Cyclopropanes from metallacyclobutanes

Table 8 Reductive formation of cyclopropanes from metallacyclobutane complexes...

Decomplexation of cyclopropanes from metallacyclobutanes is facilitated thermally or photo-chemically as well as by numerous reagents such as phosphanes, alkenes, cyanides, or oxidants [Ij, Oj, Ce(IV), etc.]. Depending on the method used, striking differences in product selectivities are observed, even with respect to stereochemical results. Thus, decomplexation of trans-1,2-... 

Calculations [28] on the formation of cyclopropanes from electrophilic Fischer-type carbene complexes and alkenes suggest that this reaction does not generally proceed via metallacyclobutane intermediates. The least-energy pathway for this process starts with electrophilic addition of the carbene carbon atom to the alkene (Figure 1.9). Ring closure occurs by electrophilic attack of the second carbon atom... 

Besides rearrangements, ligand exchange, formation of alkanes, alkenes and other products, release of cyclopropanes is one of the most important reactions of metallacyclobutanes. Of course, this latter reaction is only useful in cyclopropane synthesis if the product is not identical with the starting material used to form the metallacyclobutane. Nevertheless, the discovery of a complex formed from hexachloroplatinic acid and cyclopropane and later structural elucidations have initiated intensive investigations on the conversion of cyclopropanes to metallacyclobutanes and release of cyclopropanes from the latter. These results have been thoroughly discussed in several reviews. " Therefore in this section only some general aspects of cyclopropane formation from metallacyclobutanes and selected synthetically useful methods are discussed. 

Complex formation of cyclopropane with hexachloroplatinate initiated extensive investigations on transition-metal coordination chemistry of cyclopropanes. From normal cyclopropanes, metallacyclobutanes are usually obtained, the formation and chemistry of which have been thoroughly discussed and compiled in several reviews. ... 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

The stereoselectivities in this reaction are governed by steric interactions in the formation of metallacyclobutane 60 (35). Of two possible intermediates (Fig. 5), 61 suffers from steric interactions between the ligand and the ester functionality. Avoidance of these interactions and minimization of 1,2-interaction in the metallacyclobutane leads to the formation of the observed major enantiomer and dias-tereomer (trans). The model suggests that increased diastereoselectivity should be observed with increasing steric bulk of the diazoester, a relationship that has already been established as discussed (cf. Eqs. 24 and 26). It is interesting to note that this model loosely corresponds to the stereochemical model proposed by Aratani for the Sumitomo cyclopropanation with one important difference the Aratani model is based on a tetrahedral metal while the Evans-Woerpel model is predicated on square-planar copper. Applying the Aratani model to the Evans ligand would predict formation of the opposite enantiomer as the major product (35). 

As illustrated in Scheme 93, a metallacyclobutane formed from a Ti-carbene and an olefin oxidatively decomposes into cyclopropanes 224). 

In any chain reaction, apart from initiation steps, the termination steps are also important. In metathesis there are many possibilities for termination reactions. Besides the reverse of the initiation step, the reaction between two carbene species is also a possibility (eq. (17)). The observation that, when using the Me4SnAVCl6 system, as well as methane traces of ethylene are also observed [26] is in agreement with this reaction. Further reactions which lead to loss of catalytic activity are (1) the destruction of the metallacyclobutane intermediate resulting in the formation of cyclopropanes or alkenes, and (2) the reaction of the metallacycle or metal carbene with impurities in the system or with the functional group in the case of a functionally substituted alkene (e. g., Wittig-type reactions of the metal carbene with carbonyl groups). 

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

Metallacyclobutanes formed from cyclopropanes upon reductive elimination usually regenerate the starting material. Thus, reactions of this type are not useful in synthesis of new cyclopropanes. In some selected cases, however, strained hydrocarbons with cyclopropane subunits yield metallacyclobutanes which upon decomplexation give cyclopropanes not identical with the starting material. These conversions, however, are hard to generalize and often lead to unselective product formation. 

Nucleophilic attack of the central carbon of allyl ligands represents an important access to metallacyclobutanes independent from cyclopropanes as precursors. This reaction pathway competes with the more common attack at one of the terminal allylic carbons. However, several examples of metallacyclobutane formation followed by release of cyclopropane products have been reported, especially with allyl complexes of palladium, platinum, and iridium as stable precursors or reactive intermediates in catalytic cycles. Detailed experimental (see below) and theoretical studies considering chemo-, regio- and stereoselectivity of the crucial reaction steps with respect to influence of the metal, ligands, substituents and reaction conditions are available. ... 

Some rare cases of cyclopropane formation from metallacycles other than metallacyclobutanes have been reported. Thus thermolysis of a rhenacyclopentane 3, obtained from CpRe(CO)2H2 (1) and 1,4-diiodobutane (2), yields methylcyclopropane (4) in near quantitative yield. Based... 

In the two separate, initial reports on the reactivity of Fischer carbenes with enynes, one study found cyclobutanone and furan products [59], while the other found products due to olefin metathesis [60]. These products have turned out to be the exceptions rather than the rule, as enynes have since been found to react with Fischer carbenes to produce bicyclic cyclopropanes quite generally. The proposed mechanistic pathway is included as part of Bq. (28), in which vinylcarbene 10, produced by insertion of the alkyne into the metal carbene, may then cyclize with the pendant olefin to metallacyclobutane 11, leading to product. The first reported version of this reaction suffered from extreme sensitivity to olefin substitution [Eq. (28) compare R=H, Me] often producing side-products due to metathesis (through 11 to yield dienes) and CO insertion (into 10 to yield cyclobutanones and furans) [61]. Since then, several important modifications have been developed which improve yield, provide greater tolerance for alkene substitution, and increase chemoselectivity for the bicyclic cyclopropane... 

Several platinacyclobutane complexes have been isolated from the reaction of platinum-phosphine complexes with cyclopropanes. Those structurally characterized are listed in Table 8. No other structures of metallacyclobutane-phosphine complexes have been reported, but an... 

Reductive elimination from the metallacyclobutane can give the cyclopropane and liberate the metal that can in turn enter catalytic cycles (see Chap. 15.2). 

If the metal already has 18 valence electrons, which is the case for all the Fischer carbenes, the electrophilic carbene carbon is attacked by the olefin to develop a zwitterionic intermediate before ring closure.From the 18-electron metallacyclobutane, p-elimination, that requires a free coordination site, cannot occur. Among the above reactions, only the metathesis and reductive elimination of the metallacyclobutane to cyclopropane ean be observed, as in the two following examples ... 

The orbitals of cyclopropane C-C bonds form banana bonds , which protrude away from the bond axis between the two carbon atoms (Figure 1.2). Consequently a metal center can interact with them similarly, to some extent, to the case of a metal-olefin interaction. This interaction lowers the kinetic barrier of the C-C oxidative addition. In addition, the enlargement of the three-membered cyclopropane ring to a four-membered metallacyclobutane relieves the structural strain owing to its constrained bond angles. Thus, the use of cyclopropanes as substrates for oxidative addition of C - C bonds is advantageous both kinetically and thermodynamically. 

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