Hydrogen bonding to carbonyl

The data suggest that the hydroxyl groups of 1 are involved in hydrogen bonds to carbonyl oxygens of 32 a, as is shown in Scheme 1. Moreover, although 32 is easily oxidized to disulfide 34 in solution and in the solid state, 32a in 33 is stable. 

The relative stereochemistry of the two hydroxy-groups in verticillin was assigned on the basis that both are hydrogen-bonded to carbonyl oxygen. The absolute configuration is the same as that of chaetocin, as shown, again by c.d. comparisons. 

Pierre and Handel have studied the effect of [2.1.1]-cryptate on the lithium aluminum hydride reduction of cyclohexanone in diglyme [16]. The [2.1.1]-cryptate strongly complexes lithium ion and if sufficient cryptate is used to sequester all of the lithium ion, no reduction occurs. Apparently, lithium ion is needed as an electrophilic catalyst for the reduction to occur (see Eq. 12.8). Consistent with this interpretation is the observation that even in the presence of cryptate, reduction will occur if an excess of lithium iodide is also present. The relatively low reactivity of tetrabutyl-ammonium borohydride in benzene solution may also reflect this property, at least in part [9]. Likewise, the jS-hydroxyethyl quaternary ammonium ions may be better catalysts than non-oxygenated quaternary ions because the hydroxyl may hydrogen bond to carbonyl and provide electrophilic catalysis [5]. Similar, though less dramatic results, have been observed in the reduction of aromatic aldehydes and ketones by lithium aluminum hydride in the presence of [2.1.1]-cryptate [17]. 

N-H stretch (hydrogen bonded to carbonyl) Brunette etal (1982) Christenson etal. (1986)... 

Figure 11.12 Alternative modes of hydrogen bonding to carbonyl.

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