Substituents, size

Within a homologous series such as the -butyl-, -hexyl-, and -octyl-substituted methacrylates there is a trend for those polymers with the bulkier substituents to show smaller values of f. Note that there is also a correlation between substituent size and Tg within this series. 

An increase of the substituent size leads to an increase of the p-character of the lone pair. The higher the p-character of the lone pair in trialkylstibines or bismuthines, the more basic is the pnictine... 

Vinyl cyclopropanes tethered to an aUcyne chain 127 were also subjected to the cycloisomerisation reaction in presence of the NHC-Ni catalyst system (Scheme 5.34) [39], The product formation depends on the substrate used and the NHC hgand. When SIPr carbene is used, three different products were obtained depending on the size of the R group attached to the alkyne moiety. If R is small (like a methyl) product 128 is obtained exclusively. If R is Et or Pr a mixture of 128 and 129 is obtained in 3 2 to 1 2 ratio, respectively. However, when R is large groups such as Bu or TMS only product 130 is obtained. When IfBu carbene 131 is used as the ligand, cycloisomerisation of 127 afforded product 128 exclusively, regardless of substituent size (Scheme 5.34) [39]. 

The copper(I) complexes [Cu(L)] (786) [L as in Cu11 complex (426)], and (787) (L as in Cu11 complex (427)], were also structurally characterized.365 From the standpoint of aggregation behavior of copper(I) thiolates579 through substituent size and ancillary chelate ligands, Kaim... 

Calculations of the HOMO-LUMO gap energy as a function of alkyl substituent size are in agreement with the experimentally observed increase in lowest energy UV Amax,231 and the effects of changing from alkyl to aryl side... 

The concept of steric effects was introduced by Kehrmann over a century ago. V. Meyer ° and Sudborough and Lloyd shortly thereafter presented kinetic results supporting the steric effect explanation of rate retardation in the esterification of 2-substituted and 2,6-disubstituted benzoic and 3-c/i-substituted acrylic acids. Major early reviews of steric effects are given by Stewart and Wittig. Somewhat later reviews are by Wheland, and in a volume edited by Newman . Steric effects are proximity effects that result from and are related to substituent size. 

The first enzymatic polymerizations of substituted lactones were performed by Kobayashi and coworkers using Pseudomonas fluorescens lipase or CALB as the biocatalyst [90-92]. A clear enantiopreference was observed for different lactone monomers, resulting in the formation of optically active polymers. More recently, a systematic study was performed by Al-Azemi et al. [93] and Peelers et al. [83] on the ROP of 4-alkyl-substituted CLs using Novozym 435. Peelers et al. studied the selectivity and the rates as a function of the substituent size with the aim of elucidating the mechanism and the rate-determining step in these polymerizations. Enantio-enriched polymers were obtained, but the selectivity decreased drastically with the increase in substituent size [83]. Remarkably for 4-propyl-e-caprolactone, the selectivity was for the (R)-enantiomer in a polymerization, whereas it was S)-selective in the hydrolysis reaction. Comparison of the selectivity in the hydrolysis reaction (Fig. 10b) with that of the polymerization reaction (Scheme 8a) revealed that the more bulky the alkyl substituent, the more important the deacylation step becomes as the rate-determining step. 

According to the reaction medium the results in the third column of the Table V should be approached in a different way. A rigid polyethylene film suppresses phenol formation almost entirely by keeping the radicals in the solvent cage. Furthermore, all substituents decrease the quantum yield relative to hydrogen. This effect cannot be explained solely by a Hammett-type treatment of electronic effects. The effect of the substituent size is particularly pronounced in the case of the p-isooctyl substituted ester whose quantum yield is reduced drastically when compared to that of the p-methyl substituted... 

The observation that the yield of orrfto-rearranged product depends on the bulk of para-substituent is unique. It would be interesting to find out whether this also holds true with photorearrangement in a low viscosity solvent, e.g., in an experiment where only the substituent size would be changed preserving the electronic effect of the substituents. One can argue that the presented photorearrangement in polyethylene film is almost a photoreaction in the solid state. Such reactions occur with the minimum amount of atomic or... 

H6]dimethylcyclobulane (4) was pyrolyzed at 338 401 °C under initial pressures of 10 and 30 Torr. The results arc inconclusive, showing that there is less than 4% racemization and no geometric isomerization in the recovered cyclobutane after its 11.7% cleavage to 2-methyl-propene.74 However, it appears that the ratio of the rate constants for the thermal cracking to geometric isomerization is a function of substituent size, as evidenced by the fact that dimethyl... 

Predicting Hydrolysis Rate Difference Based on Substituents Sizes PROBLEM Based on Taft steric parameters, how much faster than ethyl acetate would you expect ethyl formate to hydrolyze ... 

The absorption maximum of alkyl-substituted polysilanes shifts linearly to lower energies with increasing substituent size. This shift comes from steric interference of the substituents, resulting in straining of the Si-Si backbone and a change in backbone conformation.75... 

