Isophorone, dimerization

Isophorone yields dimers (72H74) upon irradiation(86) ... 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

Palladium(II) acetate was found to be a good catalyst for such cyclopropanations with ethyl diazoacetate (Scheme 19) by analogy with the same transformation using diazomethane (see Sect. 2.1). The best yields were obtained with monosubstituted alkenes such as acrylic esters and methyl vinyl ketone (64-85 %), whereas they dropped to 10-30% for a,p-unsaturated carbonyl compounds bearing alkyl groups in a- or p-position such as ethyl crotonate, isophorone and methyl methacrylate 141). In none of these reactions was formation of carbene dimers observed. 7>ms-benzalaceto-phenone was cyclopropanated stereospecifically in about 50% yield PdCl2 and palladium(II) acetylacetonate were less efficient catalysts 34 >. Diazoketones may be used instead of diazoesters, as the cyclopropanation of acrylonitrile by diazoacenaph-thenone/Pd(OAc)2 (75 % yield) shows142). 

Cyclic enones participate in several other photochemical reactions. Irradiation of neat cyclopentenone417 or cyclohexenone418 leads to formation of dimers. Eaton has shown that the reaction is quenched by piperylene and that there is a strong polar solvent effect on the ratio of head-to-head and head-to-tail dimers formed.511 Similar solvent effects have been noted for the triplet-state photodimerization of isophorone.512 Head-to-head dimers are favored in polar solvents and in neat ketone, while head-to-tail dimers are favored in nonpolar media. 

Stereoselectivity. The SCF can influence the stereoselectivity as well as the regioselectivity of a cyclic enone dimerization. There are both syn and anti stereoisomers of the head-to-tail dimers of both isophorone and cyclohexenone. The differences in alignment of the cyclohexyl rings was portrayed schematically by Hrnjez et al. (31). Both HT photodimers for isophorone have similar dipole moments. Hence, a variation in solvent polarity is not expected to influence the anti/syn ratio and, indeed, these stereoisomers are formed in equal amounts regardless of the dielectric constant in liquid solvents (32). For the cyclohexenone HT stereoisomers, the anti configuration dominates in acetonitrile and benzene, but there has been limited study of stereoselectivity in other liquids. 

The rate and yields of dimerization for isophorone (51)48) has been found to be enhanced in micellar microemulsions relative to homogeneous solutions (Scheme XVIII). In homogeneous solutions, the ratio of head-to-head/head-to-tail (HH/HT) dimers increases with increasing polarity, but in micelles polarity effects alone cannot fully explain the observed ratios. The change in the regioselectivities and high yields is attributed to the localization of the substrate in the vicinity of the Stem layer where polarity and orientational effects are quite strong. 

Kinetic studies of the reaction of CO2 with radical anions generated from dialkyl fumarates and maleates showed that C-C bond formation was the rate-determining step. The pseudo first-order rate constants, kco2, for fumarate radical anions in C02-saturated DMF were found to vary between 0.35 and 1.5 s and to decrease in the same order as observed for dimerization [218]. Rate constants for maleates (kco2 varied from 32.0 s to 18.0 s ) were higher. Rather slow is the coupling of CO2 with the 4-keto isophorone in MeCN (k co2 = 0-35 s ) [219]. 

Figure 4.4-4 Anti syn ratio for the head-to-tail dimers formed in the dimerization of isophorone in scCHFa (34.5 °C) and SCCO2 (38 °C) as a function of pressure (data taken from Reference 49).

Additions to Cyclohexenones and Related Systems - A re-investigation of the photodimerization of isophorone (37) has been reported. The study examined the influence of solvent and of the concentration of the enone. Some of the results and the yields of dimers obtained are shown in Scheme 4. From this detailed study the authors suggest that supramolecular structures are involved in the dimerization. These apparently take part even at low concentrations of enone. The photocycloaddition of enones such as (39) to buckminsterfullerene (Ceo) has been studied. The outcome of the addition is the formation of low yields of furanylfullerenes. This addition occurs to the exclusion of de Mayo type of addition. Photocycloaddition of the cyanocyclohexenone derivative (40) to alkenes has been reported. ... 

Isopentenylhydroquinone, 397, 398 Isopentyl nitrite, 48 Isophorone, 287 oxide, 282, 283, 394 Isoprene dimers, 4-5 Isoprene epoxide, 79 isoprenoid quinones, 250 Isoprenylation, 397-398 N-Isopropylideneanilines, 3 Isopropylidenecarbene. 358 Isup ropy I idenecy do propanes, 358... 

Coating powders for exterior use are mainly based on the SP-TGIC system. Of some importance is the SP-PUR system, i.e., the combination of hydroxy-functional polyester resins with caprolactam-blocked isocyanate adducts (primarily based on isophorone diisocyanate). The newly developed SP-HAA and SP-glycidyl estermix systems avoid the problems of TGIC as regards toxicology. Acrylic resins are of importance mainly in the United States (AY-PUR) and in Japan (AY-DDA). The AY-PUR system is used for automotive components and in domestic appliances. The PUR systems based on uretdione (a dimerization product of isocyanates) do not... 

A particularly striking example of the use of supercritical fluids is the photochemical dimerization of isophorone in sc CO2 and in sc CHF3... 

The elusive parent diisocyanate, 0=C=N—N=C=0, is only stable at —75°C, and therefore it is not suitable as a monomer for polyurethanes (23). The least costly aliphatic diisocyanate is hexamethylene diisocyanate (HDI), which is obtained by phosgenating the nylon intermediate hexamethylenediamine (HDA). Because of its low boiling point, HDI is mostly used in the form of its derivatives, such as biurets, allophanates, dimers, or trimers (24). Isophorone diisocyanate (IPDI) and its derivatives are also used in the formulation of rigid coatings, while hydrogenated MDI (HMDI) and cyclohexane diisocyanate (CHDI) are used in the formulation of flexible coatings and polyurethane elastomers. 

Pyrazole (55) appears to be formed directly from the aldehyde, whereas its isomer (56) seems (as indicated by a separate experiment) to arise from initial formation of the aldol dimer Et-CM CMe-CHO. Of the other aj8-unsaturated carbonyl compounds treated with the hydrazone, only chalconegave apyrazole cinnamaldehyde and isophorone gave azines (see footnote ), whereas acrolein, crotonaldehyde, benzalacetone, mesityl oxide, or methyl vinyl ketone gave inseparable mixtures. 

C27H38O, Isophorone condensation trienone, 43B, 256 C28H20f Di-p-anthracene, 31B, 112 46B, 192 C28H26Br04, Norbornadiene dimer di-p-bromobenzoate, 38B, 242 C28H28 f 9,10-Benzotricyclo[3.3.2.0 ]deca-3(7),9(10)-diene dimer, 43B, 256... 

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