1.4- trans addition

The opening of the oxirane ring is accompanied by inversion except when the oxirane ring is in the terminal position of an aliphatic chain the ultimate result is equivalent to trans addition to the double bond. Thus cyciohexene yields trans-1 2- f/ohexanediol ... 

Addition of a hydroxy group to alkynes to form enol ethers is possible with Pd(II). Enol ether formation and its hydrolysis mean the hydration of alkynes to ketones. The 5-hydroxyalkyne 249 was converted into the cyclic enol ether 250[124], Stereoselective enol ether formation was applied to the synthesis of prostacyclin[131]. Treatment of the 4-alkynol 251 with a stoichiometric amount of PdCl2, followed by hydrogenolysis with formic acid, gives the cyclic enol ether 253. Alkoxypalladation to give 252 is trans addition, because the Z E ratio of the alkene 253 was 33 1. 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other paHadium-cataly2ed oxidations of olefins ia various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Bromine monofluoride [13863-59-7], BrF, can be prepared by the direct reaction of Br2 and F2, but because it readily disproportionates it has never been prepared in pure form (57). However, BrF can be prepared in situ by the reaction of Br2 with AgF in benzene (58) or by the reaction of A/-bromoacetamide and HF in ether (59). BrF adds to simple alkenes at room temperature to give products of trans-addition. Bromine trifluoride [7787-71-5], BrF, can be formed from gaseous fluorine and Hquid bromine (60). Bromine pentafluoride [7789-30-2], BrF, is formed from the reaction of BrF vapor with gaseous fluorine at 200°C (60). The tri- and pentafluorides are commercially available. As strong fluorinating agents they are useful in... 

The addition of secondary amines to acetylenes is most applicable to the synthesis of conjugated acyclic enamines (50,171,172). Particularly the addition to acetylenic esters and sulfones has been investigated (173-177) and it appears that an initial trans addition is followed by isomerization to more stable products where the amine and functional group are in a trans orientation (178). Enamines have also been obtained by addition of secondary amines to allenes (179). 

Compound 123 (R = R = H) oxidatively adds chloroform and gem-dichloro-alkanes (940M4153). With methyl iodide, trans addition occurs and species 147 is formed. With dichloromethane, the route is three-fragment and four-electron, and the result is species 148 (R = H). Dichloromelhylbenzene and methyl dichloro-ethanoate give 148 (R = Ph and COOMe, respectively). Simultaneously, the... 

Iodine azide, generated in situ from an excess of sodium azide and iodine monochloride in acetonitrile, adds to ethyl l//-azepine-l-carboxylate at the C4 — C5 and C2 —C3 positions to yield a 10 1 mixture of the rw-diazidodihydro-l//-azepines 1 and 2, respectively.278 The as stereochemistry of the products is thought to be the result of initial trans addition of the iodine azide followed by an SN2 azido-deiodination. The diazides were isolated and their stereochemistry determined by conversion to their bis-l,3-dipolar cycloadducts with dimethyl acetylene-dicarboxylate. 

The stereospecific trans addition of bromine to 5//-dibcnz[/i,/]azepine has been the subject of intense mechanistic study.230 231... 

The obvious route by trans-addition of Br2 was found to give the desired product contaminated by tri- and tetrabromo species because of substitution reactions (Figure 3.109). 

Sulfonyl bromides and iodides react similarly217-218-225 copper-salt catalysis in these cases facilitates the additions but is not absolutely necessary however, it influences the stereochemistry of the additions. Addition of sulfonyl iodides226 as well as the uncatalyzed thermal addition of sulfonyl bromides227 to alkynes leads to an exclusive trans-addition, whereas CuBr2 catalysis in the latter case causes the formation of cis-addition products to some extent (11 16%) correspondingly, copper-salt catalysis in sulfonyl chloride additions to alkynes leads to the formation of a mixture of Z,E-isomers228-229 (equation 40). 

Ethyleneimine reacts with (p-tolylsulfonyl)acetylene to give only the (Z)-product 115 via trans addition (equation 91), while primary and secondary aliphatic amines afford ( )-products76. With nonterminal acetylenes such as l-(ethylsulfonyl)-l-propyne, the reactions of ethyleneimine, n-propylamine and f-butylamine give mixtures of ( )- and (Z)-adducts. The double conjugate addition of sodium sulfide, selenide and telluride to bis(l-propynyl)sulfone (116) produces heterocycles (117) as illustrated in equation 9277. 

