Realizing 1,1‐Dehydration of Secondary Alcohols to Carbenes: Pyrrolidin‐2‐ols as a Source of Cyclic (Alkyl)(Amino)Carbenes

Abstract

Herein we report secondary pyrrolidin-2-ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC-Cu I -complexes and cyclic thiones when reacted with Cu I -salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)-center. This result demonstrates a convenient and facile access to CAAC-based Cu I -salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes-realizing after 185 years Dumas' dream who tried to prepare the parent carbene (CH 2 ) by 1,1-dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the OÀ H bond to the carbon(II)-center. This emphasizes the ability of carbon-compounds to mimic the reactivity of transition-metal complexes: reversible oxidative addition and reductive elimination of the OÀ H bond to/from the C(II)/C(IV)-centre.

Cyclic (amino)(barrelene)carbenes: an original family of CAACs through a novel synthetic pathway

“…Iminium salts 2a and 3a-d can be deprotonated with KHMDS at À78 1C to afford the corresponding carbenes Carb2a and Carb3a-d (Scheme 4). 13 C NMR spectra exhibit singlets at 306 ppm (Carb2a), 304-315 ppm (Carb3a-d), in the range observed for classical five-membered CAACs (304-319 ppm). 2 Carb2a and Carb3d demonstrate a good thermal stability with the latter being stable up to 70 1C in benzene for at least 1 h. Deprotonation of iminiums 2c 2b also occurred with KHMDS at À78 1C, but the corresponding free carbenes Carb2c and Carb2b could not be observed.…”

“…These results demonstrate the existence of a thermal equilibrium between CABC-derived hydroxy pyrrolidines and the corresponding carbenes (Scheme 5). 13 Unsurprisingly, the overall donating properties of CABCs are similar to those of CAACs, as indicated by the infrared n av CO frequency of (Carb2c)Rh(CO) 2 Cl complex (n av CO = 2038 cm À1 ) compared to CAAC analogous complexes (2036 cm À1 ). 14 Interestingly, the 77 Se NMR signal 15 of CABC selenium adducts appeared at much lower field than for CAACs as shown in Fig.…”

“…These results demonstrate the existence of a thermal equilibrium between CABC-derived hydroxy pyrrolidines and the corresponding carbenes (Scheme 5). 13…”

C−C and C−N Bond Activation, Lewis‐Base Coordination and One‐ and Two‐Electron Oxidation at a Linear Aminoborylene

“…As tricoordinate borylenes are known to activate elemental chalcogens, [48][49][50] borylene 2 was reacted with N 2 O, typically used as a selective oxygen source in reactions with silylenes, [51] as well as elemental sulfur and selenium. With both N 2 O and sulfur, the BÀ C CAAC bond was cleaved and the boron and carbene centers fully oxidized, leading to the quantitative formation of a 2 : 1 mixture of the known CAAC-chalcogen adducts 7°and 7 S , respectively, [52,53] and the known 2,4diamino-1,3,2,4-dichalcogenadiboretanes 8°(δ 11B = 28.2 ppm) and 8 S (δ 11B = 40.7 ppm), respectively, [54,55] as identified by NMR spectroscopic and X-ray crystallographic analyses (Scheme 5a). A similar complete oxidation of (CAAC Me ) 2 Si with N 2 O, yielding Chemistry-A European Journal 7°and SiO 2 , has been reported by Roesky and Stalke.…”

A Donor–Acceptor Cyclopropane by Intramolecular C(sp3)–H Activation at a Cyclic(alkyl)(amino)carbene Center and Its Fascinating Ring-Opening Chemistry

“…Similar conversions of [( R 2 CAAC)(H)] + to [( R 2 CAAC)(H)(OH)] (R 2 = Me 2 , Et 2 , Cy) are said to occur through OH − acting as a nucleophile. 19 An alternative deprotonation (CAAC formation) followed by H 2 O activation should not be ruled out considering the poor nucleophilicity but stronger basicity of OH − . 20 In fact, in the present case, 7 also serves as a pico CAAC precursor by 1,1-dehydration in the presence of Se or CuCl at 170 °C under solvent-free conditions, giving 5 or 6, respectively (Scheme 2).…”