Hydrogen bonds, formyl

The formyl C-H O hydrogen bond idea (Fig. 1.4) was first conceived for the catalyst 9 and its existence is supported by several X-ray studies of BX3 aldehyde... 

A. Side view of channel spanning the lipid layer of a planar lipid bilayer, The structure is comprised of two monomers, each in a left-handed, single stranded p -helical conformation, and joined together at the head or formyl end by means of six, intermolecular hydrogen bonds. The two formyl protons are seen at the center of the structure in this view. Replacement of these protons by methyls destabilizes the conducting dimer as shown with N-acetyl desformyl Gramicidin A (Fig. 3D). 

These catalysts are believed to function through an acyclic TS. In addition to the normal steric effects of the open TS, the facial selectivity is probably influenced by tt stacking with the aryl ring and possibly hydrogen bonding by the formyl hydrogen.152... 

The enantioselectivity of the BINOL-Ti(IV)-catalyzed reactions can be interpreted in terms of several fundamental structural principles.42 The aldehyde is coordinated to Ti through an apical position and there is also a 0-HC=0 hydrogen bond involving the formyl group. The most sterically favored approach of the alkene toward the complexed aldehyde then leads to the observed product. Figure 10.2 shows a representation of the complexed aldehyde and the TS structure for the reaction. 

The hydroxy function can be introduced by two methods by oxidation of selenopheneboronic acids or by dealkylation of ferf-butylselenophene ethers. Both methods have been used by Pastour and co-workers117,118 in their preparation of 2-formyl- and 2-acetyl-3-hydroxyselenophene. The importance of hydrogen bonding for the tautomeric equilibrium (88) is discussed. 

The CO reductions generally could likely proceed through formyl intermediates, probably at a multinuclear site (420) hydride migration to a coordinated CO [e.g., as in the hypothetical scheme outlined in Eq. (72)] has not yet been observed, although metal formyl complexes have been synthesized via other methods (422-425). A ir-bonded formyl also seems plausible (426), since 7r-bonded acyl groups have been demonstrated (427). A stoichiometric hydrogen reduction of CO to methanol under mild conditions via a bis(pentamethylcyclopentadienyl)zirconium complex is considered to go through a formyl intermediate (428, 429) ... 

To the extent that mechanistic similarities exist, it is of interest to examine several crucial transformations in catalytic CO reduction and to see whether the organoactinide carbonylation results contribute to a better understanding of what may be occurring. The insertion of CO into a surface metal-hydrogen bond to produce a formyl (eq.(18)) has been discussed at length... 

The timescale of a microwave observation is ca 10 12s so that an average of the properties of the species in equilibrium (35) is obtained if the equilibrium occurs in a time shorter than this. The X-ray photoelectron spectra of intramolecularly hydrogen-bonded species in the gas phase have been studied in an attempt to obtain an instantaneous picture of the structure of these molecules. In this technique the ionisation of core electrons which occurs within 10 16s is observed. For malondialdehyde, 6-hydroxy-2-formyl-fulvene, 2-hydroxy-1,1,1,5,5,5-hexafluoropent-2-ene-4-one, 9-hydroxyphen-alenone [19], and tropolone [20], two peaks are observed in the Ou region of the photoelectron spectrum (Brown et al., 1979). If these molecules existed in the C2v form with a symmetrical hydrogen bond and equivalent oxygen... 

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

Myrtaceae), from which the related 2, 4, 6 -trihydroxy-3 -formyl-5 -methyldihydrochal-cone was also obtained. The NMR spectra of the synthetic formyl derivative show evidence for exchange broadening, which was attributed to conformational exchange between two stable hydrogen-bonded rotamers from molecular mechanics and ab initio calculations ... 

Slightly greater effects are observed for the B-bands of m-hydroxy-benzaldehyde and ra-nitrophenol (D, H-bond +68 and +96 A), although the nitro and formyl groups may not be terminal. Greater displacements should have been expected for the B-bands of o-hydroxy benzaldehyde and o-nitrophenol, but, owing to the formation of intramolecular hydrogen bonds by these derivatives (cf. below), an experimental confirmation is, unfortunately, not possible. 

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