Other Metallacycles

Other metallacyclic carbene complexes result when 2-lithio-l,3-dithiane is added to Cr(CO)6 in ether at 0°C and the reaction mixture, after removal of the solvent in vacuo, is treated with [Et30][BF4] in CH2CI2 ... 

This product distribution has been the basis of the Phillips commercial 1-hexene process discovered in 1989 [674]. Clearly, there is another mechanism involved in addition to the traditional growth reaction. It has been suggested that a metallacyclic intermediate may be involved, as in Scheme 43. The key to the selective formation of 1-hexene lies in the relative stability toward intramolecular hydride transfer of the chromacyclopentane ring relative to that of the chromacycloheptane ring, consistent with the known behavior of other metallacycles [675-678]. 

In a following paper [110], some of the same authors reported the syntheses and reactivity of some other metallacyclic complexes related to the one previously analysed. However, these complexes do not appear to be involved in the main catalytic cycle and we will not further discussed them here. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

Similar metallacyclic species have been proposed as intermediates in other reactions of metal compounds with diazoalkanes (57,55). The presence of unfavorable steric interactions in an intermediate such as 53 could well explain the failure to observe any reaction of diaryldiazoalkane with 45 (55). 

In view of the extensive and fruitful results described above, redox reactions of small ring compounds provide a variety of versatile synthetic methods. In particular, transition metal-induced redox reactions play an important role in this area. Transition metal intermediates such as metallacycles, carbene complexes, 71-allyl complexes, transition metal enolates are involved, allowing further transformations, for example, insertion of olefins and carbon monoxide. Two-electron- and one-electron-mediated transformations are complementary to each other although the latter radical reactions have been less thoroughly investigated. 

A study of the reactions of butadiene, isoprene, or allene coordinated to nickel in a metallacycle, with carbonylic compounds, has been reported by Baker (example 11, Table IV). In the presence of phosphines, these metallacycles adopt a cr-allyl structure on one end and a ir-allyl structure on the other, as mentioned in Section II,A,1. The former is mainly attacked by aldehydes or electrophilic reagents in general, the latter by nucleophiles (C—H acids, see Table I, or amines, see Table IX). 

Carbonylation is an exceedingly broad subject, but the main reaction patterns can be easily rationalized by recalling the classification used earlier for coupling reactions involving (a) metallacycles (b) hydride-promoted reactions and (c) oxidative addition of organic halides to zero-valent nickel. In fact, one or other of these steps is necessary to form a species able to undergo carbonylation. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Using the unsymmetrically substituted acetylene Me3SiC=CPh, the kinetically favored substituted complex 8a is formed initially, cycloreversion of which gives the symmetrically substituted and thermodynamically more stable product 8b. Due to steric reasons, the other conceivable symmetric product 8c is not formed [9]. Such metallacycles are typically very stable compounds and are frequently used in organic synthesis, as shown by the detailed investigations of Negishi and Takahashi [lm], Bis(trimethylsilyl)acetylene complexes are a new and synthetically useful alternative. 

The distances and angles (70—74° at C-a and 147—150° at C-(3) in the metallacycle correspond closely to those calculated for organic cyclocumulenes such as cyclohexa- and cy-cloheptatrienes [23]. According to other theoretical calculations, titana- and zirconacyclocumulenes are thermodynamically more stable than the isomeric bis(o-acetylide) complexes [24], The calculated data are in good agreement with the obtained experimental values. All four carbon atoms of the former diyne are viewed as having p orbitals perpendicular to the plane of the cyclocumulene. The sp-hybridized internal C atoms possess additional p orbitals in that plane, which are used to establish a coordination of the relevant bond to the metal center. 

A proposed mechanism of this reaction was reported by Magnus and Principle [10], which is nowadays widely accepted (Scheme 1). Recently, negative-ion electrospray collision experiments have confirmed this mechanism in detail [11]. Starting with the formation of the alkyne-Co2(CO)6 complex 2, olefin 3 coordination and subsequent insertion takes place at the less hindered end of the alkyne. The in situ formed metallacycle 4 reacts rapidly under insertion of a CO ligand 5 and reductive elimination of 6 proceeds to liberate the desired cyclopentenone 7. It is important to note that all the bond-forming steps occur on only one cobalt atom. The other cobalt atom of the complex is presumed to act as an anchor which has additional electronic influences on the bond-forming metal atom via the existing metal-metal bond [12]. 

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

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