other metallic acetylides have also been used. A particularly convenient reagent is lithium acetylide-ethylenediamine complex, a stable, free-flowing powder that is commercially available. Alternatively, the substrate may be treated with the alkyne itself in the presence of a base, so that the acetylide is generated in situ. This procedure is called the Favorskii reaction, not to be confused with the Favorskii rearrangement (18-7). ... 

A variety of aldehyde/alkyne reductive couplings involving the stoichiometric use of early transition metals (Ti and Zr) have been developed (Scheme 27) [68-70]. The low cost and ease of handling of titanium alkox-ides render these stoichiometric processes very practical despite the lack of catalytic turnover. Recent variants of stoichiometric processes involving titanium alkoxides have demonstrated impressive scope in relatively complex applications [71-73]. 

Sodium acetylides are the most common reagents, but lithium, magnesium and other metallic acetylides have also been used. A particularly convenient reagent is lithium acetylide-ethylene diamine complex. Alternatively, the substrate may be treated with the alkyne itself in the presence of a base, so that the acetylide is generated in situ. 1,4-Diols can be prepared by treatment of aldehyde with dimetalloacetylenes. 

A highly regioselective, efficient, and clean anti-Markovnikov hydration of terminal acetylenes has been realized through the use of catalytic amounts of Ru complexes.561 Typically, [CpRu(dppm)Cl] catalyzes the reaction at 100°C to give aldehydes in high yields (81-94%). Triflic acid or trifluoromethanesulfonimide effectively catalyzes the hydration of alkynes without a metal catalyst to afford Markovnikov products (ketones).562... 

Metal acetylacetonates quench triplet species generated by flash photolysis of aromatic ketones and hydrocarbons.330-333 More recently, these reactions have been studied from a synthetic standpoint. Triplet state benzophenone sensitizes photoreduction of Cu(MeCOCHCOMe)2 by alcohols to give black, presumably polymeric, [Cu(MeCOCHCOMe)] . This reacts with Lewis bases to provide complexes of the type CuL2(MeCOCHCOMe) (L = bipyridyl/2, ethylenediamine/2, carbon monoxide, Ph3P). Disubstituted alkynes yield Cu(C2 R2 XMeCOCHCOMe) but terminal alkynes form CuQR acetylides.334 The bipyridyl complex of copper(I) acetylacetonate catalyzes the reduction of oxygen to water and the oxidation of primary and secondary alcohols to aldehydes and ketones.335... 

The reaction of alkenes (and alkynes) with synthesis gas (CO + H2) to produce aldehydes, catalyzed by a number of transition metal complexes, is most often referred to as a hydroformylation reaction or the oxo process. The discovery was made using a cobalt catalyst, and although rhodium-based catalysts have received increased attention because of their increased selectivity under mild reaction conditions, cobalt is still the most used catalyst on an industrial basis. The most industrially important hydrocarbonylation reaction is the synthesis of n-butanal from propene (equation 3). Some of the butanal is hydrogenated to butanol, but most is converted to 2-ethylhexanol via aldol and hydrogenation sequences. 

Addition of H and CO to alkenes and alkynes catalysed by transition metal complexes is called hydrocarbonylation, and is useful for the syntheses of carboxylic acids, their esters, aldehydes and ketones [1]. Oxidative carbonylation of alkenes and alkynes with Pd(II), treated in Section 11.1.5, differs mechanistically from hydrocarbonylation. Some carbonylation reactions occur at under 1 atm or low pressures, without using a high-pressure laboratory apparatus. Several commercial processes based on hydrocarbonylation have been developed. 

Hydrosilane HSiR.3 behaves similar to H2 toward transition metal complexes in some cases. When HSiR.3 is used instead of hydrogen in hydroformylation, two reactions are expected. One is a hydrocarbonylation-type reaction, by which formation of the silyl enol ethers 62 via the acylmetal intermediate 61, and the acylsilanes 64 via the acyl complex 63, are expected in practice both reactions are observed. The other possibility is silylformylation to form 65, which is unknown, even though silylformylation of alkynes is known. When Co2(CO)8 is used, the silyl enol ether of aldehyde 66 is obtained [36], However, the silyl enol ether 67 of acylsilane 68 is obtained when an Ir complex is used, and converted to the acylsilane 68 by hydrolysis [37],... 

Metal complexes enable one to employ molecules that are thermally unreactive toward cycloadditions by taking advantage of their ability to be activated through complexation. Most of the molecules activated by transition-metal complexes involve C-C unsaturated bonds such as alkynes, alkenes, 1,3-dienes, allenes, and cyclopropanes. In contrast, carbonyl functionalities such as aldehydes, ketones, esters, and imines seldom participate in transition-metal-catalyzed carbonylative cycloaddition reactions. Recently, such a transformation was reported via the use of ruthenium complexes. 

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

Metal enolate complexes have also been used to catalyze the allylation of carbonyl compounds ° °, addition of aldehydes to l,3-dienes ° and alkynes as well as the addition of alkenes to alkynes " and indoles ". In the latter study, 5 mol% of Pd(acac)2 (29) and 10 mol% of PPhs were found to be an effective catalyst system for the coupling of Ai-methylindole (93) with a variety of 2-acetoxymethyl-substituted electron-deficient alkenes, including methyl 2-(acetoxymethyl)acrylate (94) (equation 26). Substituted indoles (95) constitute an important class of biologically active natural products and synthetic routes to these valuable compounds have therefore attracted considerable attention. 

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