Molybdenum cyclopropanations with

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

SCHEME 2. Generation of molybdenum carbenes by the reaction of cyclopropane with Mo(TV)/SiC)2... 

The relative yields of 93, 94 and 95 in the molybdenum-catalyzed reactions turned out to be exceptionally sensitive towards catalyst concentration, with different characteristics for different reaction partners. For example, the following yields of 93, 94 and 95 b were obtained when oo-diazoacetophenone reacted with acrylonitrile in the presence of different amounts of Mo(CO)6 46, 2, 50% (0.2 mol-% catalyst) 68, 3, 28% (1 mol-%) 83, 4, 0% (15 mol-%). In contrast, the yield of cyclopropane... 

Based on a detailed investigation, it was concluded that the exceptional ability of the molybdenum compounds to promote cyclopropanation of electron-poor alkenes is not caused by intermediate nucleophilic metal carbenes, as one might assume at first glance. Rather, they seem to interfere with the reaction sequence of the uncatalyzed formation of 2-pyrazolines (Scheme 18) by preventing the 1-pyrazoline - 2-pyrazoline tautomerization from occurring. Thereby, the 1-pyrazoline has the opportunity to decompose purely thermally to cyclopropanes and formal vinylic C—H insertion products. This assumption is supported by the following facts a) Neither Mo(CO)6 nor Mo2(OAc)4 influence the rate of [3 + 2] cycloaddition of the diazocarbonyl compound to the alkene. b) Decomposition of ethyl diazoacetate is only weakly accelerated by the molybdenum compounds, c) The latter do not affect the decomposition rate of and product distribution from independently synthesized, representative 1-pyrazolines, and 2-pyrazolines are not at all decomposed in their presence at the given reaction temperature. 

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... 

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... 

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

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. ... 

Vinylcyclopropane complexes can be obtained by [2-F1] cycloaddition of transition-metal car-bene complexes to dienes. Thus, formation of a complexed bicyclo[3.1.0]hex-2-ene system 7 was achieved by [2-Fl] cycloaddition of pentacarbonyl(4-methoxybenzylidene)molybdenum to cyclopenta-1,3-diene. The complexed cyclopropanation product 7 upon treatment with oxygen gave end(7-6-(4-methoxyphenyl)bicyclo[3.1.0]hex-2-ene (8) in low yield. 

The reactions are stereospecific.35 The ratio of stereospecific cyclopropanes formed in the reactions of pentacarbonyl[phenyl(methoxy)carbene] complexes of chromium(O), molybdenum ) 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.31... 

Pentacarbonyl[butyl(methoxy)carbene]molybdenum(0) reacts with electrophilic alkenes under mild conditions (65 UC, 1 h, THF) to give a mixture of isomeric cyclopropanes 5 in satisfactory yields.43 The isomers with methoxy and tra/is-located electron-withdrawing substituent predominate. 

Cyclopropanation Chromium and molybdenum carbcne complexes can react with unactivated 1,3-dicncs as well as electron-rich or -poor dienes (14,91) to form vinylcyclopropancs with high regio- and diastercosclectivity. In fact, reactions with alkylmcthoxycarbenes result in a single diastereomcr. In addition, cyclopropanation occurs selectively with the least hindered double bond of the diene and more readily with 1,3-dicnes that can adopt the s-cis conformation. 

Cyclopropanation. These carbencs are particularly useful for cyclopropanation of electron-poor olefins. The reaction occurs under milder conditions and at a faster rate with molybdenum carbencs than with chromium- or tungsten-derived complexes. This cyclopropanation has been used to trap a molybdenum vinylearbene generated by intra-... 

If the photoreduced Mo03/Si02 catalyst is treated with cyclopropane (Elev 1989a), methylcyclopropane (Vikulov 1991) or cycloheptatriene (Vikulov 1992b), followed by evacuation at 300-350°C, a much more active metathesis catalyst is obtained. Molybdenum carbenes are formed, as shown by IR and UV/vis spectroscopy (Vikulov 1989a,b), and are assumed to result from a sequence of reactions such as those shown for cyclopropane in Scheme 2.4 (Shelimov 1988 ... 

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