Frans termination reactions

After 20 h, conversion was found to be higher in SCCO2 than in toluene under similar reaction conditions (Table 2) [136]. This effect is more pronounced for internal olefins, such as frans-3-hexene, compared to terminal olefins, such as 1-octene. 

Isomerization of terminal olefins was achieved with the system Ln(C5H5)3/NaH (Ln = Y, Er, Lu). The reactions were carried out at 45 °C in THF and afforded cis- and frans-2-alkenes in very good yields. The catalytic isomerization may occur via organolanthanide hydride intermediates. The proposed mechanism is depicted in Scheme 9 [73]. 

Figure 5.43 (a) Structures of the cis- and frans-thiol-terminated azobenzene-ferrocene dyads, and (b) illustration of the principle of operation of the photogated reaction in the SAM of these complexes, whereby photoinduced structural changes in the monolayer allow solution-phase ferrocyanide ions access to the electrode surface. Reprinted with permission from D. G. Water, D. J. Campbell and C. A. Mirkin, /. Phys. Chem., B, 103, 402 (1999). Copyright (1999) American Chemical Society... 

As an electrochemical reaction, Torii and co-workers demonstrated that the facile transformation of alkenes into allylic alcohols and ethers proceeded in the presence of a catalytic amount (10 mol%) of diphenyl diselenide (Scheme 15) [18]. Most of terminal co double bonds of isoprenoids undergo regioselective oxyselenenylation-deselenenylation to give frans-allylic alcohols in aqueous acetonitrile and methyl ethers in methanol. The addition of SOI salts improves chemical yields since SOI salts prevent the conversion of phenylselenenic acid (PhSeOH) into the inert phenylseleninic acid (PhSe02H) by both disproportionation and electro-oxidation. This method was also applied to intramolecular reaction to form -lactone in high yield. 

This subject has recently been reviewed. Several additional papers have appeared on the catalytic oxidation of alkenes by O2 in the presence of PdCl(MeCN)2N02(148). Terminal alkenes and frans-cyclooctene yield the corresponding ketones, cyclopentene and cyclohexene the corresponding allyl alcohol, and bicyclic alkenes the corresponding epoxide. Heterometallacy-clopentanes such as U52) have been isolated from the reaction of (148) with norbornene (dicy-clopentadiene), and characterized by X-ray crystallography. Glycol monoacetates were obtained from the reaction of (148) with terminal alkenes in acetic acid. ... 

Two of these are the cycloaddition of the methyhdene with ethylene (path E, non-productive), reaction of the methylidene with an internal olefin such that the alkyl substituent on the metallacyclobutane is in the j9-position (path H, non-productive). The other two pathways are the cycloaddition of the alkylidene with an internal olefin to give the trisubstituted metallacyclobutane (path G, frans-metath-esis, non-productive) and the reaction of the alkylidene with a terminal olefin to give the a,a -disubstituted metallacyclobutane (path F), which can be looked at as a chain transfer-type event, albeit not in the sense of a chain polymerization. In this case, the alkylidene is shifted from the end of one chain to the end of another chain. So, assuming that all pathways have somewhat similar rates, the elimination of ethylene will drive the reaction to high polymer. In the case of ADMET, these additional mechanistic pathways do not prevent the polymerization reaction, since these additional pathways are either degenerate or represent processes that do not affect the overall molecular weight distribution of the polymer. 

These reactions account for terminal vinyl-group formation, but there are other species, viz. frans-vinylene and vinylidene, that cannot be accounted for in this way. It is noted that these do not terminate the chains since re-initiation at the Ti site may occur. 

Total synthesis of a-santalol.2 a-Santalol (7) has been synthesized from (—)-ir-bromotricyclene (1) by essentially the same procedure used previously by Corey and Kirst for the synthesis of trans,frans-farnesol (2, 240-241). The starting material was converted into the terminal acetylene (2) by reaction with lithio-1-trimethylsilylpropyne followed by desilylation (2, 239-240). This was converted into the propargylic alcohol (3) by way of the lithium derivative by reaction with paraformaldehyde. /run.v-Hydroaluruination was then effected by treatment with H-buty(lithium followed by diisobutylaluminum hydride. Treatment with iodine... 

The reaction of formaldehyde with alkenes is of industrial interest and has been extensively studied. 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. For example frans-2-butene gives an 85 15 mixture of erythro- and fAreo-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). 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... 

The AD reaction was central in the preparation of (+)-cw-sylvaticin 41,27 a natural product found to have potent anti-tumor activity. The ability of this compound to inhibit ATP production by blockade of the mitochondrial complex I was thought to be the origin of this biological outcome. The AD reaction, in this example, exploited the preference of this reaction for the oxidation of 1,2-frans-alkenes over monosubstituted alkenes. The E,E-isomer of tetradecatetraene 42 could be chemoselectively dihydroxylated at both internal alkenes, while the terminal alkenes remained untouched. Thus, 43 was generated in excellent chemical yield. 

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