Determination of the Electrophilicity Parameters of Diethyl Benzylidenemalonates in Dimethyl Sulfoxide: Reference Electrophiles for Characterizing Strong Nucleophiles

Abstract

The second-order rate constants of the reactions of nine substituted diethyl benzylidenemalonates 1 a-i with the carbanions 2 a-e have been determined spectrophotometrically in dimethyl sulfoxide (DMSO). Product studies show that the nucleophiles attack regioselectively at the electrophilic C==C double bond of the Michael acceptors to form the carbanionic adducts 4. The correlation log k(20 degrees C)=s(N+E) allows the determination of the electrophilicity parameters E for the electrophiles 1 a-i from the rate constants determined in this work and the previously published N and s parameters for the nucleophiles 2 a-e. The electrophilicities E for compounds 1 a-i cover a range of six units (-17.7>E>-23.8) and correlate excellently with Hammett's substituent constants sigma(p). The title compounds are roughly ten orders of magnitude less reactive than analogously substituted benzylidene Meldrum's acids, their cyclic analogues. Due to their low reactivities, compounds 1 a-i are suitable reference electrophiles for determining the reactivities of highly reactive nucleophiles, such as carbanions with 16<N<30.

Scope and Mechanisms of Frustrated Lewis Pair Catalyzed Hydrogenation Reactions of Electron‐Deficient CC Double Bonds

“…[5,6] Thelow nucleophilicity of 7a (N = 10.04) also accounts for the failure of the stoichiometric reaction of (C 6 F 5 ) 3 BH À K + with diethyl-2-benzylidenemalonate (E = À20.55), [12] as recently described by Alcarazo and co-workers. [6,12] As the s N values of 7a-c are in an arrow range,t hus indicating that their relative nucleophilicity depends little on the reactivity of the electrophile,t he N values in Table 2c an be used for adirect comparison with other classes of hydrides. As shown in Figure 4, 7b-Et 4 N + and 7c-Et 4 N + are slightly less nucleophilic than sodium borohydride,N aBH 4 (N = 14.74), [11,13] and are among the most reactive hydride donors so far quantified by Equation (1).…”

Selective Michael additions to alkylidenemalonates using thiourea-based bifunctional organocatalysts

“…Mayr's reactivity scales indicate that both nitromethane and malononitrile are excellent nucleophiles, with nucleophilicity values (N) of 20.71 and 19.36, respectively (in DMSO). 25 We also found this to be the case, with nitromethane generating yields of up to 84%, entry 4, Table 3, and a much improved enantiomeric excess of 48%, entry 2, Table 3.…”

“…Mayr's impressive and large study on the nucleophilicy and electrophilicity of many substrates details this reduced electrophilicity when compared with other common Michael acceptors. [23][24][25] Our interest in the Michael addition of pronucleophiles with a relatively acidic hydrogen to alkylidenemalonates is due to the potential synthetic utility of the functional group-rich chiral conjugate addition products (Scheme 1).…”

The Tunable Functionality of α,β‐Unsaturated Carbonyl Compounds Enables Their Differential Application in Biological Systems

“…[33] When characterizing strong nucleophiles, Mayr et al was able to produce very weakly electrophilic Michael acceptors, namely benzylidene malononitriles, benzylidene indandiones, benzylidene barbituric acids, benzylidene Meldrum's acids, and benzylidenemalonates. [34] Here, a formal a-substitution of the a,b-unsaturated carbonyl system is present because of the symmetric nature of these compounds. When trying to classify these molecules, such a series could be considered as "a-modenones" (from a-modified enones).…”