Oxygen compounds lead-coordinated products

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

Excellent yields can be obtained when the mixture of RC=CLi and the epoxide in ammonia is allowed to stand for 12 to 24 hours [2]. With sodium acetylides, there is some risk of subsequent attack of the acetylenic alcoholate on the epoxide, especially when the latter compound is used in excess. The reaction of acetylenic magnesium halides, RCsCMgX, with epoxides also leads to homopropargylic alcohols, but this is generally not recommended as a satisfactory method, as impure products are often obtained [62], Coordination of the epoxide-oxygen atom with the Lewis acid may give rise to a Sjql-like attack of RCaC on... 

Because transition-metal anions can be prepared conveniently in tetrahydrofuran solution, this cyclic ether was often used in earlier attempts to prepare silicon-metal compounds. It is now generally realized, however, that tetrahydrofuran can frustrate these attempts in two ways. First, it promotes electrophilic attack by the silicon compound on oxygen atoms of coordinated carbonyl groups this leads to the formation of products with Si-0 bonds (54, 138, 262, 300, 306, 310, 336,337), e.g.,... 

Nucleophilic addition of the trifluoromethyl group to aldehydes, ketones and other carbonyl compounds leads primarily to the corresponding trimethylsilyl ether this must subsequently be hydrolyzed to the alcohol. Because typical reaction conditions are very mild, the method is widely applicable, even for sensitive substrates. In contrast with most other methods, fluoride-induced perfluoroalkylation via silicon compounds also works for enolizable carbonyl compounds. With c(, -un-saturated substrates 1,2-addition directly to the carbonyl group is strongly preferred [64b]. If the oxygen is coordinated to a bulky Lewis acid, for example aluminum tri(2,6-bis(tert-butyl)phenoxide (ATPH), the 1,4-addition products are obtained selectively [73f] (Scheme 2.128). 

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