Aldehydes from hydroboration-oxidation 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. 

In Chapter 5, the cis addition inherent to hydroboration is the result of a four-center transition state (sec. 5.2.A.ii), which delivered boron and hydrogen from the same face. Subsequent transformations that involve boron lead to a new group on the same face as boron. Reaction of methylcyclopentene with 9-BBN, for example, gave exclusively the trans borane (96), and that was converted to aldehyde 97 by carbonylation. The addition of the boron dictated the stereochemical relationship of the methyl group and the 9-BBN moiety. The carbonylation and oxidation reactions proceeded with net retention of configuration, maintaining the trans geometry in the isolated product. 

The exo-olefin is a precursor to a variety of chain-end-functionalized polymers via postpolymerization reaaions as shown in Scheme 37. Hydroxy-functionalized PIB has been utilized as a precursor for the preparation of RAFT macromo-lecular CTAs and carboxylic acid end-fimctionalized polymers. Telechelic PIB diols have been prepared using p-dicumyl chloride/BCU initiator system followed by dehydro-chlorination, hydroboration, and alkaline peroxide oxidation. Aldehyde end-fiinctionalized PIBs have been used to prepare carboxyl- and hydroxyl-terminated poly-mers. Carboxylic acid-functionalized PIBs have also been prepared from the oxidation of the methyl ketone-functionalized PIBs." The hydroxyl-functionalized PIBs have also been used to prepare methacrylate macromonomers by their reaction with methacryloyl chloride in the presence of triethylamine. The homopolymerization and copolymerization of these maaomonomers with methyl methacrylate were also reported. Cyanoacrylate-funaionalized PIBs prepared via the hydroxyl-functionalized PIB were also reported by Kennedy et al. ... 

Other methods for the syngas-free synthesis of aldehydes from olefins, for example, a one-pot reaction consisting of hydroboration-homologation and final oxidation (Scheme 3.1), are outside the scope of this book and are therefore not considered in detail here [6]. 

In the effort towards the synthesis of olivine, acetylene 19 was made from aldehyde 18 by the reaction of CBr4 and PPh3 to form a dibromoalkene, hydroboration/oxidation to form an alcohol, and treatment of the resulting dibromoalkene with -BuLi. 

Treatment with alkaline H202 oxidizes trialkylboranes to esters of boric acid. 1 This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles. The R group does not rearrange, and this reaction is a step in the hydroboration method of converting olefins to alcohols (5-12). The mechanism has been formulated as involving a rearrangement from boron to oxygen 311... 

This type of disconnection is mainly used for the preparation of dipeptides of type Xaai >[ , CH=CH]Gly. It allows control of the stereochemistry of the Xaa residue by starting from chiral a-amino aldehydes. For the construction of the /ram -p,y-unsaturated carboxylic acid moiety, the use of the triphenylphosphonium salt 31 (Scheme 9) derived from 3-chloro-propanoic acid was not suitable.14 Instead, the trimethylsilylprop-2-ynyl phosphonium salt 33 serves as a three-carbon unit, which can be converted into the P,y-unsaturated acid by hydroboration and oxidation. The required Boc-protected a-amino aldehyde 32 can be prepared using virtually racemization-free procedures. 37 However, at the end of the reaction sequence, racemization has been detected, especially for Boc-Phet )[ , CH=CH]Gly-OH, but not for the Ala and Pro analogues. 63 A mixture of E- and Z-enynes 34 and 35 is formed (8 2 to 9 1), which can be separated by column chromatography. 4,48 50 53 64 65 ... 

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