Azides, reaction with alkyldichloroborane

Preparation of 2°-Amines Using Alkyldichloroboranes, RBCI2. Organyl azides undergo facile reaction with alkyldichloroboranes (readily prepared via the hydroboration of alkenes with dichloroborane) to provide the corresponding secondary amines (eq 12).21... 

Secondary amines from azides. The reaction of alkyldichloroboranes with azides leads to secondary amines directly. This process has been incorporated into a polyamine synthesis. Note that the dichloroboranes can be prepared from organobis (diisopropylamino)boranes. ... 

An intramolecular version of the reaction of alkyldichloroboranes with organic azides has been employed in a highly... 

The first application of the alkyltrifluoroborate salts was the conversion into alkyldihaloboranes by silyl hahdes and subsequent reaction with alkyl azides [77]. An example of a usefid synthesis was the preparation of (S)-2-phenylpyrrolidine (141) (Scheme 8.32). (S)-DICHED (3-bromopropyl)boronate (13S) was converted into the 3-azido derivative 136 at reflux temperature under phase-transfer conditions. The usual reaction with (dichloromethyl)lithium followed by phenylmagnesium bromide to form DICHED ester 137 was followed by treatment with potassium bifluoride in aqueous methanol to provide the alkyltrifluoroborate salt 138. Neither boronic esters nor alkyltrifluoroborate salts react with alkyl azides. Reaction of 138 with trimethylsi-lyl chloride yielded (S)-2-phenylpyrrolidine (141), but reaction with silicon tetrachloride proved much faster and more efficient. At first it was thought that the intermediates 139 and 140 were probably difluoroboranes in accord with literature precedent [76], but careftil reinvestigation has revealed that reaction of alkyltrifluoroborate salts with silicon tetrachloride in coordinating solvents yields alkyldichloroboranes [78]. 

Di- and trisubstituted alkenes were converted to optically active alkylboronic esters (1) with 99% ee according to the procedure described in Section 1.23.3. The boronic esters were then treated sequentially with lithium aluminum hydride and hydrochloric acid to afford the required optically active alkyldichloroboranes 2 (>99% ee). The reaction of the alkyldichloroboranes with organic azides was best performed in 1,2-dichloromethane as the solvent to yield the required amines 3. Representative secondary amines containing one or two stereogenic centers, C2-sym-metrical and mew-amines have been synthesized by this route with >99% ee (Table 3)22. 

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