Syn-addition stereospecific

Epoxidation (Section 6 18) Peroxy acids transfer oxygen to the double bond of alkenes to yield epoxides The reaction IS a stereospecific syn addition... 

Epoxidation of alkenes is a stereospecific syn addition Which stereoisomer of 2 butene reacts with peroxyacetic acid to give meso 2 3 epoxybu tane Which one gives a racemic mixture of (2/ 3/ ) and (25 35) 2 3 epoxybutane ... 

The stereochemistry of the 1,3-dipolar cycloaddition reaction is analogous to that of the Diels-Alder reaction and is a stereospecific syn addition. Diazomethane, for example, adds stereospecifically to the diesters 43 and 44 to yield the pyrazolines 45 and 46, respectively. 

The Simmons-Smith reaction (Section 14.12) Methylene transfer from iodo-methylzinc iodide converts alkenes to cyclopropanes. The reaction is a stereospecific syn addition of a CH2 group to the double bond. 

The reaction is easy to cany out, and yields are usually high. Epoxidation is a stereospecific syn addition. 

Peroxy acid oxidation of alkenes (Sections 6.18 and 16.9) Peroxy acids transfer oxygen to alkenes to yield epoxides. Stereospecific syn addition is observed. 

Hydroboration is a stereospecific syn addition that occurs through a four-center TS with simultaneous bonding to boron and hydrogen. The new C—B and C—H bonds are thus both formed from the same face of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene it orbital with the empty p orbital on boron, accompanied by concerted C—H bond formation.158... 

Two issues are of essential for predicting the structure of 1,3-DCA products (1) What is the regiochemistry and (2) What is the stereochemistry Many specific examples demonstrate that 1,3-dipolar cycloaddition is a stereospecific syn addition with respect to the dipolarophile, as expected for a concerted process. 

The most widely used reagent for oxidation of alkenes to glycols is osmium tetroxide. Osmium tetroxide is a highly selective oxidant that gives glycols by a stereospecific syn addition.39 The reaction occurs through a cyclic osmate ester that is formed by a [3 + 2] cycloaddition.40... 

It has been demonstrated that ionic intermediates are not involved in the epoxidation reaction. The reaction rate is not very sensitive to solvent polarity.71 Stereospecific syn addition is consistently observed. The oxidation is therefore believed to be a concerted process. A representation of the transition structure is shown below. 

Another synthesis of P-D lactone that is based on an enantiomerically pure starting material is shown in Scheme 13.35. The stereocenter in the starting material is destined to become C(4) in the final product. Steps A and B served to extend the chain to provide a seven-carbon 1,5-diene. The configuration of two of the three remaining stereocenters is controlled by the hydroboration step, which is a stereospecific syn addition (Section 4.5.1). In 1,5-dienes of this type, an intramolecular hydroboration occurs and establishes the configuration of the two newly formed C—B and C—H bonds. 

A Michael-type addition reaction of phosphine generated from red phosphorus in concentrated aqueous KOH solution has been noted to provide moderate isolable yields of pure organophosphorus products.27 For example, tris-(2-cyanoethyl)phosphine is produced in 45% isolable yield from acrylonitrile, and tris-(2-[y-pyridyl]ethyl) phosphine oxide is isolated in 40% yield from 4-vinylpyridine under these conditions. Excellent yields of the tertiary phosphine oxide, tris-(2-cyanoethyl)phosphine oxide, have been reported using white phosphorus in absolute ethanol with KOH at ice/salt-bath temperatures.28 A variety of solvent systems were examined for this reaction involving a Michael-type addition to acrylonitrile. Similarly, tris-(Z-styryl)phosphine is produced from phenylacetylene under these conditions in 55% isolated yield. It is noteworthy that this last cited reaction involves stereospecific syn- addition of the phosphine to the alkyne. 

Trifluorovinylcopper reacted readily with perfluoroalkynes to give stereospecific syn addition products, which were quenched with iodine to afford the corresponding iodide [181] (Scheme 64). 

The polar 1,4-cycloaddition of alkenes with 2-hydroxy-5-nitrobenzyl chloride in the presence of tin(IV) chloride yields 6-nitrochromans (69TL5279). The quinone methide, which is protonated under these conditions, undergoes stereospecific syn addition of the alkene. Although in most cases the reaction is regiospecific, ds-pent-2-ene yields a mixture of isomers. 

