Tungsten, carbene complex

Complex 58, used in excess (3 equiv.), transforms benzaldehyde (91%), p-methoxybenzaldehyde (89%), and cyclohexanone (93%) into terminal olefins in high yields. Unlike molybdenum complexes, the reactivity of 58 is dramatically reduced when olefinations are carried out in aqueous or ethanolic solvents. 

Alkylidene-tungsten complexes, W(=CHBu )(OCH2Bu )2X2 (X = halogen) 60 

The alkylidene complex 61 readily converts a wide variety of carbonyl compounds into the corresponding olefins in good yields, with the following order of reactivity aldehydes ketones formates esters amides (Table 4.20) [134]. Replacement of two neopentoxo groups by chloro, bromo, or iodo ligands reduces 

Besides the carbene complexes, the tungsten alkyUdynes 62 [135] are also employed for carbonyl olefination. The complexes 62 react with carbonyl compounds to give 0x0 vinyl complexes 63, which are hydrolyzed with 1 N NaOH to form the 

Entry Carbonyl compound Tungsten complex Product Yield 

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Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Reaction of unsaturated chromium and tungsten carbene complexes 407 with piperidazine provides amino-carbene complexes 408 and 409. Although various solvents such as CH2C12 benzene, ether, and THF can be used in the reaction, the yields of the desired products 410 are 32-59%, and considerable amounts of by-products 409 are formed. The carbene complexes 408 are rather stable and can be stored in a refrigerator. Oxidation of both complexes with iodoso-benzene affords oxo derivative 410 in 70% (M = Cr) and 41% (M =W) yields (Scheme 64) <1994CL777>. 

Reaction of a tungsten carbene complex with alkynyllithium followed by treatment of aldehyde in the presence of Et3Al afforded trisubstituted furans in good to excellent yields <06AG(I)6874>. Dienes were the products without Ft,AI. 

Conjugate hydride abstractions have also been used for the generation of carbon-metal double bonds. An interesting reaction sequence, in which a (thermally unstable) cationic, non-heteroatom-substituted tungsten carbene complex is prepared by conjugate hydride abstraction, is shown in Figure 3.9. 

Fig. 3.9. Preparation of a cationic, non-heteroatom-substituted tungsten carbene complex by conjugate hydride abstraction [435].

Table 3.1. Cyclopropanation with stoichiometric amounts of chromium, molybdenum and tungsten carbene complexes.

Similarly, neither zirconium, tantalum, molybdenum, nor tungsten carbene complexes have been applied extensively by organic chemists for carbonyl olefination [609,727-729], probably because of the difficulty of their preparation and the high price of some of these compounds. These reagents can, however, have appealing chemo- and stereo-selectivity (Table 3.11). 

When alkynes are treated with catalytic amounts of a carbene complex, polymerization instead of metathesis can occur (Figure 3.44) [565,595,597,752-754]. The use of carbene complexes to catalyze alkyne polymerization enables much better control of the reaction than with heterogeneous or multi-component catalysts. Pure acetylene oligomers (n = 3-9) with terminal fcrf-butyl groups have been prepared with the aid of a tungsten carbene complex [755]. 

Reactions of Cjq with metal carbene complexes also yield the [6,6] methano-fullerenes [392]. These adducts are probably not formed via a carbene addition, but via a formal [2-1-2] cycloaddition under formation of a metalla cyclobutane intermediate. The Fischer carbene complex [mefhyl(methoxymethylene)]pentacarbonyl chromium can be utilized to prepare l,2-mefhyl(methoxymethano)-fullerene in 20% yield [392]. A tungsten carbene complex was primarily used to initiate the formation of a polyacetylene polymer, but it was discovered that addition of to the complex-polymer-mixture improves the polymerization and dramatically increases the catalytic activity of the carbene complex [393]. can be integrated into the polymer via carbene addition. 

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... 

In 1994, Quayle et al. reported the application of this cyclic Fischer-carbene synthesis from 3-butynols to spirolactone synthesis, although the process was stepwise and a stoichiometric amount of the complex was employed [17]. The key transformation was the chromium or tungsten carbene complex formation followed by the CAN oxidation of the complex to give y-lactone. The reaction was further applied to the synthesis of andirolactone and muricatacin, the former being shown in Scheme 5.14. 

The first enyne metathesis was reported by who used a Fischer tungsten-carbene complex. 

However, the reaction was shown to be catalyzed by a methylidene tungsten-carbene complex rather than the Fischer tungsten carbene complex. They proposed that the reaction would proceed by [2 + 2] cycloaddition of the tungsten carbene complex with the alkyne in Equation (3), ring opening, and another [2 + 2] cycloaddition with the alkene moiety to finally give the cyclized product. 

