Driving Force 3 - Steric Strain

The transformation of the porphyrin intermediate 4 into a chlorin can be achieved after introduction of a C — C double bond into the 15-propanoate side chain of 4 to yield 5. The cyclization of 5 with participation of the 15-acrylic ester side chain under acidic conditions gives the chlorin 6 which is then transformed in a multistep reaction sequence into chlorophyll a. The driving force of chlorin formation from the porphyrin is believed to be the relief of steric strain at the sterically overcrowded porphyrin periphery which gives the desired trans arrangement of the propanoate side chain and the methyl group in the reduced ring. The total... 

The CpCo complexes, on the other hand, should be more stable due to the presence of the robust and bulky Cp-shield. Unfortunately, however (tetraiodo-cyclobutadiene)CpCo is not available, and there is no obvious synthetic path to make it. But maybe another way to produce CpCo-stabiHzed tetraethynylated cyclobutadiene complexe exists It is known, that 22 a undergoes a rearrangement to 22 d when subjected to the conditions of flash vacuum pyrolysis at elevated temperatures [24]. The driving force behind this rearrangement is twofold first, the steric strain between the two adjacent TMS groups is removed in 22d and second, the TMS groups in 22d are not bound to an -hybridized center but to an sp-hybridized one, which is a more favorable situation from a thermodynamic point of view. 

The isomerization shown in Scheme 6.32 involves two intermediary dianions and not the direct conversion of the initial anion-radical into the final one. However, what is the driving force for this isomerization The molecule of l,3,6,8-tetra(tcrt-butyl)naphthalene is nonplanar the tert-butyl group and carbon atom at position 1 of the naphthalene skeleton lie off the plane of the molecule. The corresponding anion-radical has the same stereochemical peculiarity (Goldberg 1973). Such bending removes the steric strain but, naturally, decreases the degree of the n-electron delocalization over the neutral molecule. As for the anion-radical, its unpaired electron delocalizes less effectively than the anion-radical of the unsubstituted naphthalene. Bending of the naphthalene skeleton... 

The driving force in some isomerization polymerizations is relief of steric strain. Polymerization of P-pinene proceeds by the first-formed carbocation XVI rearranging to XVII... 

Molecules may be strained by a variety of modes of distortion that include angle bending, torsional strain, and steric interactions. In some cases, strain can provide an important driving force for reaction. However, it is not the strain in the molecule that is important, but rather the change in strain in a reaction. Even this is not sufficient because there must also be a mechanism for the conversion of a compound to a lower energy product that leads to a relatively low barrier for reaction. 

There are relatively few examples of nucleophilic attack upon the porphyrin system, electrophilic substitutions being far more common. One well substantiated example of nucleophilic attack was described by Woodward (62MI30700). In this example the sterically hindered porphyrin (59) suffers nucleophilic attack at the 15-position to form a phlorin cation (60), the driving force for the reaction presumably arising from release of strain accompanied by the change from sp2 (planar) to sp3 (tetrahedral) of the C-15 atom. 

Lammertsma and Cerfontain269 performed a detailed study on the ions generated from isomeric dimethylnaphthalenes. An interesting observation is the transformation upon increasing the temperature of cation 97 through an intermediate to cations 98 and 99. The driving force is the relief of steric strain in the peri position [Eq. (3.38)]. For the... 

The driving force for rearrangement is relief of steric strain between the isopropyl group and one of its adjacent methyl groups. Isomerization is acid-catalyzed. Protonation of the ring generates the necessary carbocation intermediate and rearomatization occurs by loss of a proton. 

The driving forces for electron transfer are a high-energy level of the highest occupied molecular orbital or a steric strain of the starting molecule. Complexation of oxygen by the electron-rich organic molecule has often been indicated as the first step of the mechanism. 

For pyrrolidine N-oxides, one might a priori anticipate iminium ion formation to occur by a syn elimination process, since the N—O and adjacent Co—bonds cannot become antiperiplanar to each other. However, the N-oxide (29a) of the steroid alkaloid conanine reacts with acetic and trifluoroacetic anhydride exclusively by the anti pathway to give the exocyclic enamine (30 Scheme 6). Release of steric strain between the C-16 methylene and the ring methyl substituent is undoubtedly a major driving force in this reaction. More revealing is the reaction of N-oxide (29b), which would give the same enamine product if a syn pathway is favored. The observed formation of compounds (31) and (32) argues in favor of the anti elimination. 

E2S.il (a) Ring opening alkene metathesis polymerization. ROMP can result in reduced steric strain, thereby providing a thermodynamic driving force for the reaction. 

The thermodynamic driving force for fructan biosynthesis is the sterically strained nature of the crowded glycosidic linkage in sucrose. 

As demonstrated by reaction 64, cyclic products can also be generated. by the interaction of ww-charged sites. Based on the study of labeled precursors it was shown923 that the base peak at [M —119]+ in the El mass spectra of TMS derivatives of methyl esters of ortho-dihydroxy-substituted benzoic acids is due to the successive losses of MeO and Me4Si. It was argued that the driving force for the reaction 161 - 162 is the relief of steric strain. 

Finally, the papulinone (21) proved to be a new P-lactone characterised as 4-(l-hydroxy-2-phenylethyl)-4-carbomethoxyethan-2-one this structure was confirmed by conversion of 21 into the corresponding y-butyrolactone [22, Fig. (3)] by acetylation carried out in standard conditions. The mechanism to explain this unexpected reaction, whose driving force is the basic properties of pyridine used as solvent and the high steric strain present in 21, was also hypothesised [48]. Compounds 19, 20 and 21 were indistinguishable by bioassay on bean or apple leaves. When 2-6 mg/ml of purified metabolites taken up in solution in citrate buffer (pH 6.0) were applied to both leaves, similar areas of necrosis resulted. At concentrations of less than 2 and 1 mg/ml only slight chlorosis and no toxic effects were observed [48]. 

Since singlet oxygen is a dienophile, such oxidations are believed to involve cycloaddition of oxygen to an endoperoxide (46, Scheme 14), followed by rearrangement to the hydroperoxide. Reduction in steric strain and the formation of a strong intramolecular hydrogen bond (vmax 2800 cm 1) could provide the driving force in the second step.48... 

There are 3 driving forces, which affect the course of carbocation chemistry (cation stability, ring strain and steric strain). 

The transition from the bisabolane skeleton to the cuparane is not favourable energetically. Initially a secondary carbocation is formed from a tertiary, though a hydrogen shift quickly moves the positive charge back to a tertiary centre. The five-membered ring which is formed is very cluttered and, therefore, suffers from steric strain. The driving force must, therefore, come from the protein which catalyses the reaction. The 1,2-carbon shift to the chamigrane skeleton relieves some of the steric strain but this creates a spiro centre, the strain of which is relieved by the 1,2-carbon shift which creates the thujopsane skeleton. 

The cyclisation of the acorane skeleton to the cedrane skeleton creates steric and ring strain. The driving force must come from the enzyme involved. The first 1,2-carbon shift relieves much of the ring strain but still leaves steric strain but, in doing so, forms a secondary carbocation from a tertiary one. This is reversed by the second 1,2-carbon shift and the final methyl shift helps reduce steric strain. 

---