Methyl phenylcarbamate

The structural effect of alkyl groups such as methyl, ethyl, and -butyl on the Rp is small. Alkyl 4-methyl-phenylcarbamate can be chosen as a model compound for the hard segment of poly(ether-urethane) (PEU). This group can initiate grafting reaction with Ce(IV) ion and the grafting site was proposed at the hard segment of PEU [3,15] as shown in Scheme (1). 

The binding of l,l -bi-2-naphthol racemate with tris(5-fluoro-2-methyl-phenylcarbamate) was studied by mass spectroscopy and discussed by Yashima... 

Several chiral columns and elution conditions were screened to find the best column to determine the optical purity of nine of the undeiivatized diphenylphosphine and diphosphine oxide ligands describe above. Supelcosil LC-(R)-phenyl and (R)-naphthyl urea based columns and cellulose tris(4-methyl-phenylcarbamate) coated on silica (Chiralcel OG) were very efficient in these separations. All the atropisomers could be analyzed using these three columns (98JC(A795)289,02CHR25). 

Ragaini and colleagues recently studied the influences of acid additives [20-22]. Using the palladium-phenanthroline catalyst system for the carbonylation of nitrobenzene to methyl phenylcarbamate, the addition of anthraniUc acid [20] or phosphorus acids [21, 22] can accelerate the reaction. Anthranilic acid produced higher activity compared with the use of simple benzoic acid. The 4-amino isomer does not show the same increased activity. Later on, they established an improved catalytic system for the carbonylation of nitrobenzene by adding phosphoms acids as an additive, for the first time yielding activities and catalyst fife in the range necessary for industrial applications. By pafladium-phenanthroline complexes and phosphorus acids as promoters, nitrobenzene was carbonylated to methyl phenylcarbamate with unprecedented reaction rates (TOP up to 6,000/h) and catalyst sta-bUity (TON up to 10 ). The best promoter was phosphoric acid, which is very cheap, nontoxic and easily separable from the reaction products. The catalyst system was also applied to the economically very important dinitrotoluenes reduction. 

We also remark that, in the well clarified mechanism of the carbonylation of nitrobenzene to methyl phenylcarbamate catalysed by Ru(CO)3(DPPE) (see... 

Table 1. Carbonylation of nitrobenzene to methyl phenylcarbamate catalysed by Pd(OAc)2 in the presence of chelating ligands and noncoordinating acids. (Data from [124]).

Under the best reported conditions (concerning activity) for the aforementioned catalytic systems (use of Rh(acac)(CO)2, molar ratios Rh/Phen = 1, PhNOa/Rh = 250, PhNH2/Rh = 125, 160 °C, 68 atm in methanol, total volume 75 ml) a total conversion was reached in 2 h (plus 1.5 h required to reach the final temperature), with a 80 % selectivity in methyl phenylcarbamate, 15 % in aniline, 6 % in N-methyleneaniline, and 1 % in N-methylaniline. Use of [Rh(CO)2Cl]2 as catalyst under the same conditions gave a slower, but more selective reaction (3 h at 160 °C to reach complete conversion, with a 89 % selectivity in carbamate) [164], An even higher selectivity (96 %) was reported for the pyridine-promoted reaction (Py = 25 ml), although a lower catalytic ratio (166.7) was used in a reaction run for 5.5 h at 130 °C [165] with [Rh(CO)2Cl]2 as catalyst. Unfortunately, the examples were chosen is such a way to prevent a complete comparison of the different catalyst-cocatalyst combinations imder exactly the same experimental conditions, so that it is impossible to say which is the best. 

Table 2. Carbonylation of PhN02 to methyl phenylcarbamate catalysed by [PPN][Rh(CO)4] . (Data from [166]).

XI/) The examples reported in [19] were clearly chosen so as to avoid to report the best experimental conditions with respect to all variables vide supra). With this limitation in mind, the best reported example involved the reaction of PhN02 (0.100 mol), PhNH2 (0.050 mol) and Ru3(CO)i2 (0.2 mmol) in MeOH (6.4 g, 0.200 mol) and toluene (total volume 75 ml) at 160 °C and under 68 atm CO. Complete conversion was reached in 8.5 h, with a 95 % selectivity in methyl phenylcarbamate and 4 % in additional aniline (the remaining 1 % being probably due to condensation products of aniline and formaldehyde, as evidenced by comparison with other examples). Note that, with respect to the conditions reported in [171], the addition of aniline allows the use of higher catalytic ratios without a drop in selectivity. The addition of an alkylammonium salt under these conditions should result in a further improvement of the rate. 

