Formyl complexes cations

All anionic transition metal formyl complexes described in the literature through the end of 1980 (21-47) are compiled in Table I. Since several of these have been prepared with more than one counterion, cations are not specified in the table. Geometric isomers are not assigned unless warranted by direct spectroscopic evidence. Also, the stability data in Table 1 should be regarded as qualitative, since decomposition rates have been shown to be dependent on both purity and counterion. When half-lives are specified, they are usually based upon measured rate constants. 

The sole example of a cationic formyl complex was reported recently by Thorn (67). It was obtained by the oxidative addition of formaldehyde to a coordinatively unsaturated iridium cation, as shown in Eq. (14). Characterization included IR and H, l3C, and 3IP NMR. Formyl 49 is stable as a solid to 146°C. 

As shown in Eqs. (17) and (18), the isolated formyls 19 and 24 are capable of reducing aldehydes and ketones (37, 38, 42. 47, 66). Thus there is no doubt that hydride transfer is an intrinsic chemical property of anionic formyl complexes. One reaction of a neutral formyl complex with an aldehyde has been reported addition of benzaldehyde to (i7-C5H5)Re(NO)(CO)(CHO) (38) yields the alkoxycarbonyl complex (i7-C5H5)Re(NO)(CO)(C02CH2C6Hs) (62). This transformation, which appears to require catalysis by adventitious acid, can be viewed as occurring via attack of initially formed benzyl alcohol upon the intermediate carbonyl cation [(i -C5H5)Re(NO)(CO)2]+. 

Formyl transfers" involving neutral formyl complexes, such as shown in Eq. (22), have recently been reported (68). By precipitation of the metal carbonyl cation by-product, quite pure solutions of kinetically labile neutral formyl complexes may be obtained. 

Reactions of neutral formyl complexes with alkylating agents can follow different courses. Roper has observed the O-methylation reaction shown in Eq. (23) (54). Cationic methoxymethylidene complex 53 was obtained in excellent yield. 

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

Ru( CHO)( CO)(dppe)][SbFg] have been used also " to identify more fully the radicals formed in the decomposition of cationic formyl complexes of ruthenium. The conclusion has been reached that a radical mechanism does not constitute major pathway for these decompositions ... 

Two neutral formyl complexes have been synthesized and isolated via equation (a), beginning with the cationic carbonyl compounds, CpRe(NO)(CO)L where L = CO and PPh3 . These compounds are intermediates in the BH4 reduction of coordinated... 

The rhenium formyl complex was independently synthesized by reaction of K HB(0-/-Pr)3" with Re2(CO)io in THF at 0°C, followed by aqueous basic workup and cation exchange with Et4N" Br". It was recrystallized from THF-hexane, and isolated in 32% yield as a yellow, air-stable solid. 

In compliance with this expectation, neutral formyl complexes have been synthetized ° by a reaction of Re carbonyl cations with hydrides. With very strong hydride donors, like trialkyl- or trialkoxy-borohydrides, anionic formyl complexes have been prepared even from neutral carbonyl compounds. 

The reaction of 2-formyl- and 2-acetylarylpalladium(n) bromide complexes 201 with internal alkynes and TIOTf affords indenols at room temperature, whereas cationic 2-formylarylpalladium(ll) pyridine complexes 202 do not react with alkynes at room temperature but afford indenones at 90 °C (Scheme 94). 

The Z-substituted benzene (benzaldehyde, Figure 11.2) is not activated toward electrophilic attack since the HOMO of benzene is scarcely affected. No preferred site for attack of the electrophile can be deduced from inspection of the HOMOs. The interaction diagram for a Z-substituted pentadienyl cation, substituted in the 1-, 2-, and 3-positions, as models of the transition states for the ortho, meta, and para channels are too complex to draw simple conclusions. The HOMO and LUMO of the three pentadienyl cations with a formyl substituent are shown in Figure 11.4. The stabilities of the transition states should be in the order of the Hiickel n energies. These are 6a — 9.204 / , 6a — 9.2031/ , and 6a -9.1291/ , respectively. Thus, by SHMO, the ortho and meta channels are favored over the para channel, with no distinction between the ortho and meta pathways. Experimentally, meta substitution products are usually the major ones, contrary to the SHMO predictions. Either the SHMO method fails in this case or the predominance of meta products may be attributed to steric effects. 

Other Formylations. Formyl fluoride, the only known stable formic acid derivative, can be used to perform Friedel-Crafts-type acylation to form aromatic aldehydes. The method was developed by Olah and Kuhn.105 Although a number of Lewis acids may be used, BF3 is the best catalyst. It is dissolved in the aromatic compound to be formylated then formyl fluoride is introduced at low temperature and the reaction mixture is allowed to warm up to room temperature. The aldehydes of benzene, methylbenzenes, and naphthalene were isolated in 56-78% yields. Selectivities are similar to those in the Gattermann synthesis ( toiuene benzene = 34.6, 53.2% para isomer). The reacting electrophile was suggested to be the activated HCOF BF3 complex and not the free formyl cation. Clearly there is close relationship with the discussed CO—HF—BF3 system. 

Certain unsaturated aldehydes may be converted to cyclic ketones by a related mechanism. The formyl group reacts with Rh(I) complexes to form an acyl-Rh hydride species, which undergoes intramolecular reaction with the olefinic linkage present in the same molecule (117a). Asymmetric induction is observed with a chiral diphosphine ligand (Scheme 53) (117b-d). Enantioselective cyclization of 4-substituted 4-pentanals into 3-substituted cyclopentanones in greater than 99% ee is achieved with a cationic BINAP-Rh complex. 

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