Dithianes anomeric effects

The decrease of the anomeric effect in polar solvents was also supported by quantum mechanics calculations.13 Nevertheless further studies on the anomeric effect demonstrated the limitations of the electrostatic model. In particular, Juaristi et al.14 demonstrated that, at low temperature, the dependence of conformational equilibria of 2-carbomethoxy-l,3-dithiane upon solvent shows an opposite trend to the stronger anomeric effect in less polar media observed at 25 °C (Table 4). 

The conformational behaviour of 2-phosphoryl-l,3-dioxanes and dithianes continues to be an area of interest. Molecular mechanics methods have been applied to 2-phosphoryl-l,3-dioxane (34), the corresponding dithiane (35) and 2-thiophosphoryl-l, 3-dithiane (36).15 The relative stabilities of the axial and equatorial conformers in each case vary due to a combination of differing 1,3-axial interactions and anomeric effects. The conformer calculated to be the most stable for (34) and (35) is the structure given in each case. In the case of (36) the isomers are estimated to be of... 

Comparison of the O-C-O with the S-C-S anomeric interaction has been effected (2, 4 ) as shown in Scheme 2. In this case one can tell, unequivocally, that the O-C-O anomeric effect is the larger, for the overall AG° in the two systems is nearly the same and from the known conformational energies of 2-alkyl-l,3-dioxanes and -1,3-dithianes (5) it is obvious that the countervailing... 

Third-row anomeric effects of arylselenenyl substituents were studied in 2-arylseleno-l,3-dithianes (5) (Scheme 3). This effect favours the axial conformer by 6.7-9.2 kJ/mol, and an intramolecular se interaction... 

Mikolajczyk, M.. Graczyk, P. and Wieczorek, M.W., Conformational preference in L3-dithianes containing 2-phosphoryl. -(thiophosphoryl), and -(selenophosphoryl) groups. Chemical and crystallographic implications of the nature of the anomeric effect, J. Org. Chem., 59. 1672, 1994. 

X-ray diffraction studies have suggested that the anomeric effect operates in 2-phosphinoyl derivatives of the saturated heterocycles, where the 2-substituent has no lone electron pairs. For example, in 2-diphenylphosphinoyl-l,3-dithiane, the substituent occupies an axial position, in contrast to the 1,3-dioxane and 1,3-oxathiane derivatives, where the same substituent occupies an equatorial position <84JOC3026,88JOC3609,88TL6801>. 

The majority of thermodynamic data examined in these heterocyclic systems has been concerned with their ring conformations and the anomeric effect. Much of this work is cited in Section 6.08.3. Other data has been obtained from 2-phosphoryl- and 2-thiophosphoryl-l,3-dithianes <92TL227l, 94JOCi672> and 2-phosphoryl- and 2-thiophosphoryl-1,3-dioxanes <93PS(74)3il>. 

Anomeric effects are also observed for elements in the heavier rows of the periodic table. For example, frani-2,3-dichloro-l,4-dithiane exists in the diaxial confor-mation. ... 

Tetravalent phosphoms groups prefer axial orientations in 1,3-dithianes and 1,3,5-trithianes. NMR studies have indicated a preference of about 1 kcal for the diphenylphosphoryl group. When combined with the conformational energy in a cyclohexane ring, this suggests an anomeric effect of around 3 kcal. ... 

Fuchs et al. (129). They suggested that when the molecular dipoles of the axial and equatorial conformers are of similar magnitude, the more polar double bond-no bond structure, resulting from hyperconjugative interactions in the axial conformer, 54a (see Scheme 20), will be stabilized in the more polar solvent. This hypothesis would support the importance of the hyperconjugative origin of the anomeric effect. The appearance of a large proportion of the axial conformers of 2-(arylseleno)-l,3-dithianes 37 (Scheme 13), even in... 

In the course of our studies on the anomeric effect in the C—S—C—P system we also become interested in the influence of solvent on the pertinent conformational equilibria in the hope that solvent effects could shed light on the origin of the observed anomeric effect. As models for our study 2-diphenylthiophosphinoyl-l,3-dithianes 55 and 56 were chosen (54). Confor-... 

A reversed dependence of the axial preference on solvent polarity, which is sometimes regarded as a proof for the reverse anomeric effect, is discussed together with our results on phosphonio-l,3-dithianes in Section II.I. 

In fact, however, more polar solvents may enhance axial preferences without the intervention of a reverse anomeric effect, as was shown by Lemieux (27), Fuchs et al. (129), Zefirov and Fedorovskaya (171), and Giralt et al. (172). In the course of our studies on the conformation of 2-phosphonio-l,3-dithianes 85 we also found (173) that the axial preference may increase with increasing solvent polarity, as expected based on the concept of the reverse anomeric effect. However, 2-phosphonio-l,3-dithianes 85 do not exhibit the reverse but the generalized anomeric effect (see Section V.A). Since the axial... 

A significant contribution in this area was presented by Cuevas from his theoretical analysis of the anomeric effect in S—C—P units. Thus, using computational resources and 2-dimethylphosphinoyl-l,3-dithiane as a model system the author argues that the polar nature of the P—O bond... 

Mikolajczyk M, Graczyk P, Wieczorek MW, Bujacz G (1991) Conformational preference of 2-triphenylphosphonio-l,3-dithianes competition between steric and anomeric effects. Angew Chem Int Ed Engl 30 578-580... 

Juaristi E, Cuevas G (1993) Conformational analysis of l,3-dithiane-2-yhnethylpho-sphionium chloride. Origin of the S-C-P anomeric effect. J Am Chem Soc 115 1313-1316... 

Pinto BM, Johnston BD, Nagelkerke R (1988) Solvtait and temperature dependence of the anomeric effect in 2[(4-methoxyphenyl)seleno]-l, 3-dithianes. Dtuninance of the orbital interaction component. J Org Chem 53 5668-5672... 

Alabugin IV (2000) Stereoelectronic interactions in cyclohexane, 1, 3-dioxane, 1.3-oxathiane, and 1, 3-dithian W-effect, oC-X<- o -H interactions, anomeric effect-what is really important J Org Chem 65 3910-3919... 

Alabugin, I. V. (2000). Stereoelectronic Interactions in Cyclohexane, 1,3-Dioxane, 1,3-Oxathiane, and 1,3-Dithiane W-Effect, aC-X a C-H Interactions, Anomeric Effect - What Is Really Important The Journal of Organic... 

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