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Unformatted text preview: Latent Nucleophilicity of Dichlorocarbene Robert A. Moss,* Min Zhang, and Karsten Krogh-Jespersen* Department of Chemistry and Chemical Biology, Rutgers, The State Uni V ersity of New Jersey, New Brunswick, New Jersey 08903 [email protected]; [email protected] Received February 23, 2009 ABSTRACT Rate constants and activation parameters are reported for additions of CCl 2 to methyl acrylate, acrylonitrile, and r-chloroacrylonitrile. The results reveal latent nucleophilicity in the addition of CCl 2 to r-chloroacrylonitrile, a conclusion supported by theoretical studies of the cyclopropanation reaction transition state. Dichlorocarbene (CCl 2 ) has been the iconic carbene ever since Hine demonstrated its intermediacy in the hydrolysis of chloroform 1 and Doering trapped it by addition to alkenes. 2 Subsequent research revealed that CCl 2 discrimi- nated between simple alkenes in an electrophilic fashion, 3 and it became the exemplar of electrophilic carbenes. 4 In fact, a singlet carbene like CCl 2 is intrinsically both an electrophile and a nucleophile. In terms of frontier molecular orbital (FMO) theory, philicity depends on whether the “electrophilic” carbene-LUMO (p)/alkene-HOMO ( π ) or the “nucleophilic” carbene-HOMO ( σ )/alkene-LUMO ( π *) or- bital interaction dominates in the carbene/alkene cycload- dition transition state and governs the direction of charge transfer between the carbene and the alkene; cf. Figure 1. 4- 6 Due to its relatively low-lying, readily accessible vacant p orbital (LUMO), 6 CCl 2 is an electrophilic carbene. Various other theoretical treatments concur in this characterization. 7 However, if an alkene is made very electron poor (i.e., its π * LUMO is sufficiently lowered), then even a normally electrophilic carbene like CCl 2 could add as a nucleophile. This occurs with phenylchlorocarbene ( 1 ), whose reactivity is generally similar to that of CCl 2 . 4 Although 1 selects electrophilically across a “standard set” of alkenes, 5b,8 the absolute rate constants for its additions to such strongly electron-deficient alkenes as diethyl fumarate 9 or R-chloro- acrylonitrile ( 2 ) 10 reveal nucleophilic character. Will a similar philicity reversal be manifested by CCl 2 ? The advent of a diazirine precursor for CCl 2 , dichlorodiaz- irine ( 3 ), 11 now enables us to obtain absolute rate constants and activation parameters for CCl 2- alkene additions. 12 Moreover, the absence of trichloromethide anion (Cl 3 C- ) when CCl 2 is generated by laser flash photolysis (LFP) of (1) Hine, J. J. Am. Chem. Soc. 1950 , 72 , 2438. (2) Doering, W. v. E.; Hoffman, A. K. J. Am. Chem. Soc. 1954 , 76 , 6162. (3) Doering, W. v. E.; Henderson, W. A., Jr. J. Am. Chem. Soc. 1956 , 80 , 5274....
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