Unusual Chemoselectivity

The following examples show how unusual chemoselectivities can be attained  

Reversal of Normal Reduction Sequences are Observed with Lanthanide—NaBH Systems 

Typically, selective reductions of ketones in the presence of aldehydes 

The inertness of the aldehyde group has been attributed to selective ketalization (in alcohol—water mixtures) catalyzed by the cerium cation, thus protecting the group from reduction. 

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The failure of 2,3-dihydrofuran, tetrahydropyran, 2,3-dihydro-4H-pyran, and 1,4-dioxane to undergo the reaction demonstrates the unusual chemoselectivity achieved by selective molecular adsorption on the semiconductor surface. This same adsorption equilibrium presumably allows for the much more... 

Bis(camphorquinone-a-dioximato)cobalt(II) (10) has been developed as a catalyst for enan-tioselective cyclopropanation reactions. It allows selective carbene transfer from diazoacetic esters to terminal C-C double bonds which are in conjugation with vinyl, aryl, alkoxycarbonyl or cyano groups, but not to alkyl-substituted alkenes, cycloalkenes, 1,3-dienes and al-lenes. The unusual chemoselectivity and some other experimental observations make the two mechanistic pathways proposed vide supra) questionable for these special carbene-transfer reactions. In contrast, the cobalt(II) complex 11 allows not only the cyclopropanation of styrene but also of oct-l-ene, a nonactivated alkene (ethyl diazoacetate, 35 °C, 3mol% of catalyst yield 50-60%). ... 

Glycosylation. Unusual chemoselectivity is revealed using trityl ethers as gly-eosyl acceptors. Secondary ROTr have higher reactivity than primary ethers. In this reaction, TrC104 acts as a catalyst. 

This was the first example of catalytic chemoselective reduction of a,/ -unsatu-rated ketones to allylic alcohols by hydrogen transfer and, unusually, did not require the use of a basic co-catalyst. 

An unusual case of intramolecular competition (chemoselectivity, see Chapt. 1 in [la]) between ester and oxirane occurs in the detoxification of (oxiran-2-yl)methyl 2-ethyl-2,5-dimethylhexanoate (10.49), one of the most abundant isomers of an epoxy resin. The compound is chemically very stable, i.e., resistant to aqueous hydrolysis, but is rapidly hydrolyzed in cytosolic and microsomal preparations by epoxide hydrolase and carboxylesterase, which attack the epoxide and ester groups, respectively [129], The rate of overall enzymatic hydrolysis was species dependent, decreasing in the order mouse > rat > human, but was relatively fast in all tissues examined (lung and skin as portals of entry, and liver as a further barrier). In mouse and rat lung microsomes, ester hydrolysis was 3-4 times faster than epoxide hydration, whereas the opposite was true in human lung microsomes. 

As already briefly mentioned, the oxygen-atom insertion into Si—H bonds of silanes constitutes a selective method for the chemoselective preparation of silanols, which has been much less studied compared to the CH oxidation. This unique oxyfunctionalization of silanes is also highly stereoselective (equation 35) since, like the CH insertions, it proceeds with complete retention of configuration. A novel application of the SiH insertion process is the synthesis of the unusual iron complex with a silanediol functionality, in which selectively both Si—H bonds of the silicon atom proximate to the iron ligand are oxidized in the silane substrate (equation 36). ... 

Generally, hypervalent iodine reagents are often better than traditional reagents of similar reactivity, with respect to efficiency and chemoselectivity - sometimes even stereoselectivity. Unusual reactivity is another interesting feature which has often resulted in unexpected transformation. Examples of such reactions may be found in the oxidation of nitrogen-containing compounds, the Hofmann rearrangement in acidic conditions, the acetalization of carbonyl compounds in alkali, the remote functionalization of steroids, etc. Some unique transformations were effected in the... 

This unusual stabilization effect of MAPH has been successfully utilized for discrimination between two structurally different aldehyde carbonyls, thereby allowing the chemoselective functionalization of sterically more hindered aldehydes [44]. 

Using a y-hydroxyalkynoate combines two effects - the unusual contra-electronic regioselectivity leading to a-attack and a cyclization evolving from the juxtaposition of the hydroxyl and ester functionalities. Thus, as shown in Equation 1.41, the initial adduct 35 between hydroxyalkynoate 33 and alkene 34 spontaneously lactonizes upon its formation to form butenolide 36. The presence of a second monosubstituted double bond proceeds equally facilely in like fashion such that the bis-annulated product 37 is directly formed [38]. Chemoselective addition of the less sterically... 

Remarkably, the heterogenisadon process can confer dramatically enhanced activity on the metal complex. Thus, the dimolybdenum complex [Mo2(MeCN)8][BF4]-Si02 prepared by direct reaction of silica with the metal complex is unusual in its ability to catalyse the polymerisation of norbomene in the absence of an aluminium co-catalyst and at moderate temperatures.165 Sol-gel processing of the ruthenium complex m-Cl(H)Ru(CO)(P)3 (where P is a coordinated ether-phosphine) with tetraethoxysilane and [Al(0-i-Pr)3] gives a stable material with moderate surface area which is active in the hydrogenation of fraras-crotylaldehyde with reasonable chemoselectivity to the carbonyl reduced products, cis- and trans-crotyl alcohol.166... 

A remarkable application of the stannylation procedure is the organotin-mediated monoacylation of diols with reversed chemoselectivity, by which monoesterification of unsymmet-rically substituted diols at the most substituted hydroxyl group can be achieved with acyl chlorides [60], In the reported mechanism [61], this unusual reversal of chemoselectivity rests on a fast intramolecular transesterification equilibrium in which the dibutylstannylene acetal plays the double role of reagent and catalyst. The knowledge of the reaction mechanism allows for adjustment of experimental conditions to achieve remarkable selective level which can be higher than 99% using appropriate reagents (Scheme 4). 

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