Similar methodology has been used to prepare the dialkoxyphosphinyldi-fluoromethylcopper compound [242] (equation 160) Functionalization with allyl bromide, methyl iodide, and iodobenzene occurs readily, as well as stereospecific syn addition to hexafluoro-2-butyne [243]. 

The formation of dimethyl succinates is stereospecific, syn addition of the two carbomethoxy functions taking place, as illustrated by the dicarboxylation of (Z)-2-butene to the meso diester and ( )-2-butene to the ( )-diester (equations 64 and 65). This occurs since the alkene insertion to yield intermediate (23) is syn and the CO insertion (23) — (24) takes place with retention of configuration at carbon. 

The adducts can produce silenes by thermolysis at 190 °C. Thermolysis of 40a in the presence of a 9-fold excess of methanol gave only 41a. Similar thermolysis by using a 69/31 mixture of 40a and 40b afforded a 69/31 mixture of 41a and 41b. Thus methanol adds to the silene stereospecifically69. A similar stereospecific addition of alkoxysilane to silene was reported by the same group70. The stereospecific syn addition can be explained... 

Stereospecific syn addition of methoxysilane to silacyclobutadiene was reported by Fink and coworkers (Scheme 12)72. Ethanol also adds the silacyclobutadiene stereospecifically in a syn fashion to give 42 (Scheme 12), but the ethanol adduct undergoes photoisomerization to a photostationary mixture of 42 and its stereoisomer. A similar system, but with the more bulky 2,4,6-triisopropylphenyl (Tip) groups, also gave the syn ethanol adduct 4373. The rather complicated photochemical-thermal isomerization process was discussed by Fink74. 

Irradiation of silylene 47 (Ar = Mes) with visible light resulted in the vanishing of the silylene absorption bands and the formation of a new species, which could be identified as the first example of a silacyclobutadiene 49 (Ar = Mes). Trapping of 49 (Ar = Mes) with methoxytrimethylsilane or ethanol yielded the expected products 50 and 51 in a completely stereospecific syn addition (equation 16). This result is in good agreement with reports by Jones and coworkers about syn additions of these reagents to simpler acyclic silenes62,63 (equation 16). 

The reaction has been shown to involve a stereospecific syn-addition with respect to the dienophile. For example, the reaction of 2,3-dimethylbuta-1,3-diene with maleic anhydride gives cis-l,2,3,6-tetrahydro-4,5-dimethylphthalic anhydride (Expt 7.20). An example of the use of a quinone as the dienophile is provided by the synthesis of cis-1,4,4a,9a-tetrahydro-2,3-dimethyl-9,l0-anthra-quinone which upon dehydrogenation (most simply by the action of oxygen upon its solution in alcoholic potassium hydroxide) yields 2,3-dimethylanthra-quinone (Expt 7.21). Tetraphenylcyclopentadienone (tetracyclone) readily undergoes the addition of dienophiles, such as maleic anhydride, to give an adduct, which then extrudes a molecule of carbon monoxide on heating, as in the preparation of 3,4,5,6-tetraphenyldihydrophthalic anhydride (Expt 7.22). 

The reaction is a stereospecific syn addition. The cis alkyl groups in the starting alkene remain cis in the product epoxide. 

Hydroboration-oxidation leads to stereospecific syn addition of H and OH across a carbon-carbon double bond. The regiochemistry of addition is opposite to that predicted by Markovnikov s rule. Hydroboration-oxidation of the E alkene gives alcohol A. 

An excellent example of stereospecific syn addition is seen in the hydrogenation of cis- and franv-dimethylstilbcnc with palladium catalyst (eq. 3.24).13 Under the same conditions, diethylstilbestrol (50, R = H) and its dimethyl ether (50, R = Me) were hydrogenated to the products containing, respectively, 90 and 97% of the corresponding racemic 3,4-diphenylhexane derivatives. Syn addition decreased to 86 and 70%, re-... 

The processes as well as the potential problems of hydroformylations are depicted in Scheme 1. Catalytically active is the 16-electron complex 4, which, after coordination of the olefin 1 to the r-complex 5, forms in a stereospecific syn addition the <7-complex 8. Subsequently, coordination and insertion of CO gives rise to the acyl complex 9. After addition of H2 to the Rh(III) complex 7, a reductive elimination to... 

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