The copper-catalyzed 1 1 additions of aliphatic and aromatic sulfonyl chlorides82,85 or bromides84 to acetylenes yielding mixtures of trans- and cis-/3-halovinyl sulfones have also been described. Highly polar solvents favored trans addition, while cis addition predominated in low polarity media84,85. A comparison between the thermal and the copper-catalyzed addition of sulfonyl bromides to phenylacetylene (cf. Scheme 6) enabled Amiel84 to suggest that the two stereoisomers do not have a common intermediate. That is, the trans addition product is a result of a normal radical chain, while the cis addition... 

Naumberg, Duong, and Gaudemer (729) have studied the stereochemistry of the addition of various acetylenes to [Co (DMG)2py] in methanol. They found, as already noted by Schrauzer and Windgassen 163), that the main product from the reaction with phenylacetylene in alkaline solution (pH > 10.5) was the jS-phenylvinyl complex (PhCH=CHCo), formed by the trans addition of the Co(I) anion and a proton, i.e.. 

The mechanism for the product formation was originally proposed to involve a cis transfer of OH to a metal-bound alkene.83 However, stereochemical studies carried out by Backvall84 and by Stille85 proved otherwise, demonstrating that a trans addition is involved (Eq. 3.17). 

The kinetics, products, and stereochemistry of the addition of HC1, HBr, and HI to propiolic acid in water have been studied.28 The addition is predominantly trans to give the cz s-3-haloacrylic acid. Both the rate of addition and the selectivity giving trans-addition increase with the nucleophilicity of the halide in water (i.e., I- > Br > Cl-). The order of reactivity is also consistent with the order of the softness of the nucleophiles. The reaction is first order in propiolic acid and the halide anion. It was proposed that the addition involves two mechanistic pathways a major /ra/z.v-addition via a transient carbanion formed with specific geometry and a minor cO-addition process (Scheme 10.2). 

A concerted four-center cis addition leads to (52) and a trans adduct a trans addition, possibly via protonium species, leads to (53) and a cis adduct a stepwise cationic addition leads to (54) and a mixture of cis and trans adducts. Recent studies by Marshall and Wurth strongly indicate that intermediate (54) is correct. Irradiation of octalin (55) in aqueous /-butyl alcohol (DaO)-xylene results in formation of the equatorially deuterated alcohols (56) and (57) and the equatorially deuterated exocyclic olefin (58) ... 

Hydrogen bromide undergoes photoaddition to 1-bromocyclohexene to produce czj-1, 2-dibromocyclohexane as the sole product/11 Studies of the photoaddition of HBr to other olefins indicate that essentially stereospecific trans addition occurs[Pg.272]

The product obtained from this type of decarboxylation is reported to contain only about 5% of /ra s-stilbene.5 A sample made according to the above directions can be treated with bromine in carbon tetrachloride at room temperature in the dark to give an 80-85% yield of the d/-dibromide which arises from trans addition to cw-stilbene. The meso-dibromide, which is very soluble and easily separated, is obtained only to the extent of 10% or less. Part of this latter product may arise from the action of bromine atoms on cw-stilbene rather than from trans addition to tfnms-stilbene. The cu-stilbene prepared by this method is readily and completely soluble in cold absolute ethanol. It freezes solid at about —5°. Its ultraviolet absorption coefficient (8) is 1.10 X 104 at 274 mil and 8.7 X 103 at 294 mp, quite different from h-aws-stilbene. 

Apparent trans addition of hydrogen to a carbon-carbon double bond has been the subject of intense investigations and several mechanism have been proposed the stereochemistry of hydrogenation of alkenes has been comprehensively reviewed by Siegel in 1966,66 by Molnar in 1983,67 and again by Bartok and Molnar in 1997,68 so only a brief history is related here. 

Nevertheless, during hydrogenation of the dimethylcyclohexenes, the fact that the quantities of trans isomer increase with the double bond migration ability of the various catalysts90 suggests that double bond migration sites (or the sites nearby) are involved in apparent trans addition. 

Since edges (and presumably ledges) are now associated with double bond migration,98 and since apparent trans addition is a function of the double bond migration ability of various catalysts, perhaps such locations can produce both processes. The fact that tetrasubstituted alkenes hydrogenate much more slowly than tri-, di-, or monosubsituted alkenes would allow greater... 

FIGURE 2.11 Possible mode of attack of bulk hydrogen to initiate trans addition. 

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