Carbamate catalysis

Another significant use of 3-methylphenol is in the production of herbicides and insecticides. 2-/ f2 -Butyl-5-methylphenol is converted to the dinitro acetate derivative, 2-/ f2 -butyl-5-methyl-4,6-dinitrophenyl acetate [2487-01 -6] which is used as both a pre- and postemergent herbicide to control broad leaf weeds (42). Carbamate derivatives of 3-methylphenol based compounds are used as insecticides. The condensation of 3-methylphenol with formaldehyde yields a curable phenoHc resin. Since 3-methylphenol is trifunctional with respect to its reaction with formaldehyde, it is possible to form a thermosetting resin by the reaction of a prepolymer with paraformaldehyde or other suitable formaldehyde sources. 3-Methylphenol is also used in the production of fragrances and flavors. It is reduced with hydrogen under nickel catalysis and the corresponding esters are used as synthetic musk (see Table 3). 

Friesen et al. reported that 2,3-allenols 340 can be converted to the corresponding carbamate derivatives 343, in which the terminal C=C bond of the allene moiety can be iodinated to afford diiodides 344. Under the catalysis by an Ag+ salt, compounds 344 react to give iminocarbonates 345 and oxazolindinones 346, leading to diols 347 and amino alcohols 348, respectively, after hydrolysis (Scheme 10.138) [160-162], A similar reaction was observed with trichloroacetimidates 350 [163],... 

Fig. 11.11. Mechanism postulated for hydrolysis of organic isocyanates involving electrophilic addition of H20 with general base catalysis. The product of hydration is a carbamic acid that spontaneously decomposes to the primary amine with loss of C02 [117].

The first example of anti-Markovnikov addition of O-nudeophiles to terminal alkynes was the catalytic addition of ammonium carbamates, generated in situ from secondary amines and carbon dioxide, to terminal alkynes, which regioselectively produced vinylcarbamates (Scheme 10.1) [5]. It was also the first time that a metal vinylidene was suggested as an active intermediate in catalysis [5]. 

Since base catalysis plays an important role in the hydrolytic breakdown of carbamates, this process has been investigated quite extensively. However, very few data are available on the neutral reaction, which, in some cases, may be significant at ambient pH values. The acid-catalyzed reaction, on the other hand, can generally be neglected. This is presumably because so many electron-withdrawing atoms surround the central carbon that protonation of the carboxyl oxygen insignificantly enhances its susceptibility to nucleophilic attack. 

Wang resin bound 4-nitrophenyl carbonate is a convenient intermediate for the attachment of amines to polystyrene as carbamates (see Experimental Procedure 14.2). Aliphatic amines [82-87], ammonia [88], and amino acids [89] react exothermically with this support, whereas anilines generally require catalysis and/or long reaction times (Entry 3, Table 14.7). For the immobilization of anilines as carbamates, Wang resin derived chloroformate [90-92] generally leads to better results than resin-bound 4-nitrophenyl carbonates. Amidines also react with polystyrene-bound 4-nitrophenyl carbonates to yield /V-alkoxycarbonyl amidines (Section 3.9 [93-95]). Support-bound alkoxycarbonyl hydrazines can be prepared by treating polystyrene-bound phenyl carbonates with hydrazine [96-98]. 

Polystyrene-bound allylsilanes react with /V-(alkoxycarbonyl)imincs under Lewis acid catalysis to yield /V-homoallylcarbamates (Entry 4, Table 14.9). Similarly, Wang resin bound carbamates have been successfully N-alkylated with allylsilanes and aldehydes in a Mannich-type reaction (Entry 5, Table 14.9). Resin-bound /V-(alkoxycarbo-nyl)imines can be generated either from unsubstituted carbamates ROCONH2 by... 

Catalysis by cobalt(III) has been the subject of several papers.185-187 The N.N-bis(sahcyldene)ethylnediaminocobalt(III)-catalysed oxidative carbonylation of o-, m-and -substituted primary aromatic amines in MeOH gives ureas, isocyanates, carbamates, and azo derivatives. A Hammett p value of —0.5 for the reaction indicates that electrophilic attack of CO at a nitrogen anion complexed to Co in the TS is... 

A new copper-catalyzed reaction involving imines, acid chlorides, and alkynes has been applied to the synthesis of propargyl amides 160 in a single operation by Arndtsen and co-workers. The same method allows the synthesis of N-carbamate-protected propargylamines [149]. a-Substituted amides 161 may also be prepared under palladium catalysis by substituting alkynes for vinyltin (Scheme 8.71) [150]. 

These reactions illustrate the importance of ruthenium vinylidene species, as activated forms of terminal alkynes, in catalysis, because they favor the addition of O-nudeophiles (carbamic and carboxylic acids, alcohols, water) to terminal alkynes and completely reverse the expected regioselectivity of the addition. These examples also show that the activation processes are very sensitive to the nature of the nucleophiles, and the success of the awtt-Markovnikov addition to terminal alkynes is highly dependent on both the electron richness and steric hindrance of the ancillary ligands coordinated to the active site. 

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