Combination tables alkynes

The data shown in Table 6.3 show no obvious trends that may shed light on the mechanism(s) of the metathesis reactions. In terms of overall relative reactivity (krei of Table 6.3), phenylacetylene reacts fastest and pent-l-yne is the most sluggish alkyne, while ethyne and the parasubstituted phenylacetylenes are unreactive under the experimental conditions used. When the branching ratios and the relative reaction rates are combined, the alkyne - nitrile chaimel is the most productive for propargyl alcohol and phenylacetylene and least productive for pent-2-yne and pent-l-yne. The sterically congested alkyne exhibits modest reactivity for the metathesis reaction. No obvious relationship exists between the structure of the alkyne substrate and the propensity for a metathesis reaction. With regard to which metathesis reaction is favored for unsymmetrical alkynes, while no regioselectivity operates for pent-2-yne and phenylacetylene other terminal acetylenes favor the loss of the more substituted nitrile (Eq. (6.131)). 

Reactions with mono-substituted alkynes usually give mixtures of both 5-and 6-substituted indoles, although certain combinations of substituents result in good regiosclcctivity. Table 8.2 provides some examples. 

The behavior of strained,/Zuorimiret/ methylenecyelopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but Its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies (A/f j for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotnfluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101, 102, 103]... 

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

Cycloadditions have been reported for many combinations of diazo compounds and alkynes (5). A few recent examples are given in Table 8.2. An inspection of entries 2-4 shows that the regiochemical behavior of internal sulfonylalkynes is totally reversed when the second substituent (R ) is changed from Me or Ph to SiMe3. This difference was explained in terms of a steric effect of... 

Selected examples are given in Table 9 for bromofluorinations of alkenes using A-bromosuccin-imide in combination with 70% hydrogen fluoride/pyridine (Method A) and hydrogen fluo-ride/polyvinylpyridine (Method B), respectively. Table 10 shows examples of the selective monoaddition of in situ produced halofluorides (70 % hydrogen fluoride/pyridine and A-iodo-. A-bromo- or /V-ehloro-succinimide) to symmetrically substituted alkynes. 

Thioketones reacting as dienophiles are well documented and follow the course of equation (93). Table 14 illustrates the variety of substrates and some typical yields for the reaction. Related to the simple Diels-Alder cyclizations are the [4 + 2+] cationic polar cycloadditions of equation (94) (72JA8932) and the [4+ + 2] version shown in equations (95) and (96) (81TL3773). The use of an alkyne in equation (94) to trap the thienium species leads to a benzothiopyran. It may be seen that choice of the enophile/dienophile combination leads to clean access to 5,6-dihydro- or 3,4-dihydro-thiopyrans with synthetically attractive substitution patterns. 

The catalytic dimerization of alkynes has led to the development of a variety of catalysts by new combinations of transition metals and ligands, and to a better understanding of the processes and mechanism involved, leading to improvement of selectivity and scope. In Table 1 the most relevant catalysts are compared with regard to phenylacetylene dimerization. The nature of the terminal alkyne has also a marked effect on the outcome of this reaction. 

The first direct coupling of terminal alkynes with aryl iodides or bromides without palladium was reported by Wang and Li in 2006.135 Silver iodide and triphenylphos-phane in polar solvents proved to be the best catalyst combination, while potassium carbonate proved to be the better base, giving diarylacetylenes in high yields (Table 10.8). 

The only complex in Table I with identical ligands in the two positions cis to the alkyne is the [(775-C9H7)MoL2(MeC=CMe]+ cation with L = PMe3 (72). Here the 2-butyne is parallel to one Mo—L vector and perpendicular to one Mo—L vector (Fig. 11). The 7r-ligand properties of the PMe3 ligands are probably not sufficiently dominant to create a substantial alkyne preference between the dir orbital combinations which are available. 

The simultaneous addition of fluorine and bromine to alkynes can be carried out by the use of bromine fluoride, prepared directly from the corresponding elements,553 the hydrogen fluoride/(V-bromoacetamide system (HF/AcNHBr)185 or the hydrogen fluoride/pyridine complex in combination with either A -bromosuccinimide or 1,3-dibromo-5.5-dimethylhydanloin (DBH).552 Examples arc listed in Table 44. 

In a polarographic measurement one may, in favourable cases, determine the potentials, reversibility and electron equivalents for a given cathodic process. When combined with an analysis of products, these usually provide insights into the gross mechanism of reduction. By using Ej data (Table 7) as limits, one can arrange reductions in which some group may be altered cleanly before the triple bond is touched, or vice versa. At the same time, data comprise an approximate electro-philicity scale of alkynes towards cathodic electrons, that is, a sort of solution electron affinity . The practical and theoretical uses of these data appear in several places in this chapter. 

As shown in Table 20, the ligand has a deleterious effect on the amount of the syn-adduct, and the solvent plays a significant role on the stereoselectivity, since a reaction run in benzene gave a 20 80 anti syn ratio, compared with Et20 (35 65) however, the chemical yield is poorer (48 and 72, respectively). Addition of TMSCl improves both the syn-selectivity and the combined yield. With substituted alkynes. the same effects (ligand, additive, solvent) are observed [32]. 

The CsC triple bonds of other alkynes closely follow the same pattern as ethyne including the effect of the extra QC methods. Since QC results have been collected for a significant number of compounds with C=C triple bonds and since they all show that MP2 calculations give rather poor bond lengths while the B3LYP model consistently gives bond lengths closer to experimental whether r, tq or even r, these results are presented in a different format in Table lOA for only a few combinations of method and basis set. 

In most of the palladium-catalysed domino processes known so far, the Mizoroki-Heck reaction - the palladium(0)-catalysed reaction of aryl halides or triflates as well as of alkenyl halides or triflates with alkenes or alkynes - has been apphed as the starting transformation accordingly to our classification (Table 8.1). It has been combined with another Mizoroki-Heck reaction [6] or a cross-coupling reaction [7], such as Suzuki, Stille or Sonogashira reactions. In other examples, a Tsuji-Trost reaction [8], a carbonylation, a pericyclic or an aldol reaction has been employed as the second step. On the other hand, cross-couphng reactions have also been used as the first step followed by, for example, a Mizoroki-Heck reaction or Tsuji-Trost reactions, palladation of alkynes or allenes [9], carbonylations [10], aminations [11] or palladium(II)-catalysedWacker-type reactions [12] were employed as the first step. A novel illustrative example of the latter procedure is the efficient enantioselective synthesis of vitamin E [13]. 

Examples of palladium- and rhodium-catalyzed hydroaminations of alkynes are shown in Equations 16.90-16.92 and Table 16.9. The reaction in Equation 16.90 is one of many examples of intramolecular hydroaminations to form indoles that are catalyzed by palladium complexes. The reaction in Equation 16.91 shows earlier versions of this transformation to form pyrroles by the intramolecular hydroamination of amino-substituted propargyl alcohols. More recently, intramolecular hydroaminations of alkynes catalyzed by complexes of rhodium and iridium containing nitrogen donor ligands have been reported, and intermolecular hydroaminations of terminal alkynes at room temperature catalyzed by the combination of a cationic rhodium precursor and tricyclohexylphosphine are known. The latter reaction forms the Markovnikov addition product, as shown in Equation 16.92 and Table 16.9. These reactions catalyzed by rhodium and iridium complexes are presumed to occur by nucleophilic attack on a coordinated alkyne. 

Table 16.9. Scope of the hydioamination of alkynes catalyzed by the combination of [Rh(COD)JBF and PCy,.

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