New Stable and Persistent Acyclic Diaminocarbenes

Expedient Synthesis of Thermally Stable Acyclic Amino(haloaryl)carbenes: Experimental and Theoretical Evidence of "Push-Pull" Stabilized Carbenes

A library of novel structurally related singlet carbenes, namely, acyclic amino(haloaryl)carbenes, was designed by a high-yielding two-step procedure, and their chemical stability explored both experimentally and theoretically. Thanks to a careful selection of both the amino and the aryl substitution pattern, these carbenes exhibit a wide range of stability and reactivity, spanning from rapid self-dimerization for carbenes featuring ortho-F substituents to very high chemical stability as bare carbenes, up to 60 °C for several hours for compounds carrying ortho-Br substituents. Their structure was determined through NMR and X-ray diffraction studies, and their reactivity evaluated in benchmark reactions, highlighting the ambiphilic character of this novel class of singlet carbenes. In contrast with previously reported aryl substituents incorporating o-CF3 and t-Bu groups, which were considered "spectator", the high chemical stability of some of these carbenes relates to the stabilization of the σ-orbital of the carbene center by the π-accepting haloaryl substituent through delocalization. Kinetic protection of the carbene center is also provided by the ortho-halogen atoms, as demonstrated computationally. This push-pull stabilization effect makes acyclic amino(haloaryl) carbenes among the most ambiphilic stable carbenes reported to date, holding promise for a variety of applications.

Highly selective ethenolysis with acyclic-aminooxycarbene ruthenium catalysts

Acyclic carbene–ruthenium catalysts were developed for the ethenolysis. Remarkable catalytic efficiency (turnover numbers of 100 000) and excellent α-olefin selectivity (up to 98%) were exhibited.

Acyclic diaminocarbene-based Thiele, Chichibabin, and Müller hydrocarbons

Thiele, Chichibabin and Müller hydrocarbons are considered as classical Kekulé diradicaloids. Herein we report the synthesis and characterization of acyclic diaminocarbene (ADC)-based Thiele, Chichibabin, and Müller hydrocarbons. The calculated singlet–triplet energy gaps are ΔE_S–T = −27.96, −3.70, −0.37 kcal mol⁻¹, respectively, and gradually decrease with the increasing length of the π-conjugated spacer (p-phenylene vs. p,p′-biphenylene vs. p,p′′-terphenylene) between the two ADC-scaffolds. In agreement with the calculations, we also experimentally observed the enhancement of paramagnetic diradical character as a function of the length of the π-conjugated spacer. ADC-based Thiele's hydrocarbon is EPR silent and exhibits very well resolved NMR spectra, whereas ADC-based Müller's hydrocarbon displays EPR signals and featureless NMR spectra at room temperature. The spacer also has a strong influence on the UV-Vis-NIR spectra of these compounds. Considering that our methodology is modular, these results provide a convenient platform for the synthesis of an electronically modified new class of carbon-centered Kekulé diradicaloids.

We report the synthesis of acyclic diaminocarbene (ADC)-scaffold based Thiele, Chichibabin, and Müller hydrocarbons. Studies support that the singlet-triplet energy gap depends on the π-conjugated spacer between the ADC scaffolds.

Stereoelectronic Profiling of Acyclic Diamino Carbenes (ADCs)

A library of 14 heterobis(carbene) complexes of the general formula [Au(ⁱPr₂-bimy)(ADC)]BF₄ (7-20) containing the N-heterocyclic carbene reporter ⁱPr₂-bimy and various protic acyclic diaminocarbenes (ADCs) have been prepared to estimate their stereoelectronic properties by ¹³C NMR spectroscopy and percentage buried volume (%V_bur) determinations. Their preparation was achieved by nucleophilic attack of five secondary amines on six mixed NHC/isocyanide complexes of the type [Au(ⁱPr₂-bimy)(CN-R)]BF₄ (1-6). Analyses of the ⁱPr₂-bimy carbene signals reveal that protic ADCs are stronger donors than classical and expanded-ring NHCs. On the other hand, they are weaker donating compared to NHCs with reduced-heteroatom stabilization. Moreover, stereoelectronic fine-tuning of these ligands is possible by a diverse range of substituents originating from the employed isocyanides and amines.

