Isocyanates vinyl

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. 

Perhaps the most thoroughly investigated approach is the modification of preformed polymers in particular poly(vinyl isocyanate) and polyacrylonitrile Figure 29.21). 

A new approach to indoloquinohne alkaloids from Cryptolepis sanguino-lenta has been reported based on the cyclization of an o-substituted vinyl isocyanate 244 imder microwave irradiation and further additional cyclization based on an Aza-Wittig reaction carried out in the presence of microwaves [159]. The apphcation of this synthetic scheme to the synthesis of Cryptotackienine 247 is reported in Scheme 91. 

Other notable methods involve Hetero Diels-Alder reactions [20,21], and the reaction of vinyl isocyanates with enamines [22,23]. 

The rearrangements 67 —> 70, 71 —> 72 and 74 —> 75 include the transformation of conjugated dienes to cumulenes. Nevertheless, these reactions take place with very high yields in some cases, because either an irreversible step of hydrolysis such as 69 —> 70 is involved or the very exothermic transformation from cyanates to isocyanates is used. Comparison of the energies, calculated by ab initio methods [121], shows that, for example, the energy of methyl isocyanate is lower than that of methyl cyanate by 26.8 kcal mol-1 and that of vinyl isocyanate is lower than that of vinyl cyanate by 28.1 kcal mol-1. 

The other major approach to systems of this type is cycloaddition of cumulene ylides with vinyl isocyanate and vinyl isothiocyanate. Thus, Ph3P=C=C=0 reacts with these two reagents to give 160 and 161 respectively while the corresponding reactions of Ph3P=C=C=NPh afford 162 and 163 <88T543>. By using styryl isothiocyanate with the ketene ylide, 164 was obtained. 

However, vinyl isocyanates have been nicely demonstrated by the Rigby group to be particularly useful intermediates in synthesis of pyrroli-... 

Examples of synthesis of five-membered heterocycles following this strategy are limited in number. An approach to 1,3,4-triazoles involves bonding of C-l and C-3 of vinyl isocyanate 150 to both nitrogen atoms of aromatic hydrazines (78JOC402) (Scheme 37). The reaction was run at room temperature and gave quantitatively regioisomer 151 or a mixture of 151 and 152 when p-nitrophenyl and phenylhydrazine, respectively, were employed. 

N-Vinyl heterocumulenes represent a new, highly reactive 2-azadiene species, which react, in general, with electron-rich alkenes and alkynes. Accordingly, we think it is of interest to complement the utility of electron-poor 2-azadienes in [4 + 2] cycloadditions by showing some examples involving A-vinyl isocyanates, -isothiocyanates, -carbodiimides, and -ketenimines. 

Vinyl isocyanates, readily available from the corresponding a,/3-unsaturated acids by a Curtius procedure, have been by far the systems studied in more detail, mainly by the Rigby group, e.g., in the construction of the pyridone ring (Scheme 53). Thus, heteropolycyclic compounds having 2-pyridone structure 233 were shown to be formed in moderate to... 

A direct entry into phenanthridinone and benzophenanthridinone skeletons was also disclosed by the formal [4 + 2] cycloaddition of N-vinyl isocyanates 232 and benzyne (89JOC4019). On the basis of this cycloaddi-... 

In an extension of this reaction, the same group prepared functionalized 4-hydroxy-2(l//)-pyridones by reacting vinyl isocyanates with ester eno-lates (Scheme 54). The reaction of 2-azadienes 232 with enolates derived... 

Heteroaromatic vinyl isocyanates have been used as a source of bicyclic pyridones. In the preparation of the l-methylpyrazolo[4,3-c]pyridone 153 the isocyanate 152b, obtained by Curtius rearrangement of the corresponding acid azide 152a, was heated at 220°C in diphenyl ether to furnish the product in good yield.143,144 Several 2-alkyl derivatives were also prepared.144... 

Thieno[3,2-c]pyridin-4-one (277) has been prepared by thermal cyclization of 2-thienyl-vinyl isocyanate (Scheme 73) (70BSB301). The derived chloro compound (278) can either be reduced by zinc-acetic acid to (260) or be readily converted into other derivatives by nucleophilic substitution of the halogen. The formation of 4-thiomethyl-6,7-dihydro-thieno[3,2-c]pyridine (280) by cyclization of the isothiocyanate (279) has also been reported (equation 23) (73GEP2318399). 

The reaction of vinyl isocyanates of type 123 with enamines gives intermediates containing the enaminoamide moiety, which subsequently undergoes an intramolecular cyclization and affords phenanthridine derivatives (84JOC4569). Numerous polyheterocycles were synthesized similarly (89JOC224). 

Arylazirines are carbonylated by [Rh2Cl2(CO)4] (96) to give isocyanates under mild conditions.462 The reaction can be carried out at normal pressure and at room temperature or below. Vinyl isocyanates are produced (equation 98). By addition of alcohols or amines, carbamates or ureas were obtained. 

The use of polar solvents or ligands such as 2,2 -bipyridyl decreased the yield. The complex [Pd(diphos)2] (104) was inactive, as were [Ni(PPh3)4] and [Pt(PPh3)4]. Complex (103) was believed to give the carbonyl [Pd(CO)(PPh3)3] prior to commencement of catalytic action. When [Pd(DBA)2](105) was used as catalyst, reaction (139) did not occur at all, the sole product being the vinyl isocyanate formed according to equation (140). A possible mechanism for these reactions is shown in Scheme 51. 

Phenanthridinones.1 Benzyne, generated in situ by Pb(OAc)4 oxidation of 1-aminobenzotriazole (l),2 undergoes [4 + 2]cycloaddition to cyclic vinyl isocyanates to form phenanthridinones. 

Treatment of /3-lactam 580 with rhodium acetate resulted in the formation of vinyl isocyanate 581 via electrocyclic ring opening of the intermediate iV-acylaminoazetinone 582 (Scheme 75). The latter azetinone 582 has been trapped with several alcohols, resulting in m-disubstituted /3-lactams 363 <2001CJC1259>. 

J. H. Rigby, Vinyl Isocyanates as Useful Building Blocks for Alkaloid Synthesis, Synlett 2000, 1-12. W. D. Rudorf, Reactions of Carbon Disulfide with C-Nucleophiles, Sulfur Rep. 1991, 11, 51-141. 

Rigby has developed a different approach for the construction of the hydroindolone intermediates65. These compounds were prepared by [1+4] cycloaddition of vinyl isocyanates and isocyanides (Scheme 22). 

Aza-steroids and -D-homosteroids, from vinyl isocyanates and enamines,... 

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