Alkenes, erythro/threo addition

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

The stereochemistry of the addition has been shown to be cis. This can be demonstrated by the deuteration of (E)- or (Z)-butendioic acids to form (R,R)- and (S,S)-2,3-dideuterobutandioic acid or (/ ,5 )-2,3-dideuterobutandioic acid, respectively (equations 43 and 44). The deuteration of other ( )-alkenes to threo-dSkam and (Z)-alkenes to erythro products also demonstrates that cis addition occurs. 

Hydroxylation, the addition of two hydroxyl groups across double bonds, converts alkenes and cycloalkenes into vicinal dials. Stereochem-ically. the addition may occur in the syn or the anti mode. In open-chain alkenes (with the exception of terminal alkenes for which stereochemistry is irrelevant), syn hydroxylation transforms cis alkenes into erythro (or meso) diols and trans alkenes into threo (or dl) diols. anti Hydroxylation of cis alkenes gives threo (or dl) diols, whereas anti hydroxylation of trans alkenes yields erythro (or meso) diols. syn Hydroxylation of cycloalkenes gives cis diols, whereas anti hydroxylation furnishes trans diols (Table I). 

The mechanism of phosphonate anion (135) addition to carbonyl derivatives is similar to the phosphonium ylide addition however, there are several notable features to these anion additions that distinguish the reactions fix)m those of the classical Wittig. The addition of the anion gives a mixture of the erythro (136 and 137) and threo (139 and 140) isomeric p-hydroxyphosphonates (Scheme 24). In the case of phosphine oxides, the initial oxyanion intermediates may be trapped. The anion intermediates decompose by a syn elimination of phosphate or phosphinate to give the alkene. The elinunation is stereospecific, with tile erythro isomer producing the ci.r-alkene (138), and the threo addition adduct producing the... 

NMR-spectroscopic studies and a single crystal X-ray structure determination of the reduced Co / form of 100 revealed the presence of a bridging 2,3-dibromo-3-phenyl-propionato ligand (threo dl pair). The complex bearing the erythro form of 2,3-dibromo-3-phenyl-propionate is only produced in minor yields <3%. Therefore, the bromination of the encapsulated alkene is a highly diastereoselective syu-addition. This is rather... 

Merritt et al. [124,170-174] carried out some of the earliest additions of fluorine to carbon-carbon double bonds. The fluorination of cis and trans propenyl benzene in a nonpolar solvent at low temperature gave predominantly erythro and threo difluorides respectively. More recently, Rozen [175] carried out similar reactions, but used a more polar solvent (trichlorofluoro methane, chloroform and ethanol) and a very low concentration of fluorine. Thus, in the fluorination of cis and trans 3-hexene- l-ol acetate (Fig. 72), syn addition occured to give exclusively the erythro and threo difluoro compounds respectively. Corresponding results were obtained in the addition of fluorine to other alkenes, including cyclic alkenes and cyclic enones. 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

The stereochemistry of HC1 or DC1 addition to a wide variety of alkenes has been examined. Addition of HC1 to m-2,3-dideutero-2-butene affords a mixture of erythro and threo 2-chlorobutanes.40 1-Methyl-cyclopentene37 and 1,2-dimethylcyclopentene41 give almost exclusively tertiary chlorides formed by anti... 

The stereochemistry of HBr addition to alkenes has been thoroughly studied. The reaction of cis- or fnwu-2-butene with DBr in DO Ac gives a mixture consisting of 60% threo and 40% erythro products.116 While cyclohexene117 and alkene (l)118 undergo predominant anti addition, the alkenes (4 -(6) give... 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

The normal preference for (Z) alkenes in reactions of non-stabilized phos-phoranes can be reversed by employing the Schlosser modification of the Wittig reaction (Scheme 6).19 Here, equilibration of the initially formed erythro and threo betaine intermediates is achieved by reaction with additional strong base, usually an alkyl lithium. The resulting betaine ylide then gives the (E) alkene on treatment with a proton source followed by potassium tert-butoxide. 

