Insertion of CO into

To further illustrate these points dealing with orbital symmetry, consider the insertion of CO into H2 along a path which preserves C2v symmetry. As the insertion occurs, the degenerate n bonding orbitals of CO become hi and 62 orbitals. The antibonding n orbitals of CO also become hi and 62. The <5g bonding orbital of H2 becomes ai, and the antibonding H2 orbital becomes 62. The orbitals of the reactant... 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

Carbon monoxide insertion is not restricted to transition metal-carbon bonds, although M—C is by far the most common substrate involved. Reactions have also been reported which lead to insertion of CO into M—O (114) and M—N (199) bonds. 1,1-Additions of M—H (27, 114) and M—M (104) linkages to CO have been postulated, too. However, direct replacement of CO, without rupture of the W—H bond, is indicated for the reaction between CpW(CO)3H (or -D) and PPhj (5) ... 

Attempts at the carbonylation of CpCr(NO)2Me in hexane or THF at reflux resulted only in recovery of the nitrosylalkyl (105). A recent report of the reaction between CpCr(CO)3Me and L [L = PPhj, P(p-CjH40Me)3, and PMejPh] to yield trawi-CpCr(CO)2L(COMe) furnishes the first example of insertion of CO into Cr—C bonds (19b). 

Insertion of CO into Ti—C bonds has very recently been reported for reaction of CpiTifCHjPhlj with CO and for oxidative additions of Mel and EtI to Cp2Ti(CO)2. See refs. (89a) and (90a), respectively. 

Insertion of CO into Mo—C bonds has been postulated in the reaction... 

The carbonyl [CpFe(CO)2]2 has been successfully employed as a catalyst for hydroformylation of propylene (229) and for the reaction in Eq. (55) (221). Insertion of CO into Fe—C bonds is thought to occur therein. 

No insertion of CO into a Co—C bond has been reported, although possible equilibria and reactions involving CO at a pressure of 1 atm have been examined with DMG 163, 80), salen 44), and eorrinoid complexes... 

The first one, (A), includes (b) insertion of CO into the Pd-S bond (c) insertion of the C C triple bond of the enyne into the Pd-C(0)SR bond whereby Pd binds to the terminal carbon and the RSC(O) group to the internal carbon, and (d) C-H bond-forming reductive elimination or protolysis by the thiol to form 29 (Scheme 7-7). 

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

Starting from 63, the carbonylation may proceed via coordination and insertion of CO into the vinyl-C-Pd bond to provide an a,P-unsaturated acyl complex. This complex reacts with (ArY) 2, and subsequently the C-Y bond is formed by reductive elimination to give 64 (Scheme 7-14). Because the compound 64 could be directly converted into the corresponding enal 65 by the Pd-catalyzed reduction with BujSnH, this sequence is synthetically equivalent to the regio- and stereoselective thioformy-lation and selenoformylation of alkynes (Eq. 7.49) [53, 54]. 

An alternative proposal for the propagation of this reaction is shown in Scheme 7-24, which includes (a) insertion of CO into the Pd-S bond of 118 to provide 119, and (b) reaction of 119 with 115 to give 116 with regeneration of 118 through a o-metathesis-like transition state 120. According to this mechanism, the ArS group in 118 and the RiN group of 115 are incorporated simultaneously. 

When the rhodium-catalyzed reaction is performed under a high pressure of CO in the presence of phosphite ligands, aldehyde products (159) are formed by insertion of CO into the rhodium-alkyl bond followed by reductive elimination (Eq. 31) [90]. The bimetallic catalysts were immobilized as nanoparticles, giving the same products and functional group tolerance, with the advantage that the catalyst could be recovered and reused without loss of... 

The insertion of CO into an organic Pd species is a very common procedure, and may also form part of a domino process, for example, after a Heck reaction. 

A typical second step after the insertion of CO into aryl or alkenyl-Pd(II) compounds is the addition to alkenes [148]. However, allenes can also be used (as shown in the following examples) where a it-allyl-r 3-Pd-complex is formed as an intermediate which undergoes a nucleophilic substitution. Thus, Alper and coworkers [148], as well as Grigg and coworkers [149], described a Pd-catalyzed transformation of o-iodophenols and o-iodoanilines with allenes in the presence of CO. Reaction of 6/1-310 or 6/1-311 with 6/1-312 in the presence of Pd° under a CO atmosphere (1 atm) led to the chromanones 6/1-314 and quinolones 6/1-315, respectively, via the Jt-allyl-r 3-Pd-complex 6/1-313 (Scheme 6/1.82). The enones obtained can be transformed by a Michael addition with amines, followed by reduction to give y-amino alcohols. Quinolones and chromanones are of interest due to their pronounced biological activity as antibacterials [150], antifungals [151] and neurotrophic factors [152]. 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

All the above systems represent indirect routes to metal formyl species. At present, there is no proven example of the direct insertion of CO into a metal-hydride bond ... 

Insertion of CO into the metal-methyl bond of 1 followed by reduction and elimination of water would yield a metal ethyl species (3). This latter set of reactions represents, formally at least, a possible growth sequence for the Fischer-Tropsch synthesis. 

At the present, the most straightforward mechanism for the formation of J5 from 1 is via insertion of CO into the Th-C(acyl) bond to form a ketene (H, (eq. (4)) which subsequently dimerizes. Presumably, initial CO interaction could involve coordination either to the metal ion as shown or to the electrophilic vacant "carbene p atomic orbital. Considering the affinity of the Th(IV) ion for oxygenated ligands, interaction of the ketene oxygen atom with the metal ion seems reason-... 

To the extent that mechanistic similarities exist, it is of interest to examine several crucial transformations in catalytic CO reduction and to see whether the organoactinide carbonylation results contribute to a better understanding of what may be occurring. The insertion of CO into a surface metal-hydrogen bond to produce a formyl (eq.(18)) has been discussed at length... 

Subsequent insertion of CO into the newly formed alkyl-ruthenium moiety, C, to form Ru-acyl, D, is in agreement with our 13C tracer studies (e.g., Table III, eq. 3), while reductive elimination of propionyl iodide from D, accompanied by immediate hydrolysis of the acyl iodide (3,14) to propionic acid product, would complete the catalytic cycle and regenerate the original ruthenium carbonyl complex. 

Zirconacyclopentadiene shows a different reactivity towards CO as compared with zirco-nacyclopentane and zirconacyclopentene. Zirconacyclopentane and zirconacydopentene readily react with CO at low temperature to give cyclopentanone and cyclopentenone, respectively. The different reactivity of zirconacyclopentadienes can be explained by comparing the reactivity of the Zr—Csp2 bond with that of the Zr—Csp3 bond. Insertion of CO into the Zr—C bond proceeds readily at low temperature and therefore zirconacydopentane and zirconacyclopentene, which contain Zr—Csp3 bonds, react directly with CO as shown in Eq. 2.65 [45], Zirconacyclopentadienes, on the other hand, do not. 

Although this migratory insertion of CO into a carbon—zirconium bond accounts for the majority of acylzirconocene complexes that have been reported, the CO insertion... 

The sequential double migratory insertion of CO into acydic and cydic diorganozircono-cene complexes through acylzirconocene and ketone—zirconocene species provides a convenient procedure for preparing acyclic and cyclic ketones (Scheme 5.6) [8], Thus, the bi-cydic enones from enynes can be obtained through CO insertion into zirconacyclopen-tenes followed by a subsequent rearrangement (Scheme 5.7). The scope and limitations of this procedure have been described in detail elsewhere [8d]. This procedure provides a complementary version of the well-known Pauson Khand reaction [9]. 

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