Formyl complexes, synthesis

Unfortunately, formic acetic anhydride is not a general reagent for formyl complex synthesis (29). One reason is that formylation of a transition metal monoanion would afford a neutral formyl complex. Insofar as comparisons are valid, neutral formyl complexes tend to be kinet-ically less stable than anionic formyl complexes. In cases where neutral formyl complexes are stable (vide infra), the corresponding transition metal monoanions are unknown. Whereas formic acetic anyhydride might be of greater use for the preparation of anionic formyl complexes from transition metal dianions, only a limited number of transition metal dianions [i.e., (CO)5Cr2", (t7-C5H5)(CO)3V2 J are known (49). These appear to... 

The first synthesis of a formyl complex was described in 1973 (27, 28). In a landmark paper, Collman and Winter reported that the reaction of Na2Fe(CO)4 with formic acetic anhydride [Eq. (4)] afforded the anionic formyl Na+(CO)4Fe(CHOU (subsequently isolated as its [(CeHs PjiN or PPN+ salt) in good yield. Formic acetic anhydride is an excellent formylat-ing agent. This seemingly unusual reagent was selected because many HCOX species (X = Cl, 02CH) are unstable at room temperature (4 ). 

Reagents such as LiAlH4 and KH are not effective for the synthesis of formyl complexes. LiAlH4 does react with many metal carbonyl compounds, but it can transfer more than one and usually effects the formation of metal hydride products (50). Similar results are usually found with NaBH4(50), although some neutral formyl complexes (vide infra) can be obtained under special conditions. KH will also react with some metal carbonyls. However, rates are not very rapid, and any formyl intermediates are likely to decompose faster than they form (51). 

Metal formyl complexes have been proposed as important intermediates in the metal-catalyzed reduction of CO by H2 1,2, 3, 4). While the insertion of CO into alkyl and aryl carbon-metal bonds is well known (5), the insertion of CO into a metal-hydrogen bond to give a metal formyl complex has not been observed. (The intermediacy of metal formyl compounds in the substitution reactions of metal hydrides has been considered.) To ascertain the reasons for the failure to observe metal formyl complexes in the reactions of metal hydrides with CO, we have developed a new synthesis of metal formyl complexes and have studied their properties. 

We have developed a new synthesis of metal formyl compounds from the addition of metal trialkoxyborohydrides to metal carbonyls (10,11). The formyl proton characteristically appears at very low field, 14-16 8, in the NMR spectrum of metal formyl complexes. This low field resonance has allowed us to rapidly survey the reactions of trialkoxyborohydrides with a series of metal carbonyls. Initially, Na HB(OCH3)3 was used as the borohydride reducing agent, but we have subsequently found that K HB(0-i Pr)3" is a more rapid and eflFective hydride donor (JO). We have obtained NMR evidence for the formation of metal formyl complexes in the reactions of K HB(O-f-Pr)3" with Fe(CO)5 (14.9 8) (C6H50)3PFe(C0)4 (14.8 8, d, / = 44) (C6H5)3PFe(CO)4 (15.5 8, d, / = 24) Cr(CO)6 (15.2 8) W(CO)e 5.9 8) and Re2(CO)io (16.0 8). In some cases we have isolated the metal formyl complexes. In other cases, such a Cr(CO)6, the maximum observed conversion to (CO)5Cr-CHO was 76% after 25 min at room temperature, and the formyl complex underwent subsequent decomposition with a half-life of 40 min at room temperature. 

The versatility of CO as a synthon also stems from its ability to undergo insertion reactions into a variety of metal-heteroatom bonds. The migratory insertion of CO into transition metal-hydride bonds, while thermodynamically unfavorable, generates metal-formyl complexes M-C(0)H (Equation (19)), a few examples of which have been isolated independently. This reaction is assumed to be a key step in both the homogeneous and heterogeneous catalytic hydrogenation (i.e., reduction) of CO, including the Fischer-Tropsch synthesis of hydrocarbons and... 

The synthesis and further transformation of formyl complexes via hydrido-acyl intermediates into ti2-acetaldehyde complexes has been reported.S7 The molybdenum(II) carbonyl complex, [Mo(CO)3Me(Ti5-Cp)l, was found to react with Li[BHEt3l in THF at -66 °C to yield the formyl complex, [Mo(CHO)(CO)2Me(Ti5-Cp)]-. Slow warming allowed the spectroscopic observation of successive rearrangements of the formyl complex into hydrido-acyl and, finally, ti2-acetaldehyde derivatives. Scheme 10.9. 

