Carbene tungsten

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-... 

Scheme 8 Addition reactions with tungsten carbenes...

A combination of cyclizations and a cycloaddition was observed by Aumann and coworkers [308] by reaction of a cycloalkenyl-ethynyl tungsten-carbene 6/4-109 with an alcohol to give 6/4-112 (Scheme 6/4.27). The last step in the sequence is a jt-cyclization with insertion of CO of the intermediate cycloadduct 6/4-111 formed from 6/4-110 and 6/4-109. 

When the o-ethynylphenyl isopropyl ketone 6/4-113 was treated with 1 equiv. of W(CO)5-THF for 2 h in the presence of 4equiv. of 1,1-diethoxyethylene, the bridged compound 6/4-118 was obtained in 73% yield, and not the expected 3-ethoxy-l-isopropylnaphthalene (Scheme 6/4.28). Iwasawa and coworkers [309] proposed a mechanism for their new domino reaction, which includes a [3+2] cycloaddition followed by insertion of the resulting tungsten carbene moiety into the neighboring C-H bond, with 6/4-114-6/4-117 as possible intermediates. These authors have further shown that 10-20 mol% of W(CO) THF is sufficient, and... 

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. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

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. 

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