Isocyanates special

Other isocyanates have also been utilized for the synthesis of p-lactams [31]. However apart from the low yields often produced in the cycloaddition step and the lack of stereoselectivity, most isocyanates, specially CSI [32], are reactive towards several functional groups and such a process is not usually feasible. 

A special problem arises in the preparation of secondary amines. These compounds are highly nucleophilic, and alkylation of an amine with alkyl halides cannot be expected to stop at any specifle stage. Secondary amides, however, can be monoalkylated and lydrolyzed or be reduced to secondary amines (p. 11 If.). In the elegant synthesis of phenyl- phrine an intermediate -hydroxy isocyanate (from a hydrazide and nitrous acid) cyclizes to pve an oxazolidinone which is monomethylated. Treatment with strong acid cleaves the cyclic irethan. 

The dimeri2ation and trimeri2ation of isocyanates are special cases of the cycloaddition reaction ia that they iavolve reageats of the same type. The uacataly2ed carbodiiaiidi2atioa of isocyanates likely iavolves a labile 2 + 2 cycloadduct (12) which Hberates carboa dioxide. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Special reactions of hydrazides and azides are illustrated by the conversion of the hydrazide (205) into the azide (206) by nitrous acid (60JOC1950) and thence into the urethane (207) by ethanol (64FES(19)105Q) the conversion of the same azide (206) into the N-alkylamide (208) by ethylamine the formation of the hydrazone (209) from acetaldehyde and the hydrazide (205) and the IV-acylation of the hydrazide (205) to give, for example, the formylhydrazide (210) (65FES(20)259). It is evident that there is an isocyanate intermediate between (206) and (207) such compounds have been isolated sometimes, e.g. (211). Several of the above reactions are involved in some Curtius degradations. 

Typical of these materials are the poly(vinyl thioethers), the poly(vinyl isocyanates), the poly(vinyl ureas) and the poly(alkyl vinyl ketones). Methyl isopropenyl ketone and certain vinylpyridine derivatives have been copolymerised with butadiene to give special purpose rubbers. 

Few of the many other inorganic compounds of Ge, Sn and Pb call for special comment. Many pseudo-halogen derivatives of Sn, Pb and Pb have been reported, e.g. cyanides, azides, isocyanates, isothiocyanates, isoselenocyanates and alkoxides. ... 

This chapter, therefore, ends the monograph with a potpourri of reactions all of which occur without a change in oxidation state. In many cases, the reaction is one of nucleophilic attack at an electrophilic C-atom. The result is often hydrolytic bond cleavage (e.g., in carbohydrate conjugates, disubstitut-ed methylene and methine groups, imines, oximes, isocyanates, and nitriles, and various ring systems) or a nucleophilic substitution (e.g., hydrolytic de-halogenation of halocarbons and chloroplatin derivatives, and cyclization reactions). The formation of multiple bonds by dehydration is a special case to be discussed separately. 

The intramolecular nitrile oxide-alkene cycloaddition sequence has been used for the assembly of a great variety of natural products. A target that has received special attention is that of taxol (156), undoubtedly due to its unique structural features, its potent anticancer activity, and its hmited availability from natural sources (318,319). In 1984 Kozikowski et al. found that the treatment of nitro dienone 158 (obtained from the p-benzoquinone derivative 157) with p-chlorophe-nyl isocyanate under Mukaiyama conditions afforded the unexpected eight-mem-bered ring 159, which is related to ring B of taxol (156) (Scheme 6.79). 

The advanced applications for nitrocellulose plastisol propellants require that they be integrally bonded to the motor case. Successful case bonding for the multiyear storage life of a rocket calls for special adhesives and liners which are completely compatible with these highly plasticized propellants. Best results have been obtained with a combination of an impervious rubber liner and a crosslinked adhesive system with a limited affinity for the plasticizers used in the propellants. Examples of effective liners are silica-filled butyl rubber and chlorinated synthetic rubber. Epoxy polyamides, isocyanate-crosslinked cellulose esters, and combinations of crosslinked phenol-formaldehyde and polyvinyl formal varnishes have proved to be effective adhesives between propellant and impervious liners. Pressure curing of the propellants helps... 

The esters 35 are obtained by reaction of hydrazides 34 with chloroformate esters or carbonic diesters 3 23 28 42 as previously discussed for the special case of the activated esters (see Section 10.4.2). The amides 36, on the other hand, are formed from hydrazides 34 with isocyanic acid 3,43,44 or with trimethylsilyl isocyanate 42 (Scheme 10). 

Other important tests are for acid and alkalinity number and for water content (266), because water content and alkalinity of the polyether glycol can influence the reaction with isocyanates. The standard ASTM test for acid and alkalinity number, ASTM D4662 (267), is not sensitive enough for the low acidity and alkalinity numbers of PTMEG, and special methods have been developed. A useful alkalinity number (AN) has been defined as milliequivalents KOH per 30 kg of PTMEG, as titrated in methanol solution with 0.005 N HC1 (268). Other useful nonstandard tests are for heavy metals, sulfated ash, and peroxide. The peroxides formed initially in oxidations are quickly transformed into carbonyl groups, which are detectable by infrared spectroscopy. On oxidation, a small C—O peak develops at 1726 cm-1 and can be detected in thick (0.5-mm) films. A relative ratio of this peak against an internal standard peak at 2075 C—O is sometimes defined as the carbonyl ratio. 

Abstract The main computational studies on the formation of (3-lactams through [2+2] cycloadditions published during 1992-2008 are reported with special emphasis on the mechanistic and selectivity aspects of these reactions. Disconnection of the N1-C2 and C3-C4 bonds of the azetidin-2-one ring leads to the reaction between ketenes and imines. Computational and experimental results point to a stepwise mechanism for this reaction. The first step consists of a nucleophilic attack of the iminic nitrogen on the sp-hybridized carbon atom of the ketene. The zwitterionic intermediate thus formed yields the corresponding (3-1 actant by means of a four-electron conrotatoty electrocyclization. The steroecontrol and the periselectivity of the reaction support this two-step mechanism. The [2+2] cycloaddition between isocyanates and alkenes takes place via a concerted (but asynchronous) mechanism that can be interpreted in terms of a [n2s + (n2s + n2s)] interaction between both reactants. Both the regio and the stereochemistry observed are compatible with this computational model. However, the combination of solvent and substituent effects can result in a stepwise mechanism. 

Because of their much higher costs, aliphatic diisocyanates find use mainly in specialized areas where their special properties such as nonyellowing in light are of great importance. The nonyellowing is a result of the aliphatic structure of the isocyanate. There are no series of double bonds that cause the yellowing. 

However, one of the main points to be considered in the making of a polyurethane elastomer (apart from the reactivities of the isocyanate and curative) is the water absorption. Special precautions must be taken to keep a diol curative (such as butanediol) sufficiently dry for use. Diols have to be kept dry with molecular sieves and dry nitrogen blanketing. 

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