Isomerization steric interaction

These various photoproducts are all valence isomers of the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the t-butyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the t-butyl groups are coplanar with the ring in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent t-butyl groups. 

The photolysis of 2 -azido-2,4,4, 6-tetramethylbiphenyl (47) in diethylamine gives 10% of a mixture of A,iY-dicthyl-4-methyl-7-mesityl-3//-azcpin-2-amine (48) and the isomeric 3H-azepine 49, together with the unexpected 4//-azepine SO as the major product." Formation of the 4//-azepine is attributed to severe steric interactions in all conformers of the 3H-tautomers. 

Variable-temperature HNMR spectroscopic studies on l-phenyl-3/7-naphtho[l,2-c]azepine and the isomeric l-phenyl-37/-naphtho[2,l-c]azepine indicate that in the [l,2-c]-isomer severe steric interactions between the phenyl group and the adjacent naphthalene ring prevent ring inversion at room temperature.39... 

The very efficient di-rr-methane rearrangement from the triplet state in this case is probably due to two factors (a) the energetics is favorable for cleavage of bond 3-4 and formation of bond 2-4 [see Eq. (8.26)] and (b) rotation about bond 1-2, cis-trans isomerization, should be restricted because of steric interactions. 

These results point to a significant effect of the configuration of the diol moiety on the intracomplex forces involved in the isomeric [C/j-M/j/j], [C/j-M/js], and [C/j-Mss] adducts. The OH- - -O hydrogen bonding in these complexes is responsible for the bathochromic shifts observed in the corresponding spectra. " Different spectral shifts for diastereomeric complexes are often due to the superimposing effects of attractive dispersive (polarization) and repulsive (steric) interactions. ... 

The exclusive loss of H2O from MIKE/CID of [(14-H-As)-H20]" indicates that its formation process 32 is also regioselective in the sense that it is the amino group of the As nucleophile that exclusively attacks the activated [14-H-As]" precursor. Steric interactions in the isomeric [(14 H-As)-H20]" ions are responsible for the differences observed in the relevant MIKE/CID spectra of Fig. 20. 

In homogeneous systems the tight ion pair 12 is formed, the collapse of which depends on the electronic effects and the steric interactions of the substituents. The stability of this ion pair determines the intramolecularity of the rearrangement.121 For example, when (+)-3-phenyl-l-butene was isomerized in tert-BuOK-tert-BuOD, the recovered starting material was deuterium-free and exhibited the same rotation as the starting material, and the product m-2-phenyl-2-butene contained 0.46 deuterium at C(4). This indicates that the isomerization is at least 54% intramolecular.126 Proton migration in perdeutero-l-pentene in ferf-BuOK-DMSO was demonstrated to proceed almost exclusively in an intramolecular manner.127... 

In contrast, the meta isomer is formed under isomerizing conditions (A1C13, FeCl3) in yields up to 67%, which reflects the thermodynamically controlled isomer distribution.138 As a result of severe steric interaction, little or no ortho isomer is formed. 

The kinetically controlled product in the reaction of 99a,b with a large excess of ethyl vinyl ether was the thermodynamically less stable diastereo-mer 137 (Ph and OR trans) (Scheme 37). Complex 137 was present in solution as two conformers that rapidly interconverted. At room temperature in CDC13 the complexes 137 isomerized completely within several hours to form the diastereomer 138. In 138 the substituents OEt and Ph occupy mutual cis positions, thus minimizing steric interaction with the bulky W(CO)5 fragment on the heteroatom.250... 

This C—H substitution process results in a Markovnikov orientation, with the H that is allylic to the more substituted end of the alkene preferentially abstracted. The stereochemistry of the resulting ir-allyl complex does not represent the stereochemistry of the starting alkene, as the complexes are capable of isomerization under the conditions in which they are formed. Typically, a thermodynamic mixture is obtained, with the syn form of the complex predominating over the anti form (equation 1). The syn form is more stable due to unfavorable steric interactions that the anti form encounters with the coordination sphere of the palladium. 

Shibasaki and co-workers have described a regioselective Heck cyclization of aryl triflate 12.1, which ultimately provides tricyclic enone 12.4, a key intermediate in a number of diterpene syntheses (Scheme 8G.12) [25], Treatment of 12.1 under typical cationic conditions resulted in preferential 6-exo closure to give 12.2 and 12.3 as a 3 1 mixture in 62% overall yield and with 95% ee for both products. The complete selectivity for 6-exo cyclization is noteworthy because 6-endo, 5-exo, and 7-endo cyclization modes were also possible. An analysis of the steric interactions involved in the various cyclization modes was presented and was used to rationalize the observed selectivity. Non-conjugated diene 12.2 could be isomerized to the fully conjugated diene 12.3 in quantitative yield by using catalytic naphthalene Cr(CO)3. Both Heck products could be converted to the enone 12.4,... 

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