ZnEt₂ as a Precatalyst for the Addition of Alcohols to Carbodiimides

Abstract

We report here the use of commercially available ZnEt2 as an efficient precatalyst for the addition of alcohols to carbodiimides to obtain a wide range of isoureas under mild conditions. In an initial screening using methanol and commercial carbodiimides as substrates, the bulky isourea (OMe)(NHDipp)C(NDipp) (Dipp = 2,6-iPr2C6H3) was prepared for the first time using a catalytic method, and its structure confirmed by an X-ray diffraction analysis. Then, the efficiency of the precatalyst was tested with two carbodiimides, C(NiPr)2 and C(Np-tol)2, toward a series of alkylic and arylic alcohols and diols, with different steric and electronic properties, including the presence of other functional groups, usually with excellent conversions, especially for the more reactive aromatic carbodiimide. Some of the new isoureas thus prepared have also been isolated and characterized. Kinetic and stoichiometric experiments allowed us to propose a plausible mechanism for these transformations.

Zinc Catalyzed Hydroelementation (HE; E = B, C, N, and O) of Carbodiimides: Intermediates Isolation and Mechanistic Insights

“…Based upon the above results and previously reported mechanisms in the literature, ,, the most probable catalytic cycles have been proposed for C–H, N–H, and O–H addition to CDIs in Schemes , S1, and S2.…”

“…A recent literature study confirms that catalytic insertion of O−H (addition of alcohols) into carbodiimides is an exciting synthetic route to prepare the isoureas. 22 With our present CBG zinc hydride precatalyst (1), we explored that the catalytic addition of benzyl alcohol (8a) and p-methoxy benzyl alcohol (8b) to CDIs (2a and 2f) led to the quantitative formation of isourea derivatives 9a−9c with no trace of any starting material (Scheme 4).…”

“…Evan, Eisen, and Panda groups independently reported Th, U, and Ti metal-catalyzed hydroelementation of heterocumulenes (Figure B). However, the previous reports on the hydroelementation reaction using zinc complexes usually involve one or two kinds of E–H nucleophiles and are limited to a narrow substrate scope. ,,, Therefore, developing a zinc-based versatile catalyst that can exhibit the addition of various E–H (E = B, C, N, O, P, and S) moieties to carbodiimide is highly demanding.…”

“…11,20 However, zinc-catalyzed hydroalkynylation of carbodiimides is limited (Figure 1A). 21 Roesky 5a and Carrillo−Hermosilla 22 research groups recently established diethylzinc as a precatalyst for P−H and O−H addition to heterocumulenes, respectively. As far as the E−H (E = C, N, O, P, and S) bond insertion into carbodiimide is concerned, only a handful of reports are known.…”

Linear not cyclic – unravelling an anionic initiation pathway for Lewis pair polymerization of lactones

“…Furthermore, self-initiated polymeriz-ation of εCL by Zn(C 6 F 5 ) 2 or Al(C 6 F 5 ) 3 have been reported to yield low number average molecular weight polymers attributed to uncontrolled side reactions. 3,6,7 It should be noted that ZnEt 2 has very diverse chemical features and has been used for the addition of alcohols to carbodiimides, 26 reductions of imines, 27 reduction and nucleophilic addition of and to ketones and aldehydes, [28][29][30][31] reduction of acylsilanes, 32 and deprotonation of β-keto esters, 33 clearly testifying to its high and diverse reactivity. In addition, when we tried to employ ZnEt 2 to synthesise cyclic poly(εCL), ZnEt 2 was found proficient in the self-initiated ring-opening polymerization of εCL without requiring any additional initiator (Table S1, † entries 1 and 2) which is in agreement with literature.…”