Steric features

One interesting phenomenon was the effect of the boron substituent on enantioselectivity. The stereochemistry of the reaction of a-substituted a,/ -unsatu-rated aldehydes was completely independent of the steric features of the boron substituents, probably because of a preference for the s-trans conformation in the transition state in all cases. On the other hand, the stereochemistry of the reaction of cyclopentadiene with a-unsubstituted a,/ -unsaturated aldehydes was dramatically reversed on altering the structure of the boron substituents, because the stable conformation changed from s-cis to s-trans, resulting in production of the opposite enantiomer. It should be noted that selective cycloadditions of a-unsubsti-tuted a,/ -unsaturated aldehydes are rarer than those of a-substituted a,/ -unsatu-... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Novel cationic methyl and neutral methyl, chloro Pd11 complexes have been synthesized with various functionalized imidazole ligands. For the first time, structurally characterized examples of this type of Pd complex could be obtained. The chelate ligand is shown to adopt a boat conformation. Inversion of the chelate ring was established by NMR spectroscopy. Depending on electronic and steric features of the ligand, the complexes can act as catalysts in the CO/ethylene as well as the Fleck reaction.186... 

Where such alignment is prevented by structural or steric features, the expected stabilisation may not take place. Thus while pentan-2,4-... 

The cis-to-trans ratios of the isomeric 4-tert-butyl-l-methylcyclohexanes derived from treatment of 4- < r(-butyl-l -methylenecyclohcxanc with trifluoroacetic acid vary with the steric features of the organosilicon hydrides that are used (Eq. 76).204 The ratio is 0.04 with n-butylsilane, 0.09 with diethylsi-lane, 0.11 with triethylsilane, 0.10 with triisopentylsilane, and 0.19 with either tri-.s< c-butylsilanc or di-tert-butylsilane. 

In contrast to these examples, the antibacterial agent piromidic acid (5.91, Fig. 5.24) is not converted to a lactam derivative but yields only the cw-ami-no acid derivative [188]. Steric features might underlie this metabolic difference. Bulky groups in close proximity to the N-atom seem to favor the formation of the amino acid metabolite, whereas substrates with an unhindered N-atom seem to favor the formation of a lactam metabolite. 

The data in Table 10.1 suggest that the reactivity of epoxide hydrolase toward alkene oxides is highly variable and appears to depend, among other things, on the size of the substrate (compare epoxybutane to epoxyoctane), steric features (compare epoxyoctane to cycloalkene oxides), and electronic factors (see the chlorinated epoxides). In fact, comprehensive structure-metabolism relationships have not been reported for substrates of EH, in contrast to some narrow relationships that are valid for closely related series of substrates. A group of arene oxides, along with two alkene oxides to be discussed below (epoxyoctane and styrene oxide), are compared as substrates of human liver EH in Table 10.2 [119]. Clearly, the two alkene oxides are among the better substrates for the human enzyme, as they are for the rat enzyme (Table 10.1). 

For each of the following reactions, develop a transition-state structure using molecular models which would account for the observed stereoselectivity. Identify important conformational and/or steric features of the proposed transition state. 

Benzoflavone (166) is non-planar with a torsion angle of 23° between the phenyl group and the remainder of the molecule, in keeping with the proximity of H-3 and H-2 (80MI22202). However, 5,6-benzoflavone (167) is approximately planar despite similar steric features. It is postulated that interaction between the carbonyl group and H-10 increases the contribution of the canonical form (168), which leads to a more planar structure than would otherwise he exneeteri... 

Other cases are not always so obvious, yet any structural or steric feature which influences the close approach of the it bond and the free-radical center will influence file rate of cyclization and hence the yield of cyclized product. For example, trans-fused cyclopentyl systems are much higher in energy than cis-fused ones thus file trans-fused cyclopentyl compound does not cyclize effectively and gives only reduction,... 

Alcohols and phenols. Both these classes of compounds are characterised by the strong absorption resulting from the O—H stretching modes the position and shape of the bands are sensitive to the electronic and steric features of the compound and also to the physical state of the sample. Absorption bands arising from C—O stretching and O—H bending vibrations are also of diagnostic value. 

Of course, the fascinating steric features of lanthanide elements are most impressively expressed in the lanthanide contraction [76]. In textbooks, lanthanide contraction is often simply explained as being the phenomena responsible for similar chemical properties, in particular between the pairs of the d-transition metal homologues Zr/Hf, Nb/Ta and Mo/W. However, what does the contraction mean to the lanthanide elements (and compounds derived from them)... 

The mechanical, thermal, optical, and other properties of a polymer depend on the structure of the monomer units. Where a copolymer is used, it depends additionally on the relative amounts and distribution of the two monomeric building blocks. Metallocene catalysts have four main advantages over the conventional polymerization catalysts. They can polymerize a very wide variety of vinyl monomers irrespective of their molecular weights and steric features. They also can polymerize mixtures of monomers to give polymers of unique properties. 

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