Decarbonylation, solid state reactions

The utility of the solid-state photodecarbonylation of crystalline ketones was recently demonstrated in the total syntheses of two natural products, where the key step is the solid-state reaction. The first example involves the synthesis of the sesquiterpene ( )-herbertenolide [80] by the solid-state decarbonylation of cyclohexanone 189, followed by cyclization of the photoproduct 190 (Scheme 2.46). With precursor 189 obtained by simple methods and a solid-state reaction carried out to 76% conversion, herbertenolide was obtained in good overall yield in a record number of steps from commercial starting materials. With a similar synthetic strategy, samples of the natural product (i)-a-cuparenone were obtained in about 60% overall yield from 191 by a very succinct procedure that included four simple steps and a solid-state reaction at — 20 °C [81]. 

Figure 7.24. Solid-state photochemical decarbonylation model for ketones. The dashed path corresponds to the experimentally determined energies of acetone (in kcal/mol). The effects of substituents with radical stabilizing energies (RSEs) are illustrated by the solid line in the reaction coordinate. See color insert.

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

State decarbonylation reaction in total synthesis was reported recently in the case of natnral prodnct (+)-herbetenolide, which farther illustrates the exquisite control that the solid state may exert on the chemical behavior of the otherwise highly promiscuous reactive intermediates. As word or caution, it should be mentioned that intramolecular quenching effects known to act in solution can also affect that reaction in the solid state. Recently reported examples include the well-known intramolecular P-phenyl and electron transfer quenching. ... 

Such pentacarbonyl species can be further decarbonylated when the sample is heated to 373 K under an inert gas stream and under reduced pressure. This slow decarbonylation process provides the surface Mo(CO)3 species depicted in Figure 9.4, which is stable up to 473 K [14]. In contrast with the relevant chemical behavior in solution (9.1 and 9.2), in the solid state, where the species are somewhat diluted and present low mobility, no dimeric species have been identified as resulting from penta- or tricarbonyl species. Heating to 673 K gives rise to the evolution of H2, CO, CO2 and CH4, due to redox reactions between the metal center and the OH surface groups. The resulting oxidation states, as determined by XPS measurements, are mainly II and IV, besides some Mo(0) species ]20]. It is worth underHn-... 

Singlet-state ntt reactions that occur in the gas phase and in solution may not be observed in crystals because the acyl-alkyl radical pair (E) is subject to the perfect cage effect . Radical-radical recombination in the singlet-state acyl-alkyl biradical (from E =>A) may occur within the time scale of a single bond vibration. However, because many decarbonylation reactions have been documented in crystals, conditions may be satisfied for a-cleavage to compete with excited-state decay and for decarbonylation to compete with recombination of the acyl-alkyl radical pair. In this chapter, we review various aspects of the a-cleavage and decarbonylation steps in the reaction, describe recent theoretical advances, and conclude with examples that reflect our current understanding of the reaction in the solid state. 

Although the reports by Quinkert and Turro suggested the feasibility of the decarbonylation reaction in crystals and demonstrated a very high chemo- and stereoselectivity, the factors that may facilitate or impede solid-state reactivity remained unexplained. Recognizing that the efficient a-cleavage and rapid acyl-radical decarbonylation of ketones with radical stabilizing a- and a -phenyl substituents may determine the reactivity of ketones 54 to 57 and 64 (Schemes 15 and 17), Choi et al. carried out the first systematic search for a correlation between the reactivity of solids and the stability of the radicals involved in the reaction (Scheme 18). 

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