Compression, steric

Shifts are also affected by steric compression of any kind on the atom nnder study. The effect on a C atom can... 

Na or Li and ammonia, excellent yields. " A dissolving metal reduction can be effected without cleavage of a sulfur-carbon bond. Note also the unusual selectivity in the cleavage illustrated below. This was attributed to steric compression. ... 

The steeply increasing steric compression shown when passing from one to two adjacent methyl groups is dramatically illustrated in the case of peri-hydrogens. Annelation of abenzo ring has the usual rate-increasing effect (see Section IV, C, 4), i.e., on going from 4-chloro-... 

Steric factors fall into four main categories 27 (a) The release or occurrence of steric compression due to rehybridization in the transition state where the attacking radical and site of attack are each undergoing rehybridization (from sp1 - sp1 and sp1 - sp respectively for aliphatic carbons - refer Figure 1.6). As a consequence, substituents on the attacking radical are brought closer together while those at the site of attack... 

It was postulated that the rate decreased as the basicity of the halogen decreased and/or steric compression increased in f-BuX, and as the polarizability of halogen in MeX increased. The objective of the present research was to extend this model study to isobutylene polymerization systems, in particular to investigate the effect of reagent addition sequence and that of the nature of the halogen in f-BuX and MeX on the polymerization rate and PIB yield using Me3 Al coinitiator. 

Ede, R. M. Main, L. Ralph, J. Evidence for increased steric compression in anti compared to syn lignin model quinone methides. J. Wood Chem. Technol. 1990, 10, 101-110. 

Steric acceleration of the isomerization can apparently occur it is almost certainly Cla of 65 which is removed electrolytically, yet the major electrolysis product is 66(66 67 = 3.2 1) 27>, probably because steric compression between Clb and the nearby aromatic ring in 65 can be relieved through isomerization (inversion). 

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

If the above hypothesis is correct, the basic assumption of the extrather-modynamic treatment, namely, that there is exact cancellation of chemical contributions in the quantity 8AH, no longer holds. Reduction of the contribution to the activation enthalpy from steric compression (AH sc) should be accounted for by introducing an appropriate 0A//+sc-term besides a0AH° into (62), giving (68). Now, if we make the reasonable assumptions that the... 

Steric compression of enediyne moiety by ortho substituents is further illustrated in Fig. 14, which clearly shows that steric destabilization is accompanied by decrease in the distance between carbons Cl and C6. 

The isopropoxy compound 25f reacts about an order of magnitude faster than 25a, 25c and 25g. A measurably larger AS is consistent with additional relief of steric compression in the transition state the protonated intermediate (Fig. 14a) would be sp3 hybridised at nitrogen while the alkoxynitrenium ion (Fig. 14c) would be sp2 hybridised at nitrogen. The isobutoxy compound 25g, in which the branching is one methylene removed from the oxygen atom has similar parameters to straight chain substrates 25a and 25c. 

Based on the discussed acylpalladium 7i-allylic complex (Scheme 5.22) and the reported X-ray structure of the (R)-MOP—Pd 7i-allylic complex [31], the acylpalladium (R)-MOP Ti-allylic complex C (Scheme 5.24) is proposed for the formation of the (R)-product. Complex D, which would give the (S)-product, suffers from steric compression between the MeO-naphthyl ring and the acyl group, while there is no such steric interaction in complex C. Thus, reductive elimination of Pd(0) from C would preferentially yield the... 

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