Hydroxymethyl complexes, formation

This mechanistic scheme was supported by the observation that the formyl complex reacts with BHa-THF to yield the methyl complex. When a tetrahydrofuran/water mixture is used as the solvent, the reaction can be stopped at each stage, including the step of hydroxymethyl complex formation. Scheme 10.13 ... 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Chapters 7 to 12 deal with factors affecting conformational equilibria and complex formation. Angyal explains the effect of calcium chloride on the anomeric equilibria of certain sugars by formation of especially stable complexes. These involve an axial-equatorial-axial sequence of three hydroxyl groups on a six-membered ring or a cis-cis sequence on a five-membered ring. Lemieux and Brewer use model compounds to study solvation effects on the orientation of the hydroxymethyl... 

Hydroxymeihylene would also be responsible for methanol formation (62) by a further reduction to an hydroxymethyl complex and hydtogenolysts to CHjOH. Methane would be the result of further dehydration of a hydroxymethyl hydride complex to a methylene followed by hydrogenation to give a methyl group and hydrogenolysis to give methane (63) ... 

Proton NMR spectroscopy was employed to follow complex formation on titration of V,7V-bis[ 6-(hydroxymethyl)-pyridine-2-yl methyl]-p-tosylamide (= L) with Cu(I) and Zn(II) salts (giving (L)Cu1, (L)Zn11).1163 31P NMR spectra were used to follow oxidative-addition reactions of (L)AuX + X2 (X = Cl, Br L = phosphines, phosphates), showing a reluctance by very bulky L ligands to undergo such reactions.1164 Similar data were obtained for redox reactions of (R3P)2AuBr with diselenides (R = Me, Et).1165... 

Ampicillin, cephalexin, and bacampicillin are stabilized by complex formation with aldehydes such as benzaldehyde and furfural,517-522 although this stabilization involves reversible formation of covalent species. Even though these interactions involve covalent bond formation, they follow Scheme 76, because the covalent association and dissociation (defined by the equilibrium constant K) is fast relative to k, and kf. The greater stability of N-nitrosoureas in Tris buffers than in carbonate buffers has been ascribed to complex formation with tris(hydroxymethyl)aminomethane.523... 

Based on these experiments, mechanistic steps were suggested, which explain, independent of the metal used (M = Co, Rh), the formation of the typical reaction products such as formaldehyde, methyl formate, methanol, and the desired ethylene glycol (Scheme 6.115) [11]. A key role in this mechanism is played by formaldehyde, which is produced by the hydrogenolysis of a metal formyl intermediate. Either it reacts afterwards to methyl formate via a methoxy complex or, alternatively, a transient hydroxymethyl complex is formed which becomes the starting point for next transformations. In the absence of CO, preferentially methanol is released. Only by coupling with a further CO equivalent the C2-unit is constructed. Upon hydrolysis, ethylene glycol is released. 

The formyl complex reacts with dihydrogen to give 4.51, a complex where formaldehyde acts as a ligand. Two different insertion reactions of formaldehyde into the metal hydrogen bond leading to the formation of 4.52 or 4.53 are possible. The hydroxymethyl complex 4.53 can undergo further CO insertion to give 4.54. 

The pathways for thiamine biosynthesis have been elucidated only partiy. Thiamine pyrophosphate is made universally from the precursors 4-amino-5-hydroxymethyl-2-methylpytimidinepyrophosphate [841-01-0] (47) and 4-methyl-5-(2-hydroxyethyl)thiazolephosphate [3269-79-2] (48), but there appear to be different pathways ia the eadier steps. In bacteria, the early steps of the pyrimidine biosynthesis are same as those of purine nucleotide biosynthesis, 5-Aminoimidazole ribotide [41535-66-4] (AIR) (49) appears to be the sole and last common iatermediate ultimately the elements are suppHed by glycine, formate, and ribose. AIR is rearranged in a complex manner to the pyrimidine by an as-yet undetermined mechanism. In yeasts, the pathway to the pyrimidine is less well understood and maybe different (74—83) (Fig. 9). 

It forms a complex with dioxane contg 2 moles of TNMe to one of dioxane, mp 44—4.5°, bp at 8mm, 61—2° (Ref 19a, p 33). It reacts with aromatic dlazonium salts to give compds of the type ArN NC(N02)3. The compds are relatively unstable and their expl props have not been examined (Ref 12). It reacts with N-hydroxymethyl compds to form adducts of the type RNHCH2C(N02)3. The same compds are formed from TNMe, formaldehyde, and the amine or from trinitroethanol and the amine (Ref 31). It forms complexes with N-contg heterocyclics whose expl props have not been examined (Ref 42). It forms complexes with benzene and methylbenzenes. The formation constants for these complexes vary from 8.46 for the benzene complex to 279.4 for the hexa methylbenzene complex (Ref 49)... 

The dihaptoformaldehyde complex 0s(Tj2-CH20)(C0)2(PPh3)2 reacts with hydrogen halides, affording hydroxymethyl species. Further reaction leads to the formation of halomethyl complexes, probably via the intermediacy of methylene complexes (60) (73) ... 

---