Isocyanates nucleophilic additions

Because of their relative instabiUty, primary phosphine oxides caimot be isolated and must be converted direcdy to derivatives. Primary and secondary phosphine oxides undergo reactions characteristic of the presence of P—H bonds, eg, the base-cataly2ed nucleophilic addition to unsaturated compounds such as olefins, ketones, and isocyanates (95). 

Polyurethane Formation. The key to the manufacture of polyurethanes is the unique reactivity of the heterocumulene groups in diisocyanates toward nucleophilic additions. The polarization of the isocyanate group enhances the addition across the carbon—nitrogen double bond, which allows rapid formation of addition polymers from diisocyanates and macroglycols. 

The isocyanate formed on rearrangement adds water in a nucleophilic addition step to yield a carbamic acid. 

A urethane is typically prepared by nucleophilic addition reaction between an alcohol and an isocyanate (R—N = C=0), so a polyurethane is prepared by reaction between a cliol and a diisocyanate. The diol is usually a low-molecular-weight polymer (MW 1000 amu) with hydroxyl end-groups the diisocyanate is often toluene-2,4-diisocyanate. 

Problem 31.9 Show the mechanism of the nucleophilic addition reaction of an alcohol with an isocyanate to yield a urethane. 

Reactions of stable mesito- and duronitrile oxides with 1-chloroalkyl isocyanates R R2CC1NC0 (R1 = CF3, R2 = Ph, 4-MeC6H4 R1 = CC13, R2 = H) gave oxadiazolones 176. The double adducts are formed by the cycloaddition of one nitrile oxide molecule at the isocyanate C=N bond and the nucleophilic addition of the chloroalkyl moiety to a second nitrile oxide molecule (344). 

It was found that the 15-membered macrocyclic complexes were significantly less acidic. Similar reversible protonations have been shown to occur in related macrocyclic complexes36 37 and this work has been developed by Busch into a major study of ligand reactions,38 which are summarized in equations (16)—(19). It is significant that these reactions include not only typical substitution reactions such as acylation and nitration (equations 16 and 19), but also nucleophilic addition to isocyanates (equation 17) and to a, 3-unsaturated esters (equation 18). 

Urea derivatives are of general interest in medidnal chemistry. They may be obtained either from urea itself (barbiturates, see p. 306) or from amines and isocyanates. The latter are usually prepared from amines and phosgene under evolution of hydrogen chloride. Alkyl isocyanates are highly reactive in nucleophilic addition reactions. Even amides, e.g. sulfonamides, are nucleophilic enough to produce urea derivatives. 

Less satisfactory results have been frequently encountered in the related asymmetric cycloaddition of a vinyl epoxide to an isocyanate as in Scheme 8E.30 [160]. The modest enantioselectivities of this process are indicative of the competitive intramolecular nucleophilic addition with enantioface exchange. When the oxazolidinone was generated from an achiral substrate, somewhat higher enantioselectiviites were obtained presumably due to superposition of the enantioselection obtained in the ionization step. 

The mechanism for isocyanate reactions also consists of the nucleophilic addition of an active hydrogen compound, AH, to the electron-poor carbonyl atom ... 

The reactions of 3-amido-l-hydroxyazetidin-2-ones 327 with />-toluenesulfonyl chloride in the presence of triethyl-amine provided the ring-expanded 4-imidazolin-2-ones 328 (Equation 114) <1995TL1617>. The key step in the mechanism of this reaction involved the ring opening by cleavage of the C(2)-G(3) bond, which is followed by double bond migration and an intramolecular nucleophilic addition of the amide nitrogen to an intermediate isocyanate. 

A urethane (also called a carbamate) is a compound that contains a carbonyl group bonded to both an OR group and an NHR (or NR2) group (Section 28.6). Urethanes are prepared by the nucleophilic addition of an alcohol to the carbonyl group of an isocyanate, RN=C=0. 

Several syntheses of aminodeoxypentoses have employed a similar approach in which a three-carbon starting material is condensed with a two-carbon entity. For instance, the nucleophilic addition of methyl isocyanate to R)-2A is highly diastereoselective giving a mixture of erythro and threo adducts (Scheme 13.36). The major adduct is then converted into methyl 2-amino-2-deoxy-o-arabonate [77]. 

The high reactivity of CSI is probably associated with the powerful electronwithdrawing character of the chlorosulfonyl moiety, which activates the adjacent isocyanate group with respect to nucleophilic addition. The [2 + 2] cycloadditions with alkenes may therefore be depicted as in Scheme 33. 

Several new complexes of iminophosphoranes have been reported by Cavell and co-workers (Scheme 9). Reaction of dimethyl zinc with bis- trimethylsilyl-imino-diphenylphosphorano methane (61) yields zinc complex (62) via elimination of methane. Complex (62) did not react with adamantyl nitrile or isonitrile but does undergo nucleophilic addition reactions with hetereroallenes such as carbodiimides and isocyanates. Thus, treatment of (62) with adamantyl isocyanate produces the novel tripodal alkyl zinc complex (63). Iminophos-phorane (61) reacts with [Sm(NCy2)3(THF)] and tetrakis(benzyl)zirconium(IV)... 

Polyurethanes are produced by the reaction of diols with diisocyanates. Nucleophilic addition of an alcohol to an isocyanate produces the urethane functional group. Although no (small molecule) by-products are produced, the urethane bonds are formed in discrete steps. 

---