Steric hindrance phenyl substituents

A still more intricate pattern of potential energy may be expected if the repeat units of the polymer chain carry other substituents, such as the phenyl groups in polystyrene, but these examples establish the general method for quantitatively describing the effects of steric hindrance on rotation. 

The interactions between bulky phenyl substituents in the polymer chain can give more steric hindrance than the deformation of the valency angles in the four membered ring. Similar interactions prevent the polymerization of 1,1-diphenylethylene and 2,2-diphenyloxirane (16). Thus, octaphenylcyclotetrasilane can be thermodynamically more stable than linear perphenylpolysilane and no initiator exists capable of converting this cycle to the linear polymer. 

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... 

A comparison of the structures of products and initial 1,3,5-diazapho-sphorinane reactants reveals a correlation between the number of added borane molecules and the predominant conformation of the initial compounds. l,3-Dibenzyl-5-phenyl-l,3,5-diazaphosphorinane (45) adopts a chair conformation. The substituents at the phosphorus atom and at one of the nitrogen atoms are equatorial, and another nitrogen possesses an axial substituent. In other 1,3,5-diazaphosphorinanes, all the substituents are equatorial. The number of borane molecules added is likely to be determined by the same factors as their conformational stability. There is steric hindrance when the three BH3 groups are axial. 

The related copolymer 94, synthesized in the 1980s by Feast et al. [144], presents a rare example of a PPV-containing phenyl substituents on the vinylene unit. Apparently, the steric hindrance caused by phenyl substituents in 94 is not dramatic, and the optical properties of 94 are similar to those of other PPVs (green emission, APL AEL 530 nm). An internal QE of up to 1% was reported with multilayer 94-based PLEDs containing PPV 1 and PVK as HTL [145]. 

The synthesis of isoxazolines and pyrazolines via the Michael addition of hydro-xylamine and phenyl hydrazine to chalcones and related enones was also reported with activated Ba(OH)2 as a basic catalyst (293) (Scheme 45). In both cases, reactions were performed at reflux of ethanol, and excellent yields (65-80%) with 100% selectivity to the heterocyclic compounds were observed. Steric hindrance associated with the carbonyl compound as well as the electronic character of the substituents in the aromatic ring slightly affected the yields of the heterocyclic compounds. 

Wilcox and co-workers (145) reported that the stereoselectivity of 1,3-dipolar cycloaddition reactions can be controlled in a predictable manner when ion pairs are located at a proper position near the reaction site (Scheme 11.40). He has employed an A-phenylmaleimide substrate having a chiral center in the substituent at ortho position of the phenyl group. Due to serious steric hindrance, this phenyl group suffers hindered rotation around the aryl-nitrogen bond (rotation barrier 22 kcal/mol). Four diastereomeric cycloadducts are possible in the cycloaddition step with a nitrile oxide. When the cycloaddition reaction is carried out in... 

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