Carbene complexes with amines

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... 

Ruthenium complexes B are stable in the presence of alcohols, amines, or water, even at 60 °C. Olefin metathesis can be realized even in water as solvent, either using ruthenium carbene complexes with water-soluble phosphine ligands [815], or in emulsions. These complexes are also stable in air [584]. No olefination of aldehydes, ketones, or derivatives of carboxylic acids has been observed [582]. During catalysis of olefin metathesis replacement of one phosphine ligand by an olefin can occur [598,809]. 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Table 28 summarizes pAf value determined in acetonitrile " along with pAf values in 50% MeCN-50% water for the complexes where these are available (values in parentheses). They were obtained spectrophotometrically from measurements of the equihbrium constants of the reaction of the carbene complex with strong amine bases such as TMG (1,1,2,2-tetramethyguanidine), DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), BEMP (2-(tert-butylimino)-2-(diethylamino)-... 

These exchange reactions of Fischer carbene complexes occur through tetrahedral intermediates, just like the transesterification of esters. Reaction of a Fischer carbene complex with an alcohol in the presence of base generates the same type of anionic intermediate that an ester does. This intermediate breaks down like an ester to form a new alkoxy-substituted carbene complex. Reactions of amines with Fischer carbene complexes occur similarly, but the high basicity of amines and the high electrophilicity of the carbene complexes alleviates the need for any additional base. 

Reactions of carbene complexes with tertiary amines, di-, or trialkylphosphines give ylid-type compounds ... 

Allylsilanes result from allyl sulphides which have siloxy subsdtuents with regio and stereochemically using allyl-lithium, from alkenyl Fischer carbene complexes with silanes through addition of Si-H to the carbene, and opdcally active y-silylallylamines result frnn a n-allylPd intermediate obtained from the carbonate using amines or azide, while allyldichlorosilane can be prepared by the direct method at 220 - 320 C as the main product. The protodesilyladon of allylsilanes provides a route to vinyl sulphones while silylmethyl allyl sulphones result from silylmethyl cuprates and sulphonylalka-1,2-dienes, and are used in the... 

The reactions of carbene complexes with most primary and secondary amines give the expected substitution products in high yield. However, the reaction of (CO)5CrC(OCH3)CH3 with diisopropylamine unexpectedly gave the monoisopropylaminocarbene complex (Connor and Fischer, 1969). Apparently, elimination of propylene from the very hindered diisopropylamino-carbene complex can occur readily. 

Ni(Gl)Gp(carbene) complexes with the carbenes l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro imidazol-2-ylidene, l,3-bis(2,6-diisopropylphenyl)-4,5-dihydorimidazol-2-ylidene, and l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene have also been prepared, structurally characterized, and shown to have catalytic activities in the aryl amination and aryl halide dehalogenation reactions. Structurally similar carbene complexes have also been reported with the novel carbene formed by aryl cyclohexadienyl-ylidene rearrangement, as shown in Equation (11) the solid-state structure of this complex has shown an Ni-Gcarben bond of ca. 189 pm. 

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). 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Fig. 4.15), are active for ATRP of both styrene and methylmethacrylate (MMA) [46]. Polymerisation was well controlled with polydispersities ranging from 1.05 to 1.47. The rates of polymerisation 1 x 10 s ) showed the complexes to be more active than phosphine and amine ligated Fe complexes, and were said to rival Cu-based ATRP systems. It was quite recent that Cu(I) complexes of NHCs were tested as ATRP catalysts [47]. In this work, tetrahydropyrimidine-based carbenes were employed to yield mono-carbene and di-carbene complexes 42 and 43 (Fig. 4.15), which were tested for MMA polymerisation. The mono-carbene complex 42 gave relatively high polydispersities (1.4-1.8) and a low initiation efficiency (0.5), both indicative of poor catalyst control. The di-carbene complex 43 led to nncontrolled radical polymerisation, which was ascribed to the insolubility of the complex. 

The reaction of the gold(I) pentafluorophenyl isocyanide complexes with primary and secondary amines as well as alcohols leads to the corresponding gold(I) [62, 65] carbenes (Table 3.2). The addition of amines leads to the corresponding carbenes... 