In many cases, the synthesis of formyl complexes is performed in tetrahydrofuran using strong monohydride donors, typically alkali metal trialkylborohydrides. Although LiLBHEtsl is commonly employed in preparative coordination chemistry, it is not always possible to convert carbonyl complexes into formyl complexes with this reagent. In such cases a stronger hydride donor, such as K[BH(Bu-sec)3l, is required. 9... 

Gladysz has noted 3 that K[BH(OPr-/)3] is a valuable reagent for the synthesis of metal formyl complexes. Potassium hydride O is not so useful in such syntheses reaction rates are low, and any formyl intermediates are likely to decompose as fast as they form. Lithium aluminum hydride may function as a donor of more than one hydride and so reactions with this reagent may result in the formation of metal hydride products.63... 

Ozonization of A -steroids usually gives complex mixtures (however, see ref. 48). Ozonolysis became a practical step in the general synthesis of B-norsteroids with the discovery that added methanol" (or formaldehyde ) improves yields significantly. Thus, Tanabe and Morisawa prepared 5/ -hydroxy-6/ -formyl-B-norsteroids (74) from cholesterol acetate, dehydroepiandrosterone acetate and pregnenolone acetate in overall yields of 64-74% by the reaction sequence represented below. 

The Madelung indole synthesis has been employed in the preparation of some complex indole systems. Uhle et al. reported the conversion of N-formyl-5,6,7,8-tetrahydronaphthylamine 28 into 1,3,4,5-tetrahydrobenz[c,ti]indole 29 with t-BuOK in 11% yield in regard to synthesis of ergot alkaloids. ... 

Enantiomerically enriched formyl irimclhyleiiemethane -irontricarbonyl complexes add to allylzinc bromides with high stereoselectivity. This was used, after photochemical deprotection, in an enantioselcctive synthesis of the insect pheromone (-)-ipsdienol4s. 

Complex (51b) was prepared according to Equation (11). The dangling formyl groups can be used for further derivatization (e.g., (517) shown in Figure 16), and (516) serves as a valuable starting material for the synthesis of macrocyclic dinuclear species (compare Section 6.3.4.12).1367... 

Transition metal complexes have been used in a number of reactions leading to the direct synthesis of pyridine derivatives from acyclic compounds and from other heterocycles. It is pertinent also to describe two methods that have been employed to prepare difficultly accessible 3-alkyl-, 3-formyl-, and 3-acylpyridines. By elaborating on reported194,195 procedures used in aromatic reactions, it is possible to convert 3-bromopyridines to products containing a 3-oxoalkyl function196 (Scheme 129). A minor problem in this simple catalytic process is caused by the formation in some cases of 2-substituted pyridines but this is minimized by using dimethyl-formamide as the solvent.196... 

The synthesis of the purine ring is more complex. The only major component is glycine, which donates C-4 and C-5, as well as N-7. All of the other atoms in the ring are incorporated individually. C-6 comes from HCOa . Amide groups from glutamine provide the atoms N-3 and N-9. The amino group donor for the inclusion of N-1 is aspartate, which is converted into fumarate in the process, in the same way as in the urea cycle (see p. 182). Finally, the carbon atoms C-2 and C-8 are derived from formyl groups in N °-formyl-tetrahydrofolate (see p. 108). 

The approach to polyketide synthesis described in Scheme 5.2 requires the relatively nontrivial synthesis of acid-sensitive enol acetals 1. An alternative can be envisioned wherein hemiacetals derived from homoallylic alcohols and aldehydes undergo dia-stereoselective oxymercuration. Transmetallation to rhodium could then intercept the hydroformylation pathway and lead to formylation to produce aldehydes 2. This proposal has been reduced to practice as shown in Scheme 5.6. For example, Yb(OTf)3-cata-lyzed oxymercuration of the illustrated homoallyhc alcohol provided organomercurial 14 [6]. Rhodium(l)-catalyzed hydroformylation of 14 proved successful, giving aldehyde 15, but was highly dependent on the use of exactly 0.5 equiv of DABCO as an additive [7]. Several other amines and diamines were examined with variation of the stoichiometry and none proved nearly as effective in promoting the reaction. This remarkable effect has been ascribed to the facilitation of transmetallation by formation of a 2 1 R-HgCl DABCO complex and the unique properties of DABCO when both amines are complexed/protonated. 

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