Thermal racemization

The relative ease with which aryl benzyl sulfoxides undergo homolytic dissociation (Rayner et al., 1966) as compared to aryl benzyl sulfides or sulfones is supportive of this idea that ArSO radicals are easier to form than ArS or ArS02 radicals. Another interesting set of observations is the following. Booms and Cram (1972) found that optically active arene-sulfinamides ArS(0)NRPh (R = H or CH3) racemize thermally very readily at room temperature and that this racemization is the result of a free radical chain reaction (160) that is initiated by the dissociation of some of the sulfinamide into an ArSO and a PhNR radical (159). While the length of the inhibition... 

Perhaps the most intriguing observation of helicenes is the unexpected ease with which these compounds racemize thermally. According to Martin 1031 three pathways for the thermal racemization can be considered 1) via bond breaking 2) via an internal double Diels-Alder adduct, and 3) via a direct inversion. Martin rejected the first possibility because it is not in accordance with kinetic data for the racemization. He could exclude the second possibility in an elegant way by using appropriately substituted derivativesl03>. 

Nonetheless, the experimental evidence overwhelmingly favors S-0 homolysis in most instances. Sometimes 0-C cleavage would go unnoticed because most of the reactions studied are cyclic systems derived from sulfoxides. However, Guo found that photolysis of benzyl benzenesulfenate did not lead to phenyl benzyl sulfoxide [60], even though the latter compound is known to racemize thermally through homolytic cleavage and reclosure of the sulfi-nyl/benzyl radical pair [71]. 

Disubstituted allenes are chiral, and when resolved, can be racemized thermally in a first-order reaction. Roth determined log 13.61 — 46 170/2.3Rr for the racemization of 2,3-pentadiene at 1-2 in a pressure-independent reaction. The pathway most likely involves a 90° rotation around one rr-bond to give an allylic biradical where the electrons are in mutually perpendicular orbitals (Scheme 4.1). 

L-Glutamic acid does not racemize in neutral solution, even at 100°C. Deviation of pH from neutral to greater than 8.5 results in thermal racemization with loss of taste characteristics. Racemization in neutral solution occurs at 190 °C after formation of the lactam, 5-oxo-L-proline, pyroglutamic acid [98-79-3]. 

A mechanism for this reaction has been proposed [75], The first key intermediate in the reaction is the copper(I) acetylide 42. The additional ligand may be solvent or H2O. The acetylene moiety in 42 is activated for a 1,3-dipolar cycloaddition with the nitrone to give intermediate 43, with introduction of chirality in the product. A possible route to ris/traws-41 might be via intermediate 44. Finally, the cis isomer is isomerized into the thermally more stable trans-41. It should be mentioned that the mechanism outlined in Scheme 6.32 was originally proposed for a racemic version of the reaction to which water was added. 

A new brush-type CSP, the Whelk-0 1, was used by Blum et al. for the analytical and preparative-scale separations of racemic pharmaceutical compounds, including verapamil and ketoprofen. A comparison of LC and SFC revealed the superiority of SFC in terms of efficiency and speed of method development [50]. The Whelk-0 1 selector and its homologues have also been incorporated into polysiloxanes. The resulting polymers were coated on silica and thermally immobilized. Higher efficiencies were observed when these CSPs were used with sub- and supercritical fluids as eluents, and a greater number of compounds were resolved in SFC compared to LC. Compounds such as flurbiprofen, warfarin, and benzoin were enantioresolved with a modified CO, eluent [37]. 

Several applications of this methodology to the synthesis of racemic a-substituted allylboronates are provided in refs 2-4. It is noted that reagents 6 (X = Br) and 7 are unstable with respect to ailyl rearrangement of the halide ions, either thermally or in tile presence of halide ion, and so care must be exercised in the preparation and handling of a-haloallvlboronates. 

Isothermal and dynamic studies [1286] of the thermal deamination and racemization reactions of (+)5 9-[Cr(en)3](NCS)3 gave activation energy values 113 and 100 kJ mole"1, respectively. The mechanisms of these and... 

The numerous examples of optically active sulfoxides reflect their configurational stability. Optically active sulfoxides resist thermal racemization by pyramidal inversion, so... 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow (K 10-5 s-1)194. The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur238, and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

In order to account for the unusually facile thermal racemization of optically active allyl p-tolyl sulfoxide (15 R = p-Tol) whose rate of racemization is orders of magnitude faster than that of alkyl aryl or diaryl sulfoxides as a result of a comparably drastically reduced AH (22kcalmol- ), Mislow and coworkers44 suggested a cyclic (intramolecular) mechanism in which the chiral sulfoxide is in mobile equilibrium with the corresponding achiral sulfenate (equation 10). 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

As typical examples of crystal-to-crystal thermal reactions, the cyclization of allene derivatives to four-membered ring compounds and the transformation of a racemic complex into a conglomerate complex are described. 

The tandem-Knoevenagel-ene reaction is a powerful tool to synthesize five-and six-membered carbocycles.2 5 The process is exemplified by the diastereoselective synthesis of 4a. Compound 4a has been obtained In both enantiomeric forms and as a racemate according to the procedure described here. The sequence includes the Knoevenagel reaction of citronellal, 1, and dimethyl malonate, 2, followed by the intramolecular ene cyclization of the chiral 1,7-diene 3 to yield the trans 1,2-disubstituted products 4a and 4b. Whereas the thermal cyclization of 3 at 160°C provides 4a and 4 b in a ratio of only 89.7 10.3, the Lewis acid... 

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