Oxidative acetylenic coupling

Apparently, the oxidative acetylene coupling can overcome a drastic strain increase as demonstrated by the highly efficient cyclization of octamethyltetra-deca-l,4,7,10,13-pentayne 36 and its perspirocyclopropanated analogue 61 to the corresponding 14-membered macrocycles 181 (67%) and 62 (45%), respectively (Scheme 34) [18]. 

It must be emphasized that current mechanistic understanding of copper-mediated oxidative acetylenic couplings is unsatisfactory. Several studies have shown the strong dependency of the mechanism on the experimental setup, suggesting highly complex coherences and interactions. Nevertheless, the mechanistic idea of Bohlmann et al. described above still provides the most accepted picture for Glaser-type oxidative acetylenic homocouplings. 

Figure 23.11 Schematic setup for the continuous-flow oxidative acetylene coupling.

MacrocycHzation for the synthesis of macrolactones was also achieved by C-C bond formation using a variety of techniques (Scheme 6.5). Oxidative acetylene coupling for exaltolide 12, as reported by Bergelson et al. [61], nickel-carbonyl-mediated coupling of allyhc dibromides by Corey and Kirst [62], palladium-catalyzed... 

The key step in the synthesis of 168 and 169 is an oxidative acetylenic coupling reaction of open-top precursor cavitands 170 and 171. Intramolecular coupling of 171 affords the basket 169 along with a dimeric structure and the switchable tube 172, in a ratio of 2/6 10 1.The two products can be separated by preparative high-performance gel-permeation chromatography (2007ACE260). 

Oxidative acetylenic coupling to produce 1,3-diynes is an important reaction with broad applications. Despite tremendous improvements in homocoupling processes, notably the Hay procedure, the construction of un-symmetrical conjugated diynes from two different terminal allqmes has only been recently reported. In 2009, Lei and co-workers described a copper/ nickel-co-catalyzed aerobic cross-coupling method that tolerated a variety of functional groups such as amides, halides and free propargylic alcohols (Scheme 9.24). ... 

Quantitative aluminum deterrninations in aluminum and aluminum base alloys is rarely done. The aluminum content is generally inferred as the balance after determining alloying additions and tramp elements. When aluminum is present as an alloying component in alternative alloy systems it is commonly deterrnined by some form of spectroscopy (qv) spark source emission, x-ray fluorescence, plasma emission (both inductively coupled and d-c plasmas), or atomic absorption using a nitrous oxide acetylene flame. 

Although an attempted twofold alkynylation of a difunctional Cj-building block with the deprotected pentayne 61 could not be achieved, intramolecular acetylene coupling under oxidative conditions was successful and gave the eye-... 

Many cyclization reactions via formation of metallacycles from alkynes and alkenes are known. Formally these reactions can be considered as oxidative cyclization (coupling) involving oxidation of the central metals. Although confusing, they are also called the reductive cyclization, because alkynes and alkenes are reduced to alkenes and alkanes by the metallacycle formation. Three basic patterns for the intermolecular oxidative coupling to give the metallacyclopentane 94, metallacyclopentene 95 and metallacyclopentadiene 96 are known. (For simplicity only ethylene and acetylene are used. The reaction can be extended to substituted alkenes and alkynes too). Formation of these metallacycles is not a one-step process, and is understood by initial formation of an tj2 complex, or metallacyclopropene 99, followed by insertion of the alkyne or alkene to generate the metallacycles 94-96, 100 and 101-103 (Scheme 7.1). 

Because all currently known mechanisms of oxidative acetylenic homocouplings are very specific to single reaction conditions, e.g. pH or oxidation state of the used copper salt, this section summarizes the most reasonable mechanistic ideas proposed for the commonly utilized coupling procedures. 

Altogether, mechanistic understanding of both coupling processes, oxidative and non-oxidative, still requires much work. Improvements here may be promising for development of new coupling methods, enriching the already wide scope of acetylenic coupling reactions. 

The element may be determined at 196.0 nm by A AS, using a nitrous oxide-acetylene flame (which is more transparent than air-acetylene at this low wavelength), or by AFS in a variety of flames.46,47 The detection limit of both techniques for selenium is around 1 mg 1 1, too low to be useful for environmental analyses. The element is therefore invariably determined by hydride generation techniques, coupled to AAS or AFS detection, as discussed in Chapter 6, section 2, or by furnace AAS, or occasionally by solution spectrofluorimetry using 2,3-diaminonaphthalene as a reagent. If direct flame AAS or AFS are to be used for some reason, then pre-concentration by solvent extraction is necessary.1 However, this approach is rarely used nowadays. 

Atomic absorption spectroscopy is commonly used to determine Fe, Al, Mn, Cr and other metals. Standard solutions should be prepared with the same acid concentration as that of the test solutions. Apart from Al which requires a nitrous oxide/acetylene flame, these cations may be measured using an air/acetylene flame. These metals may also be measured by inductive coupled plasma analysis (ICP). 

Cuprous ammonium chloride. The combination of cuprous chloride and ammonium chloride in a slightly acidic aqueous solution catalyzes oxidative (air) coupling of terminal acetylenes to diacetyienes. - The groups NHa, OH, COjH, and COjR do not interfere, in the synthesis formulated, cross coupling was accomplished in a mixture of ethanol and 0.08 A hydrochloric acid containing the catalyst. 

Trace amounts of titanium can be determined by X-ray fluorescence spectrometry, neutron activation analysis (NAA), atomic absorption techniques (AAS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). In case of AAS, a high-temperature flame (nitrous oxide, acetylene) is essential, and the optimum wavelengths are 364.3 and 365.4 nm the sensitivity is low. With the graphite furnace, a lower detection limit of approximately 0.5 xg L can be achieved. ICP-OES is especially sensitive, and is the recommended instrumental... 

The separation of yttrium from the lanthanides is performed by selective oxidation, reduction, fractionated crystallization, or precipitation, ion-exchange and liquid-liquid extraction. Methods for determination include arc spectrography, flame photometry and atomic absorption spectrometry with the nitrous oxide acetylene flame. The latter method improved the detection limits of yttrium in the air, rocks and other components of the natural environment (Deuber and Heim 1991 Welz and Sperling 1999).Other analytical methods useful for sensitive monitoring of trace amounts of yttrium are X-ray emission spectroscopy, mass spectrometry and neutron activation analysis (NAA) the latter method utilizes the large thermal neutron cross-section of yttrium. For high-sensitivity analysis of yttrium, inductively coupled plasma atomic emission spectroscopy (ICP-AES) is especially recommended for solid samples, and inductively coupled plasma mass spectroscopy (ICP-MS) for liquid samples (Reiman and Caritat 1998). 

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