Alkenes bond strengths

Thermochemical data correspond with a decrease in the platinum(0)-alkene bond strength ir the sequence C2H4 > PhCH=CH2 > cis-PhCH=CHPh > frans-PhCH==CHPh. Displacemem reactions show an expanded stability order for platinum(O) complexes to be TCNE> PhCfeCH > alkenes.801 The relative weakness of alkene complexes relative to alkyne com plexes of platinum(O) is the reverse of that found with platinum(II).802... 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene k bond strengths vary with the level of fluonnabon (Table 18) Both CHF=CF2 and CF2=Cp2 have significantly weaker 7t bonds than CH2= CH2, CH7=CHF, and CH2=Cp2, consistent with other data indicatuig that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low n bond energy in Cp2=CF2 underlies its propensity to undergo thermal biradical [2-1-2] cycloaddibons [103] (see p 767 )... 

A second factor that contributes to alkene stability involves bond strengths. A bond between an sp2 carbon and an sp3 carbon is somewhat stronger than a bond between two sp3 carbons. Thus, in comparing 1-butene and 2-butene, the monosubstituted isomer has one sp -.sp3 bond and one sp3-sp2 bond, while... 

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. 

The central bond of [l.l.l]propellanes (1) is the center of their reactivity, and in many ways, it is useful to think of it as somewhat akin to the n bond in an alkene. The strengths of the two are comparable, and both are susceptible to electrophilic and radical attack. The main difference is that the central bond in la is apparently somewhat susceptible to nucleophilic attack as well, whereas the n bond in unsubstituted ethylene is not. In both cases, introduction of electron-withdrawing groups enhances reactivity towards nucleophiles. 

Why is ethyne so much less stable than ethene or ethane First, C-C bonds are not as strong as C-H bonds. Therefore a gain in stability usually is to be expected when C-H bonds are made at the expense of C-C bonds ethene and ethane each have more C-H bonds than ethyne has. Second, ethyne has six electrons held between the two carbons and these electrons experience considerable mutual interelectronic repulsion. This accounts for the fact that the average C—C bond strength for the triple bond of an alkyne is 200/3 = 67 kcal, compared to 146/2 = 73 for the double bond of an alkene and 83 kcal for a normal single bond of an alkane. 

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

There has been considerable effort directed towards obtaining a fundamental understanding of the factors that govern the reactivities of carbon-centered radicals in bimolecular reactions, particularly with respect to their addition to alkenes [84]. From early liquid and gas phase studies, reactivity in such addition reactions was concluded to derive from a complex interplay of polar, steric, and bond-strength terms [85], which is much influenced by the nature and position of substituents on both the radical and the alkene. 

Always bear this in mind bond strength is only a guide to selectivity in radical reactions. As we shall see shortly, it s not the only factor involved. Indeed, you ve already seen steric effects in action when the Br radical added to the less hindered end of the alkene in the first radical reaction of this chapter, and you will later see how front/erorfj/fa/ effects can ope rate too. 

It is proposed that the observed stereoselectivity seen in the reactions involving this bidentate ligand is a result of the difference in bond strength between Co-P and Co-S. The more labile Co-S bond is able to dissociate, leaving a free coordination site on one of the cobalt atoms onto which the alkene is able to coordinate, leading to a stereoselective reaction. 

IR, Raman and 13C NMR spectroscopic studies have been performed on various [Ir(acac)(L)2] complexes (L = ethylene, propene, vinyl chloride, vinyl acetate, methyl acrylate, styrene) for the elucidation of the bonding between Ir and the alkene ligand.142 Also, the square planar iridium(I) acetylacetonate complexes [Ir(LL)(L )2], where LL is a /J-diketonate and L is CO or ethylene, have been studied by UVPES.143 The enthalpies of reaction of the crystalline [Ir(acac)(L)2] complexes with gaseous CO (reaction 28) have been determined by differential scanning calorimetry. The enthalpies for the gaseous reaction have been derived from these results and Ir—L bond strengths estimated.143... 

A key feature of alkene coordination is the loss of planar symmetry. Indeed, it is this loss of symmetry that allows the vc=c IR absorption of alkene complexes to be observed (IR inactive for the free molecule). Consistent with the synergic bonding description, coordination leads to a decrease in the C=C bond strength and accordingly a decrease in the value of vc=c. The case of prochiral alkenes is of particular importance,... 

Little related work on the technetium analog has been completed. Much of the synthetic work with high-valent Tc has centered on imido complexes,54 or on complexes for use in radiopharmaceuticals.55 Methyl technetium trioxide has been prepared.8 This species is thermodynamically a powerful enough oxidant to cycloadd to alkenes cyclohexene gave (after hydrolysis) c/s-l,2-cyclohexanediol. No chemistry with H202 has been reported it is expected to behave in a similar manner to MTO, unless the change in the metal-oxo bond strength perturbs the overall thermodynamics. 

---