Jason W. Runyon

Carbene-Based Lewis Pairs for Hydrogen Activation

A series of Lewis acid–base pairs containing sterically demanding carbenes were investigated for hydrogen activation that could potentially be reversible for use in hydrogen storage applications. When electron-rich boranes are employed as electrophiles, the imidazolium cation is reduced to a 2H-imidazoline (aminal). The aminals were synthesized independently by reduction of imidazolium cations with strong reducing agents. Carbocations were also found to act as electrophiles for hydrogen activation. Preliminary results revealed that it is possible to reduce an alcohol to an alkane using hydrogen gas as a reducing agent in these systems. Finally, it was demonstrated that a transition metal can be used as an electrophile to activate hydrogen through heterolytic cleavage.

Synthesis and Complexes of Fluoroalkoxy Carbenes

tropositive equatorial substituent (C or N, respectively), the electronegative axial substituents (R C(CF3)2 O), the formation of favorable fiveand six-membered rings, and the Thorpe–Ingold effect all contribute to the efficacy of the Martin ligand. The use of Martin-type ligands for d-block metals is limited, partially owing to the relatively poor aryl– metal bond. The abundance of adducts of hypervalent main-group elements with carbenes and of transition-metal–carbene complexes suggests that replacement of the central aryl group in the Martin-type structures 2 and 3 with an imidazol2-ylidene (as in 4) as the central binding moiety should provide a more suitable ligand for d-block metal complexes while taking advantage of all of the other attractive aspects of the Martin ligands. A number of popular tridentate ligands already exist, but the architecture of 4 offers a remarkably robust structure. Introduction of an CH2C(CF3)2OH group on an azole is simplified by the availability of hexafluoroisobutylene oxide (5). In an effort to avoid ring-opening polymerization of the epoxide, initial attempts to prepare the desired tridentate ligands involved the use of the iodohydrin 6 (derived from epoxide 5) as an alkylating agent. When a solution of sodium imidazolide in THF was heated at reflux with two equivalents of the iodohydrin over a period of days, the imidazolium salt 7 was formed (Scheme 2).

Xylochemistry—Making Natural Products Entirely from Wood

The first total synthesis of the dimeric berberine alkaloid ilicifoline (ilicifoline B) is reported. Its carbon skeleton is constructed from ferulic acid, veratrole, and methanol. The synthesis reported herein employs starting materials solely derived from wood. The natural product is thus constructed entirely from renewable resources. The same strategy is applied to a formal total synthesis of morphinan alkaloids. The use of wood-derived building blocks (xylochemicals) instead of the conventional petrochemicals represents a sustainable alternative to classical synthetic approaches.

Reactions of imidazol(in)-2-ylidenes with electron deficient fluoroolefins

GRAPHICAL ABSTRACT ABSTRACT 1,3-Bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene forms stable 1:1 adducts with tetrafluoroethylene (2), hexafluorocyclobutene (3), and octafluorocyclopentene (4). Adduct 2 shows properties typical for nonpolarized olefins, as indicated by NMR spectroscopy and X-ray crystallography. By contrast, adducts 3 and 4 are best described as ylides with a significant charge separation between the imidazoline ring and the perfluorocycloalkyl unit. Similarly, 1,3-di-1-adamantylimidazol-2-ylidene reacts with tetrakis(trifluoromethyl)allene to form a polarized trimethylenemethane derivative, and bis(trifluoromethyl)ketene to form an imidazolium enolate zwitterion. The synthesis and characterization of a number of fluorinated methyleneimidazolines are described herein.

Synthesis of Stabilized Phosphinidenoid Complexes Using Weakly Coordinating Cations

The formation of phosphinidenoid complex salts having weakly coordinating cations (WCCs) is reported via treatment of P-functional phosphane complexes with N,N’-di-tert-butyl imidazol-2-ylidene or a P4-t-Bu phosphazene base. The thermal stability of phosphinidenoid complex salts is dependent upon the P–X substituents and the nature of the WCC. The complexes were characterized by multinuclear NMR spectroscopy and confirmed by single-crystal X-ray structure.