Lithium carbene complexes

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

Dimesitylimidazolium chloride with nickelocene gives the carbene complex [(T) -Cp)NiCl(L)] (L= l,3-dimesitylimidazol-2-ylidene), in which the chloride ligand can be substituted by a methyl group by reacting the product with methyl-lithium (OOJOM(596)3). 

Deprotonation of l-methyl-3-ferrocenylimidazolium tetrafluoroborate or iodide (98JOM(552)45) by lithium di-Mo-propylamide and subsequent reaction with W(C0)5-THF gives the carbene complex 107 and bis-carbene 108, even when excess W(CO)j THF is applied (99JOM(572)177). Numerous ferrocenyl benzimidazoles are known (97RCR613, 99JOM(580)26). 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

When the Au(I) complex [Au(CGF5)(tht)j reacts with lithium l-methylimidazol-2-yls the organic compound is protonated or alkylated and the carbene complexes are obtained [67] (see Figure 3.3). 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

Treatment of (tht)AuC6F5 with l-methyl-imidazol-2-yl-lithium gives an ionic product which can be converted into the neutral carbene complex upon A-methylation with Mel (Equation (29)).146... 

Compounds of the same type are accessible by reaction of l-methyl-imidazol-2-yl lithium with components XAu(tht) (X = CN, C6F5). Treatment of the complex salts thus obtained as intermediates with strong methylating agents affords the carbene complexes (Scheme 61).257,260... 

Monomeric carbene complexes with 1 1 stoichiometry have now been isolated from the reaction of 4 (R = Bu, adamantyl or 2,4,6-trimethylphenyl R = H) with lithium l,2,4-tris(trimethylsilyl)cyclo-pentadienide (72). The crystal structure of one such complex (R = Bu) revealed that there is a single cr-interaction between the lithium and the carbene center (Li-C(carbene) 1.90 A) with the cyclopentadienyl ring coordinated in an if-fashion to the lithium center. A novel hyper-valent antimonide complex has also been reported (73). Thus, the nucleophilic addition of 4 (R = Mes R = Cl) to Sb(CF3)3 resulted in the isolation of the 1 1 complex with a pseudo-trigonal bipyramidal geometry at the antimony center. 

The first of these are called carbene complexes and the latter are referred to as carbyne complexes.61 The first carbene complex was reported in 1964 by Fischer and Maasbol62 and was prepared by reaction of hexacarbonyltungsten with methyl or phenyl lithium to generate an acyl anion which was then alkylated with diazomethane. 

For most methoxycarbene complexes a one pot modification of the above method is utilized. This involves direct alkylation of the initially formed lithium acylate carbene complex with trifluoromethanesulfonate or with methyl fluorosulfonate. The method is successfully employed for preparation of chromium140 as well as molybdenum and tungsten monocyclic uation 65), bicyclic (equation 66) and tricyclic carbene complexes... 

Chromium carbene complexes, 82 Chromium carbonyl, 51 Chromium(II) chloride, 84 Chromium(III) chloride-Lithium aluminum hydride, 84 Chromium(VI) oxide, 338... 

The first example of lithium-NHC complexes, in which the lithium is coordinated only to carbon centres, was reported by Arduengo and coworkers.10 Stable NHCs were reacted with lithium 1,2,4-n-A(trimethylsilyl) cyclopentadienide to give 2 (Fig. 2). A single crystal X-ray structure reveals a complex in which the lithium centre is coordinated in a r 5-fashion to the cyclopentadienyl ring, with a single cr-interaction between the lithium and carbene centre. The lithium centre lies 2.155(4) A from the carbene centre hence has a closer contact than in the previous example, possibly as a result of the carbene interacting with only one lithium centre. 

