P-tautomer

Conclusion As expected, the double cyclisation of bicyclo-phosphanes induces a strong ring constraint. The marked tendency of these compounds to oligomerization by rupture of the P-0 bond is certainly favoured because it minimizes this tension. The driving force of the addition reaction of X-H molecules is the great stability of the bicyjJjphosphorane structure. Depending upon the nature of X, the P tautomer forms of the last compounds could be observed or not. 

Examples of the various practical applications of dipole moments include, but are not limited to differentiation between isomers cis and trans, o, m, and p, tautomers, etc ), conformational analysis, studies of molecular geometry, supporting evidence for resonance hybrids, information about the polar character of molecules (important for solubility in different solvents and permeability through membranes), information about electrical effects of substituents (inductive, resonance), studies of hydrogen bonding, and studies of donor-acceptor interactions (e g., charge transfer complexes). Practical cases describing the use of dipole moments for different types of structural studies mentioned above can be found in numerous publications mentioned in this chapter. 

The cyclic tetramine NH(CH2)3NH(CH2)2 2 gives a mixture of products when it reacts with P(NMc2)3, in which the phosphorane tautomer (40) predominates, in contrast to the P" tautomers which arise from the isomer -fNH(CH2)3[NH(CH2)2]2NH(CH2)3. ""... 

The 4-hydroxy-THISs are extremely hygroscopic hydrolysis affords the parent thiobenzimide (3). When R" = Me and R = p-ClPh or Ph, but not p-NOjPh the nondipolar tautomer 3 (X = 0, Y = CH2) is present according to NMR and infrared spectra, the latter exhibiting carbonyl absorption at 1710 to 1720 cm ... 

These observations can be extrapolated to the pyrrole series the 2-amino derivatives are very unstable whereas 3-aminopyrroles appear to be more stable. 3-Amino-l-tritylpyr-role (162) appears to exist in solution exclusively in the imino-A -pyrroline form (163) (83JCS(P1)93). 2-Aminoindole (164) is unusual in that it exists mainly as the 3//-tautomer (165). 4-Alkylaminoindoles (166) undergo an unexpected rearrangement to 4-amino-1-alkylindoles (167) when heated with p-toluenesulfonic acid hydrate (82CC1356). 

The reaction of aliphatic primary amines with alkyl a-hydrogenoperfluorocar-boxylates leads to the corresponding P-alkyl iminoesters as the major or the sole tautomers, depending on the length of the perfluoroalkyl chain [ 103] (equation 89). 

Another series includes the cyc/o-metaphos-phimic acids, which are tautomers of the cyclO polyphospha/enc hydroxides (p. 541). Similarly, halogen atoms in PX3 or other P- X compounds can be successively replaced by the iscx lcctronic groups -NH . -NHR, -NR2, etc., and sometimes a pair of halogens can b>e replaced by - NH or -NR. These, in turn, can be used to prepare a large number of other derivatives as indicated schematically opposite for P(NMe2)3 ... 

Treatment of dimethyl sulfoxide (DMSO) is reported to lead to 2-phenylisoindole (17) in yields up to 40%. An interesting possibility in this case is involvement of the isoindole valence tautomer (67) as an intermediate. P. A. Barrelt, R. P. LinsteacI, G. A. P. Tuey, and J. M. Robertson, J. Chem. Soc. p. 1809 (1939). 

Generally the name of a compound should correspond to the most stable tautomer (76AHCS1, p. 5). This is often problematic when several tautomers have similar stabilities, but is a simple and reasonable rule whose violation could lead to naming phenol as cyclohexadienone. Different types of tautomerism use different types of nomenclature. For instance, in the case of annular tautomers both are named, e.g., 4(5)-methylimidazole, while for functional tautomerism, usually the name of an individual tautomer is used because to name all would be cumbersome. In the case of crystal structures, the name should reflect the tautomer actually found therefore, 3-nitropyrazole should be named as such (97JPOC637) and not as 3(5)-nitropyrazole. 

Although not a heteroaromatic compound, the case of citrinin studied by Destro and Luz ([97JPC(A)5097] and references therein) is so significant that it deserves mention here. Citrinin exists in the crystal as a mixture of the p-quinone 5a and o-quinone 5b tautomers (Scheme 3). The equilibrium ii temperature dependent and by using CPMAS NMR (Section VI,F) and, more remarkably. X-ray crystallography, the authors were able to determine the AH and AS values (the rate is extremely fast on the NMR time scale, >10 s ). 

Pyrazolinones and other five-membered compounds (functional tautomerism). These studies have often used the most simple but rather experimentally neglected pyrazolin-5-one 19. Its four tautomers are called CH 19a, NH 19b, 5-OH 19c, and 3-OH 19c (Scheme 10) [76AHCS1, p. 313]. 

Interpolation methods based on N chemical shifts require the use of the general equations.Those reported in the previous edition (76AHCSl,p. 29, see also 82JOC5132) have been slightly modified for the present purpose. We call / x the observed average property, and the property of the individual tautomers (A or B), / ma and / mb a corresponding property that can be measured (in a model compound or in the solid state) or calculated theoretically, and P and / b the correction factors defined as P = -... 

The possible use of Si NMR for the study of the prototropy and sily-lotropy of pyrazoles has been explored (98MRC110). Another paper reports the use of P NMR for establishing that 3-phenyl-4-benzoylisoxazol-5-one is hydrogen-bonded to tri-n-octylphosphine as the NH-tautomer [96MI(14)653j. 

The predominance in the gas phase of the CH tautomers in the case of several pyrazolinones including one 3-aminopyrazolin-5-one was demonstrated by PES [88JCS(P2)641] this result is consistent with previous findings (76AHCS1, p. 325). The same technique was applied to the study of the amino/imino equilibrium in 9-amino and 9-(methylamino)acridines [91MI(61)166] although both tautomers have similar stabilities, the experimental data are in better agreement with the amino tautomer. 

This technique provides quantitative information about tautomeric equilibria in the gas phase. The results are often complementary to those obtained by mass spectrometry (Section VII,E). In principle, gas-phase proton affinities, as determined by ICR, should provide quantitative data on tautomeric equilibria. The problem is the need to correct the measured values for the model compounds, generally methyl derivatives, by the so-called N-, 0-, or S-methylation effect. Since the difference in stability between tautomers is generally not too large (otherwise determination of the most stable tautomer is trivial) and since the methylation effects are difficult to calculate, the result is that proton affinity measurements allow only semi-quantitative estimates of individual tautomer stabilities. This is a problem similar to but more severe than that encountered in the method using solution basicities (76AHCS1, p. 20). 

The first column corresponds to six-membered transition states where two types can be differentiated situation A, where the proton transfer leads to a neutral tautomer, and situation B (often Tinuvin P or TIN 75), where the proton transfer leads to a zwitterionic tautomer (Scheme 25). 

Our previous treatment (76AHCS1, p. 12) contained a section called Chemical Methods to Study Tautomerism where the relationship between tautomerism and reactivity was discussed. Today, nobody uses chemical methods to study tautomerism. However, a great many reactions are carried out on tautomeric heterocycles, although few papers contain new insights on that topic. Authors desiring to explain reactivity results based on tautomerism must take great care to verify that the substrate is in the neutral form AH and not as a conjugated anion A or cation HAH, which are usually devoid of tautomerism. They must also realize that most frequently the reaction path from tautomers to products in-... 

---