Oxidative attack carbamates

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

C(l) in 355 is in the oxidation state of an aldehyde or a ketone in 1-alkylated products. The necessary solvolytic attack of the enol carbamate double bond requires substoichio-metric amounts of a catalyst such as mercuric acetate or palladium chloride and one equivalent of acid (e.g. methanesulphonic acid) for binding the liberated diisopropylamine. 

Padwa has shown that rhodium-catalyzed oxidation of indolyl carbamate 67 employing either Phl(OAc)2 or Phl=0 follows a path similar to that of the D-aUal carbamate (Scheme 17.26) [95]. In principle, indole attack of the putative rhodium-nitrene generates zwitterion 68, which is trapped subsequently by an exogenous nucleophile. Spiro-oxazolidinone products (for example, 69) are isolated as single diastereomers in yields ranging from 50 to 85%. As an intriguing aside, Padwa has found that certain carbamates react with Phl=0 in the absence of any metal catalyst to furnish oxazoHdinone products. This result may have implications for the mechanism of the rhodium-catalyzed process, although it should be noted that control experiments by Espino and Du Bois confirm the essential role of the metal catalyst for C-H amination [57]. 

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... 

Pd(0)/phosphine complexes, or their precursors, in the presence of a suitable co-base, have also been shown to promote, in good yields (66-100%), the formation of allylic carbamates from various primary and secondary aliphatic amines, pressurized C02 and allylic chlorides, in THF, at ambient temperature [87a]. The choice of the added co-base (Base), used for generating the carbamate salt RR NC02 (BaseH)+, was found to be critical for high yields of O-allylic urethanes. The use of a guanidine (CyTMG) or amidine (DBU) base was optimal for this system (see also Section 6.3.1). ft is assumed that this chemistry passes catalytically through a mechanism similar to that illustrated in Scheme 6.19. This involves nucleophilic attack by carbamate anion on a (tt-allyl) palladium species, formed by the oxidative addition of the allylic chloride to a palladium(O) intermediate. 

In the synthesis of carbamates, R NH.C02R, from A iV -dialkylureas, (R NH)2CO, and dialkyl carbonates, (RO)2CO, dibutyltin oxide, Bu2SnO, acted as an efficient catalyst. The proposed mechanism (Scheme 13) involves addition of the dialkyl carbonate to Bu2SnO to give an adduct (43), which is attacked by the urea to yield a new tin complex (44) and one molecule of carbamate. Attack by dialkyl carbonate upon this complex (44) yields a further molecule of carbamate and regenerates the original tin complex (43), which can continue the catalytic cycle.42... 

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