Pentafluorosulfuranyl group

It has been known since Sheppard s original work [43b] that the hydrolytic stability of aromatic pentafluorosulfuranyl groups equals or exceeds that of trifluoromethyl groups (Scheme 2.182) this is considered sufficiently stable for ubiquitous use as a structural motif in medicinal chemistry. Aromatic SF5 derivatives tolerate... 

Scheme 2.182 Examples of conversions of 4-nitropentafluorosulfuranylbenzene demonstrate the stability of the aromatic pentafluorosulfuranyl group, w/hich is comparable with that of the trifluoromethyl function [43b, 49a]. The Achilles heel of the SF5 group is its susceptibility toward reduction by some organometallic species, for example n-butyllithium in THF.

Scheme 2.183 Conversions of l-fluoro-4-nitro-2-pentafluorosulfuranylbenzene into other functionalized derivatives demonstrate the hydrolytic stability of the pentafluorosulfuranyl group even in the presence of bulky ortho-substituents [48].

Since the 1950s perfluoroahphatic SF5 compounds have been made by a variety of methods, including fluorination with cobalt trifluoride and direct and electrochemical fluorination [42a]. Selective introduction of a pentafluorosulfuranyl group into more complex aliphatic compounds, on the other hand, remains still a challenge. 

The electronegativity of the SFj group is 3.62 [55], compared to 3.45 for CF3 [56]. A particular attractive property in the design of functional organic materials, for example liquid crystals, is the strong dipole moment which can be achieved by use of the SF5 group. For example, the dipole moment of pentafluorosulfuranyl benzene (PhSFs) is 3.44 D (25 °C) [43c] compared with only 2.6 D for benzotrifluoride (PhCFj) [57]. 

---