Olefin chemoselective

An understanding of olefin chemoselectivity in CM is also crucial when homologating 1,3-dienes, which represent a particularly challenging substrate class. In 2005, Grubbs and co-workers demonstrated that, by employing either an electronic or steric barrier to reaction, one of the olefins in a conjugated diene could be deactivated relative to the other for CM. For example, in the reaction of ethyl sorbate with 5-hexenyl acetate in the presence of 5 (I0mol%),... 

A major concern in H2O2 oxidation is the alcohol/olefin chemoselectivity (Rao, 1991). This biphasic oxidation was initially developed for olefin epoxidations but with an (aminomethyl)phosphonic acid additive for high selectivity (Rudolph et al., 1997). Now, the removal of this additive has been found to significantly increase the rate and selectivity of alcohol oxidation. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The high chemoselectivity for the Baeyer-Villiger process was utilized in the synthetic elaboration of another hetero-bicyclic substrate. The biooxidation only provides the expected unsaturated lactone in a desymmetrization reaction without compromising the olefin functionality. The biotransformation product was then converted to pivotal intermediates for C-nucleosides like showdomycin, tetrahydro-furan natural products like kumausyne, and goniofufurone analogs in subsequent chemical operations (Scheme 9.17) [161]. 

The Markovnikov regioselectivity of the gem-alkenes is associated with a chemoselectivity. in favour of methanol attack, significantly greater than that observed for the other alkenes. If no sodium bromide is added to the reaction medium, no dibromide is observed for this series. Therefore, these alkenes behave as highly conjugated olefins, as regards their regio- and chemo-selectivity. In other words, the bromination intermediates of gem-alkenes resemble P-bromocarbocations, rather than bromonium ions. Theoretical calculations (ref. 8) but not kinetic data (ref. 14) support this conclusion. 

The chemoselectivity of bromination going through bromocarbocations (highly conjugated olefins and also gem-alkenes ) is 100 % in favour of methanol, a nucleophile stronger than bromide ions. However, when the intermediates are bromonium ions, the chemoselectivity is poor. Branched substituents seem to favour the dibromide over the solvent-incorporated adduct, although the bromide ion is considered to be a bulkier nucleophile than methanol. 

Reduction of carbon-carbon double bond Microalgae easily reduce carbon-carbon double bonds in enone. Usually, the reduction of carbonyl group and carbon-carbon double bond proceeds concomitantly to afford the mixture of corresponding saturated ketone, saturated alcohol, and unsaturated alcohol because a whole cell of microalgae has two types of reductases to reduce carbonyl and olefinic groups. The use of isolated reductase, which reduces carbon-carbon double bond chemoselectively, can produce saturated ketones selectively. 

The rate of hydrozirconation of a terminal olefin is much faster than the reduction of the oxirane ring. This chemoselectivity was used in the preparation of various cycloalkylmethanols by hydrozirconation of alkenyloxiranes (Scheme 8-28) [217]. 

The reductive coupling/silylation reaction was extended to more complicated polyenes, such as the triene-substituted cyclopentanol 73, which cyclizes to provide 74 with a 72% yield and 6 1 dr after oxidation (Eq. 10) [44], The reaction is chemoselective the initial insertion occurs into the allyl substituent, which then inserts into the less hindered of the two remaining olefins, leaving the most hindered alkene unreacted. 

The carboxylic acid 150 (Scheme 10.15) was prepared from commercial (-)-carvone (68) via a chemoselective reduction of the terminal olefin using tris... 

To summarize, when the kinetic data predict that only bromonium ions or only bromocarbocations are formed, the bromination products are obtained stereospecifically and regiospecifically, respectively, whatever the solvent. Olefin brominations involving open intermediates lead to more solvent-incorporated products in methanol or acetic acid than those involving bridged ions. This chemoselectivity can be interpreted in terms of the hard and soft acid and base theory (Dubois and Chretien, 1978). Methanol assistance to intermediate formation also plays a role in determining product-selectivity (Ruasse et al, 1991). 

Figure 8.4. Chemoselectivity as a function of system pressure during the hydroformylation of long chain olefins with Rh/PEt3 catalysts. The change in chemoselectivity has been correlated with the transition from biphasic to monophasic reaction conditions...

Palladium-catalyzed cyclization of alkenes and alkynes were reported by Balme and co-workers.143 144 Intramolecular carbopalladation occurs to give polycyclic compounds. It has been shown that the nucleophile type has a large influence on the cyclization process. Both 5-exo- and 6-endo-cyclization are observed for substrates with nitrile (116 and 118) and ester (120, 122, and 124) substituents, respectively (Scheme 36). When a mixed nucleophile (CN and C02Me) is used, a mixture of 5-exo and 6-endo products is obtained. The chemoselectivity is controlled by the size of the nucleophile used. The stereochemistry of the initial double bond plays an important role on the stereoselectivity of the cyclization. (Z)-olefins (118 and 120) and (/. )-olefins (116 and 124) afford as- (119 and 121) and trans-cyclization products (117 and 123), respectively. 

These initial forays verified the utility of the process and provided mechanistic information which set the stage for further exploration into the scope of the process. Successful reactions with a variety of substrates demonstrated the generality of the process, with 6- and 7-membered cyclic enol ethers being most accessible (Scheme 19). Preservation of the trisubstituted olefin present in the complex substrate 132 indicates the chemoselective nature of the process [34a]. 

RuCl2(PPh3)3 has been shown to catalyze the reduction of several aldehydes, but does not have widespread scope. This catalyst is not chemoselective and, in the presence of alkenes, would favor olefin reduction over that of the aldehyde. Noyori and coworkers showed that chemoselectivity is easily introduced by the addition of ethylene-diamine as a ligand (Scheme 15.6) [29, 30]. This system requires the presence of co-catalytic KOH/i-PrOH as an activator. 

Subsequently, high chemoselectivity and enantioselectivity have been observed in the asymmetric epoxidation of a variety of conjugated enynes using fructose-derived chiral ketone as the catalyst and Oxone as the oxidant. Reported enantioselectivities range from 89% to 97%, and epoxidation occurs chemoselectively at the olefins. In contrast to certain isolated trisubstituted olefins, high enantioselectivity for trisubstituted enynes is noticeable. This may indicate that the alkyne group is beneficial for these substrates due to both electronic and steric effects. 

Several aspects are particularly noteworthy. Good chemoselectivity is noted in the compatibility with epoxides, esters, olefins, and alcohols. Entries 44 and 45 demonstrate the chemoselectivity between an unsaturated and saturated ketone. 

In Baldwin s formal total synthesis of haliclamines A and B, a Suzuki coupling of 3-bromopyridine was the central operation [52], Chemoselective hydroboration of diene 66 employing 9-BBN occurred at the less hindered terminal olefin. Suzuki coupling of the resulting alkylborane with 3-bromopyridine then furnished alkylpyridine 67 as a common intermediate for the synthesis of haliclamines A and B. 

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