On the proposed existence of a ketene derived from carbon monoxide and 1,3-di-1-adamantylimidazol-2-ylidene

Cyclic (Alkyl)(Amino)Carbene Complexes of Rhodium and Nickel and Their Steric and Electronic Parameters

“…Whereasc lassical NHCs are not electrophilic enough to react with CO, [20] relatedd iamidocarbenes [21] and alkyl(amino)carbenes [22,23] add to CO to give the corresponding ketenes I. [24] Bertrande tal.…”

CO Fixation to Stable Acyclic and Cyclic Alkyl Amino Carbenes: Stable Amino Ketenes with a Small HOMO–LUMO Gap

“…[13] In 1994 it was claimed that the imidazol-2-ylidene 1 a reacts with carbon monoxide to give the stable diamino ketene 2 a (Scheme 1). [14] However, a year later Arduengo et al [15] were not able to duplicate these experimental results. They demonstrated computationally that the parent compound 2 b is not even a transition state, and found that there is no stable structure associated with the combination of 1 b and CO, other than "a non-bonded weakly interacting (van der Waals) complex" 3 b (scheme 1).…”

“…Moreover, the calculations showed that the CO addition leading to 2 b is not favored thermodynamically [DH(298 K) = + 15.9 kcal mol À1 ]. [15] Herein we report that, in marked contrast with cyclic diamino carbenes 1, stable acyclic 4 a [16] and 4 b, and cyclic alkyl amino carbenes (CAACs) 6 [17] react with CO to afford amino ketenes 5 a,b and 7, respectively (see Scheme 2), which are indefinitely stable at room temperature both in solution and in the solid state. We show that the ring structure forces the planarization of the amino fragment of 7, and therefore causes the destabilizing n-p donation from the amino group.…”

The quest for observation and isolation of oxyallyl derivatives

“…In the case of oxyllallyls, the introduction of +M donating groups seems the most appropriate, since it strongly stabilizes the cationic allylic moieties in the zwitterionic resonance form (scheme 14). 37-41 The scope of this reaction is limited to few stable electrophilic carbenes and doesn't include the classical cyclic di(amino)carbenes, which don't react with CO. 42 Furthermore, the steric bulk of stable carbenes usually preclude the addition of a second equivalent, which would afford the corresponding oxyallyl. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Another synthetic strategy could rely on the reaction of carbenes with ketenes, which is a known method for generating transient oxyallyls.…”