Physico-Chemical Study of Some Cross Conjugated Systems , Part II

A series of cross-conjugated enones [1-10] are condensed with menthone under (PTC) technique. The structures of the new products [11-20] were identified by spectral analysis. The probable reaction mechanism for each condensation is investigated by theoretical approach using both quantum mechanic (AM1) and molecular (MM2) computational methods. The unique behavior of these cross conjugated enones toward nucleophilic addition orientation, substituent effect on the electronic distribution, and the polarity of the molecules have been examined. Physico-Chemical Study of Some Cross Conjugated Systems, Part II. 2 Introduction Molecules with cross conjugated bond systems are of great interest for both theoretical and applied chemistry 1-2 . Extended cross-conjugated enones (,, , and ) system is an important system due to its presence in biologically active compounds 1-4 . Some of cross conjugated dienones are in pre-clinical trials as an anticancer agents, Also extended enones are well known as common flavor constituent in tea, tobacco & foods 4 . In addition,this system has been used as useful substances or intermediates in the synthesis of vitamins 5 and natural products 6 . Several theoretical investigations have carried out on a series of cross conjugated compounds 1,7-9 . Semi-empirical calculations on cyclopentadienone and cyclohexadienone have been reported 7 . The calculation of geometries by empirical force field showed that the rings are planer for both compounds. Electrochemical data and AM1 calculations on a series of cross-conjugated p-phenylene vinylidene with one to four double bonds revealed that electronic interaction between the repeating C=C bond unit is not very strong 8 . Theoretical calculations by the ZINDO method on cross conjugated 2,6-di-(phenylethynyl)pyridine & 2,6-d-(phenylbutadienyl)pyridine showed that the central group in cross-conjugated compound is conjugated with one or the other double bond, but not with both simultanously 9 . In conjunction to our interest in the synthesis and condensation reactions of  -unsaturated carbonyl compounds with various C-H acids under phase transfer catalysis (PTC) 10-12 , the condensation of a series of cross-conjugated enones, dienones [1-3], trienones [4-7], and tetraenones [8-10] with menthone have been investigated. Theoretical calculations using (AM1) 13 and (MM2) 14 computational method on both reactants & products are performed. Experimental 1. Instrumental Melting points were measured using electro thermal (IA9000) digital-series M.P. apparatus. IR spectra were recorded on Tensor-27 Bruker (FT.IR) and U.V spectra were measured by Schimadzu UV-160 spectrophotometer. 2. The Computation Chemoffice (version 5.0) software package was used for theoretical calculations. Geometrical optimization of dihedral angle, heat of formation (H.F), net atomic charge & bond distance were calculated using Semi-empirical methods (AM1 with package MOPAC). Whilst, MM2 were used for molecular mechanics calculations of steric energy (S.E) and atomic steric energy (A.S.E). M. Y. Shandala & M. M. Al-Shamamry & H. A. A. Al-Wahab & R. G. Salih 3 Preparations 1. Preparation of the starting materials The cross-conjugated enones [1-10] were prepared by standard methods 15-17 where the desired benzaldehyde was condensed with the proper ketone in aqueous ethanolic sodium hydroxide. These enones were identified by their physical & spectral data 11-12 (IR, UV,NMR). These data are reported & published 12 in our study of the same series of enones in part(I). 2. General procedure for the condensation of the enones [1-10] with menthone under (PTC) 18 . A mixture of (3ml) 50% NaOH, (25ml) benzene, (0.3gm) benzyl tributylammouium chloride (TEBA) and ((0.0025 mole) of menthone is stirred magnetically for (5min). The enone [1-10] (0.0025 mole) was added dropwise, stirring is continued for certain intervals at 30-40 o C until no further change in color is observed. The benzene layer was separated, washed with water, dried over MgSO4 and evaporated. The residue obtained is recrystallized from (95%) ethanol. Table (1) illustrates weight of reactants, time of reactions, names of the products, [11-20], M.P and % yields. Table (1): Weight of reactants, time of reactions, names of the products, melting points and percentage yields [11-29] Compd . No. Wt. of Reactants (gm) X Time of reaction (min) Name m.p & color Yield % Uns. Ketone Menthone H3C CH(CH3)2 O CH CH2 O


Introduction
Molecules with cross conjugated bond systems are of great interest for both theoretical and applied chemistry [1][2] .Extended cross-conjugated enones (,, , and ) system is an important system due to its presence in biologically active compounds [1][2][3][4] .Some of cross conjugated dienones are in pre-clinical trials as an anticancer agents, Also extended enones are well known as common flavor constituent in tea, tobacco & foods 4 .In addition,this system has been used as useful substances or intermediates in the synthesis of vitamins 5 and natural products 6 .
Several theoretical investigations have carried out on a series of cross conjugated compounds 1,[7][8][9] .Semi-empirical calculations on cyclopentadienone and cyclohexadienone have been reported 7 .The calculation of geometries by empirical force field showed that the rings are planer for both compounds.Electrochemical data and AM1 calculations on a series of cross-conjugated p-phenylene vinylidene with one to four double bonds revealed that electronic interaction between the repeating C=C bond unit is not very strong 8 .Theoretical calculations by the ZINDO method on cross conjugated 2,6-di-(phenylethynyl)pyridine & 2,6-d-(phenylbutadienyl)pyridine showed that the central group in cross-conjugated compound is conjugated with one or the other double bond, but not with both simultanously 9 .

