Stereoselectivity acyclic alkenes

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

It is found in practice with (5), and with other simple acyclic alkenes, that the addition is almost completely stereoselective, i.e. 100% ANTI addition. This result also is incompatible with a one-step pathway, as the atoms in a bromine molecule are too close to each other to be able to add, simultaneously, ANTI. 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

The stereoselective cyclopropanation of chiral alkenes can be divided into two classes cyclic and acyclic alkenes. Furthermore, within each class, a subdivision exists involving those that contain a proximal basic group that can direct the cyclopropanation reaction of zinc carbenoids and the others that do not. The discrimination of reactivity between alkenes that possess a proximal basic group and those that do not was first highhghted early on when Simmons and Smith noticed that the cyclopropanation of l-(o-methoxyphenyl)-l-propene was more efficient than that of the related meta and para isomers (equation 46). ... 

Cyclopropanation mediated by zinc organometallics 2. Stereoselective cyclopropanation of acyclic alkenes... 

The stereoselectivity of metal catalysed intermolecular cyclopropanation has been examined in detail. With acyclic alkenes, EIZ ratios of substituted cyclopropanes in the... 

The stereoselectivity of this hydration process has been studied on a number of alkenes. Acyclic alkenes and most cyclic alkenes give exclusively trans addition (equations 214 and 2 1 5).331,332 However, some strained cyclic alkenes, such as rrans-cyclooctene and trans-cyclononene, give cis adducts.332... 

Control over regioselectivity and stereoselectivity in the formation of new C-C a-bond is required to utilize the Heck reaction in complex molecule synthesis. For the intramolecular Heck reaction, the size of the ring formed in the insertion step controls the regiochemistry, with 5-exo and 6-exo cyclization favoured. A mixture of regioisomers is formed from Heck insertions of acyclic alkenes, whereas cyclic alkenes such as cycloalkenes as a Heck substrate produce a a-arkylpalladium(II) intermediate A, which has only one syn-P-hydrogen. Syn-elimination of the hydrogen provides only product B (Scheme 5.6). 

An alternative procedure for the preparation of /J-alkoxy selenides involves treatment of cyclic or acyclic alkenes with phenylselenenyl cyanide in the appropriate alcohol in the presence of catalytic amounts of copper(ri) or nickel(ll) halides. The reaction using methanol as the alcohol with cyclic alkenes is tram stereoselective giving /1-methoxy selenides 23 usually in high yield34. 

Free-radical hydrosilylation of alkenes with tris(trimethylsilyl)silane proceeds in high yields and with good stereoselectivity. In the case of cyclic alkenes the thermodynamically less stable c/.y-products (resulting from anti attack) arc formed preferentially, while in the case of acyclic alkenes the products are formed with opposite stereochemistry33. 

Acyclic Alkenes. Acyclic alkenes also undergo stereoselective hydrogenation with catalysis by (1) (Table 3). It is sig-... 

The cycloadditions proceed stereoselectively with the geometry of ( )-alkenes being preserved, but that for (Z)-alkenes being partially lost (equations 1 and 2), Excellent diastereofacial selectivity has been observed in cycloadditions to cyclic alkenes, the major product resulting from the addition of the TMM-PdL2 complex to the least-hindered face of the alkene (equations 3-5). Modest (4 1) to high (>99 1) diastereoselectivity is also observed when enantiomerically pure acyclic alkenes are used (equations 6,7). ... 

For acyclic alkenes, the facile rearrangement process obviates most possibilities of stereoselective transformation. The one exception is that of stereospecific isotopic labelling, for which hydrozirconation is a potentially powerful technique compounds of type Bu CHDCHDX (36), useful for NMR-based mechanistic studies, are conveniently obtained as in Scheme 9. This requires that four separate processes — hydrozirconation of alkyne, cleavage of alkeiiylzirconium, hydrozirconation of alkene and cleavage of alkylzirconium — all be completely stereospecific. 

The issue of stereoselectivity of alkene formation from acyclic tosyl- or trisyl-hydrazones has been addressed in only a few cases. For straight chain ketones, there is a preference for m-alkene formation (see Scheme 67). A few other examples appear to give lower ratios, which is surprising in view of several reports of highly cis selective reactions in which the vinyllithium intermediate was trapped with electrophiles.Hydrocarbon branching in the a -position degrades the stereoselectivity considerably (Scheme 67). 

Ketones and 1,2-diketones are readily converted into alkynes, which can subsequently be reduced to alkenes. Since there are a number of methods to reduce alkynes stereoselectively, this route offers a method of controlling the stereochemistry of acyclic alkenes derived from ketone starting materials. The most common approach for the ketone to alkyne transformation has been conversion of the starting ketone into a vicinal dichloride and subsequent bis elimination induced by strong base. ... 

In contrast to the stereospecific epoxidation of acyclic alkenes with peroxy acids, oxidation of acyclic a,(3-unsaturated ketones with alkaline hydrogen peroxide is stereoselective in that only one stereoisomer is formed from cis- and trans-enones ... 

The synthesis of stereodefined acyclic alkenes via 3-elimination reactions—such as (1) dehydration of alcohols, (2) base-induced eliminations of alkyl halides or sulfonates (tosyl or mesyl esters), and (3) Hofmann eliminations of quaternary ammonium salts—often suffers from a lack of regio- and stereoselectivity, producing mixtures of isomeric alkenes. 

Few examples exist in the literature concerning the stereoselective addition of acyl radicals to a radical acceptor in an acyclic manner. Equation (13.1) shows the efficient 1,2-asymmetric induction in the addition of aliphatic or aromatic acyl radicals to chiral acyclic alkenes 1 [7]. The corresponding a-hydroxy ketones 3 were produced with high syn selectivity (Table 13-1). This acyl radical addition is very exothermic, and it is hypothesized that Hammond s postulate can be invoked to predict a transition state that is very close in energy to the starting alkene 1. The X-ray structure of 1 was then used to rationalize the stereochemical outcome of this radical addition by determination of the least sterically hindered path for the approaching radical. 

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