Synthesis and Comparison of the Reactivity of Allyl Fluorides and ...

n R R’ Alkaloid ee (%)
1 H Me (DHQ)2PYR 60
1 CH2Ph Me DHQB 85
1 CH2Ph Me (DHQ)2PYR 96
2 H Me DHQPE 30
1 H Ph (DHQ)2PYR 87
Table 2.3 Results of enantioselective fluorodesilylation of prochiral allylsilanes.
Figure 2.2 (DHQ)2PYR
42
Chapter Two
Though numerous alkaloids were screened, the reagent derived from hydroquinine 2,5-
diphenyl-4,6-pyrimidinediyl diether ((DHQ)2PYR) (Figure 2.2) was the most successful,
affording the fluorinated benzyl-substituted allylsilane derived from indanone with a 96 %
ee. Enantioselectivities were also influenced by substrate and starting material, with greater
enantiomeric excesses for substrates substituted by large groups (R= CH2Ph, 85 % ee vs R=
Me, 60 % ee). Increased enantiomeric excesses were achieved when indanones were used
in comparison with tetralones. The results also showed that when the three methyl groups
attached to the silicon are replaced by phenyl groups higher enantiomeric excesses (R’= Ph,
87 % ee vs R’= Me, 60 % ee) were achieved.
2.1.3 Allylic chlorides from Allylic Alcohols
Allylic chlorides are an important class of compounds which can undergo an array of
transformations. They are also important in reactions such as metal-catalysed allylation
reactions, with recent examples including nickel-catalysed Negishi reactions [21] and
stereospecific zirconium-mediated SN2’ substitutions. [22] Over the years numerous methods
have been developed to convert allylic alcohols into the corresponding chlorides, including;
a) Reaction with conventional halide-producing reagents like SOCl2 [23-25] or PX3 [26-28]
b) Reaction with dimethyl sulphide and an N-halosuccinimide [29]