Formyl complexes anions

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... 

There is substantial hydride mobility associated with homogeneous formyl complexes (particularly those which are anionic) [10,11,12,13]. Therefore, the generation of small quantities of catalyst-bound formyls (a step which based upon homogeneous precedent is likely uphill thermodynamically) might be accompanied by a similar electrophile-induced disproportionation. 

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 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 ). 

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... 

As mentioned above, appropriate hydride nucleophiles are capable of attacking coordinated CO [Eq. (3)]. This route to anionic formyl complexes was reported in 1976 by Casey, Gladysz, and Winter (29-34). All of the anionic formyl complexes in Table 1, including 22 [Eq. (4)], can be prepared by hydride attack on neutral metal carbonyl precursors. 

A special class of anionic formyl complexes are those with two formyl ligands. Examples are given in Eqs. (6) and (7). These complexes have analogy in anionic bisacyls previously synthesized by Lukehart (2J). Very recently, evidence for the bisformyl cluster Ir4(CO)i0(CDO)l has been obtained (46). 

IR spectra of both anionic and neutral formyl complexes show tv=(> between 1530 and 1630 cm 1 which are medium in intensity relative to... 

The involvement of trialkylboranes in these reactions was probed by use of the optically active trialkylborohydride 52, shown in Eq. (16) (59). In previous work, 52 had been demonstrated to reduce the prochiral ketone acetophenone to 1-phenylethanol of 17% optical purity (80). Compound 52 was then used to generate the unstable anionic formyls 6, 12, and 26 (Table I) subsequently, acetophenone was added to these reaction mixtures. If Eq. (16) were reversible and 52 were the active hydride transfer agent, 1-phenylethanol of 17% optical purity would be expected. In practice, optical purities of 3.1-11.7% were obtained (39). This indicates some type of trialkylborane involvement in the hydride transfer (the exact role cannot be readily determined by experiment). Therefore, it became important to attempt similar reactions with isolable, purified formyl complexes. 

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]+. 

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... 

Reactions with organometalllc substrates. One useful reaction of this and other trialkylborohydrides is cleavage of metal carbonyl dimers to metal carbonyl anions (equation I). The by-products are Hj and B(C2Hs)3. Another useful reaction is the generation of anionic formyl complexes (equation II). Sulfur (Sg) can be cleaved to give either LijS or Li2S2 (equations III and IV). Disulfides are cleaved by this reaction to lithium mercaptides. 

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. 

Of particular interest are phosphido-bridged derivatives of iron carbonyls, such as (OC)3 Fe(/i-PR2)2 Fe(CO)3. Upon reduction, these compounds give anions of the type [(OC)3 Fe(/i-PR2)2 Fe(CO)3] , in which the Fe —Fe bond is cleaved. The reduction with [BEt3H] leads to the formation of formyl complexes and the reaction with LiR furnishes acyl coordination compounds. In the reaction of... 

Nowhere, perhaps, is this phenomenon better illustrated than in the phenothiazine class. The earlier volume devoted a full chapter to the discussion of this important structural class, which was represented by both major tranquilizers and antihistamines. The lone phenothiazine below, flutiazin (130), in fact fails to show the activities characteristic of its class. Instead, the ring system is used as the aromatic nucleus for a nonsteroidal antiinflammatory agent. Preparation of 130 starts with formylation of the rather complex aniline 123. Reaction with alcoholic sodium hydroxide results in net overall transformation to the phenothiazine by the Smiles rearrangement. The sequence begins with formation of the anion on the amide nitrogen addition to the carbon bearing sulfur affords the corresponding transient spiro intermediate 126. Rearomatization... 

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