The first group 1 carbene complex with an N-bound anionic functional group was reported in 2004.12 An alkylamino carbene is readily deprotonated using //-butyl lithium to afford 4 (Fig. 3). The solid state structure comprises a discrete dimer via bridging amido groups. Although there is severe distortion of the lithium-NCN bond (147.9° compared to the closer to linear 161.8° in 3), the lithium-NHC bond distance of 2.124(4) A is still short, suggesting that the interaction is predominantly ionic. 

Arnold and co-workers also reported the deprotonation of alkoxy imi-dazolium iodides with -butyl lithium to yield lithium alkoxide carbenes (Scheme 3).14 Single crystals of one of the complexes were grown from a diethyl ether solution, and revealed a dimer of LiL with lithium iodide incorporated to form a tetramer of lithium cations (7). The lithium-NHC bond distance of 2.131(6) A is similar to that of the lithium amide carbene 4. Also as in 4 there is distortion of the lithium-NCN bond which has an angle of 152.3°. The C2 carbon resonates at 200 ppm in the 13C NMR spectrum which is a relatively high-frequency, possibly as a result of the incorporated lithium iodide. The lithium salts were able to act as ligand transfer reagents and react with copper (II) chloride or triflate to afford mono- or bis-substituted copper(II) alkoxy carbene complexes. 

Addition of aryl lithium compounds to a (l-alkynyl)carbene complex la,b affords Michael adducts 90 in up to 45% yield, together with cyclopen-... 

The observation by Fischer et al.18 that the 4,1-addition of dimethylamine to compound la is thermodynamically controlled at 20°C, whereas 2,1-addition/elimination is kinetically controlled at -115°C, turned out to be limited to few cases.20 It has been shown9a 9b 42 112 113 that for most cases, three competing reaction paths must be considered (i) 2,1-addition/elimina-tion with formation of (l-amino)alkynylcarbene complexes (= 2-amino-l-metalla-l-en-3-ynes) 98 (ii) 4,1-addition to give [(2-amino)alkenyl]carbene complexes (= 4-amino-l-metalla-l,3-butadienes) 96 and (iii) 4,1-addition/ elimination to (3-amino)allenylidene complexes (= 4-amino-l-metalla-1,2,3-butatrienes) 99 (Scheme 33, M = Cr, W). The product ratio 96 98 99 depends on the bulk of substituents R and R1, as well as on the reaction conditions. Addition of lithium amides instead of amines leads to predominant formation of allenylidene complexes 99.112 Furthermore, compounds 99 also can be generated by elimination of ethanol from complexes 96 with BF3 or AlEt3114 and A1C13,113 respectively. 

Silver carbene complexes are the most commonly used carbene transfer complexes [83], Other carbene transfer agents include lithium adducts [56], potassium complexes [53], molybdenum carbene complexes [83,84] or chromium carbene complexes [85],... 

Synthesis of amide functionalised NHC ligands is facile and follows a standard protocol of unsymmetrically substituted imidazolium salts (see Chapter 1). In the present case, Arnold et al. reacted iV-fcrt-butyl-imidazole with iV-tcrt-butyl-aminoethyl bromide hydro-bromide [34]. Subsequent reaction with potassium hydride yields the amino functionalised carbene, probably in the form of a hydrogen bond stabilised zwitterion (see Figure 4.28). Stepwise reactiou of the amino functionalised imidazolium salt hydrobromide first strips off the hydrobromide and then deprotonates the imidazolium ring (not the secondary amino group) to form a lithium chelate complex. 

The reaction of the amino imidazolium salt hydrobromide with silver(I) oxide yields the corresponding silver(I) carbene complex (see Figure 4.29) that has the same central structural unit as the lithium complex [35]. 

Reaction of the commercially available [TiCKOlVy with a lithium adduct of an amide functionalised carbene results in the substitution of an isopropyloxy substituent on titanium(lV) by the amide sidearm of the carbene. Simultaneous coordination of the carbene unit results in a five-coordinate chelate titanium(lV) complex [111] (see Figure 4.33). The choice of the lithium carbene adduct determines the number of OPi groups still present on the metal centre. 

---