Formyl complexes protonation

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. 

Formyl complexes show varying behavior when treated with protonating agents. Reaction of (CO)4Fe(CHO)" (22) with acid gives formaldehyde in 13-20% yield (27-29). Homologous (CO)4Fe(COR) acyls afford excellent yields of aldehydes when protonated, presumably via... 

Two reports of H2 formation upon acidification of anionic formyls 6 (31) and 19 (38) could not be reproduced (32, 47). Thus there are no documented examples of H2 evolution upon protonation of anionic formyl complexes. It is clear, however, that rapid reactions ensue in all cases (32, 47, 66) and that good yields of neutral metal carbonyl (H loss) products are obtained. 

Other anionic formyl complexes decompose by more complex pathways. Unstable formyl 6 (Scheme 4) yielded approximately equimolar amounts of (CO)5Mn, (CO)5Mn(COC6H5), and (after protonation) benzyl alcohol (31, 32). The rate of decomposition was first order, accelerated by... 

Treatment of 348 with Na[EIBEt3] has also been reported, affording initially the Pt(II) formyl complex anion [Tp PtMe C(0)H ] (355 ), as determined by 1EI and 13C NMR spectroscopic data that again reveal a chiral metal center. At 193 K two formyl environments (2 1 ratio) are apparent, which coalesce at 201 K. These are attributed to restricted rotation about the Pt—C linkage, and AG for conversion of the minor to major isomer was calculated at 8.8 kcal mol-1.120 Protonation of 355 affords exclusively the Pt(IV) formyl complex Tp PtMe C(0)H H (356), with no evidence for protonation of either pyrazole or the formyl, or... 

Different mechanisms for production of the rhodium formyls are followed in different reaction media. In benzene solution where [Rh(OEP)]2 and the metalloradical [Rh(OEP)] are present, a radical chain reaction is involved in CO activation and production of the formyl complex . In pyridine solution where [Rh(OEP)] dispropor-tionates to [Rh(OEP)(py)2] and [Rh(OEP)] , interaction of the metalloanion [Rh(OEP)] with CO is believed to be the dominant pathway for CO activation and production of the metalloformyl species by protonation . 

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. 

Both chemical shifts and coupling constants have been used to characterize the novel Pt pyrazolyl borate formyl complex, (16). The observed coupling constant from the Pt atom to the formyl proton, 327 Hz, is relatively large. 

Interest in the stepwise reduction of coordinated CO continues. Further work on determining whether surface methylenes could arise from formyl intermediates has been reported using 0s3(C0)i2 as a model system. On hydride reduction, 033(00)12 yields [OS3 (C0)u (GH0)] which can be converted into [OS3(C0)u(U-CH2)] by protonation. The methylene complex eliminates CHi, on heating in H2 gas, and forms [033112(00)9(113-000)] in the absence of H2. The first stable neutral formyl complex of a 3d-transltion metal has been claimed. Reduction of trans-[Mn(C0) 1,(P(0R) 3 2 ] or mer-[Mn(C0) 3(P(0R) 3 3 ] yields [Mn(CO)3(OHO)(P(0R)3 2], and the crystal structure of the complex with R = Ph was determined. Other reports on the reduction of Or-, Mo- or Fe-coordinated 00 are referenced below. ... 

The formyl complexes tran -[lrX(CHO)(dppe)2(X=H,C1) are protonated by strong acids to yield the dicationic hydroxycarbene complexes trans-[lrX (CHOH) (dppe) 3. The dichlorocarbene... 

Intermediate formation of formyl chloride is not necessary since the actual alkylating agent, HCO", can be produced by protonation of carbon monoxide or its complexes. However, it is difficult to obtain an equimolar mixture of anhydrous hydrogen chloride and carbon monoxide. Suitable laboratory preparations involve the reaction of chlorosulfonic acid with formic acid or the reaction of ben2oyl chloride with formic acid ... 

Complexation of indoles with chromium hexacarbonyl, which reduces the electron density of the heterocyclic system, promotes nucleophilic attack at the 7-position and, to a lesser extent, also at the 4-position of the indole ring and provides a viable synthetic route to 7-formyl-l-methylindole (78CC1076). Curiously, although the benzenoid ring is rendered susceptible to nucleophilic attack, the reaction of the chromium complex with butyllithium results in abstraction of the proton from the 2-position. However, if this position is... 

A review of formylation reactions involving methyl formate in a hydrogen fluoride-boron trifluoride medium has appeared.60 Regioselectivity and kinetic data have been reported for Gattermann-Koch formylation in superacids and provide evidence for an intra-complex reaction where the formylation electrophile HCO+ is generated by protonation of CO by the arenium ion.61 The observed selectivity results from... 

Thus, the calculated values of total energies of cationic cr-complexes formed by a- and /1-protonation of model molecules of N-phenylsulfonyl-and N-(4-nitrophenyl)pyrroles are consistent with experimental data concerning preferable reactivity of their a-positions, while the role of steric factors and charges on atoms should be considered for interpretation of the increase in percentage of jS-substituted derivatives on formylation of N-alkylpyrroles in the sequence Me[Pg.169]

---