Seven-membered intramolecular hydrogen bonds

Chen KY, Cheng YM, Lai CH et al (2007) Ortho green fluorescence protein synthetic chromophore Excited-state intramolecular proton transfer via a seven-membered-ring hydrogen-bonding system. J Am Chem Soc 129 4534 -535... 

Fig. 6 Postulated seven- (A) and six-membered intramolecular hydrogen bonds (B) in sugar thioureas.

The pA(,2-values for dissociation of the diprotonated diamines [50] to [53] in aqueous solution are shown in Table 14. These values are lower than for the corresponding acyclic diamines and the values decrease in the order to be expected if the monoprotonated species form intramolecular hydrogen bonds. The strongest intramolecular hydrogen bond is expected for the seven-membered ring as in [53]. 

By contrast, the X-ray crystal structures of both 2-formylbenzeneboronic acid (13) and its O-methyl oxime (14) reveal an intramolecular hydrogen bond in which one hydroxyl of the B(OH), unit truly acts as a hydrogen bond donor to a heteroatom of the ortho side chain <94MI621>. The hydrogen bond distance in the seven-membered ring is 1.562 A in 13 and 1.614 A in 14. 

IHB-7 Seven-membered ring intramolecular hydrogen bonding (-2.36)... 

F2i2i2x Z = 4 Dx = 1.608 R = 0.035 for 643 intensities. The pyranose ring, approximately is distorted by the anhydro bridge, °, which forms a boat-shaped, seven-membered ring. There is no intramolecular hydrogen-bond, and the repulsion of the syn-axial hydroxyl groups also distorts the pyranose ring. 

As a consequence, the extent of intermolecular hydrogen bonding is decreased. Decreased intermolecular hydrogen bonding allows molecules to escape more readily and lowers the boiling point. The probability of an intramolecular hydrogen bond in 5-hydroxy-2-pentanone is far less than in 3-hydrox-yl-2-pentanone because a seven-membered ring would be required. 

An intramolecular hydrogen bond is unlikely in a seven-membered ring. 

The other type of carbamoyllithiums IIIc can also be prepared by reaction of CO with (V-lithioketimines, resulting from the addition of rert-butyllithium to aryl cyanides 10477,102. These intermediates 105 underwent selective cyclization to give 177-isoindole derivatives 10677 and six- (107)102 or seven-membered (108)102 cyclic products (Scheme 27). Compounds 107 result either by insertion of the carbene structure into the benzylic carbon-hydrogen bond, as in the case of carbamoyllithiums96, or by intramolecular protonation. 

Since the transition state is a seven-membered ring, this intramolecular process is expected to be reasonably fast. This proton-transfer step is between two groups with about the same pA"a, for a proton-transfer gq of approximately 1. The water solvent can also mediate this proton-transfer step. The water that is hydrogen-bonded to the anion can be deprotonated, then in turn deprotonate the alcohol. 

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