Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0367 421

The source of cyanide ions is the Meisenheimer adduct formation between trinitrobenzene and cyanide ions in equilibrium 25. The constant of this equilibrium 25 is well known; if the equilibrium concentration of free cyanide ions is negligible, the concentration of the strong red 137 provides a proof of equilibrium 24. Also, in the case of equilibrium 24, DMSO enhances the reactivity of the anionic nucleophile compared with protic solvents.

The addition of nucleophiles to the carbonyl group may be catalysed by acids obtained by the protonation of the carbonyl oxygen (equilibrium 26). Acid catalysis can also occur during the elimination step which follows the addition step. For example, the reactions of aldehydes with amines (and of all the ammonia derivatives) to form imines are generally assumed to occur in two steps: the first is the addition of nucleophile to yield a gem amino alcohol, the second includes the elimination of water from the tetrahedral adduct 138 (see Scheme 45). This elimination is usually thought to be catalysed by electrophiles171,212.

SCHEME 45

Depending on the experimental conditions either the addition or the elimination step may be rate determining.

The formation of the tetrahedral intermediate 138 may be preceded by the zwitterionic complex 139, as the first step of the nucleophilic attack on the partially positive carbon of the carbonyl group.

O− H

C N+ H

H

(139)

While Scheme 45 is generally accepted, only very few direct observations of 1,1-amino alcohols (138) or of the zwitterionic intermediate 139 were reported213.

Direct inspection (by 1H NMR) of mixtures of 4-nitrobenzaldehyde, or pyridine-4- carbaldehyde, in hexadeutero DMSO and piperidine reveals the presence of 140 in the reaction mixtures214.

The starting aldehyde H˛ signal (υ D 10.36) was shifted to υ D 5.33 as required for a change of sp2 to sp3 hybridization by passing from 4-nitrobenzaldehyde to derivative 140 (equation 27). Similar behaviour was observed for mixtures of aromatic aldehydes and of some primary amines (butylamine, t-butylamine): the presence of the 1,1- aminoalcohol (141) was observed for short times, after the imine (142)215 had been formed (equation 28).

Equilibrium 29 regarding the tautomerism of pentane-2,4-dione (143) was found to have been completely shifted towards the enol form by a tertiary amine. The predominance of the enol form can be attributed to strong hydrogen-bonding interactions with the amine as shown in 145 and 146. Low polarity of solvents also favours the enol form 144.

The interaction shown in 146 was investigated by X-ray diffraction216 of the adduct between diethylamine and 143. This dimeric complex (147) arises mainly from hydrogenbonding interactions and shows a three-centred hydrogen bond.

The interactions between the carbonyl carbon of 143 and triethylamine shown in 148, 149 and 150 appear217 to be a better explanation than the reported (and usually accepted) hydrogen-bonding interactions of 143 in triethylamine. IR and 1H-NMR investigations217 show the presence of a labile adduct between 143 and triethylamine which reduces theCH2 1H-NMR signal of the keto form. By using an excess of 143 or reducing the temperature or diluting the 143/triethylamine mixtures with CDCl3, there is an enhancement of the CH2 signal of the keto form.

The use of tributylamine or of diisopropylethylamine favours the keto form, as expected if a nucleophilic interaction of the amine on a CDO carbon atom is the driving force for enolization217. This conclusion is supported also by ab initio calculations217 on the

C C

O O

H

(151)

malonodialdehyde/NH3 system: the carbon/nitrogen interaction (151) presents an energy state that is lower than those for the more usual hydrogen-bonding interactions.

1,1,1-Trifluoroacetylacetone (152) reacts by a condensation reaction 30 with aromatic aldehydes218 in the presence of piperidine and acetic acid, in benzene. Triethylamine does not catalyse this reaction. Probably, the product 153 is obtained via the carbinolamine (154), which in turn yields the carbonyl group of the aldehyde. Obviously, the tertiary amine cannot displace a proton and the interaction forms a zwitterionic adduct (see 148).

