Stereospecific reactions steric

The complexes will effect oxyamination reaction with alkenes in a stereospecific reaction (Scheme 8).290 After reductive cleavage of the intermediate alkanolaminato complex (I) (see below) vicinal amino alcohols (II) are formed. The reaction is unusual in that the new C—N bond is always formed at the least substituted terminal alkenic carbon atom, and there is a clear preference for the imido complex to use its NR group for coordination to the osmium despite the steric restraints of R.299,300 However, the least sterically hindered part of the alkene moiety is attached to. the nitrogen atom.290,291,300 The yields of amino alcohols in the reaction can be improved by addition of tertiary alkyl bulkhead amines (see below). 

In contrast with other electrophilic additions, the peracid epoxidation is syn-stereospecific. With sterically strongly hindered alkenes the reaction takes place on the less sterically hindered side. In other cases, the stereochemistry of the reaction is affected by polar effects or the geometry of the transition state. Important conclusions regarding the mechanism of the reaction can be drawn from the steric pathways in the synthesis of the oxiranes. This has been dealt with comprehensively by Berti, who reviewed the topic up to 1971, with special emphasis on the peracid oxidation. A noteworthy account of the topic of peracid epoxidation is given in a review by Rebek. ... 

Degradation of the product to carve out the chiral methyl group and convert it into a compound suitable for configurational analysis, using only stereospecific reactions of known steric course. 

The Cope rearrangement is an example of valence isomerization, which has been extensively studied in recent years. Valence isomerizations are iso-merizations in which only single bonds are broken and formed whilst double bonds change their site. They have a high degree of stereospecificity, the steric result depending on the number of double bonds and on whether the reaction is initiated thermally or photochemically.160... 

Chiral phosphines are valuable as ligands in numerous important catalytic asymmetric processes. Intense research efforts have produced many classes of chiral phosphines with diverse steric and electronic properties, and researchers have examined how these properties influence activity and stereoselectivity in catalysis. Given the rapid development of new methods in asymmetric catalysis and the prominence of chiral phosphines in this area of ehemistry, it is surprising that few catalytic asymmetric syntheses of chiral phosphines have been described. Instead, enantiopure phosphines are most commonly prepared either via stereospecific reactions of resolved starting materials or through routes which require an additional resolution step, such as fractional crystallisation of diastereomers. On the other... 

Narrowing Diels-Alder reaction products to one enantiomer requires a chiral influence either as part of the dienophile [40], as in Equation 8.29 [41], or as part of a Lewis acid catalyst [42] as in Equation 8.30 [43]. In many Diels-Alder reactions, steric hindrance or intramolecular restrictions limit the number of isomers. Altogether, stereospecificity, regiospecificity, and endo/exo control make most Diels-Alder reactions quite practical. 

Note that the endo product is predicted by the concerted mechanism. It is the major product, even though it is sterically more congested than the exo isomer. This stereospecificity is characteristic of the reaction. 

Some advantages of this reaction are high yield if the tosylate is in a sterically accessible position excellent isotopic purity of the product (usually higher than-95%) and perhaps most important, access to stereospecifically labeled methylene derivatives. For example, deuteride displacement of 3j -tosylates (183) yields the corresponding Sa-d derivative (185) in 96-98% isotopic purity. Application of this method to the labeled sulfonate (184), obtained. by lithium aluminum deuteride reduction of a 3-ketone precursor (see section HI-A) followed by tosylation, provides an excellent synthesis of 3,3-d2 labeled steroids (186) without isotopic scrambling at the adjacent positions. The only other method which provides products of comparable isotopic purity at this position is the reduction of the tosyl-hydrazone derivative of 3-keto steroids (section IV-B). 

This reaction, now termed hydroboration, has opened up the quantitative preparation of organoboranes and these, in turn, have proved to be of outstanding synthetic utility. It was for his development of this field that H. C. Brown (Purdue) was awarded the 1979 Nobel Prize in Chemistry . Hydroboration is regiospecific, the boron showing preferential attachment to the least substituted C atom (anti-Markovnikov). This finds ready interpretation in terms of electronic factors and relative bond polarities (p. 144) steric factors also work in the same direction. The addition is stereospecific cis (syn). Recent extensions of the methodology have encompassed the significant development of generalized chiral syntheses. 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

Diels-Alder reaction and. 494-495 El reaction and, 392 E2 reaction and, 387-388 R.S configuration and, 297-300 S 1 reaction and, 374-375 S -2 reactions and, 363-364 Stereogenic center, 292 Stereoisomers, 111 kinds of, 310-311 number of, 302 properties of, 306 Stereospecilic, 228, 494 Stereospecific numbering, sn-glycerol 3-phosphate and, 1132 Steric hindrance, Sjvj2 reaction and, 365-366 Steric strain, 96... 

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