The majority of gold(I) carbene complexes are pure organometallic compounds and the are out of the scope of this work. Some halide or triphenylphosphine carbene complexes are known and they will be considered here. Nucleophilic addition of alcohols or amines to gold-coordinated isocyanides is one of the best-established methods to obtain gold carbene derivatives. The reaction of H[Au(CN)2] with propene oxide and estirene oxide yields (cyano)carbene complexes (380) avoiding the intermediate step.2257 A cyclic carbene compound is obtained by reaction of a dinuclear isocyanide with amine (Scheme 32).2258... 

Transition metal isocyanide complexes can undergo reactions with nucleophiles to generate carbene complexes. Pt(II) and Pd(II) complexes have been most extensively investigated, and the range of nucleophilic reagents employed in these reactions has included alcohols, amines, and thiols (56) ... 

Alvarez-Toledano has investigated149 the reactivity of tertiary amines toward vinylketene complexes of type 221 and found that a number of unusual transformations take place. The formation of 237 and 238 is not directly linked to the presence of the amine, but instead relies on a thermo-lytic coupling of two carbene complexes formed from the vinylketene ligand, with additional loss of carbon monoxide. It is thought that 239 is formed by the deprotonation of the labile C-4 proton, followed by subsequent coordination of a capping Fe(CO)3 group. 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Electron-rich carbyne complexes can react at the carbyne carbon atom with electrophiles to yield carbene complexes. Numerous examples of such reactions, mostly protonations, have been reported [519]. Depending on the nucleophilicity of the carbyne complex, such reactions will occur more or less readily. The protonation of weakly nucleophilic carbyne complexes requires the use of strong acids, such as triflic [533], tetrafluoroboric [534] or hydrochloric acid [535,536]. More electron-rich carbyne complexes can, however, even react with phenols [537,538], water [393,539], amines [418,540,541], alkyl halides, or intramolecularly with arenes (cyclometallation, [542]) to yield the corresponding carbene complexes. A selection of illustrative examples is shown in Figure 3.25. 

As shown in Figure 4.1, the initial step of the conversion of an ylide into a carbene complex is an electrophilic attack at the ylide. Reactions of this type will, therefore, occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex L M. Complexes which efficiently catalyze the decomposition of even weakly nucleophilic ylides can easily react with other nucleophiles also, such as amines or thiols. This has to be taken into account if reactions with substrates containing such strongly nucleophilic functional groups are to be performed. 

Electrophilic carbene complexes can react with amines, alcohols or thiols to yield the products of a formal X-H bond insertion (X N, O, S). Unlike the insertion of carbene complexes into aliphatic C-H bonds, insertion into X-H bonds can proceed via intermediate formation of ylides (Figure 4.7). 

Ylide formation, and hence X-H bond insertion, generally proceeds faster than C-H bond insertion or cyclopropanation [1176], 1,2-C-H insertion can, however, compete efficiently with X-H bond insertion [1177]. One problem occasionally encountered in transition metal-catalyzed X-H bond insertion is the deactivation of the (electrophilic) catalyst L M by the substrate RXH. The formation of the intermediate carbene complex requires nucleophilic addition of a carbene precursor (e.g. a diazocarbonyl compound) to the complex Lj,M. Other nucleophiles present in the reaction mixture can compete efficiently with the carbene precursor, or even lead to stable, catalytically inactive adducts L M-XR. For this reason carbene X-H bond insertion with substrates which might form a stable complex with the catalyst (e.g. amines, imidazole derivatives, thiols) often require larger amounts of catalyst and high reaction temperatures. 

Tertiary amines can react with electrophilic carbene complexes to yield ammonium ylides which usually undergo Stevens rearrangement (Figure 4.8) leading to products of a formal carbene C-N bond insertion. 

The starting Fischer-type carbene complexes 1 were obtained by Michael addition of dimethylamine to the carbon-carbon triple bond of the corresponding ethoxy-(phenylethynyl)carbenes. In this regard, de Meijere and co-workers observed that the reactions of several primary and secondary amines with this sort of carbenes, in particular chromium derivatives 3 containing bulky substituents at the terminal carbon of the acetylenic unit, result in formation of the aminoallenylidene derivatives 5 as by-products of the expected Michael adducts 4 (Scheme 2) [20-24]. 

---