In order to circumvent the problem of the use of selenium, analogous systems based on the use of sulphur compounds have been developed [85-88]. Aromatic nitro compounds can be reduced by CO in water/methanol media at 120-150 °C and 1-1.5 bar pressure [85, 86]. From nitrobenzene, aniline was obtained with selectivity over 97 % at 100 % PhN02 conversion. The reaction proceeds in the presence of a multicomponent catalyst consisting of a base (preferably a strong base such as sodium hydroxide or methoxide) and sulphur compounds. The ratio of catalytic effectiveness of sulphur compounds is as follows S CS2 H2S COS = 1 1.3 10 10. Vanadium(V) compounds can be added to improve selectivity in aniline formation. Aromatic dinitro derivatives undergo this reaction and selectivity to one of the two main products (phenylenediamine and nitroaniline) can be switched by the choice of reaction conditions. The main byproduct of the reaction of nitrobenzene is PhNHCOOMe [85, 86]. It has been shown that, under the catalytic conditions, methyl phenylcarbamate can be hydrolysed to afford aniline. More forcing conditions (up to 300 bar CO) have also been employed in order to increase the activity [87]. The same catalytic system has been used to reduce nitrophenols to the corresponding aminophenols [88]. 

Up to ca. 30 % azoxybenzene could also be obtained in the [Rh(CO)4] -catalysed carbonylation of nitrobenzene in the presence of methanol [115]. However, the selectivity in azoxybenzene could not be further increased and methyl phenylcarbamate remained the main product under all conditions. 

The kinetic law when PdCl2L2 (L = pyridine, isoquinoline) is used as a catalyst in the absence of any metal promoter has also been reported [44] and is similar to the one previously mentioned. Again the rate is first order in palladium and CO pressure, but zeroth order in nitrobenzene. A complex has been isolated after the reaction with w-chloronitrobenzene as substrate [45] and proposed to be an intermediate in the catalytic cycle. However, the proposed structure, Pd(CO)(Py)(ArNO)Cl2, is inconsistent with the spectroscopic data reported, since a vco = 1920 cm value is much too low to be due to a terminal CO group coordinated to a Pd" complex. Moreover there is no evidence that the isolated complex, whatever it is, is an intermediate in the reaction pathway. The kinetics of a model PdCl2/FeCl3 catalytic system for the carbonylation of nitrobenzene to methyl phenylcarbamate has also been investigated [46], but the paper is only available in Russian. 

The isocyanate formed in the previous step of the catalytic cycle reacts then in solution with aniline or methanol to afford respectively diphenylurea or methyl phenylcarbamate. The reaction with aniline is much fester then the one with methanol and, if both the reagents are present in solution, only urea is formed, unless the aniline/isocyanate ratio is lower than one. Since, however, the carbamate is the thermodinamically fevoured product, urea alcoholysis may then occur at a variable extent, depending on the reaction conditions. Under typical catalytic reaction conditions, where an aniline is added in large amount, urea is consequently inferred to be the exclusively primary product of the reaction of the isocyanate, although it may not be present at all at the end of the reaction. 

It should be noted that, by changing the chelating phosphine, other complexes and catalytic cycles may be possible. In one paper of the series here discussed [147], a dimeric complex, Ru2(p-DMPM)2(p-CO)(CO)4 (DMPM = 1,2-bis(dimethylphosphino)methane), has been reported to be a catalyst for the carbonylation of nitrobenzene to methyl phenylcarbamate. The complex appears (by high-pressure IR spectroscopy) to retain the dinuclear structure during the reaction. A protonation equilibrium exists in solution (eq. 25) ... 

Chankvetadze B, Yashima E, Okamoto Y. Chloro-methyl-phenylcarbamate derivatives of cellulose as chiral stationary phases for high performance liquid chromatography. J Chromatogr A 1994 670 39-49. 

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