Aliphatic carbons primary carbon centers

A mild one-pot procedure based on a platinum-catalyzed diborylation of 1,3-butadienes (see Eq. 30) gives doubly allylic boronate 144, which adds to an aldehyde to form a quaternary carbon center in the intermediate 145 (Eq. 105). The use of a tartrate auxiliary in this process leads to good levels of enantiose-lectivity in the final diol product, which is obtained after oxidation of the primary alkylboronate intermediate. Although examples of aliphatic, aromatic, and unsaturated aldehydes have been described, enantioselectivities vary widely (33 to 74% ee), and are good only for aliphatic aldehydes. An intramolecular variant of this interesting tandem reaction is also known. ... 

Unfortunately, even using this optimized procedure, we were not able to improve the conversion of primary alcohols into the corresponding aldehydes. However, close examination of the oxidation behavior of several primary aliphatic alcohols revealed intriguing features (Table VII). Whilst poor conversion of 1-decanol 23 to decanal 24 was achieved (Table VII, Entry 1), dibenzyl leucinol 25 and Boc-prolinol 27 were quantitatively transformed into the corresponding aldehydes (Table VII, Entries 2 and 3). The enhanced reactivity of 25 and 27 could be due either to an increased steric effect at the a-carbon center, to an electronic influence of the a-nitrogen substituent or to a combination of both. To test the importance of steric hindrance, the aerobic oxidation of cyclohexane methanol 29 and adamantane methanol 31 was carried out. Much to our surprise, oxidation of 29 afforded 30 in 70% conversion (Table VII, Entry 4) and transformation of 31 to 32 proceeded with 80% conversion (Table VII, Entry 5). Clearly increased substitution at the a-position favors the oxidation of primary aliphatic alcohols, although the conversions are still not optimum. 

The Favorskii reairangement of a-monohalo ketones is of primary utility for the synthesis of bran-ched-chain aliphatic acid derivatives, and a typical example is shown in equation (1). With increasing substitution at the a-carbon center, the rearrangement is favored since competing side reactions are suppressed increasing the substitution at the a -carbon center disfavors the rearrangement due to steric hindrance. 

II. NUCLEOPHILIC REACTIONS AT ALIPHATIC CARBONS A. Primary Carbon Centers... 

AMYL ALCOHOLS. Amyl alcohol describes any saturated aliphatic alcohol containing five carbon atoms. This class consists of three pentanols, four substituted butanols, and a disubshtuted propanol. i.e.. eight structural isomers CvHdO four primary, three secondary, and one tertiary alcohol. In addition, 2-pentanol, 2-methyl-l-butanol. and 3-methyl-2-butanol have chiral centers and hence two enantiomeric forms,... 

The slopes of the lines obtained from the Taft correlations of aliphatic primary, secondary and tertiary chlorides obtained at different temperatures by the extrapolation p%2lP%i T IT2 indicate that the positive nature at the carbon reaction center of the C—Cl bond in the transition state increases from a primary to a tertiary carbon atom (Table 8)70. An additional fact is that for each type of alkyl halide, the degree of positive charge at the carbon reaction center tends to decrease as the temperature increases. This means that the pyrolytic eliminations tend to be more concerted and less polar at very high temperatures. These data support Maccoll s theory on the heterolytic character of the alkyl halides pyrol-yses in the gas phase1. 

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