Substituent effects aliphatic

Since substituent effects in aliphatic systems and in meta positions in aromatic systems are essentially inductive in character, cr and cr values are often related by the expression cr = 0.217cr — 0.106. Substituent effects fall off with increasing distance from the reaction center generally a factor of 0.36 corresponds to the interposition of a —CHj— group, which enables cr values to be estimated for R—CHj— groups not otherwise available. 

In an effort directed at developing a racemization catalyst which works uniformly for all the substrates at room temperature, we designed and synthesized a novel aminocyclopentadienyl ruthenium chloride complex 5. The DKR of aromatic as well as aliphatic alcohols could be conducted at room temperature. In case of aromatic alcohols, the substituent effects were found insignificant in the DKR however, aromatic alcohols have comparatively faster conversion rates than their ahphatic counterparts. This is the first ever report of a catalyst... 

KSIEs for the reaction of aromatic olefins, 1,1-diphenylethylene and a-methylstyrene (Table 21) are significantly smaller they can be related to transition states earlier than those in the aliphatic series. Unfortunately, for the reactions of highly reactive aromatic olefins or enol ethers, whose low sensitivity to solvent and substituent effects indicates very early transition states, there are not enough KSIE data to confirm this conclusion. 

Duddeck, H., Substituent Effects on 13C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry, 16, 219. 

In his work on the analysis of substituent effects in aliphatic ester reactions, Taft87,89 derived a values, from which 07 values may be calculated as follows NH2, 0.10 Me2N, 0.10 AcNH 0.28. Thus there are some discrepancies as between the results of Charton109 and of Taft87,89. 

The substituent effects on the photochemistry between benzene and secondary aliphatic amines53 were studied. Irradiation of toluene or chlorobenzene with diethylamine results in the formation of mixtures of addition and substitution products (equations 34 and 35). Irradiation of anisole or benzonitrile with diethylamine gives the substitution product 7V,7V-diethylaniline (equations 36 and 37). Irradiation of benzylfluoride with diethylamine results in a side-chain substitution (equation 38). The photoreaction of p-fluorotoluene with diethylamine gives both substitution and reduction products (equation 39). 

Substituent Effects on l3C Chemical Shifts in Aliphatic Molecular Systems. 

In the preceding section substituent effects on aliphatic carbon atoms, mostly in monosubstituted molecules, were examined by comparing their, 3C chemical shifts with those of the respective unsubstituted parent compounds. If one proceeds to molecules M bearing two substituents (X, Y Scheme 43) it often happens that SCS(X) (140 — 141) or SCS(Y) (139 — 141) differ considerably from the values expected for the monosubstituted prototypes (138 — 139 and 138 — 140, respectively). 

In the previous sections the main focus of the discussion of substituent effects was their dependence on structure (especially stereochemistry) and the underlying transmission mechanisms. Admittedly, it would still be veiy difficult to predict 13C chemical shifts in more complicated and highly substituted aliphatic compounds with sufficient precision even if our knowledge about the physical back-... 

SUBSTITUENT EFFECTS ON, 3C CHEMICAL SHIFTS IN ALIPHATIC MOLECULAR SYSTEMS. DEPENDENCE ON CONSTITUTION AND STEREOCHEMISTRY 219... 

The Hammett equation has been further extended to include substituents of aliphatic systems too. In this case the constants that quantify the inductive effects, which are represented as o, have different values compared to those concerned with aromatic systems and are defined as Taft constants. Some values of these latter are also reported in Table 1. 

Since the total steric effect of component substituents R1, R2 and R3 on the reaction center in the transition state of ester hydrolysis is similar, if not identical, to that of three N-substituents of aliphatic amines on a certain electron acceptor or electrophile, we attempted to separate electronic and steric effects of N-substituents of amines NR3R2R3 for various reactions by Eq. 24 where is either the rate or equilibrium constant, Scr is the sum of a values for R1, R2 and R3 and E (R ) E (R2) Si E (R3), and showed that Eq. 24 is generally applicable 22). 

Taft separated the resonance from the inductive substituent effects and proposed Equation 2.15. The inductive parameter, ah is based on a obtained from aliphatic systems (see p. 68).14... 

---