Isocyanates, reaction with alcohols

Alkyl carbamates (urethanes) ate formed from reaction of alcohols with isocyanic acid or urea (see Urettpane polymers). 

Reactions with Isocyanates. The reaction of alcohols with isocyanates to form carbamates is well known and similar reactions with poly(vinyl alcohol) would be expected. Until recently, the only available reaction conditions were to use a heterogeneous reaction mixture or to run the reaction in a poor solvent for poly(vinyl alcohol). The best poly(vinyl alcohol) solvents, water and formaide derivatives, react rapidly with isocyanates. Nevertheless, several such reactions have been run in the past and we will cite only a few of them. A potentially photosensitive polymer was made by the reaction of allyl isocyanate with poly(vinyl alcohol) (57) and several workers have crosslinked poly(vinyl alcohol) with hexamethylene diisocyanate (58.59). 

Regioselective C-2 amination of 2,3-epoxy alcohols can be realized by intramolecular oxirane ring opening of the carbamates prepared by the reaction of the epoxy alcohols with isocyanates." -" In some cases, however, acyl transfer is a serious side reaction (Scheme 59)." ... 

Carbamates of tertiary alcohols. The reaction of tertiary alcohols with isocyanates catalyzed by base usually results in dehydration as well as carbamate formation. However, carbamates of tertiary alcohols can be obtained in yields as high as 90% by use of organotin catalysts (stannous octoate, stannous oleate, dibutyltin dilaurate) and an amine. 

These reactions can be divided into two categories. They are additions to compounds with active hydrogens and self-condensations. The reactions are well described in organic chemistry textbooks, so there is little reason to describe them here. It is noteworthy, though, that the uncatalyzed reactions of isocyanates with various active hydrogen compounds are probably broadly similar. Among them, the most investigated reactions are those of alcohols with isocyanates... 

Abbate FW, Ulrich H (1969) Urethane organometallic catalysis of the reaction of alcohols with isocyanates. Journal of Applied Polymer Science 13 1929-1936. 

The reaction of amines with isocyanates is analogous to the reaction of alcohols with isocyanates, and the same rules should be observed. However, the reaction with amines takes place at a much higher rate than with alcohols, so that it can also be carried out in an alcoholic medium. This is not recommended, however, in the case of phenylisocyanate. Therefore, although the reagent gives derivatives with suitable properties, the use of a-naphthy-lisocyanate should be given preference, as it is less sensitive toward water, so that an aqueous medium can also be used for the preparation of derivatives. 

The detailed mechanism of the reaction of alcohols with isocyanates is not known. In particular, the steps in the attack of the strongly polarized carbon atom of the isocyanate group by the oxygen atom of the hydroxyl are not well understood. 

Similarly, thioalcohols and thiophenols react with isocyanates to form thiocarbamates. Although these reactions are generally found to be much slower than that of the corresponding alcohol, alkoxide catalysts have successfully been used to provide moderate levels of rate enhancement (68). 

Reaction of the glycol, 70, affords an oxazolidinone rather than the expected carbamate (71) on fusion with urea. It has been postulated that the urea is in fact the first product formed. This compound then undergoes 0 to N migration with loss of carbon dioxide reaction of the amino alcohol with the isocyanic acid known to result from thermal decomposition of urea affords the observed product, mephenoxolone (74) this compound shows activity quite similar to that of the carbamate. An analogous reaction on the glyceryl ether, 75, affords metaxa-lone (76). 

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

I. S. Bechara, The Mechanism of Tin-Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols, in Urethane Chemistry and Applications, ACS Symposium Series 172, K. N. Edwards, (Ed.), American Chemical Society, Washington, DC, 1981. 

Methyl 3-(triphenylphosphoranylideneamino)pyrazine-2-carboxylate 33 has been synthesised and used to obtain pteridines 34, 35 by reaction with isocyanates followed by adding alcohols or amines (Scheme 5)<96JCS(P1)247>. 

Bonded phases have been prepared by other general methods besides those indicated so far [64-66]. Reaction of silica with an alcohol or isocyanate resulted in the formation of silicate esters (estersils), but these phases were too hydrolytically unstable to be generally useful. Bonded phases with an Sl-R or Sl-NHR structure are more hydrolytically stable than the estersils but... 

Recently, poly(vinyl alcohol)-isocyanate reactions have been run successfully in dimethylsulfoxide solutions by several lab- oratories, including ours. While there are reports of solvent derived products from the reactions of isocyanates in DMSO (60), these reactions do not seem to pose a problem in the reaction with poly(vinyl alcohol). This solvent has been used to react PVA with methoxymethyl isocyanate (61.62) and a series of isocyanates including methyl, ethyl, isopropyl, phenyl and 1-naphthyl (63). This reaction is illustrated in Equation 17 where R is... 

A theoretical study of the reaction of water and methanol with HNCO has led to a prediction of a four-centred transition state for both reactions. The interactions of water and of alcohols with alkyl isocyanates have been the subject of both experimental and theoretical study. In the case of hydration, evidence for initial interaction of water and water clusters (n = 1-3) across the N=C bond of the alkyl isocyanate... 

The weak nucleophilic nature of polynitroaliphatic alcohols is also reflected in their slow reactions with isocyanates to yield carbamates. These reactions often need the presence of Lewis acids like ferric acetylacetonate or boron trifluoride etherate. The reaction of bifunctional isocyanates with polynitroaliphatic diols has been used to synthesize energetic polymers.33°... 

To ensure a strong bond between liner and insulation as well as propellant to liner, it is necessary that liner as well as propellant cure well at the interfaces. This means that in many cases the rubber insulation must undergo some treatment to remove substances which may interfere with the liner cure. Such substances are usually low molecular weight compounds and can often be removed by heating—e.g., water, which would otherwise react with isocyanate in a polyurethane liner. In addition the insulation and/or the cured liner surface may be washcoated with a cure catalyst which will increase the reaction rate of alcoholic hydroxyl groups over the rate of reaction of water with isocyanate to such an extent that the latter reaction can no longer compete with the cure reaction. 

Isocyanates readily react with primary alcohols at 25°-50°C, whereas secondary alcohols react 0.3 as fast, and tertiary alcohols approximately 0.005 as fast as the primary [2]. Thus, the effect of steric hindrance shows a pronounced effect on these reactions. For example, triphenylcarbinol has been reported to be completely unreactive [3a, b]. Other tertiary alcohols such as f-butanol may react under uncatalyzed conditions with isocyanates to give olefin formation, as shown in Eq. (4), using phenyl isocyanate [4, 5]. 

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