Carbene complex chromium anionic

Addition of halogens and pseudohalogens to the cyclopropylthiocarbene chromium complexes 122 affords the 1,4-dihalo-1-phenylthio-l-alkenes 123 stereoselectively [65]. Electrophilic halogen is likely to activate the carbene complexes, followed by the homo-Michael addition of halide anion. (Scheme 44)... 

Aldol reactions.1 The anion generated (BuLi) from chromium carbene complexes undergoes aldol reactions with aldehydes or ketones activated by a Lewis acid. Best results are obtained with ketones in the presence of BF3 etherate, whereas TiCl4 is the preferred catalyst for aldehydes and acetals. 

Alkylation,4 Alkylation of anions of the usual alkoxy carbene complexes is not generally attractive because of low reactivity. However, the anion of dialkylamino chromium carbenes such as 1 (R = H) can be alkylated readily and in a useful yield. 

Narasaka and Sakurai found that chromium carbene complexes, when exposed to a copper(II) reagent, generate acyl radicals by a one-electron oxidation, and these then undergo addition to electron-deficient alkenes (Scheme 4-27) [50J. The resulting copper(I) species reduces the resulting radical to an anion, and subsequent protonation leads to the addition product. This redox type acyl radical transfer reaction works particularly well for aromatic acyl radical systems, for which decarbonylation is not a problem. Related work has also recently appeared [51]. 

Addition of ArLi followed by Et30 to Cr(CO)4(nbd) results in the carbene complexes Cr(CO)3 =C(OEt)Ar nbd. The corresponding carbenes are also formed from W(CO)4(cod).306 it is possible to protect carbene complexes towards nucleophilic attack by the temporary formation of anions either at the position a to the carbene or at the oxygen substituent. Chiral (through deuterium incorporation) glycine is available via oxazolidinone chromium carbene complexes.398... 

Chromium or tungsten hexacarbonyl react with the dilithiated dianimis, LiNRCH2C(R )CHLi, to give lithiated butyrolactams. Initial attack of the dianion on a coordinated carbonyl results in an anionic carbene complex which undergoes an intramolecular cyclisation to the product. ... 

The reaction of the iron-carbene complex [[Bu C CC(OEt)=)Fe(CO)4] with 1,3-cyclohexadiene afforded223 (32), which was characterised by X-ray diffraction. The addition of the anionic derivatives of i-(diphenylmethane)-il-fluorene- and ii-(9,10-dihydroanthracene)-bis tricarbonylchromium to the hydrocarbon moiety (alkene, benzene, cyclohexadienyl, cycloheptadienyl and cycloheptatrienyl) in various cationic compounds of manganese, rhenium, iron, chromium, molybdenum, tungsten and cobalt was reported224 to provide a new route to hydrocarbon-bridged heteronuclear species such as (33). 

The single electron transfer (SET) reduction of Fischer alkynyl carbene complexes 95 with potassium graphite (CgK) results in the formation of cyclopentadienes 100 (Scheme 5.20) [31], The radical anion intermediates 96, generated by the SET reduction of Fischer alkynyl carbene complexes 95, undergo tail-to-tail dimerization to form bis-carbene anions 97. The intermediates 97 are protonated with a strong acid to produce the intermediates 98, which convert in situ into chromium cyclopentadienylcarbene 99. After the demetallation of the cyclopentadienylcarbene 99, cyclopentadienes 100 are obtained. 

The anionic complexes (50) are obtained from the group VI carbonyl and the dilithium derivatives of the hydroxyalkynes HC=CCR2(OH) subsequent protonation or acylation affords propadienylidene (51) or carbene (52) complexes by complex cyclization and addition reactions (Scheme 3) (28). Attempts to obtain the dimethyl complex by reaction of the chromium alkynolate dianion with COCl2 gave only polymeric material. More stable complexes were obtained with R = aryl (73). 

The reaction of the chromium and tungsten carbene anions [(CO)j-M=C(OMe)CH2 ] (M = W, Cr) with coordinated alkenes has been studied by Beck and co-workers (195,196), as well as by Geoffroy and coworkers (197). The reaction of the carbene anions with cationic complexes having a coordinated ethylene ligand leads to the formation of... 

Alternatively, demetallation to give enol ethers 839 can be achieved by treatment with pyridine (Scheme 8.14). Under these conditions, as pyridine is a weak base, an equilibrium is established between the carbene 8.35 and its anion 837. The anion, however, can also reprotonate on chromium to give a chromium hydride 838. This is followed by reductive elimination. The enol ether 839 is obtained as its Z-isomer, a consequence of the carbene anion having S-geometry to keep the alkyl group away from the bulky Cr(CO)s moiety the chromium is converted into a pyridine complex. 

Alternatively, the arene can be left complexed so that the chemistry of the Ti -arene complex can be exploited (see Section 10.3). " The Dotz reaction between the cyclohexenyl carbene 8.52 and the benzylic alkyne 8.53 in the presence of a silylating agent gave the ti -complex 8.54 (Scheme 8.18). Treatment of this complex with LDA resulted in formation of the anion a- to sulfur 8.55 that cyclized by nucleophilic attack onto the chromium-complexed aromatic ring. Oxidative work-up then gave the tetracycle 8.57. 

Metal carbene centers may be coimected via either the carbon or the heteroatom carbene chain [47]. The first example of a homodimetallic biscarbene complex has been synthesized by trapping a metal carbene anion - generated in situ by a-de-protonation of chromium oxacyclopentylidene 73 - by its exo-methylene congener 74 formed upon the addition of the same chromium carbene anion to formaldehyde in a Michael-type addition reaction [48]. A more satisfactory yield of bischro-mium biscarbene 75 has been obtained by using the formaldehyde equivalent CICH2OCH3 as source of the Cj-bridge and Lil (Scheme 11.23). 

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