Isocyanates with phosphoranes

Addition of phosphinomethyl spirophosphorane 260 to isocyanates and azides presumaby generates anionic nitrogen intermediates that cyclize with the phosphorane to give spirophosphoranides 96, 97, and 261 (Scheme 35) <1996PS493>. 

Formate esters behave as typical carbonyl compounds in reactions with a number of ylides, eliminating phosphine oxide and forming vinyl ethers, e.g. (33).35 Stabilized phosphoranes are able to condense with the carbonyl group of cyclic thioanhydrides (34).38 Quinoline derivatives, e.g. (35), are obtained from the condensation of dicar-boalkoxy-ylides with isocyanates.37 Benzoyl isothiocyanates and keto-phosphoranes give quantitative yields of (36), which are unreactive in Wittig reactions but can be readily oxidized by selenous acid.38 The products obtained from reactions (Scheme 9) with the triazolinedione (37) depend upon the stability of the ylide used.39... 

A recent review on four-membered heterocycles formed from imino-phosphoranes concentrates on the preparation and the reactivity of 2,4-diimino-l,3-diazetidine and related compounds (93JPR305). As an example, the synthesis via bisiminophosphorane 85 is described in Scheme 42. The bisiminophosphorane has both a heteroaryl and a styryl site. From a mechanistic view, the reaction of the bisiminophosphorane proceeds with aryl isocyanate formation via an aza-Wittig mechanism. Intermediate car-bodiimide formation (86) occurs directly on the iminophosphorane moiety... 

The synthesis of 4//-pyrano[2,3-d]pyrimidines, a class of compounds important in crop protection, is presented in Scheme 91. Dichlorotriphenyl-phosphorane affords iminophosphorane 247 with ethyl 2-amino-4//-pyran-3-carboxylate (246). Phenyl isocyanate cyclizes under alkoxide migration to afford pyrano[2,3-d]pyrimidines (248) (90LA995). 

Additional supporting evidence for the mechanism includes the formation of pyrylium salts when arylacetyl chlorides, which yield ketenes by loss of HC1, react with the phosphorane (63CR(257)926). It is also known that diphenylketene reacts in a similar manner to give a pyran, whilst aryl isocyanates yield pyran-4-one derivatives. 

The metathetical reaction of C02 with metal (Ti(IV), U(V)) arylimido-complexes led to an arylisocyanate and a metal-oxo species [4g, 115a]. In these processes, which take place under very mild conditions (ambient temperature and pressure), the metal center acts as the sink for unrequired oxygen. A common feature which characterizes these transformations from a mechanistic point of view is the intermediate formation of a metal carbimato-species through the [2+2] cycloaddition of C02 with the metal-imido complex (Scheme 6.23). By cycloreversion, the four-membered aza-metalla-cycle then converts into isocyanate and a stable metal-oxo complex. These processes, at least formally, are reminiscent of the reaction of imino-phosphoranes with C02 to give isocyanates and carbodiimides [115b,c]. 

Isocyanates have provided one carbon atom in transformations of the general type (296) - (290). Thus, azide (343) reacts with triphenylphosphine in a Staudinger reaction giving phosphorane (344) which is then treated with isocyanates (RNCO R = benzyl or aryl) giving naphthyridines (346) (50-59% yield) via the carbodiimide (345). The cyclization (345)->(346) occurred in toluene at 160°C (92T4601). Azide (347) undergoes a similar sequence of reactions yielding a 1 1 mixture of isomeric naphthyridines (348) and (349) (44—47% yield). 

Similar bicyclic phosphoranes 51 are formed in the reactions of phosphorus isocyanates and isothiocyanates with HFA (97,176,239) via intermediate formation of oxazaphospholinones and -thiones 50. 

Acylylides. The reagent reacts with a free carboxylic acid to form the phosphorane 2. When heated, 2 is converted into the acyl ylide 3 with loss of phenyl isocyanate. These... 

In the same manner 2-phenylthiazole and 2,3-substituted thiophene derived imino-phosphoranes react with one eqivalent of aryl isocyanate to give fused 1,3-diazepines in 50-72 % yield, while with two equivalents aryl isocyanate the [2-1-2] cycloadducts are obtained in 50-61% yield. ... 

Tandem nucleophilic/intramolecular addition also occurs in the reaction of the imino-phosphorane 32 (R = C02Me, CN) with isocyanates and nucleophiles (X = O, S, NR) to give 33. ... 

The year has seen a considerable extension of the principles of hypervalent phosphorus chemistry into the fields of other elements notably Si, Sn, Ge and Sb and in this context the well-known Martin ligand has proved to be especially valuable. Particularly interesting contributions have been provided by Ju et al on the relative rates of reaction of pentoses and hexoses with pentacoordinate phosphorus, by Bentrude et on the mechanism of the reaction of hy-dridophosphoranes with dimethyl disulfide and by Kawashima et alP on the isolation of two carbaphosphatranes containing covalent P-C bonds. Holmes et al report on the fluxional properties of propeller shaped phosphoranes and Buono et have also demonstrated the utility of hydridophosphoranes in some highly diastereoselective reactions with isocyanates. In summary, although the number of reports was small, the year was noteworthy for some very sophisticated contributions to the field. 

Vinyliminophosphoranes derived from 1,6-methanol[10]-annulenes react with isocyanates to give 1,6-methanol[10]-annulenopyridines. Bis(vinyliminophosphoranes) combine with isocyanates to provide azolofused l,3-diazepines. Differences on the periphery of the fused ring systems occur based on isocyanato (alkyl vs aryl) substituents. The thermal reaction of [(2-azulenyl)imino] phosphoranes on [(1-azaazulen-z-)imino]-phosphoranes with 2-bromotropone allow for the preparation of 6-aza- and 6,7-diazaazuleno[1,2-a]azulenes. °... 

In 2012, Grushin s team developed a palladium-catalyzed carbonylative coupling of aryl iodides with sodium azide [103]. This catalytic reaction occurs smoothly at temperatures as low as 25-50 °C and 1 bar to cleanly produce aroyl azides from the corresponding aryl iodides, CO and NaNs (Scheme 4.52). The reaction exhibits high-functional group tolerance and can also be conveniently used for one-pot, two-step procedures furnishing primary benzamides, imino-phosphoranes, isocyanates, and ureas in high yield without isolation of the primary benzoyl azide product. 

Silylmethyl-Substituted Heterocumulenes. (Trimethylsilyl-methyl)iminotriphenylphosphorane is obtained from a one-pot reaction of the azide and triphenylphosphine (eq 3). The reactions of the phosphorane with carbon dioxide or carbon disulfide (eq 5) give trimethylsilylmethyl isocyanate or trimethylsilylmethyl isothiocyanate in 68 and 94% yields, respectively. ... 

A-Phenyl keteniminylidene phosphoranes 191 react with two equivalents of phenyl isocyanate to give [2+2+2] cycloadducts 192. ... 

Also, vinyl isocyanate reacts with oxovinylidenetriphenylphosphorane or N-phenylimino-vinylidenetriphenyl-phosphorane to give the [4+2] cycloadduct 505. ... 

---