Stereospecific substitution reactions

A complicating factor associated with experimental application of the Skell Hypothesis is that triplet carbenes abstract hydrogen atoms from many olefins more rapidly than they add to them. Also, in general, the two cyclopropanes that can be formed are diastereomers, and thus there is no reason to expect that they will be formed from an intermediate with equal efficiency. To allay these problems, stereospecifically deuteriated a-methyl-styrene has been employed as a probe for the multiplicity of the reacting carbene. In this case, one bond formation from the triplet carbene is expected to be rapid since it generates a particularly well-stabilized 1,3-biradical. Also, the two cyclopropane isomers differ only in isotopic substitution and this is anticipated to have only a small effect on the efficiencies of their formation. The expected non-stereospecific reaction of the triplet carbene is shown in (15) and its stereospecific counterpart in (16). 

Stereospecific nucleophilic substitution in chiral ion—dipole complexes. Chiral molecules can be discriminated in the Cl source of a CIMS instmment by specific ion-molecule reactions induced by chiral reagent gas. This method has been applied with success to distinguish between enantiomeric and diastereomeric forms of menthols ((lR,2S,5R)-(— )-14, (lS,2R,5S)-(+)-14, and (lR,2R,5S)-(—)-14 in Scheme 11) through the nucleophilic displacement of their hydroxyl group by (5)-2-amino-l-butanol Self-protonation of As... 

Scheme 10.5 The stereospecific allylic substitution approach to a stereotetrad using a linchpin cross-coupling reaction.

Walden inversion (Section 8.4) Originally, a reaction sequence developed by Paul Walden whereby a chiral starting material was transformed to its enantiomer by a series of stereospecific reactions. Current usage is more general and refers to the inversion of configuration that attends any bi-molecular nucleophilic substitution. 

The Sn reactions of cyclohexanone acetals substituted at C(2) with sulfur, iodine, or chlorine are thought to occur when the nucleophile attacks the oxocarbenium ion intermediate with the substituent on C(2) in an axial conformation.104 The most stereospecific reactions (i.e. >92% trans), were when the substituent at C(2) was sulfur. This mechanism (Scheme 26) is supported by HF/6-31G calculations that show the oxocarbenium ion (66) with the sulfur at C(2) in the axial position to be the most stable, and by the high yield of the trans-isomer (67) in the products. 

The Sn2 displacement is a good example of a stereospecific reaction one in which different stereoisomers react to give different stereoisomers of the product. To study the mechanism of a nucleophilic substitution, we often look at the product to see if the reaction is stereospecific, with inversion of configuration. If it is, the Sn2 mechanism is a good possibility, especially if the reaction kinetics are second order. In many cases (no asymmetric carbon or ring, for example), it is impossible to determine whether inversion has occurred. In these cases, we use kinetics and other evidence to help determine the reaction mechanism. 

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). 

The property that the stereochemical result of an electrocyclic reaction is absolutely predictable is called stereospecificity. A stereospecific reaction will give you one stereochemical result when a cis starting material is used, and the opposite result when a trans starting material is used. Other examples of stereospecific reactions include Sn2 substitutions, catalytic hydrogenation of alkynes or alkenes, and dihydroxylation and bromination of alkenes. 

Examples are shown in 24-26. The stereospecificity of substitution reactions at silicon indicates a viable route to other enantiomerically pure silicon compounds. All examples of chiral molecules of the type R R2R3R4Si and R1R2R3SiH have high configurational stability, and in addition halides such as PhSi(Me)(Et)Br do not readily racemize (see Corriu et aid). 

Some arylcarbenes, prepared by photodecomposition of the corresponding diazo compounds, undergo intersystem crossing so effectively that the alkenes present are mainly attacked by triplet-excited carbenes. This type of reactivity has, in particular, been associated with nitro-substituted arylcarbenes. The reaction between photochemically generated 4-nitrophenyl-carbene and (Z)- and ( )-but-2-ene, which has been studied in particular detail, has shown that nonstereospecific carbene addition may become just as important as the stereospecific reaction. ... 

Numerous tetrahedral optically active organosilicon compounds have now been obtained, and various resolution procedures have been successfully employed. They include resolution through separation of diastereomers as well as kinetic resolution and asymmetric synthesis. Moreover, the stereospecificity of substitution reaction at silicon makes possible the synthesis of various optically active compounds starting from resolved organosilicon compounds. 

This reaction of Pt(II) is general for a range of coordinated amines apart from ammonia, and appears to yield exclusively the trans geometric isomer (which is called, because of this exclusivity, a stereospecific reaction). Even chelated diamines can be substituted, as they are inevitably more volatile than any anions present thus chelated 1,2-ethanediamine can be replaced by two chloride ions in [Co(en)3]Cl3 to form m-[CoCl2(en)2]Cl, and by... 

Stereoselectivity is intimately related to the mechanism of the reaction. Some reactions are stereospecific, that is reactions in which stereoisomeric reactants each provide stereoisomeric products. For example, the Sjv2 substitution reaction results in an inversion of the configuration. It is a stereospecific reaction. The I -reactant gives the 5-product and the 5-reactant gives the / -product (assuming the priority order remains unchanged). 

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