Tungsten cyclopropanations with

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

In organic chemistry one surely thinks at once of the construction of cyclopropane derivatives from olefins and carbenes. Indeed, it has been shown that this also is possible with our complexes and with C=C double bonds that are electron-poor and arc either polarized or easily polarizable (77-81). As an example of this, I would like to cite the reaction of penta-carbonyl[methoxy (phenyl) carbene]chromium (0), -molybdenum (0), or -tungsten(0) with ethyl vinyl ether (79). One obtains the corresponding cyclopropane derivatives in this case, however, only when one removes... 

The reactions are stereospecific. The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), mo-lybdenum(O) or tungsten(O) with methyl ( )-but-2-enoate, diethyl (Z)-but-2-enedioate and ethoxyethene depends on the type of transition metal, which is consistent with a carbenoid transition state. 

Similarly, benzylidene(pentacarbonyl) complexes of chromium and tungsten react with cyclo-pentadiene, by stereoselective transfer of the benzylidene moiety to one of the C-C double bonds of the diene, to form coordinated e do-6-phenylbicyclo[3.1.0]hex-2-ene 9 with the metal-ligand unit complexing from the exo side. This method was applied to several other dienes. Reaction with cyclohexa-1,3-diene gave the corresponding cndo-7-phenylbicyclo[4.1.0]hept-2-ene complex. The cyclopropane-containing ligands obtained in this way were liberated from the complex to form the free hydrocarbon system. 

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-... 

Nickel(II) acetylaeetonate has been recommended as a very efficient homogeneous catalyst for intramolecular cyclopropanations for unsaturated diazoketones 171 The yields were better than with activated CuO as catalyst (see Table 10 for examples). The authors of this study seem to combine routinely thermocatalytic with photochemical (tungsten lamp) decomposition of the diazoketones. The benefit of this procedure (higher yields, shorter reaction times) has been communicated in the CuO case, but not for the Ni(acac)2-eatalyzed reaction. 

Strong evidence exists for the intermediacy of a tungsten ethoxycarbonyl carbene 425 in cyclopropanation of various enol ethers, 1,3-dienes and cyclohexene with ethyl diazoacetate in the presence of catalytic amounts of (CO)5W = C(OMe)Ph 413). The following equations could account for the obtained products ... 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

W(CO)6 in toluene at room temperature gave the tricyclic adduct 190a in 94% yield after acidic workup. This reaction forms the tungsten-containing azomethine ylide 191, which undergoes the [3 + 2]-cycloaddition with 189a. The rhodium(n)-catalyzed cyclization of the ene-yne-aldimine 192 with alkene 193 into the cyclopropane 194 was reported by Uemura and Ohe (Scheme 32).42c... 

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. 

Both W(CO)5[C(C6Hs)2] and the analogous di-p-tolylmethylene complex have been used in model studies of the olefin metathesis reaction.2 3 In contrast to heteroatom-stabilized carbene complexes such as W(CO)s [C(OCH3)(C6Hs)], pentacarbonyl(diphenylmethylene)tungsten(0) reacts with alkenes to give cyclopropanes and 1,1-diphenylalkenes.2 The compound W(CO)5 [C(C6H5)2] is the best reported catalyst for the metathetical polymerization of 1-methylcyclo-butene.4... 

The corresponding reaction of 2-methoxyfuran with chiral tungsten(O) alkenylcarbene complexes also proceeded via a similar Michael intermediates which collapsed to provide trisubstituted cyclopropanes, as shown below <07CEJ1326>. 

Several useful inter- and intramolecular reactions of chromium, molybdenum, tungsten, iron, or manganese Fischer carbenes with aUcenes to form cyclopropanated products have... 

Intramolecular cyclopropanations of pendant alkenes are more favorable. Heteroatom-substituted 2-aza- and 2-oxabicyclo[3.1.0]hexanes, together with 2-oxabicyclo[4.1.0] heptanes, can be prepared from chromium and tungsten Fischer carbenes having a tethered alkene chain. An interesting carbene formation via a cationic alkylidene intermediate, nucleophilic addition (see Nucleophilic Addition Rules for Predicting Direction), and intramolecular cyclopropanation is shown in Scheme 59. An intramolecular cyclopropanation via reaction of alkenyl Fischer carbene complex (28) andpropyne was used in a formal synthesis of carabrone (Scheme 60). 

Lyclooctyne (800 mg, 7.33 mmol) was added at rt to a solution of tetracarbonyl[(3,4-/j)-but-3-enyl(ethoxy)-carbene]tungsten(0) (1.0 g, 2.45 mmol) in hexane (30 mL). After 12h the solvent was evaporated, the residue taken up with light petroleum and adsorbed on a column of silica gel. After a further 12 h, eluting with light petroleum and finally light petroleum/EtjO 92 8 gave the cyclopropane yield 145 mg (31%). 

The phenylcyclopropanes are formed consistently as stereoisomeric mixtures, but the configuration of the predominant isomer depends on the alkene. For monosubstituted alkenes there is a progressive change of the cisjtrans ratio to lower values as the substituent becomes sterically larger. Thus, when the tungsten complex is allowed to react with propene, but-l-ene, 3-methyl-but-l-ene and 3,3-dimethylbut-l-ene, the ratio for the cyclopropane 3 drops from 1.8 to 0.01. ... 

When the donor character of the amino substituent at the transition-metal-carbene carbon atom is reduced, it should be possible to influence the thermal stability and reactivity in favor of the desired cyclopropanation process. Indeed, pyrrol-1 -ylcarbene complexes 18 of chromium, molybdenum and tungsten do exhibit the desired reactivity. In the last step, the pyrrole ring of 19 can be converted to the NH2 function in 20 after oxidative cleavage with ozone.In this respect, the pyrrole heterocycle represents a synthetic equivalent of the amino function. 

Pyrrol-1-ylcarbene complexes 18 effect the cyclopropanation of electron-deficient alkenes in good to very good yield. The C-H insertion products usually observed with aminocarbene complexes are only formed in low yields. Molybdenum and tungsten complexes are more favorable for cyelopropanation reactions than the analogous chromium complexes. The diaste-reoelectivity of the cycloaddition, however, was only moderate cisjtrans ratio of 1 1.6 to 3.4). 

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

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