1-Hydroxyphosphonium salts and 1-(trimethylsiloxy)phosphonium salts are obtained219 from the addition of small-sized phosphines (PMe3 and PEt3) to a carbonyl group of ketones and aldehydes, in the presence of chlorotrimethylsilane or of the mixture acetone/bromine, which is a source of anhydrous HBr.

X-ray diffraction provides a structural analysis of the adduct obtained between acetone and triethylphosphine (155) in the presence of bromine. The process of obtaining 155 is an acid-catalysed process, while the adducts obtained in the presence of chlorotrimethylsilane may be ascribed to neutral attack of the nucleophile leading to the zwitterionic complex, as shown in Scheme 46. In Scheme 46 triethylphosphine may be substituted by trimethylphosphine. When triphenylphosphine or tributylphosphine were used, no reactions were observed, owing to the reduced nucleophilic power of the bulky phosphines and to the insolubility of the obtained adducts.

R1 = H, Me; R2 = Me

R1 = H,; R2 = Ph

SCHEME 46

Usually, the reactions of carbonyl compounds and derivatives of ammonia are considered to be concluded with the formation of the imino derivative (156), but there is evidence that the CDN double bond may react faster than the CDO group with nitrogen nucleophiles to form 1,1-diamino derivatives (Scheme 47).

When Ar1 (Scheme 47) is an electron-withdrawing system, such as 4-nitrophenyl, 2- pyridyl, 2-thiazolyl and 2-benzothiazolyl, aminals (157) are obtained in high yields from the reaction mixture of aldehydes and heteroaryl amines in methanol or in DMSO (without

SCHEME 49

position 2 of the thiazole ring) on the carbonyl group is illustrated by Scheme 47 and is clearly reversible.

A recent instance225 of reaction 22 (X D NH) involves the reactions of some (heteroarylchloromethyl)lithium (165) reagents with imines (166) to form 167 and to produce the heteroaryl aziridines (168) as depicted in Scheme 50. Aziridines (167) are obtained in the preferential (or exclusive) conformation E. A tentative explanation of this behaviour is the different steric compression in the transition states affording isomeric E or Z aziridines.

Ar = 2-pyridyl, 2-quinolyl, 2-benzothiazolyl

R = Ph

R1, R2 = cyclopentylidene, cyclohexylidene, fluorenylidene

R1 = H, R2 = Ph

R1 = R2 = Me

R1 = Ph, R2 = Me

SCHEME 50

VII. REFERENCES

1.J. E. Leffler and E. Grunwald, Rates and Equilibria of Organic Reactions, Wiley, New York, 1963.

2.C. Reichardt, Solvent Effects in Organic Chemistry, Verlag Chemie, New York, 1979.

3.V. Gutmann, Coordination Chemistry in Non-Aqueous Solvents, Springer, Wien, 1968.

4.V. Gutmann. The Donor Acceptor Approach to Molecular Interaction, Plenum Press, New York, 1978.

5.C. Reichardt, Chem. Rev., 94, 2319 (1994).

6.M. Patz, H. Mayr, J. Maruta and S. Fukuzumi, Angew. Chem., Int. Ed. Engl., 34, 1225 (1995).

7.J. K. Kochi, Angew. Chem., Int. Ed. Engl., 27, 1227 (1988).

8.G. A. Jeffrey and W. Soenger, Hydrogen Bonding in Biological Molecules, Springer-Verlag, Berlin, 1991.

9.H. K. Hall and A. B. Padias, Aldrichim. Acta, 28, 37 (1995).

10.A. D. Attie and R. T. Raines, J. Chem. Educ., 72, 119 (1995).

11.S. L. Schreiber, Chem. Eng. News, 22 October 26 (1992).

12.W. P. Jencks, Acc. Chem. Res., 13, 161 (1980).

13.L. Pauling, The Nature of the Chemical Bond, Cornell Press, New York, 1959.

14.G. C. Pimentel and A. L. McClellan, The Hydrogen Bond, W. H. Freeman Co., San Francisco, 1960.

