True alkynes

The answer is D. Alkynes have higher boiling points than corresponding alkanes. So Choice A is true. Alkynes have sp hybridized carbons. So Choice B is true. Choice C is also trae, since a terminal alkyne like acetylene can form acetylide ions when exposed to strong bases. 

Let s also recall that 1-alkynes ( true alkynes ) give silver salts. These derivatives exhibit an acidic character (pKa of acetylene 25 to compare to the pKa of methane >= 42). It is ascribed to the sp hybridization of carbon atoms bringing the mobile hydrogen. Let s also note that, according to the European pharmacopeia, 17-ethynylsteroids (which are also 1-alkynes) are titrated by sodium hydroxide in tetrahydrofiiran as solvent in the presence of silver nitrate (see Chap. 11). 

Two new sections on the protection of phosphates and the alkyne-CH are included. All other sections of the book have been expanded, some more than others. The section on the protection of alcohols has increased substantially, reflecting the trend of the nineties to synthesize acetate- and propionate-derived natural products. An effort was made to include many more enzymatic methods of protection and deprotection. Most of these are associated with the protection of alcohols as esters and the protection of carboxylic acids. Here we have not attempted to be exhaustive, but hopefully, a sufficient number of cases are provided that illustrate the true power of this technology, so that the reader will examine some of the excellent monographs and review articles cited in the references. The Reactivity Charts in Chapter 10 are identical to those in the first edition. The chart number appears beside the name of each protective group when it is first introduced. No attempt was made to update these Charts, not only because of the sheer magnitude of the task, but because it is nearly impossible in... 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

The poor regioselectivity of alkyne insertion in our polycychc aromatic hydrocarbon synthesis (Scheme 17) suggested to us that perhaps the palladium intermediate in that process was actually undergoing migration from one aromatic ring to the other, perhaps by a Pd(IV) hydride intermediate, to establish an equilibrium mixture of two regioisomeric arylpalladium intermediates under our reaction conditions (Scheme 18). This, indeed, appears to be true as... 

A combination of a metathesis and a Diels-Alder reaction was published by North and coworkers [263]. However, this is not a true domino reaction, as the dienophile (e. g., maleic anhydride) was added after the in situ formation of the his-butadiene 6/3-89 from the fois-alkyne 6/3-88 and ethylene. The final product is the fois-cycloadduct 6/3-90, which was obtained in 34% yield. Using styrene as an un-symmetrical alkene instead of ethylene, the mono-cycloadduct 6/3-91 was formed as a mixture of double-bond isomers, in 38% yield (Scheme 6/3.26). 

Another particularly convenient preparative method is the reaction of the corresponding gold(l) acetylacetonate complex with the alkyne, which requires no auxiliary base.42 68-71 This reaction is also useful for the simple acetylides (L)AuC=CH.72 The acetylacetonates can be isolated and introduced as the true reagents, or prepared in situ using the corresponding gold(l) halide complex and Tl(acac) (Equations (16) and (17)).73... 

A wide variety of five-membered zirconacydes 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cydization takes place via the r 2-complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/p-hydrogen elimination) (path b). Two additional routes to zirconocene r 2-complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

The isomerization of the smallest alkynes 80 with halogens in a propargylic position has been described for chlorine [151, 152], bromine [153] and iodine [154] (Scheme 1.35), but often might proceed by an SN2 -type substitution rather than a prototropic rearrangement [155-159]. On the other hand, transformations such as 82 —> 83 [160] or 84 —> 85 [161] are clearly prototropic (Scheme 1.36). This is also true for propargylic halides such as 86 with its additional ester group assisting the prototropic isomerization [162,163] (Scheme 1.37). 

Often, selectivity for a vinylidene-mediated pathway is heavily dictated by substrate structure. It is especially true in the case of hetero-atom substituted alkynes that Jt-alkyne/vinylidene rearrangement is driven by a reduction in steric interactions at the metal center. 

Many Rh(I)-complexes are capable of dimerizing or oligomerizing alkynes to some degree. Seemingly small changes in reaction conditions can affect the stereo- and regio-selectivity of dimerization. The formation of (Z)-head-to-head dimers, however, can be indicative of metal vinylidene intermediates. This correlation was observed for Ru( 11)-catalyzed dimerizations (Chapter 10) and also holds true for the Rh(I)- and Ir (I)-catalyzed processes described herein. 

Pyrrole and 1-alkylpyrroles generally react with ir-deficient alkenes and alkynes to give Michael addition products (see Section 3.05.1.2.6), but Diels and Alder (3lLA(490)267) reported that 1-methylpyrrole also gave a 1 2 adduct with DM AD formulated as (229), which could be derived from a [w4+w2] cycloaddition of a second molecule of DM AD with the initially formed Michael adduct (cf. Section 3.05.2.3). Subsequent work, however, has shown structure (229) for the 1 2 adduct to be incorrect, the true structure being (230) (63AHC(i)i25, 78AHC(23)265). The formation of the dihydroindole (230) requires the initial... 

Whereas a number of 5-aryl thiatriazoles have been reported, the only previously known true aliphatic and alicyclic representatives were 5-tert-butyl31 and cyclohexyl thiatriazole.32 These are unstable oils that decompose at 0° with nitrogen evolution and formation of sulfur. They are prepared from the corresponding thioacylhydrazides and nitrous acid, but the method is not generally applicable because of difficulties in obtaining the required aliphatic thioacylhydrazides.1 Wijers et o/.17 have found that aliphatic thiatriazoles can be prepared from 1-acetylthio-l-alkynes. Thus a substance believed to be 5-pentylthiatriazole was isolated from the reaction between 1-acetylthio-l-hexyne and ammonium azide. It is an oil that solidifies at about —16° and could not be analyzed because of its explosive character and poor stability at room temperature. Its formation is explained by the following scheme [Eq. 

The above system could further be applied to the epoxidation of allylic alcohols [35, 36], monoterpenes [37] and oc,P-unsaturated carboxylic acids [38], the oxidation of alcohols [35, 36], amines [39] and alkynes [40], and the oxidative transformation of diols [35, 41-44]. The subsequent spedroscopic and kinetic studies by many researchers show that [P04 W0(02)2 4]3 is a true catalytically adive species formed by the readion of H3[PW12O40] with excess H202 (Figure 6.3) [45-48],... 

In the total synthesis of harveynone (152), reaction of the iodide 150 bearing labile functionality took place with the tin acetylide 151, and it was claimed that no reaction occurs with the free amine [70]. However, this claim is not always true, and the coupling of the similar iodide 153 was carried out with the corresponding free terminal alkyne in the presence of diisopropylamine [71]. The coupling of the Mg acetylide 155 with vinyl carbamate 154 to give the enyne 156 is catalysed by a Ni complex [72]. It is true that free alkynes give better results than the corresponding metal acetylides in some cases [73]. 

Kinetic isotope effects have been reported that suggest that the thermal cyclization of enyne-allenes proceeds through a stepwise diradical mechanism (Scheme 121). This is even true if steric bulk at the alkyne terminus is large, contrary to theoretical predictions.182... 

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