Mechanisms browning

Scheme 24 Brown mechanism for the conjugate addition of organoboranes to methyl vinyl ketone...

Batt and Gowenlock27 heated CH3NO to 254-396°C and found that the addition of NO enhanced the removal of CH3NO. A product was formed that could be trapped at — 196°C. On heating the product, a gas which was noncondensable at — 196°C was liberated. They presumed the product to be the intermediate in the Brown mechanism. 

Kato, H. Yamada, Y. Izaka, K. Sakurai, Y. Studies on browning mechanisms of soybean products. I. Separation and identification of 3-deoxyglucosone occurring in soy sauce and miso. Nippon Nogei Kagaku Kaishi 1961, 35, 412-14. 

In this article, we will compare the energetics of the conventional Halpern mechanism with that of the Brown mechanism. The basis functions used are the 3-21G for ethylene and hydrides, the ST0-2G for spectator ligands, PH and Cl, and valence double zeta basis functions for Rh with effective core potential replacing the core electrons (up to 4p) (5a.b.6). In addition, we carried out the MP2 calculations at selected, RHF-optimized structures with a larger basis set, which consists of uncontracted (3s,3p,4d) functions from the above valence DZ set for Rh, 4-31G for the ethyl group,... 

Figure 5. Optimized structures (in A and deg) of some important species in the Brown mechanism.

In the Brown mechanism, setting aside the high energy required for isomerization from 3 to 3b, the final step of the reductive elimination would require the highest activation energy. Olefin insertion, the rate-determining step in the Halpern mechanism, is an easy process in this mechanism. The kinetics of the Brown mechanism is thus expected to be completely different from that of the Halpern mechanism. Therefore, in the cases in which olefin insertion has been found to be rate-determining, the Halpern mechanism is clearly more consistent and acceptable. 

The above feature of the Brown mechanism that reductive elimination is more difficult than olefin insertion may be related to the nature of catalyst having a chelating bidentate ligand such as DIPHOS. Halpern have also investigated the hydrogenation (Equation 3) (2), where isomerization from the trans- to cis-biphosphine complex is not necessary. 

In this work, we have compared the potential energy profiles of the model catalytic cycle of olefin hydrogenation by the Wilkinson catalyst between the Halpern and the Brown mechanisms. The former is a well-accepted mechanism in which all the intermediates have trans phosphines, while in the latter, proposed very recently, phosphines are located cis to each other to reduce the steric repulsion between bulky olefin and phosphines. Our ab initio calculations on a sterically unhindered model catalytic cycle have shown that the profile for the Halpern mechanism is smooth without too stable intermediates and too high activation barrier. On the other hand, the key cis dihydride intermediate in the cis mechanism is electronically unstable and normally the sequence of elementary reactions would be broken. Possible sequences of reactions can be proposed from our calculation, if one assumes that steric effects of bulky olefin substituents prohibits some intermediates or reactions to be realized. 

Alder, Roger W., Ray Baker, and John Michael Brown. Mechanism in Organic Chemistry. New York Wiley-Inter-science, 1971. 

Fig. 2 PES of the Brown mechanism for hydrogenation with Wilkinson s catalyst...

Lynn jr., W. S., and R. Brown Mechanism of in vitro steroid oxidation. BiocWm. biophys. Acta (Amst.) 21, 403 (1956). 

Brown mechanical pulp board is made predominantly from recycled fibers and has a brown colored cover and reverse. It is used for the production of maihng boxes. 

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