Zwitterionic carbene complexes

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

In 1991 Fischer et al. observed the interesting phenomenon that treatment of benzyUdene tungsten carbene complex 9 with triphenylketeneimine at —70 °C in CH2CI2 gave zwitterionic intermediate 11, which was derived from the rearrangement of the initially formed zwitterioinic intermediate 10 [5]. Careful analysis of... 

Wulff et al. recently reported another unique example of this inverse transformation [35]. Thus, treatment of a, P-unsaturated Fischer carbene complexes 138 with an isopropoxy group on the carbene carbon with ketene acetal 139 at 80 °C in THF under CO pressure gave 4-pentynoate derivatives 140 in good yield. The reaction was proposed to proceed through 1,4-addition of ketene acetal to the carbene complex to give a zwitterionic intermediate 141. This underwent internal... 

Fig. 3.17. Two reactions that demonstrate the stereospecificity of Rh-catalyzed cis-cyclo-propanations of electron-rich alkenes. — The zwitterionic resonance form A turns out to be a better presentation of the electrophilic character of rhodium-carbene complexes than the (formally) charge-free resonance form B or the zwit-ter-ionic resonance form (not shown here) with the opposite charge distribution ( adjacent to the C02Me groups, on Rh) rhodium-carbene complexes preferentially react with electron-rich alkenes.

Reactions of carbene complexes with phosphorous compounds are topics of current research.164-167 (l-Alkynyl)carbene complexes (CO)5M= C(OEt) —C=CR 1 (M = Cr, W R = Ph, r-Bu, SiMe3) readily add tertiary phosphanes PPhR2 (R = Ph, Me) and form stable zwitterionic phosphonium allenide complexes (CO)5M —C(OEt)=C=C(R)—P+PhR2 145, which have been characterized by X-ray diffraction (Scheme 57).55 In previous studies on Michael-type additions of nitrogen and oxygen nucleophiles to (l-alkynyl)carbene complexes 1, zwitterionic allenide complexes of type 145 have been postulated as intermediates.21,23,503 62 112,168,169 Unambiguous structural evidence of the tendency to form such intermediates is given by isolation of a stable zwitterionic adduct, such as compound 145. Zwitterionic... 

The presence of a base is not necessarily mandatory for the formation of carbene complexes, as has been demonstrated in the reaction between Pd(OAc)2 with [Mes2im]Cl, shown in Scheme 6.2. However, instead of the expected symmetric carbene complex a zwitterionic species was isolated which turned out to be a significantly more active catalyst than the bis-carbene species.1241... 

This approach makes the synthesis of neutral zwitterionic transition metal carbene complexes possible, with valuable advantages over cationic complexes [230]. The concept was previously applied to N, P and S ligands [231,232] and has a carbene precursor in Roesler s backbone boron substituted NHC ligands [233] the latter has not been coordinated to transition metals yet. 

A very interesting amido functionahsed carbene was prepared by Legault et al. [116] from A-mesitylimidazole and 0-(2,4-dinitrophenyl)hydroxylamine, an electrophilic ami-nation reagent [117]. The exo-amino group is subsequently acylated to afford a zwitterionic amido functionalised carbene (see Figure 4.38). Reaction with silver(l) acetate and sodium carbonate [a rare variant of the silver(I) oxide method] yields the silver(l) carbene complex as a dimer with a Ag-Ag bond. The silver(l) carbene complex can be used as a carbene transfer reagent to synthesise the homoleptic monomeric copper(Il) carbene complex. 

In acetonitrile all reactions of pyrrolidine addition exhibited AV values between -15 and -17cm3mol-1.262 However, upon decreasing the solvent polarity, AV became markedly more negative and a good correlation with the solvent parameter qv (i.e. the pressure derivative of q, the polarisability of the solvent) could be demonstrated. It was concluded that the addition of pyrrolidine followed a two-step process with a polar transition state leading to a zwitterionic intermediate. Addition of / ara-substituted anilines to a similar Fischer carbene complex in acetonitrile was characterised by more negative values of AV (-21 to -27cm3mol-1) than those for addition of pyrrolidine. 263 Second-order rate constants for the addition reaction exhibited an excellent correlation with the basicity of the aniline derivatives used. The trend in activation volumes could be correlated with an early or late transition state for the fast and slow addition reactions, respectively Scheme 12. 

With neutral nucleophiles such as phosphines and amines, the nucleophilic attack on the carbene complex leads to zwitterionic intermediates such as 47-49 if there is an acidic proton on the intermediate as in the case of 49, the reaction can proceed to products by elimination of ROH, otherwise the reaction stops at the adduct stage. 

The aminolysis of carboxylic esters to which the reaction of Fischer carbene complexes with amines has frequently been compared, typically proceeds by a similar mechanism with rate-limiting deprotonation of the corresponding zwitter-ionic tetrahedral intermediate (59) and so do many SnAt reactions where the deprotonation of the zwitterionic Meisenheimer complex (e.g., 60) is rate... 

It was concluded that the addition of pyrrolidine follows a two-step process with a polar transition state leading to a zwitterionic intermediate. The addition of a series of p-substituted anilines to a Fischer carbene complex as shown in Scheme... 

Cumulene-type carbene complexes of M(CO)s have been studied, including 149 and 150 (Figure 13). In fact, their negative solvatochromism suggests that their structures are better represented by zwitterionic cr-alkyne resonance structures (Figure 13). As with the carbenes 142-148, Cr and W species show similar non-linearities for example, 149 and 150 show /3 (/3o) values of 100 (31) x 10 °esu and 102 (31) x 10 °esu, respectively (HRS, 1,064 nm). 

A new synthetic route to j8-lactams has recently been reported " involving the photolytic reaction of chromium carbene complexes with imines. Detailed analysis now establishesa mechanism involving the initial photolytic formation of a ketene complex (24). This is followed by nucleophilic attack by the imine at the ketene carbonyl to produce a zwitterion intermediate, which undergoes a conrotatory ring closure to form the j8-lactam as shown in Scheme 15,... 

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