Dihalides from aldehydes

The Formation of gem-Dihalides from Aldehydes and Ketones Dihalo-de-oxo-bisubstitution... 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

Carboxylic acid halides/aluminum halides 1,1-Dihalides from aldehydes s. 18,603 RCOHaUAlHals GHO GHHalg... 

It was found that the system tcrt-BuOH/0.1 mol% 18C6/petroleum ether is also very useful in alkyne synthesis, starting from 1,2-dihalides (from alkenes) or 1,1-dihalides (from aldehydes or ketones). Advantages are high yields and fairly mild reaction conditions [232,233]. 

Dehydrohalogenations can be performed under mild conditions using solid potassium t-butoxide in petroleum ether in the presence of catalytic amounts of 18-crown-6. These conditions are particularly effective for converting 1,2-dihalides (from terminal alkenes) and 1,1-dihalides (from aldehydes) into terminal acetylenes, and gem-dihalides from symmetrical ketones into internal acetylenes. 

This method is well-suited to preparing tram double bonds from aldehydes and geminal dihalides. The Takai reaction leads to FJ7 se-lcctivities >90 10. often even 99 1 (for further details see Chapter 13). 

Preparation of em-Dihalides. em-Dichlorides (23) and e/n-dibromides (24) are conveniently prepared from aldehydes and ketones using BTBSH and a copper(II) halide (eq 10). ... 

The preparation of heteroatom-substituted alkenes from the corresponding gem-dihalides and aldehydes was also mediated with chromium(II) chloride. In Scheme 8.46, representative examples of the preparation of alkenylborane, [51] -silane, [52] and -stannane [53] are shown. In each case, the high -selectivity is observed. As these compounds are very important substrates for Suzuki-, Hiyama-, and Stille coupling, the stereoselective formation of these compounds heightens the value of the chromium(II)-chloride-mediated reactions. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Thus, for example, the direct conversion of an ether into an acetal or ketal is difficult to achieve whereas the oxidation of an alcohol to an aldehyde or ketone (or the reverse process) is a trivial transformation. Similarly, the transition from an oxidation level of 2 to level 1 is problematic in the case when one tries to convert dihalides into monohalides while the transformation of alkynes into alkenes may be safely considered a viable route to carry out this transition. 

The mechanism of the TaA az-olefination is not yet fully understood. A possible mode of action proceeds via the formation of a geminal chromium(III) species 48 which is formed from the geminal dihalide 47. Nucleophilic attack of this intermediate to the aldehyde moiety in 49 leads to p-oxychromiumspecies 50. A formal elimination of the two cromium-containing substituents via antiperiplanar conformation depicted in 50 leads to the -alkene 51 as the major product. 

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