The Computation
Chemoffice (version 5.0) software package was used for theoretical calculations.Geometrical optimization of dihedral angle, heat of formation (H.F), net atomic charge & bond distance were calculated using Semi-empirical methods (AM1 with package MOPAC).Whilst, MM2 were used for molecular mechanics calculations of steric energy (S.E) and atomic steric energy (A.S.E).

Preparations 1. Preparation of the starting materials
The cross-conjugated enones [1-10] were prepared by standard methods [15][16][17] where the desired benzaldehyde was condensed with the proper ketone in aqueous ethanolic sodium hydroxide.These enones were identified by their physical & spectral data [11][12] (IR, UV,NMR).These data are reported & published 12 in our study of the same series of enones in part(I).

Results and discussion
The condensations of the cross conjugated enones [1-10] with menthone under (PTC) conditions were carried out and the products [11-20] were obtained.The products were identified by the analysis of their IR and UV spectral data.The spectral data of the condensation products are illustrated in table (2).As a representative model of the condensation products, compound [20] is selected.The IR spectrum shows a broad absorption band at (1705-1680cm -1 ) which is attributed to the stretching vibration of the cyclic & acyclic C=O group overlapped with each other.The absorption at (1600-1585cm -1 ) belong to (C=C) of the olefinic and aromatic double bond.The isopropy group shows a strong doublet at (1397-1370 cm -1 ) due to the inplane and out of plane deformation of the two methyl of menthone fragment.
The UV spectrum of the product [20] exhibits a maximum absorption at  max (348)nm) / with a blue shift when compared with  max of the reactant [10] ( max 387nm).The decrease in  max is due to the decrease in conjugation of the product [20] (table 2).The proposed mechanism for these reactions is illustrated in Scheme (1).The condensation of 2-methyl-l,9-diphenylnona-l,3,6,8tetraen-5-one [10] with menthone is selected as a representative model for the possible pathways of the nucleophilic addition process.
In a strong basic medium two anions from menthone can be obtained, anion Men1 and anion Men2 (scheme 1).The heat of formation of anion (Men. 1) is (-82.905Kcal/mole), while that for anion (Men.2) is (-88.936Kcal/mole).The anion (Men. 1) is less stable than anion (Men.2), but due to the steric hindrance, attack of the anion (Men. 1) is more probable than the attack of the anion (Men.2).Moreover, theoretically, the products obtained from reaction of anion (Men. 1) with the cross conjugated ketone are more stable than the products obtained from the reaction of the cross conjugated ketones with anion (Men.2).This can be observed when the H.F values are compared for all the possible reactions of the two anions (Scheme 1) (Table 4).
Inspection of scheme 1 and table (4) shows that the theoretical parameters H.F, S.E min2 AM1 and S.E min2 MM2 for the products do not give a clear distinction between the stability of addition products of rout A or rout (B) i.e their values are very close.However the physical parameters which were calculated for the reactants 11,12 (table 3) clarified which route is preferable.The atomic steric energy (A.S.E) values and the extended Huckel charge (E.H.C) for C 4 and C 4` of the central chain of the dienone [1-3] indicate that C 4` is more liable to nucleophilic attack than C 4 .On the other hand C 4 of the dienone,trienone & tetraenone ` [4-10] is the preferred site for the nucleophilic attack than C 4, C 6 or C 6`.
These data indicate that the effect of ring substituent whether e-withdrawing (p-Cl [2], [6]) or e-denoting (p-CH 3 [3], [7]) on the addition of Men1 is weak.This unexpected behavior can be attributed to the nonplanarity of the phenyl ring (A) with the enone chain which is supported by the values of the dihedral angle for all the reactants (Table 3) and 3D structure 11,12 .The data obtained from the minimized geometry for compounds [11-20] indicate that these final products are not planer.As a representative model the 3D structure of compound [20] is selected Fig (1).The dihedral angle ph A = + 66.6 and ph B = + 12.8 which indicates its non-planarity.The dihedral angle between menthone molecule and the central system is -48.4 which means that the menthone ring is not the same plane with the starting material.