Stereospecificity, in the

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

When the lactone silyl ketene acetal 18-1 is heated to 135° C a mixture of four stereoisomers is obtained. Although the maj or one is the expected [3,3] -sigmatropic rearrangement product, lesser amounts of other possible C(4a) and C(5) epimers are also formed. When the reaction mixture is heated to 100° C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic triene ester. Suggest a structure for the triene ester and show how it can be formed. Discuss the significance of the observation of the triene ester for the lack of complete stereospecificity in the rearrangement. 

The higher degree of stereospecificity in the singlet reaction relative to that of the triplet is not unexpected in light of the greater lifetime of the triplet bira cal allowing bond rotation or bond breaking to yield acetone and an isomerized olefin. 

It was shown that the fragmentation of 143 occurs stereospecifically in the sense of a trans-elimination erythro-145 gives the olefin (E)-144 while threo-745 furnishes (Z)-14495K... 

These reactions are nearly always stereospecific and the stereospecificity in the product has been confirmed by n.m.r. studies. The interest in these reactions has grown during the last about... 

Moore RE, Pettus JA Jr (1971) Isolation and structure determination of dictyopterenes C and D from Dictyopteris. Stereospecificity in the cope rearrangement of dictyopterenes A and B. J Am Chem Soc 93 3087-3088... 

The high solvolytic stereospecificity of the tosylate 91 together with the unexpectedly fast reaction rates was tentatively interpreted by Cram 88b> in terms of /8-phenyl participation in the ionization step to produce a highly strained bridged carbonium ion 96 which is opened in a second reaction step to give the final product. Both the formation of 96 and its opening must involve complete inversion in order to ensure retention of stereospecificity in the overall solvolytic process. 

When the rate of polymerisation is dependent on the propene concentration, but site isomerisation is not, the concentration of propene effects the properties of the polymer. A faster polymerisation will lead to a higher stereospecificity in the polymer. 

A basically new type of synthesis of dihydrooxazines was applied for the preparation of 177 and 179 (86JOC3248). Boron trifluoride catalyzed the intramolecular [4+2] cycloaddition of the A-acyliminium compounds derived from 176 and 178, resulting stereospecifically in the rrani-fused cyclo-pent[d][l,3]oxazines and partly saturated 3,1-benzoxazines 177 and 179, respectively. The steric orientation of the 4-methyl substituent of the product is controlled by the Z and geometry of the starting bisamides 176 and 178 (86JOC3248). 

Similarly, the [3-1-4] annulation of the E- and Z-isomers of /3-hetero-substituted acryloylsilanes 52 with lithium enolates of a,-unsaturated methyl ketones 54 gave stereospecifically the c -6,7-cyclopentyl-5-trimethylsilyl-3-cycloheptenone 55 (equation 20). The stereospecificity in the annulation was explained by an anionic oxy-Cope isomerization of the 1,2-divinylcyclopropanediol intermediate 56, which was generated through the Brook isomerization of the initial 1,2-adduct (equation 20). 

Suggest a structure for the triene ester, and show how it could be formed. Discuss the significance of the observation of the triene ester for the incomplete stereospecificity in the rearrangement. 

The high regio- and stereospecificity in the rhodium-catalyzed system does not seem to be compatible with the catalytic asymmetric synthesis using a chiral rhodium catalyst, and thus, there have so far been very few reports on the use of chiral rhodium catalysts for the asymmetric allylic alkylation. In 1999, Pregosin and his co-workers first reported asymmetric rhodium-catalyzed allylic alkylation of allylic esters (Equation (48)). Use of optically active... 

As already stated in the first chapter, several catalytic systems show a certain stereospecificity in the a-olefin polymerization. 

Oxymercuration-reduction of alkenes preparation of alcohols Addition of water to alkenes by oxymercuration-reduction produces alcohols via Markovnikov addition. This addition is similar to the acid-catalysed addition of water. Oxymercuration is regiospecific and auft -stereospecific. In the addition reaction, Hg(OAc) bonds to the less substituted carbon, and the OH to the more substituted carbon of the double bond. For example, propene reacts with mercuric acetate in the presence of an aqueous THF to give a hydroxy-mercurial compound, followed by reduction with sodium borohydride (NaBH4) to yield 2-propanol. 

The hydroperoxy functionality can be introduced into an alkene by a singlet oxygen ene reaction and subsequently reduced quantitatively to an allylic alcohol, by addition of reducing agents such as PPh3, MeiS or NaBH419°. In addition, the allylic hydroperoxides can be transformed stereospecifically in the presence of Ti(OPr-i)4 to an epoxy allylic alcohol, where epoxide and hydroxyl functionalities are cis to each other (e.g. substrate 160, Scheme 58)191192. 

In order to find a highly stereospedfic, homogeneous catalyst for the polymerization of propylene oxide, we selected the organometallic compound-primary amine catalyst system which exhibited excellent stereospecificity in the polymerization of acetaldehyde. 

B. Fraser-Reid, Z. Benko, R. Guiliano, K. M. Sun, and N. Taylor, Complete stereospecificity in the intramolecular Diels-Alder reaction of an ester derived from diacetone glucose, J. Chem. Soc. Chem. Commun. p. 1029 (1984). 

Two consecutive reactions of the citric acid cycle (Fig. 10-6), the dehydration of citrate to form czs-aconi-tate and the rehydration in a different way to form isocitrate (Eq. 13-17), are catalyzed by aconitase (aconi-tate hydratase). Both reactions are completely stereospecific. In the first (Eq. 13-17, step a), the pro-R proton from C-4 (stereochemical numbering) of citrate is removed and in step c isocitrate is formed. Proton addition is to the re face in both cases. 

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