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

Scheme 1.16 Fluorination of (32)
18
Chapter One
In 2001 further work was conducted by Bruns and Haufe; [50] reactions of various fluoride
sources for the asymmetric ring opening of cyclohexene oxide were examined in order to
overcome some of the drawbacks of KHF2, such as low solubility. [49] However, in this case
(R,R)-(-)-(salen)chromium chloride complex (35) was examined as the catalyst (Scheme
1.17).
Scheme 1.17 Asymmetric ring opening of cyclohexene oxide (32)
Initially several different fluorinating reagents were tested such as TREAT HF, Olah’s
reagent and silver fluoride (AgF). However, after optimisation of reaction conditions in
order to lessen the formation of chlorohydrin, which was occurring as a result of direct
transfer of chloride from (35) to the epoxide, AgF was utilised as the fluoride source with
MeCN as the solvent in order to increase the solubility of AgF. The reaction of cyclohexene
oxide and other meso- and racemic epoxides with AgF in MeCN with varying amounts of
catalyst (35), led to the formation of the fluorinated compound as the sole product, in
excellent yields (75-90 %) and moderate to good enantioselectivities (44-74 %). Not all of
the epoxides tested gave the desired product. Cyclooctene oxide gave no reaction at all,
while racemic tetrahydronapthalene oxide formed both the trans and cis fluorohydrins in a
2:1 ratio. The trans product gave 23 % ee, whilst the cis only 2 % ee, this was due to the
fact that two competing reaction pathways are occurring; the cis product is formed
following an SN1- like reaction path, whilst the trans follows the SN2 process.