Steric hindrance introduction

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

Despite several attractive features in this method of direct halogen introduction and the obvious applications in the synthesis of deoxy sugars, its uses have not been further exploited by other groups of workers. Some new related methods have become available which reportedly eliminate the difficulties previously encountered such as rearrangement, unreactivity due to steric hindrance, and phosphonate ester formation. The reaction is based on the observation (28) that triethylphosphine reacts with ethanol and carbon tetrachloride to give ethyl chloride, chloroform, and triethylphosphite. In a new adaptation (76, 77) of this... 

The changes with steric hindrance and temperature must be due to conformational effects along the polymer chains. It is now generally agreed that the thermochromism results from an increase in the proportion of trans- to gauche conformations in the polymer chain as the temperature is decreased. Similarly, the introduction of sterically hindering substituents could increase the amount of trans junctions. Evidence in favor of this model is presented in several recent papers, (28i) as well as the chapter by Michl in this volume. 

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

Structural investigations of individual stereoisomers and equilibrium mixtures revealed some general features (83M11). A phenyl group at phosphorus is predominantly in an axial position when there is no steric hindrance. The introduction of bulky substituents onto carbon atoms of the ring shifts the equilibrium toward an equatorial form, and one more twist conformer appears. The influence of equatorial substituents on the equilibrium position is clearly pronounced in the case of P(III) derivatives as well as for a P(IV) compound. The substitution of a phenyl group by a... 

The reduction of 4-chloro-4-methyltetrahydropyran with triethylsilane requires more than a catalytic amount of aluminum chloride. No 4-methyltetrahydropyran is obtained after 20 hours at room temperature even when 0.75 equivalents of the catalyst is used, but a 92% yield is obtained after only 30 minutes when two equivalents of catalyst and three equivalents of triethylsilane are used.136 146 This is presumably a result of the ability of the Lewis acid to coordinate at the ring oxygen as well as at the chlorine. The introduction of alkyl groups at C2 appears to introduce enough steric hindrance near the ring oxygen to enable less than one equivalent of aluminum chloride to effect reduction, but also makes the products unstable to the reaction conditions so that the synthetic yields decline compared with the unsubstituted substrate.136... 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

Wilson et al. [27] showed that the introduction of steric hindrance in the 6-position of one or two of the pyridine groups was sufficient to fine-tune the ligand field and obtain crossover compounds. These systems have been investigated using a number of different techniques, both in the solid and solution phases. Thermodynamic parameters have been derived from variable temperature magnetic susceptibility data for the single methyl-substituted (23 ) (AH°=19.7 kj mol"1, A5°=39.8 J mol"1 K"1), the double methylsubstitut-... 

Introduction of chiral centers in the nonleaving groups can achieve interesting binding differences for enantiomers toward DNA. For example, [PtCR-DACH)Cl2] binds to DNA twice as strong as the S-enantiomer (143). Pt(II) complexes containing the meso form of 1,2-bis(2-hydroxyphenyl) ethanediamine exhibit the lowest antitumor activity and reactivity because of the steric hindrance provided by the aromatic rings (144). 

In reality, the polydispersity of the hyperbranched polymer even in the absence of core is lower than that predicted for the ideal case. Cyclization and steric hindrance during polymerization can be the reasons. Polydispersity can also be lowered intentionally, for instance, by introduction of core molecules or by programmed addition of the monomers. 

An interesting phenomenon concerns the introduction of atropisomerism in the bisphenol substituents (Figure 8.21). Due to steric hindrance one of the isomers will be preferentially formed and thus it seemed worthwhile to enforce certain combinations of chirality, i.e. backbone and substituent chirality. 

Much attention has been devoted to the acid- and nucleophile-catalyzed racemization of thiosulfinates. As a result of the extensive studies by Kice and his co-workers (112) and by Fava (281), it is clear now that the easy racemization of thiosulfinates caused by acids and bases (e.g., pyridine) is related to the scission of the sulfur-sulfur bond and the formation of sulfenic acid or its anion as an achiral intermediate. As expected, introduction of steric hindrance... 

In order to allow for a closer approach of the boron centers, the introduction of flat cyclic boryl moieties with reduced steric hindrance has also been pursued. Thus, the reaction of 26 with 9-chloro-9-borafluorene and 5-bromo-10,l 1-di-hydrodibenzo[b,f]borepin resulted in the formation of diboranes 31 and 32 which bear two different boryl moieties at the pen-positions of naphthalene (Scheme 13). " These diboranes have been characterized by multinuclear NMR spectroscopy and X-ray single-crystal analysis. In 31, the boron center of the borafluorenyl moiety is 7i-coordinated by the zp o-carbon of a mesityl group with which it forms a contact of 2.730(3) A (Fig. 8). As a result of this interaction, the boron center involved in this contact is slightly pyramidalized (Xangie = 355.7°). In the case of 32 (Fig. 9), the distance between the boron center of the boracylic moiety and the zpio-carbon of... 

There is abundant evidence that steric hindrance to coordination will decrease the formation constant of any complex formed and may even inhibit complex formation altogether. Numerical data are available for transition metals and substituted ethylenediamines,27 or substituted oxines.-3 for 1,10-phenanthroline and its 2- and 2,9-methyl-substituted derivatives28 and for other systems.29 Since the introduction of a 2-methyl substituent into oxine diminishes 0 while increasing pK2, it is not surprising that the effective concentration [Al(2-MeOxine)3] may not be great enough to cause precipitation, although this is achieved in non-aqueous media such as ethanol. 

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