Carbamate complexes

All these complexes react further with CO2 to form the corresponding N,0-bound carbamate complexes, which, in turn, extrude Bu NCO (above —25 °C for the methylcyclopentadienyl complexes and at room temperature in the case of the... 

H NMR data has been reported for the ethylzinc complex, Zn(TPP—NMe)Et, formed from the reaction of free-base N-methyl porphyrin H(TPP—NMe) with ZnEti. The ethyl proton chemical shifts are observed upheld, evidence that the ethyl group is coordinated to zinc near the center of the porphyrin. The complex is stable under N2 in the dark, but decomposed by a radical mechanism in visible light.The complex reacted with hindered phenols (HOAr) when irradiated with visible light to give ethane and the aryloxo complexes Zn(TPP—NMe)OAr. The reaction of Zn(TPP—NMe)Et, a secondary amine (HNEt2) and CO2 gave zinc carbamate complexes, for example Zn(TPP—NMclOiCNEti."" ... 

Zinc carbamate complexes are well known, and the structural types and stabilities can be compared with thiocarbamates and dithiocarbamates which are discussed in Sections 6.8.11.1.3 and 6.8.7.1.4482 Carbamates of zinc can be formed from the reaction of carbon dioxide with alkylzinc alkyl amides and further reaction with alkylzinc can give a distorted cubane structure.483 The tetrameric diethylcarbamate species initially formed can also be used to produce monomeric or dimeric carbamate structures in reaction with amines tetramethylethylenediamine forms a monomer [(Me2NCH2)2Zn(02CN(C2H5)2)2] with an octahedral zinc center and pyridine forms a dimer[CsH5NZn2Me(02CN(C2H5)2)3] with tetrahedral zinc centers.484... 

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. 

The antidote for carbamate poisoning is atropine only. 2-PAM will stabilize the enzyme-carbamate complex and so exacerbate the poisoning. 

Manoussakis et al,46 have synthesized a series of selenocarbamate six-coordinate complexes of arsenic (20 and Table 8 cf. As—S analogues). By reacting the piperidyl derivative with halogens dissolved in CCU a series of haloselenato carbamate complexes are produced which are probably pentacoordinate (equation 13). 

In the case of amine nucleophiles, the products from the reaction with co-ordinated cyanates are carbamates or ureas (Fig. 4-34), and this provides a particularly convenient method for the preparation of carbamate complexes. An example of this behaviour is seen in the reaction of 3,5-dimethylpyrazole with cyanate in the presence of copper(n) salts (Fig. 4-35). Similar reactions are observed with co-ordinated thiocyanates and other heterocumulene s. 

Hintelmann, H. and Falter, R. (1996) Traps and pitfalls of the distillation/ethylation and distillation/carbamate-complexation methods for MeHg determination. In Proceedings of the Fourth International Conference on Mercury as a Global Pollutant, Hamburg, September 1996, Book of Abstracts, p. 284. 

Two vanadium(iv) ethylxanthate complexes have been prepared 440 their e.p.r. parameters are practically identical with those of the corresponding diethyldithio-carbamate complexes and on this basis analogous structures were proposed, i.e. a dodecahedral eight-co-ordinate structure with four bidentate ethylxanthate ligands and a distorted tetrahedral structure with four unidentate ligands. Addition of phosphorus(iii) ligands (L) such as trialkylphosphites to solution of the ethyl xanthate... 

Not only is the methodology non-specific, but the determination itself can be questionable because of the instability of the enzyme-carbamate complex. As with cholinesterase determinations for organophosphates, the method can be highly variable even within the same laboratory. 

Alkyl carbamates such as these cannot be deprotonated in the absence of sparteine or TMEDA - the reaction is truly an instance of ligand accelerated catalysis (neither can they be deprotonated by rc-BuLi or r-BuLi-(-)-sparteine). Furthermore, if the j -BuLi-H-sparteine complex is first treated with a non-deprotonatable O-isopropyl carbamate, it becomes inactive towards deprotonation of further carbamates presumably a BuLi-sparteine-carbamate complex is formed irreversibly, and we can deduce that such a complexation is the first step in the deprotonation of other alkylcarbamates.176... 

R = alkyl, aryl) undergoes insertion reactions with C02, COS, CS2, and CSe2 to form carbamate complexes and reacts with alcohols to form alkoxide complexes. The structural motif of the amido complexes is highly dependent on the steric bulk of the R group. When R = phenyl, the complex is... 

Some monothio-carboxylate, -carbonate and -carbamate complexes have also been synthesized but these all contain P donor ligands (see Section 45.5.4.8.v). 

In the reaction of iridium complexes 54 with two equivalents of carbodiimides, mixtures of cyclic carbamate complexes 55 and organometaUic guanidinates 56 are obtained. ... 

Complexes derived from Co and Rh afford only the cyclic carbamate complexes. ... 

Carbamate complexes are synthesized by carbonation of metal dialkylamides (equation 20), or by reaction of the appropriate amine, carbon dioxide, and a metal halide (equation 21). In both cases, it has been suggested that the reaction involves combination of the amine and carbon dioxide to form the carbamate anion, followed by anion exchange. ... 

Organozinc carboxylates formally derived from carboxylic acids are prepared via protonolysis of diorganozinc compounds by carboxylic acids (equation 20). Although dimeric carbamate complexes of the general formula [RM(E2CNR )]2 (E = S or Se, R = Me or Et, R = alkyl, M = Zn or Cd) are readily formed by reaction of CE2... 

A solution of both ammonium pyrrolidinedithiocarbamate and diethyldithio-carbamate has been used for eomplexing iron (Co, Ni, Cu, Zn, Cd, Pb) in sea water. The metal carbamate complexes are extracted from 500 ml of sea water at ca. pH 5 into 30 ml of Freon TF (l,l,2-trichloro-l,2,2-trifluoroethane) and back-extracted into 10 ml of 0.3 M nitric acid. The main advantage of this method is the transfer of the metals to a solution in which their concentrations do not change with time 70). 

Carbamate complexes of the formula M(02CNR2)4 (M = Zr, Hf R = Et,445 Prl491) have been prepared by reaction of the M(NR2)4 with C02. Crystallographic studies have confirmed eight-coordinate geometry about Zr or Hf. 

Other suitable catalysts for the carboxymethylation of aromatic amines are zinc or copper carbamates. In 1997 it was patented a process " for the preparation of carbamates Ifom an aromatic amine and DMC in the presence of N,N-substituted carbamate complexes of zinc and copper. Table 6 summarises results reported in some of the patent examples, high selectivity was reported with aniline, while with MDA a considerable amount of N-methyl derivatives was foimd. 

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