The reaction is akin to the Ritter reaction, with activation achieved by nitration, rather tiian protonation, and the products accordingly retain the nitro group. Additions to 1-phenylcyclohexene (59%) and to rrnR5-stilbene (72%) are stereospecific (trans) c -stilbene gives the expected threo product (39%) plus some erythro (6%). Reactions of nitrogen dioxide with alkenes are very complex and rarely useful. A recent mechanistic paper gives many key references. Addition of N2O3 is occasionally useful, as with dicyclopentadiene (Scheme 49). °... 

One of the major aspects of these additions is the stereoselection which is achieved to create ( )- or (Z)-vinylmetals from alkynes, or to create erythro or threo structures from alkenes. The latest developments of this approach provide a route to non-racemic, chiral, metallated structures from common organometallics (RLi, RMgX), either by transmetallation with asymmetrically ligated transition metals (as Zr complexes) or simply... 

As a consequence of the presumed irreversibility of die first step of the Peterson reaction, the stereochemistry of the elimination was determined solely by the relative rates (ki and ki ) of formation of threo- and erythro- -sHyl alkoxides (141) and (142), as indicated in Scheme 66. Support for the irreversibility of this step comes from the experiments on the nucleoidiilic opening of the diastereomeric epoxides (143) and (144). Thus, the syn- and onri-epoxides, when treated widi lithium dipropyl cuprate, yielded the threo- and erythro- -sHy alcohols (145) and (146), respectively. Treatment of die threo isomer with base gave the (Z)- ene exclusively and the erythro isomer gave the ( )-alkene. The high levels of selectivity for this elimination indicated diat die initial addition is not revenible. The elimina-... 

A study carried out by Kocienski and Lythgoe flrst demonstrated the trans selectivity of the Julia coupling process. The authors found the i uctive elimination could best be carried out with the acet-oxy or benzoyloxy sulfones. If the lithio sulfone derivative is used for addition to the carbonyl, the reaction can be worked up with acetic anhydride or benzoyl chloride to obtain the alkene precursor. In cases where enolization of the carbonyl is a complication, the magnesium derivative can frequently be used successfully. A modification of the reductive elimination was found to be most effective. Methanol, ethyl acetate/methanol or THF/methanol were the solvents of choice and a temperature of -20 C was effective at suppressing the undesired elimination of the acetoxy group to produce the vinyl sulfone. With these modifications of the original procedure, the ability of the reaction to produce dienes as well as rran.r-disubstituted alkenes was demonstrated, llie diastereoisomeric erythro- and threo-acetoxy sulfones could be separated and it was demonstrated that both isomers were converted to the rrans-alkene. It... 

The nonstereospecific products are formed in both the reaction of stable thiiranium ions with nucleophiles and the addition of arenesulfenyl chlorides to alkenes. The reaction of the isomeric stable ions (158) and (159) with acetic acid at room temperature gives the same mixture of erythro-(160) and threo-( 161) products (Scheme 59) <75TL245i, 76IZV1318). Nonstereospecific products are also formed by the addition of 2,4-dinitrobenzenesulfenyl chloride to ( )- and (Z)-anisole <72JOC3086> and 4-methoxy-/ -deutereostyrene <84TL4983>. 

The reaction of formaldehyde with alkenes is of industrial interest and has been extensively studied.12 Reaction of excess formaldehyde as formalin with an alkene and aqueous acid gives 1,3-dioxanes (2) in 40-90% yield. Reaction of 2-butenes with paraformaldehyde and hydrogen chloride at -65 C gives a mixture of diastereomeric y-chloro alcohols rich in the isomer formed by trans addition to the alkenyl double bond.14 For example frans-2-butene gives an 85 15 mixture of erythro- and threo-3-chloro-2-methyl-1 -butanol (equation 2). Reaction of 1 -alkenes under similar conditions gives 3-alkyl-4-chlorotet-rahydropyrans (5) in 50-80% yield (Scheme 2).15 Initial reaction occurs via addition of formaldehyde to the terminal carbon of the double bond, followed by loss of a proton to give the 3-alken-l-ol. Reaction of... 

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