Readily Available Onio‐Substituted Methyleneiminium Salts: Single Precursors for a Variety of Aminocarbenes

Abstract

Due to their strong s-donor properties, N-heterocycliccarbenes (NHCs) have found numerous applications as ligands for transition-metal catalysts.[1] More recently, it has been found that acyclic diaminocarbenes A (see Scheme 1) [2] or even amino aryl carbenes B[3] also behave as strong-sdonor weak-p-acceptor ligands and therefore could be considered as alternative ligands for catalysts; [4,5] to date, A is the most basic carbene ligand known.[4c] However, the range of acyclic amino carbenes (AAC) known is far surpassed by that of their NHCs counterparts. Obstacles to extending the range of available AACs reside in their supposed inherent instability, [6] but also in the lack of a general and convenient synthetic route for their preparation. We have recently reported that substitution reactions occur at the carbene center of the phosphonio-substituted carbene D, thus giving rise to new stable carbenes. [7] However, the synthesis of D requires several steps, which includes the preparation of a stannyl phosphane; a very fragile carbene intermediate C; [8] and most importantly, is limited to (methyl)di(tert-butyl)-phosphane as a leaving group. Indeed, to assure the stability of carbene C, bulky groups at phosphorus are required, and the addition of an organic group to the phosphane to form a quaternary P center is limited to methylation. (Methyl)di(tertbutyl)phosphane is 1) very basic, therefore not the perfect leaving group; 2) bulky, thus preventing the substitution reaction with sterically hindered nucleophiles; 3) features acidic methyl protons, which often react with the nucleophile (base) to afford undesirable products.Herein, we report an easy entry to amino(phosphonio)-carbenes by using simple phosphanes and readily available reagents, and the subsequent substitution reactions at the carbene center.As one of the most common synthetic routes to stable carbenes is based on the deprotonation of the corresponding iminium salt, [1] phosphonio iminium salts 3 were the obvious precursors to amino(phosphonio)carbenes. Based on the elegant work by Weiss and co-workers, we predicted that dications such as 3 should be accessible and stable as compounds that feature bis(onio)substituted sp 3 -and even sp 2 -carbon atoms have been isolated.[9] Indeed, we have found that Alders dimer 1 [2,10] cleanly reacts at room temperature with the basic tricyclohexylphosphane, but also with very simple phosphanes such as triphenylphosphane, thus leading to the formation of the desired dications 3 a,b in 94 and 93 % yield, respectively (Scheme 2). Although 3 a,b are water sensitive, giving back diisopropylamide when exposed to moisture, they are highly thermally stable (m.p.: 3 a: 217 8C; 3 b: 172 8C). Importantly, although the trifluoromethane sulfonate salts 3 a,b are barely soluble in most classical organic solvents, they are soluble in acetonitrile; thus, their purification is quite easy and large quantities can be prepared in a one-pot reaction from the amide (it is not necessary to isolate Alders dimer 1). However, in this re...

New Synthetic Routes to C‐Amino Phosphorus Ylides and their Subsequent Fragmentation into Carbenes and Phosphines

“…We have already shown that highly thermally stable phosphonio-substituted aldiminium salts 1 [11] are readily prepared in excellent yields by addition of phosphines to Alder's dimer, [12] or alternatively by treatment of the corresponding chloro aldiminium salt with the in situ generated phosphine/trimethylsilyl triflate adduct. [13] Interestingly, basic phosphines, such as tricyclohexylphosphine, are not required, triphenylphosphine can be used.…”

“…After evaporation of the solvent, the residue was extracted with pentane, and ylide 2a was isolated in near quantitative yield. We then used dication 1b, [11] featuring the less basic triphenylphosphine. Under the same experimental conditions, but with tetrakis (dimethylamino)ethylene (TDAE) as a reducing agent, we observed the quantitative formation of triphenylphosphine and alkene 3b, the expected dimer of the (diisopropylamino) (hydrogeno)carbene (Scheme 3).…”

“…The suspension was stirred for 1 hour at room temperature. After evaporation of the solvent, the solid residue was washed with THF (20 mL) affording 1a and 1b [11] …”

Structural Elucidation of tmeda‐Solvated Alkali Metal Diphenylamide Complexes

“…Of particular interest in this study, another amido reagent, the thermally highly stable lithium diphenylamide (LiNPh 2 ), has shown promise in a range of synthetic transformations. As well as regioselective deprotonation reactions, [6][7][8][9][10] it has also been used in catalytic aldol reactions involving silyl enol ethers and aldehydes; [11,12] in elimination applications; [13,14] in metathetical reactions; [15][16][17][18][19][20][21][22][23][24][25][26][27] during the preparation of amino-containing carbenes, [28] and as an initiator in the polymerisation of methyl methacrylate. [29] utilised as the metallating agent.…”

New synthesis of functionalized tertiary phosphines

“…Meanwhile, they are valuable subjects for solution of a number of structural problems 1,2 and development of the organophosphorus synthesis. [3][4][5] In particular, they are of use in the synthesis of stable aminophosphine carbene ligands for metal com plexes with potential catalytic activity. 5 In the present communication, a new reaction that is promising for the synthesis of complex phosphine compounds is presented.…”