15.R. F. Foster, Organic Charge Transfer Complexes, Academic Press, New York, 1969.

16.P. A. Kollman, Acc. Chem. Res., 10, 365 (1977).

17.P. Hobza and R. Zahradn´ık, Chem. Rev., 88, 871 (1988).

18.A. D. Buckingham and P. W. Fowler, Can. J. Chem., 63, 2018 (1985).

19.P. Hobza and R. Zahradn´ık, Intermolecular Complexes, Chap. I, Elsevier, Amsterdam, 1988.

20.D. J. Sandman and A. F. Richter, J. Am. Chem. Soc., 101, 7079 (1979).

21.N. Heinrich, W. Koch, G. Frenking and H. Schwarz, J. Am. Chem. Soc., 108, 593 (1986).

22.K. N. Houk, Acc. Chem. Res., 8, 361 (1975).

23.A. B. Padias and H. K. Hall, J. Org. Chem., 52, 4536 (1987).

24.D. Philp, V. Gramlich, P. Seiler and F. Diederich, J. Chem. Soc., Perkin Trans. 2, 875 (1995).

25.G. A. Derwish, R. S. Salman and M. S. Al-Marsoumi, J. Inclusion Phenom. Mol. Recognit.

Chem., 20, 123 (1994).

26. J. F. Frey, A. M. Andrews, D. G. Ankoviac, D. N. Beaman, L. E. Du Pont, T. E. Elsner, S. R. Lang, M. A. Oosterbaan Zwart, R. E. Seagle and L. A. Torreano, J. Org. Chem., 55, 606 (1990).

27.E. F. Hilinski, J. M. Masnovi, C. Amatore, J. K. Kochi and P. M. Rentzepis, J. Am. Chem. Soc., 105, 6167 (1983).

28.E. M. Kosower, Prog. Phys. Org. Chem., 3, 81 (1965).

29.T. Kim, H. Sarker and N. L. Bauld, J. Chem. Soc., Perkin Trans. 2, 577 (1995).

30.N. Mataga, Biomolecules, Japan Sci. Soc. Press, Tokyo; Elsevier, Amsterdam, 1985, p. 127.

31.J. D. Simon and K. S. Peters, Acc. Chem. Res., 17, 277 (1984).

32.B. K. Adams and W. R. Cherry, J. Am. Chem. Soc., 103, 6904 (1981).

33.J. L. Goodman and K. S. Peters, J. Am. Chem. Soc., 107, 1441 (1985).

34.H. Hayashi and S. Nagakura, Chem. Phys. Lett., 53, 201 (1978).

35.W. Hub, S. Schneider, F. Dorr,¨ J. D. Oxman and F. D. Lewis, J. Am. Chem. Soc., 106, 701 (1984).

36.W. Hub, S. Schneider, F. Dorr,¨ J. D. Oxman and F. D. Lewis, J. Am. Chem. Soc., 106, 708 (1984).

37.L. L. Rodina and A. V. Ryzhakov, Heterocycles, 40, 1035 (1995).

38.L. Mathew, B. Varghese and S. Sankararaman, J. Chem. Soc., Perkin Trans. 2, 2399 (1993).

39.N. Kornblum, P. Ackermann and R. T. Swiger, J. Org. Chem., 45, 5294 (1980)

40.L. Eberson, M. P. Hartshorn, F. Radner and W. T. Robinson, J. Chem. Soc., Chem. Commun., 566 (1992).

41.L. Eberson and F. Radner, J. Am. Chem. Soc., 113, 5825 (1991).

42.C. P. Butts, L. Eberson, M. P. Hartshorn, O. Persson and W. T. Robinson, Acta Chem. Scand., 49, 253 (1995).

43.A. Guy, M. Lemaire and J-P. Guette,´ Tetrahedron, 38, 2339 (1982).

44.A. Guy, M. Lemaire and J-P. Guette,´ Tetrahedron, 38, 2347 (1982).