Paracyclophane-1,9-diene is an interesting cyclic diene monomer with a very strained ring. It has been easily polymerised with a tungsten carbene complex [148] to low molecular weight poly(p-phenylene vinylene) [148] ... 

Another attractive route to the synthesis of highly reactive tungsten carbene complexes involves alkylidene transfer from phosphoranes. Arylimido tungsten... 

A variety of isolated pentacoordinate tungsten-carbene complexes are known to be active metathesis catalysts [53]. At least one of these systems has been proposed to be living based primarily on 1H NMR identification of the propagating alkylidene [53 e]. To date, verification of the living nature of these catalysts through GPC determination of polydispersities are still pending. The solution NMR studies do confirm the mechanism of the metathesis reaction, but do not insure that all of the requisite factors for a living system are met. 

More than half a century ago it was observed that Re207 and Mo or W carbonyls immobilized on alumina or silica could catalyze the metathesis of propylene into ethylene and 2-butene, an equilibrium reaction. The reaction can be driven either way and it is 100% atom efficient. The introduction of metathesis-based industrial processes was considerably faster than the elucidation of the mechanistic fundamentals [103, 104]. Indeed the first process, the Phillips triolefin process (Scheme 5.55) that was used to convert excess propylene into ethylene and 2-butene, was shut down in 1972, one year after Chauvin proposed the mechanism (Scheme 5.54) that earned him the Nobel prize [105]. Starting with a metal carbene species as active catalyst a metallocyclobutane has to be formed. The Fischer-type metal carbenes known at the time did not catalyze the metathesis reaction but further evidence supporting the Chauvin mechanism was published. Once the Schrock-type metal carbenes became known this changed. In 1980 Schrock and coworkers reported tungsten carbene complexes... 

Trimethylsilyloxyfuran reacted stereoselectively with chiral tungsten carbene complexes in a Mukaiyama-Michael addition fashion to provide -products, as shown in Equation (18) <2005AGE6583>. The metal carbene in the butenolide product serves as a useful functional group for further transformations. 

Chromium and tungsten carbene complexes containing an alkynyl or alkenyl substituent afford moderate to high yields of cyclobutene or cyclobutane complexes respectively via formal [2 - - 2]cycloaddition with... 

Tungsten carbene complexes contain W=C linkages, and if they also contain alkyl substituents on the carbene carbon,... 

In addition to cationic cyclizations, other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol. For example, treatment of (259) and (260) with Mn(III) and Cu(II) afford the D-homo-5a-androstane-3-ones (261) and (262), respectively, in approximately 30% yield. In this cyclization, seven asymmetric centers are established in one chemical step (226,227). Another intramolecular cyclization reaction of iodo-ene poly-ynes was reported using a carbopaUadation cascade terminated by carbonylation. This carbometalation—carbonylation cascade using CO at 111 kPa (1.1 atm) at 70°C converted an acycHc iodo—tetra-yne (263) to a D-homo-steroid nucleus (264) [162878-44-6] in approximately 80% yield in one chemical step (228). Intramolecular aimulations between two alkynes and a chromium or tungsten carbene complex have been examined for the formation of a variety of different fiised-ring systems. A tandem Diels-Alder—two-alkyne annulation of a triynylcarbene complex demonstrated the feasibiHty of this strategy for the synthesis of steroid nuclei. Complex (265) was prepared in two steps from commercially available materials. Treatment of (265) with Danishefsky s diene in CH CN at room temperature under an atmosphere of carbon monoxide (101.3 kPa = 1 atm), followed by heating the reaction mixture to 110°C, provided (266) in 62% yield (TBS = tert — butyldimethylsilyl). In a second experiment, a sequential Diels-Alder—two-alkyne annulation of triynylcarbene complex (267) afforded a nonaromatic steroid nucleus (269) in approximately 50% overall yield from the acycHc precursors (229). 

While unsubstituted aryl complexes of chromium react with alkynes to give six-membered ring products, the tungsten carbene complex (lb) was reported to react with diphenylacetylene to give the indene... 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Photolysis or thermolysis of the tungsten carbene complex (204) generated the aminocyclopropane (205) (equation 49). Triphenylphosphine proved to be an efficient catalyst for the decomposition of 204. ... 

The anion 117 prepared by deprotonation of the carbene complex [Cr(CO)s C(OEt)CH2CHS(CH2)3S ] (116) also rearranges spontaneously by carbonyl insertion to give a neutral six-membered chelate (118) after alkylation. The analogous tungsten carbene complex affords stereoselec-tively a vinylcarbene complex (119) by opening of the 1,3-dithiane ring (705). Reaction of 118 with methyl hydrazine produces mainly an NMe- or... 

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