Thermodynamic stability dienes

What is relevant to this chapter is that a conjugated diene has higher thermodynamic stability than one would expect in the absence of conjugation. Leaving off all hydrogens and substituents in the name of simplicity, several interrelated definitions that document... 

Considering the thermodynamic stability of isomeric acetylenes and dienes 1,3-dienes, by far the most stable compounds involved in propargylic rearrangements should be formed in the highest amount. They are, however, rarely observed since they are formed in a slow reaction. The product distribution, consequently, is kinetically controlled. 

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

Lanthanide hydride derivatives are commonly synthesized by hydrogenol-ysis of lanthanide alkyl complexes [212], In order to further exploit the thermodynamic stability of the Al-N bond dizsobutylaluminum hydride (DIBAH), a common cocatalyst in diene polymerization mixtures and well-established reducing agent in organic synthesis, was used in the hydrogenol-... 

The kinetic as well as thermodynamic stability of these dienemetallocene isomers is influenced markedly by variations of the metallocene unit (Table I) (44), and the nature and position of substituents on the particular diene ligand involved (Table II). For the butadiene complexes. 

Since the isomerization of the 1,3- to the 1,4-diene represented by eq 12 leads to an approximately 1 1 mixture, these isomers must be of comparable thermodynamic stability. It appears that the isomerization requires the presence of a fluoro acid, A (AsF, was present in twofold molar excess of the original 1,3-dtene), hence it is likely that the first step in the isomerization involves P abstraction. Formation of cation XI... 

The great importance of group VIII complexes in homogeneous catalysis has focused more attention on their insertion chemistry, but the number of authenticated insertions of monoalkenes into M-H bonds is small. Insertions of dienes and allenes are more numerous because of the greater thermodynamic stability of the product xt-allyl complexes. Some examples of insertions of complexes of group VIII are listed in Table... 

The driving force for these reactions is considered to be the thermodynamic stability of the l,3-diene-Fe(CO)3 intermediate. 

The diene ligands transform to the cisoid form because of the thermodynamic stability of the complex. Because the bond distances of Cj—and C2—Cy in the butadiene ligand are 1.45 and 1.46 A, respectively, the double bond character between C2—C3 and the bond order alternation is still insignificant [43]. This may be due to the strong K-acidity of three carbonyl ligands which reduces n-back donation from iron to the butadiene ligand. (Cyclohexadiene)tricarbonyliron complexes are also noteworthy. Fe(l,3-cyclo-hexadiene)(CO)3 was prepared by the reaction of 1,3-cyclohexadiene with Fe(CO)5 [44] (eq (13)) and more stable Fe(l,4-cyclohexadiene)(CO)3 was also prepared [45]. 

The bridgehead diolefins 210 and 211, which, like 55, are formally alkylene-bridged derivatives of trans, frans-cycloalkenes, have been reported (165) (Scheme 10). The stability of 211 toward heat, air, and moisture is in contrast to that of the unsubstituted bicyclo[4.2.2]deca-1,5-diene (210). The latter diene was obtained by a kinetically controlled reductive deiodination of 209, which leads to the cleavage of the cyclobutano bridge in preference to the ethano bridge, despite the greater thermodynamic stability of the monocyclic diene to be expected from the latter reaction. 

In the first step, cyclooctatetraene undergoes slow valence tautomerism to form a bicyclooctatriene. Due to similar thermodynamic stability, these two isomers will exist in equilibrium. In step 2, in the presence of Cz (CN) t, the bicyclooctatriene undergoes addition to yield a three-ringed product. This type of addition, involving the 1,4 addition of an alkene to a conjugated diene, is the Diels-Alder reaction. A simple scheme for the Diels-Alder mechanism is given below ... 

The H-n.tn.r. spectrum is best interpreted in terms of structure (X). With methanol it is converted into bicyclo[6.1.0]nona-2,4-diene [56]. Calorimetric studies indicate that this dianion has a much lower thermodynamic stability than the cyclooctatetraenide dianion [58],... 

The insertions of the monomers are believed to occur in two steps [268, 269]. In the first one, the incoming monomer coordinates with the transition metal. This results in formation of a short-lived a-allylic species. In second one, the metal-carbon bond is transferred to the cocrdinated mmiomer with formation of a 71-butenyl bond. Coordination of the diene can take place through both double bonds, depending upon the transition metal [270] and the structure of the diene. When flie mmiomer coordinates as amonodentate ligand, then asyn complex forms. If however, it coordinates as abidentate ligand, then an anti complex results [271]. In the syn complex, carbons one and four have the same chirality while in the anti complex they have opposite chiralities [268]. Due to lower thermodynamic stability the anti complex isomerizes to a syn complex [268]. If the aUyUc system does not have a substituent at the second carbmi, then the isomerization of anti to syn usually occurs spontaneously even at room temperature [268]. 

Probably, the dramatic distinction in stereo-selectivity between cometathesis products obtained from Z-cyclodecene and from cycloolefins C5-C9 (Table 3), is connected with the fact that Z-cyclodecene gives the cyclic transition state having eleven atoms in the cycle (10 C-atoms + 1 Mo-atom). Meanwhile, it is known that the thermodynamic stability of E-isomers in the range of unsaturated cycles of different size dramatically increases beginning from cycloundecene [33]. So, Z-cycloolefins C5, and C7-C9 in cometathesis reaction with a-olefins allow predominantly Z-isomer new double bonds to be obtained (in l,A-dienes). This radically differentiates them from cycloolefins with number of C-atoms of 10 and more, as well as from open-chain olefins. 

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