Alkynes relative stabilities

The relative stabilities of 1-phenylvinyl cations can be measured by determining the gas-phase basicity of the corresponding alkynes. The table below gives some data on free energy of protonation for substituted phenylethynes and 1-phenylpropynes. These give rise to the corresponding Yukawa-Tsuno relationships. 

The reaction of (TPP)Rh with terminal alkenes or alkynes is of special interest due to the cleavage of the carbon-carbon bond adjacent to either the alkene or the alkyne functionality and results in the ultimate formation of (TPP)Rh(R). This overall reaction implies activation of a relatively inert carbon-carbon bond, especially for the case of the terminal alkene. However, the ultimate formation of (P)Rh(R) is not surprising if one considers the relative stability of the rhodium carbon bond in this species(17). 

Because penta-1,2-diene has a larger heat of hydrogenation than penta-1,4-diene, we conclude that the cumulated double bonds of allenes are less stable than isolated double bonds and much less stable than conjugated double bonds. Figure 15-1 summarizes the relative stability of isolated, conjugated, and cumulated dienes and compares them with alkynes. 

The relative stability of vinyl and saturated cations in solution can in principle be evaluated by following three approaches (a) from the competitive formation of vinyl and saturated cations in electrophilic addition to allenes (b) from the relative rates of electrophilic addition to alkynes and alkenes (c) from the relative rates of solvolysis of vinyl and saturated derivatives. 

A study (32) of the reactions of various alkynes with Co2(CO)e indicated there was no correlation between the rates of reaction and any electronic effect attributable to the substituents on the alkyne. This claim was disputed in a subsequent investigation (45). It is clear that electronic factors do have a significant effect (30) on the relative stabilities of the complexes Co2(CO)g(RC2R ) and there are indications (32) that the size... 

The relative stabilities of vinyl cations also can be given by the gas-phase basicity AAG(cc)h+ of alkynes, namely as the standard free energy change for the protonation reaction (28). 

Lewis acid-catalyzed reactions of readily ionizable alkyl halides with alkynes yield vinyl halides [207]. The regioselectivities of these additions can be rationalized by the relative stabilities of the intermediate vinyl cations. Unlike the situation described for additions to alkenes, there is no preference for anti-additions, and the stereoselectivities can be explained by the intermediacy of nonbridged species [208]. The site of nu-... 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

Although zirconium is only one out of over 50 potentially usable metals in this class (including the lanthanides and actinides), virtually all synthetic applications of hydrometallation with transition metals involve zirconium Why is this so The primary reason derives from the near requirement of a d -metal center for hydrometallation of a general alkene or alkyne. For later transition metals, hydrometallation to give a stable organometallic product can usually be achieved only for special cases—conjugated dienes, alkenes with electronegative substituents, etc. This is due to the relative stability of the ti -complex, as discussed previously. 

Because the nonbonding orbital is occupied, stability increases with s character, the converse of the situation for carbocations. The order of stability of carbanions is sp < sp < sp. The relative stability of gas phase carbanions can be assessed by the energy of their reaction with a proton, which is called proton affinity. The proton affinities of the prototypical hydrocarbons methane, ethene, and ethyne have been calculated at the MP4/6-31+G level/ The order is consistent with the electronegativity trends discussed in Section 1.1.5, and the larger gap between sp and sp, as compared to sp and sp, is also evident. The relative acidity of the hydrogen in terminal alkynes is one of the most characteristic features of this group of compounds. 

We have seen that a carbon-carbon triple bond is shorter and stronger than a carbon-carbon double bond, which in turn, is shorter and stronger than a carbon-carbon single bond. We have also seen that a carbon-carbon tt bond is weaker than a carbon-carbon cr bond (Section 1.14). The relatively weak tt bonds allow alkynes to react easily. Eike alkenes, alkynes are stabilized by electron-donating alkyl groups. Internal alkynes, therefore, are more stable than terminal alkynes. We now have seen that alkyl groups stabilize alkenes, alkynes, carbocations, and alkyl radicals. 

Scheme 4-42) [134]. Werner s group has shown that alkynylsilanes also undergo 1,2-silyl migration in the same system via an intermediate 7i-complex [135]. Alternatively, the rearrangement may be catalyzed by base [136] or induced by sequential deprotonation/protona-tion [137]. The relative stability of the alkyne and vinylidene complexes is dependent on the electron density and the d-electron count of the metal, as illustrated by the behavior of the d -Mo complexes 84 in which the alkyne is a four-electron donor addition of CO causes the 84 85 conversion whereas tautomer 84 is favored with the phosphite ligand (Scheme 4-43)... 

Figure 10-17 Ring-opening metathesis polymerization (ROMP) [111, 112]. represents a cyclic olefin or an alkyne. Any of the steps may be reversible, depending in part on the relative stabilities of the metallacycle and metallacarbene, and on the ring-strain of the ring in the monomer which is opened.

In addition to the utility of a process that allows alkenes and alkynes to be converted to alkanes, measurement of the heat of hydrogenation (rAH°) allows the relative stabilities of alkenes to be gauged and relationships to be explored. 

In contrast to C(2)-linked terminal alkynes 145, gold-catalyzed alkylation of C(3)-linked tetrahydrofurans bearing terminal alkyne functions 148 mainly led to the formation of major product exo-methylene cyclopentanes 149 and minor products 150 (Scheme 55). This reversed selectivity might be explained by the relative stability of intermediates III and V. Steric constrains should be weaker for the fused bicyclic intermediate V (in Scheme 55) than intermediate HI (in Scheme 54), thus allowing a rapid [l,2]-hydride shift, which leads to VI rather than a [1,2]-alkyl shift, which leads to III. 

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