Diborane imine reduction

Bell and Hall have incorporated an organometallic unit into a crown by using the ferrocenyl unit as part of the ring or as a third strand. The unit is incorporated either as the 1,1 -diformylferrocene or the corresponding acid. In the former case, the bis-imine is prepared and reduced to give the saturated crown (see structure 24). In the latter case, the acid is converted into its corresponding chloride and thence into the diamide by reaction with a diamine. Diborane reduction affords the saturated amino-crown. Structure 24 could be prepared by either of these methods but the dialdehyde approach was reported to be poor compared to the amide approach which afforded the product in ca. 60% yield . 

Reduction of a 20-N-benzyliminopregnane with diborane gives the 205- and 20R -benzylamines in 55 45 ratio. The corresponding imine (231) derived from the chiral (S)-(-)-a-phenylethylamine was reduced asymmetrically to give only the 20S-amino-derivative, which afforded the pure 20S-amine (232) after debenzyla-tion.201... 

Reduction191 of the imine (367) with diborane followed by a catalytic hydrogenation produced 20a(20S)- and 20/3(20R)-amines, (370) and (371), in the ratio 55 45. Reduction employing (+ )-di-3-pinanylborane gave a similar mixture. However, when chirality was introduced into the imine as in (368) [prepared from (S)-phenylethylamine] diborane reduction yielded uniquely the 20a -amine (370) (funtuphyllamine A). The imine (369), prepared from ( + )-(R)-phenylethylamine, gave (370) (8%) and (371) (92%). 

A variety of reducing agents have been used for the conversion of imines to amines.The examples in equations (50) and (51) are typical. The relatively slow destruction of sodium cyanoboro-hydride in slightly acid aqueous solutions allows the process of imine formation to be carried out simultaneously with reduction. Apparently, the protonated imine is the species that is reduced and this process occurs significantly faster than reduction of the original carbonyl compound by this reagent. A recent report by Nose indicated that diborane in methanol will reduce only imines and no other functionality. Apparently, the diborane reacts rapidly with methanol to form a derived reagent. 

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