Olefination with Sulfonyl Halides and Esters: E‐Selective Synthesis of Alkenes from Semistabilized Carbanion Precursors

Abstract

Carbanions of sulfonyl halides and activated sulfonates add to carbonyl compounds, and so‐formed aldol‐type adducts spontaneously fragment into olefins. This transformation mimics the one‐pot Julia olefination with (hetero)aryl sulfones, but the mechanism of fragmentation involves a four‐membered intermediate, typical for reactivity of phosphorus reagents. Moreover, in contrast to the reactions of sulfones, sulfonates of fluorinated alcohols (TFE and HFI) produce byproducts that are easily removed during workup. In our report, we focus on reactions of unstabilized and semistabilized carbanion precursors: alkylsulfonates, and allyl‐ and benzylsulfonates, respectively. In particular for semistabilized systems, olefins were synthesized as predominant E isomers in good yields. The presented studies reveal that optimal reaction conditions, including the type of base and alcohol groups of the sulfonates, are different depending on stabilization of the carbanion precursors and structure of the carbonyl substrates. The practical synthetic guide is supplemented with a discussion of the mechanism, based on reactivity studies of intermediates and identification of side‐products.

Directed ortho-Metalation of Arenesulfonyl Fluorides and Aryl Fluorosulfates

“…2,8,9 So-formed sulfonyl fluorides are valuable substrates for the preparation of sulfonamides, 7,10 sulfon-ates, 11 and alkenes. [12][13][14][15][16] Importantly, in the absence of activators, the fluorosulfonyl group displays an impressive resistance toward reduction, cross-coupling conditions, bases, acids, and radical precursors. 1 We wondered if the SO 2 F substituent in arenesulfonyl fluorides can act as a directing ortho-metalation (DoM) group, 17,18 mimicking reactivity of sulfonamides (ArSO 2 NR 2 ), 19 sulfonates (ArSO 3 R), [20][21][22][23] sulfones (ArSO 2 R), 24 and triflones (ArSO 2 CF 3 ).…”

C–H‐Alkenylation of Arenes in a One‐Pot VNS – Julia‐Kocienski Reaction

“…Concerning other sulfones typically used in the J‐K olefination processes, 2‐pyrimidynyl or phenyltetrazolyl chloromethyl sulfones gave poor results already at the VNS step (low conversion and formation of unidentified numerous side products). Another approach to VNS‐based alkenylation of arenes has also been attempted which was based upon an innovative sulfone‐based olefination protocol developed recently by the Barbasiewicz group . Unlike the J‐K and similar olefination reactions, this reaction is based upon addition of deprotonated sulfonyl esters to carbonyl compounds, followed by sulfaoxetane formation in a manner resembling the mechanism of the Wittig reaction.…”

Olefination with sulfonyl halides and esters – sulfur-based variant of the Horner-Wadsworth-Emmons reaction

“…In the next paper we focused on controlling of E/Z selectivity and application of more challenging, enolizable carbonyl compounds. 17 Attempts to control the stereoselectivity of the olefination began with a series of experiments subjecting a set of alkanesulfonates into the reactions with benzaldehyde under previously reported Barbier conditions (t-BuOLi, THF, 16 h, r.t.). Structures of the precursors varied with type of the leaving group (halides, esters), and stabilization of the carbanion center with alkyl (non-stabilized), phenyl (semi-stabilized) and ester (stabilized) functions (Scheme 9).…”

“…Alkenes obtained using different types of carbanion precursors (E/Z ratio was given in parentheses, only selected cases were shown). 17 For stabilized carbanion precursors the only success was achieved after switching the base to DBU. In the reaction of ethyl (1,1,1,3,3,3-hexafluoroisopropoxy)sulfonylacetate with model 2-naphthaldehyde (THF, r.t., 16 h), the α,β-unsaturated ester was obtained in a moderate 44% of yield.…”

“…Alkenes obtained using fluoroalkyl sulfonates and DBU as a base (E/Z ratio was given in parentheses). 17,18 Since the procedure utilizing DBU was particularly successful for 1,1,1,3,3,3-hexafluoroisopropyl phenylmethanesulfonate this protocol was selected to examine influence of substituents in the arene ring of both substrates on the yields of stilbenes. Strongly electronically-altered benzaldehydes, such as pentafluorobenzaldehyde and p-(dimethylamino)benzaldehyde, gave only traces of desired products, whereas moderately tuned ones were well-tolerated (Scheme 11).…”