Two new carbene−rhodium complexes are
reported that are active hydroformylation catalysts,
giving very high selectivities for the branched isomer
(>95:5) when vinyl arenes are used as substrates. The
carbene analogue of Wilkinson's catalyst, [RhIMes(PPh3)2Cl] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene), and the related carbonyl complex
[RhIMes(PPh3)(CO)Cl] were both prepared, and the
latter was fully characterized using spectroscopic and
crystallographic means.
The catalyzed hydroboration of vinyl arenes was carried out using pinacol borane instead of catechol borane, as the former reagent and the product boronates are significantly easier to handle. By careful choice of catalyst, either the branched or the linear product can be obtained in greater than 96% selectivity. Interestingly, common ligands such as BINAP and Josiphos give opposite asymmetric induction with pinacol borane as compared with catechol borane, while P,N-ligands such as Quinap gave the same sense of induction. The hydroboration of 6-methoxynaphthalene proceeded with the greatest regio- (95:5) and enantioselectivity (94:6) of all vinyl arenes examined. The hydroboration product was then employed in a concise synthesis of the nonsteroidal antiinflammatory agent, Naproxen.
The Baeyer-Villiger reactions of acetone and 3-pentanone, including their fluorinated and chlorinated derivatives, with performic acid have been studied by ab initio and DFT calculations. Results are compared with experimental findings for the Baeyer-Villiger oxidation of aliphatic fluoro and chloroketones. According to theoretical results, the first transition state is rate-determining for all substrates even in the presence of acid catalyst. Although the introduction of acid into the reaction pathway leads to a dramatic decrease in the activation energy for the first transition state (TS), once entropy is included in the calculations, the enthalpic gain is lost. Of all substrates examined, pentanone reacts with performic acid via the lowest energy transition state. The second transition state is also lowest for pentanone, illustrating the accelerating effect of the additional alkyl group. Interestingly, there is only a small energetic difference in the transition states leading to migration of the fluorinated substituent versus the alkyl substituent in fluoropentanone and fluoroacetone. These differences match remarkably well with the experimentally obtained ratios of oxidation at the fluorinated and nonfluorinated carbons in a series of aliphatic ketones (calculated, 0.3 kcal/mol, observed, 0.5 kcal/mol), which are reported herein. The migration of the chlorinated substituent is significantly more difficult than that of the alkyl, with a difference in the second transition state of approximately 2.6 kcal/mol.
Since it was first reported in 1899 the Baeyer–Villiger reaction has become an important reaction in organic synthesis. The stereoelectronic components of this reaction have been difficult to observe because of the conformational mobility of the Criegee intermediate. Simply placing a fluorine substituent such that it interacts with the O−O bond of the peroxyester in this intermediate permits the observation of the primary stereoelectronic effect. Lactone 1 (see reaction scheme) is formed with 90 % selectivity, which demonstrates preferential migration of the equatorially substituted carbon and is in agreement with the primary stereoelectronic effect.
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