In isomers

Although compound 14 may be preferred at equilibrium, C-l in isomer 18 is particularly susceptible to nucleophilic attack because it is much less hindered than either C-2 or C-3 in 14. Once 18 is produced, it reacts selectively with hydroxide ion to give triol 19. The irreversible conversion of 18 to triol 19 drives the equilibrium process forward. 

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

The inherent lability of alkene- and hydroxyalkanesulfonates, variations in isomer composition, and the presence of the disulfonates are features which complicate AOS analyses. Improper sample handling, such as exposure to high temperatures, can also alter active matter composition. Consequently, analytical procedures have been developed that minimize potential sources of error. 

HIPS in combination with stabilizers. Four basic types of HBCD are produced by all the HBCD producers, i.e. low melt, medium range, high melt and thermal stabilized HBCD. Analysis of the HBCD manufactured by several producers in given in Table 1 (ref. 3). The melting point of HBCD varies with changes in isomer ratio and composition of impurities. 

Another type of stereoisomerism, called out-in isomerism, is found in salts of tricyclic diamines with nitrogen at the bridgeheads. In cases where k, , and m >6, the N—H bonds can be inside the molecular cavity or outside, giving rise to three isomers, as shown. Simmons and Park ° isolated several such isomers with k, I, and m varying from 6 to 10. In the 9,9,9 compound, the cavity of the in-in isomer is large enough to encapsulate a chloride ion that is hydrogen bonded to the two N—H... 

The ratio of ortho- to the meta- and /faru-products of monoalkylbenzenes with 1 decreased as the size of the substituents on benzene ring increased. No ortho-alkylation product was found in the case of i-propylbenzene due to the sterie interaction between /-propyl and the incoming allyl groups. Sterie hindrance arising from the size of the alkyl groups at ortho positions of the substituted benzenes appeared to be the principal cause of the differences in isomer product ratio." " ... 

The enamines derived from cyclohexanones are of particular interest. The pyrrolidine enamine is most frequently used for synthetic applications. The enamine mixture formed from pyrrolidine and 2-methylcyclohexanone is predominantly isomer 17.106 A steric effect is responsible for this preference. Conjugation between the nitrogen atom and the tt orbitals of the double bond favors coplanarity of the bonds that are darkened in the structures. In isomer 17 the methyl group adopts a quasi-axial conformation to avoid steric interaction with the amine substituents.107 A serious nonbonded repulsion (A1,3 strain) in 18 destabilizes this isomer. 

The reduction of A9(10)-octalin to cis- and /ran.v-decalins occurs with cis to trans stereoselectivities that vary with the nature of the organosilicon hydride employed. The ratios are 0.28-0.59 with n-butylsilane, 0.67 with diethylsilane,204 0.34212 or 0.72204 with triethylsilane, 0.67 with diphenylsilane, 0.77 with diphen-ylmethylsilane,212 1.3 8204—1.80127,212 with triphenylsilane, 0.54 with triisopen-tylsilane, 1.17 with tricyclopentylsilane, 2.57 with tri-.vcc-bulylsilane, 3.35 with di-ferf-butylsilane, 4.88 with di-ferf-butylmethylsilane, and 13.3 with tri-rm-butylsilane.204 Opinions differ about the mechanistic significance of these changes in isomer ratios.204,212... 

For cobalt as catalyst, variations in reaction parameters have been studied as a means of controlling the product composition (or isomer ratio). Thus, variations in isomer ratio from 1 1 to about 4 1 were observed under widely differing conditions of temperature, catalyst concentration, partial pressure of hydrogen, and partial pressure of carbon monoxide. 

The diamines and their mono- and di-protonated ions can exist in various conformations, in which the nitrogen lone pairs and the protons on nitrogen are directed in (i or i+) or directed out (o or o+) from the molecular cavity. Diprotonated l,10-diazabicyclo[8.8.8]hexacosane, for example, may exist in either of the three forms in equation (79). When the out-out isomer of l,10-diazabicyclo[8.8.8]hexacosane bis hydrochloride (o+ o+) is dissolved in aqueous acidic solution, isomerisation to the in-in isomer (i+ i+) occurs so... 

Table 3. Propene insertion for the homoxantphos system, energies and geometrical parameters of transition states. Energies in kJ.mol 1, distances in A and angles in degrees, (a) The CW B in isomer could not be localized (b) Dihedral angle Hhydnde-Rh-Caikenc-Caiiiene for defining alkene rotation.

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