Evaluation of benzthiazolidine-based formamidinium salts for the synthesis of penem-type β-lactams by uncatalysed carbonylation of acyclic diaminocarbenes

The reaction of [iPr2N=CHCl]Cl with 2-phenylbenzthiazolidine (Ph-BtzH) and subsequent anion metathesis afforded the formamidinium salt [(Ph-Btz)CH(NiPr2)][PF6] ([1aH][PF6]). The reaction of [1aH][PF6] with (Me3Si)2NNa in the presence of grey selenium furnished the selenourea derivative 1aSe as trapping product of the acyclic diaminocarbene 1a. In the absence of selenium, a 2H-1,4-benzothiazine derivative (2) was obtained, which is an isomer of 1a, whose formation is plausibly initiated by deprotonation of [1aH]+ not at the cationic N2CH unit, but at the neighbouring PhCH position. CO was ineffective as trapping reagent for 1a. [1aH][PF6], 1aSe and 2 have been structurally characterised by X-ray diffraction.

The reaction of a particularly electrophilic acyclic diaminocarbene with carbon monoxide: formation of β- and γ-lactams

The reaction of carbon monoxide with the acyclic diaminocarbene iPr2N–C–TMP (TMP=2,2,6,6-tetramethylpiperidino), generated in situ from the formamidinium salt [(iPr2N)CH(TMP)][PF6] and NaN(SiMe3)2, unexpectedly afforded a mixture of β- and γ-lactams.

Stable and Persistent Acyclic Diaminocarbenes with Cycloalkyl Substituents and Their Transformation to β-Lactams by Uncatalysed Carbonylation with CO

Four new acyclic diaminocarbenes (ADACs), viz. [(cyclo-C_n H_2n-1 )₂ N]₂ C (n=5-7) and iPr₂ N-C-N(cyclo-C₆ H₁₁ )₂ , were synthesised by reacting the corresponding formamidinium hexafluorophosphates with NaN(SiMe₃ )₂ . Their nucleophilicities and electrophilicities were respectively judged from the ¹ J_CH values determined for the N₂ CH unit of the corresponding formamidinium cations and from the ⁷⁷ Se NMR chemical shifts of the selenourea derivatives obtained from the reaction of elemental selenium with the corresponding ADACs. An ambiphilic profile essentially identical to that of the "Alder carbene" (iPr₂ N)₂ C was found in each case. Similar to the latter carbene, the new ADACs undergo a well-defined thermal decomposition by β-fragmentation, affording an alkene and a formamidine. The stabilities of [(cyclo-C_n H_2n-1 )₂ N]₂ C depend strongly on the value of n, following the order 6>5>7, with the latter congener being too unstable for isolation. [(cyclo-C₆ H₁₁ )₂ N]₂ C shows no thermal decomposition at room temperature in solution and is thus significantly more stable than (iPr₂ N)₂ C. The stability of iPr₂ N-C-N(cyclo-C₆ H₁₁ )₂ is intermediate between that of (iPr₂ N)₂ C and [(cyclo-C₆ H₁₁ )₂ N]₂ C, its β-fragmentation selectively affording propene and iPrN=CH-N(cyclo-C₆ H₁₁ )₂ . [(cyclo-C_n H_2n-1 )₂ N]₂ C (n=5-7) react readily with CO under mild conditions, selectively affording trisubstituted spirocyclic β-lactam derivatives with an antimicrobial activity spectrum similar to that of penicillin G.