The influence of substituent size, polarity, and location on the thermotropic properties of synthetic phosphatidylcholines has been studied by Menger et al. [18], The effect of increasing membrane curvature on the phase transition has been investigated by DSC and FTIR [19]. In addition, a data bank, LIPIDAT, on lipid phase transition temperatures and enthalpy changes is available [20, 21],... 

A short review of the origins and early development of steric effects is given elsewhere1. Steric effects are proximity effects that result from and are related to substituent size. 

The simple diastereoselectivity of the photocycloaddition of electronically excited carbonyl compounds with electron rich olefins was studied as a function of the substituent size—at identical starting conditions ignoring the electronic state involved in the reaction mechanism [123], The [2+2] photocycloaddition of 2,3-dihydrofuran with different aldehydes in the nonpolar solvent benzene resulted in oxetanes 118 with high regioselectivity and suprising simple diastereoselectivites the addition to acetaldehyde resulted in 45 55 mixture of endo and exo diastereoisomer, with increasing the size of the ot-carbonyl substituent (Me, Et, i-Bu, t-Bu), the simple diastereoselectivity increased with preferential formation of the endo stereoisomer (Sch. 37). 

By substituting SIPr for an N-alkyl-substituted NHC ligand (IfBu), the skipped-triene product (2) could be prepared selectively from cyclopropylen-yne substrates, regardless of substituent size (e.g., R) (Eq. 3) [11]. Thus, skipped-triene products were formed exclusively under mild conditions (room temperature, 2 h). 

A barrier decrease is indeed observed in a series of o,o -substituted sterically crowded acetone-anils 120> as the size of the substituents increases. Similarly, increasing substituent size decreases the isomerization barrier in o,o -disubstituted N-aryl-guanidines in contrast, a barrier increase is observed in the corresponding salts where only rotation about the C=N+ bond is possible 124>. However, such steric effects do not seem to be conclusive. They are complicated by the fact that o,o -disubstituted N-aryl compounds act also on the conjugation between the nitrogen site and the aryl group. 

Variable-temperature NMR spectroscopy is a well-established technique in the study of conformational exchange kinetics in liquid amides. [7,58] Using DNMR spectroscopy, it is possible to measure the temperature dependence of interconversion rates in order to yield a complete set of kinetic parameters characterizing the internal rotation about the carbon-nitrogen bond. In the last several years, the development of gas-phase NMR methods has provided a valuable tool for the elucidation of the magnitude of intrinsic and environmental contributions associated with this conformational exchange. In order to characterize these contributions, a series of molecules with systematic variations in substituent size and polarity must be analyzed both in the liquid and gas phase. 

The asymmetry parameter in 1,2-disubstituted 5-nitroimidazoles decreases with increasing the substituent size. The n-electron density and N-l bond population is calculated according to the Townes-Dailey theory and described in detail by Dr. Lucken [1408] and Dr. Dolgushin [1409,1410] based on height with lengthening of the substituent on N-l [1411]. 

TABLE 4. Selected statistical data for the absolute values of angular out-of-plane parameter, xNi (deg), and twist parameters, tni C2 and tc2 C3 (deg), together with pyramidality coefficients P (%) for the N1, C2 and C3 atoms of the enamine grouping both for a sample containing enamines with unrestricted substituents (size 474) and with carbon as the first atom in each of the RJ-R5 substituents (sample size 178)... 

We hope to work with a stereoisomeric pair of olefins of type 2 where the substituent sizes are sufficiently different to permit an evaluation of the influence of olefin stereochemistry on the reaction. Similar considerations will have to be kept in mind when choosing a more complex olefin of type 3. 

TS energies are likely to be adequately estimated, especially the trends among related inward versus outward TS for a series of substituted cyclobutenes. They first examined the ring opening of iran -3,4-dimethylcyclobutene (116) and irani -B.d-dihydroxycyclobutene (117).2° °2 The outward rotation is favored over the inward for both cases. More importantly, while the inward barriers are similar, the outward barrier for 117 is 17 kcal mol lower than for 116, a difference not readily amenable to rationalization based solely on substituent size. 

The QSARs do bring out flexibility of the ligand-enzyme interaction. Steric effects are modelled by the variables MR, Es, etc., which show that contact between enzyme and ligand does not destroy activity completely but simply reduces it in proportion to the substituent size. Probably both hgand and enzyme are gradually moved from their preferred positions. 

The predicted increased solvent sensitivity of Tc with decreasing substituent size was tested further by comparing the transition temperatures of a series of di-n-alkyl polymers in n-hexane and toluene solutions. The results (Table II) are again in good qualitative agreement with the theoretical prediction. In particular, the difference between Tc in toluene and that in n-hexane solution increases monotonically with decreasing side-chain length 6 C, heptyl 10 C, hexyl and 16 °C, pentyl. 

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