Promoted CO insertion

Merlic et al. were the first to predict that exposing a dienylcarbene complex 126 to photolysis would lead to an ort/zo-substituted phenolic product 129 [74a]. This photochemical benzannulation reaction, which provides products complementary to the classical para-substituted phenol as benzannulation product, can be applied to (alkoxy- and aminocarbene)pentacarbonyl complexes [74]. A mechanism proposed for this photochemical reaction is shown in Scheme 54. Photo activation promotes CO insertion resulting in the chromium ketene in-... 

The simple second-order kinetics of equation 5a apply for Mn(CO)5(0113) when L=CO at subatraospheric pressures. It is under this set of conditions that we have studied the Lewis acid promoted CO insertion reaction, see Figure 2. 

Prior to our studies it was recognized that ion pairing with anionic metal carbonyls could promote CO insertion and related reactions (14-16). Both kinetic and non-kinetic evidence suggests the importance of ion pairs in these types of reactions (14,17). For example, a small cation was found to greatly accelerate the CO insertion reaction relative to the same reaction with a large cation, equation 6 (14). 

Investigations in our laboratory by Rebecca Stimson have demonstrated that it is possible to combine the borane reduction of a metal acyl with the Lewis acid promoted CO insertion reaction which has been discussed earlier in this paper (29). In this reaction, which is presumed to proceed by equation 17, the... 

The Lewis acid was proposed to promote CO insertion into a metal-nitrogen bond in a postulated imido complex intermediate [185]. The role of the amine should be to moderate the activity of the acid. However, the data reported does... 

Our work on the bifunctional activation of CO insertion was prompted by the thought that strong molecular Lewis acids should be more effective and more general than simple cations. It already had been observed that molecular Lewis acids would promote a molecular Fischer-Tropsch type reaction (5), and that iron diene complexes can be converted to polycyclic ketones by the action of aluminum halides, equation 7,(18), but information on the course of these reactions was sketchy. 

The CO insertion process also can be promoted by proton acids (20). The only compound to be studied in detail is Mn(CO)5(CH3), for which very weak acids such as acetic acid bring... 

BH3 acts both as a reducing agent for the acyl carbonyl and as a promoting agent for subsequent CO insertion into the metal-alkyl bond. As yet the process has been carried as far as C Hg, with Mn(CO)5(CH3), CO, and I B THF as reactants. 

Two separate computational investigations of the Rh/dppms catalyst and related systems have appeared in the literature. One study [40] concluded that steric effects were important in promoting migratory CO insertion in [Rh(CO)(dppms)l2Me], while the other [41] proposed that an electronic effect, arising from the sulfur donor atom of dppms, was responsible. It is likely that a combination of steric and electronic effects result in the observed reactivity. 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

All these reactions are promoted by Pd(II) species, and can be stoichiometric (Eq. 10) or catalytic (Eqs. 11-13, in the presence of Cu(II) salts or other oxidizing agents). 3-Chloropropionyl chloride from ethylene is conceivably formed through PdCl2 addition to the double bond followed by CO insertion and reductive elimination (Scheme 2). 

PdCl2-promoted stoichiometric dichlorocarbonylation of acetylene (Eq. 20) is the first example of oxidative carbonylation of an alkyne that appeared in the literature [69,70], and presumably occurs through the mechanism shown in Scheme 13, involving addition of PdC to the triple bond followed by CO insertion, reductive elimination, oxidative addition to the C - Cl bond, further CO insertion and reductive elimination (Scheme 13, path a). 

The first step is oxidative addition to the Cl-09 bond to make a Pd % allyl complex. Both Cl and C3 are rendered reactive by this step. At this point, we can either make the C1-C10 bond by CO insertion, or we can make the C3-C7 bond by insertion of the C7=C8 n bond into the C3-Pd bond. The first alternative would be followed by displacement of Pd from CIO, requiring a new activation step to incorporate Pd into the substrate and allow the formation of the other bonds. After insertion of the C7=C8 K bond into the C3-Pd bond, though, we get a C8-Pd bond. This can insert CO to give the C8-C10 bond. The C1=C2 k bond can now insert into the ClO-Pd bond, giving a Cl-Pd bond. A second equivalent of CO then inserts. Finally, displacement of Pd from C10 by MeOH gives the product. The mechanism by which the Pd displacement proceeds is written as acid-promoted because the by-product of the reaction is AcOH. 

The acylation and subsequent "CO insertion steps may be catalysed by the promoter. Thus the formation of CH3OH (or CH3OAC) via a methoxide intermediate is suppressed. Finally, the CO-labilising ability of the acetate ion ( ) almost certainly plays an important role in these reactions. 

As reviewed by Ponec,18 the formation of alcohols is observed when a metal is promoted by a transition metal oxide. Kiennemann et al,19 has associated the presence of anion vacancies at the metal-support interface with the capability to dissociate CO and allow CO insertion to produce higher alcohols. This model can be used to explain our results on tungsten carbides. 

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