Aminkomplexe des Goldes, Teil 9: Gold(I)-halogenid-Komplexe mit primären und azyklischen sekundären Aminen und ihre Oxidation zu Gold(III)-Derivaten

The reaction of (tht)AuX (X=Cl or Br; tht=tetrahydrothiophene) with various primary amines L leads to products of the form [L2Au]+X−. Packing diagrams of the corresponding structures are dominated by N–H···X hydrogen bonds and (in some cases) aurophilic contacts. The cyclohexylamine derivative was already known as its dichloromethane ⅔-solvate; we have isolated the solvent-free compound and its pentane ¼-solvate, which all show different packing patterns. With acyclic secondary amines, the products are more varied; LAuX and [L2Au]+[AuX2]− were also found. These gold(I) products were generally formed in satisfactory quantities. The attempted oxidation to Au(III) derivatives with PhICl2 or Br2 proved impossible for the primary amine derivatives [although isopropylamine-trichloridogold(III) was obtained unexpectedly from the corresponding cyanide] and unsatisfactory for the secondary amine derivatives. Products LAuX3 and [L2AuX2]+[AuX4]− were identified but were formed in disappointing yields. In isolated cases protonated products (LH)+[AuCl4]−, (LH+)3[AuCl4]−(Cl−)2 or [(Et2N)2CH]+[AuBr4]− were formed, presumably by involvement of the dichloromethane solvent and/or adventitious water. Here also the yields were poor, and some products arose as mixtures. Direct reaction of amines with AuCl3 or (tht)AuX3 was also unsuccessful. All products were characterized by X-ray structure analysis.

N-Heterocyclic germylenes and stannylenes of the type [Fe{(η 5-C5H4)NR}2E] with bulky alkyl substituents

The 1,1′-diaminoferrocene derivatives [Fe(η 5-C5H4–NHAd)2] (Ad=2-adamantyl) and [Fe(η 5-C5H4–NHtBu)2] were investigated in terms of their suitability for the synthesis of N-heterocyclic tetrylenes of the type [{Fe(η 5-C5H4–NR)2}E] (E=Ge, Sn). The synthesis of these target compounds was easily achieved with R=tBu, but failed with R=Ad. In the latter case, the stannylene was not sufficiently stable for isolation and decomposed to the aminoiminoferrocene derivative [Fe(η 5-C5H4–NHAd)(η 5-C5H4–N=Ad′)] (Ad′=adamant-2-ylidene). Attempts to synthesise [{Fe(η 5-C5H4–NAd)2}Ge] afforded intractable material, from which the unusual compound [μ 2-{Fe(η 5-C5H4–NAd)2}Ge2(μ 3-O)(GeCl2)] was obtained by serendipity. It contains GeO, stabilised by adduct formation with GeCl2 and the target germylene. [Fe(η 5-C5H4–NHtBu)2], [Fe(η 5-C5H4–NHAd)2], [Fe(η 5-C5H4–NHAd)(η 5-C5H4–N=Ad′)], [μ 2-{Fe(η 5-C5H4–NAd)2}Ge2(μ 3-O)(GeCl2)] and [{Fe(η 5-C5H4–NtBu)2}E] (E=Ge, Sn) were structurally characterised by single-crystal X-ray diffraction.

Redox-Active N-Heterocyclic Germylenes and Stannylenes with a Ferrocene-1,1'-diyl Backbone

We describe ferrocene-based N-heterocyclic germylenes and stannylenes of the type [Fe{(η⁵ -C₅ H₄ )NR}₂ E:] (1 RE; E=Ge, Sn; R=neopentyl (Np), mesityl (Mes), trimethylsilyl (TMS)), which constitute the first examples of redox-functionalised N-heterocyclic tetrylenes (NHTs). These compounds are thermally stable and were structurally characterised by means of X-ray diffraction studies, except for the neopentyl-substituted stannylene 1 NpSn, the decomposition of which afforded the aminoiminoferrocene [fc(NHCH₂ tBu)(N=CHtBu)] (2) and the spiro tin(IV) compound (1 Np)₂ Sn (3). DFT calculations show that the HOMO of the NHTs of our study is localised on the ferrocenylene backbone. A one-electron oxidation process affords ions of the type 1 RE^+. . In contrast to the NHC system 1 RC, the localised ferrocenium-type nature of the oxidised form does not compromise the fundamental tetrylene character of 1 RE^+. .