Isocyanates extrusion

Hydroxy-THISs add to the C-C bond of diphenylcyclopropenethione (181. Inner salts without substituents in 5-posnion react similarly with diphenylcyclopropenone (Scheme 10) (4, 18). Pwolysis of the stable adducts (9) leads to rupture of the R-C-CY bond. Subsequent ring closure yields 10. When Y = O. 10 eliminates COS. producing 2-pyridone. When Y = S. 10 is isolated together with its isocyanate extrusion product, a thiopyran-2-thione (18). 

Reactions of the cyclopentadienyl-amidinate-supported imidotitanium complexes with CO2 proceed via initial cycloaddition reactions, but depending on the imido Af-substituent go on to yield products of either isocyanate extrusion or unprecedented double CO2 insertion (Scheme 89). ... 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

A similar product is obtained from the reaction of anhydro-4(5)-hydroxy-l,2,3-triazolium hydroxide (398). In this case reaction with DMAD occurred in 1 hour in boiling benzene. Extrusion of methyl isocyanate from the initial 1 1 cycloadduct (399) occurred during the reaction giving (400). 

As already described for the all-carbon-Diels-Alder reaction, a hetero-Diels-Alder reaction can also be followed by a retro-hetero-Diels-Alder reaction. This type of process, which has long been known, is especially useful for the synthesis of heterocyclic compounds. Sanchez and coworkers described the synthesis of 2-aminopyridines [48] and 2-glycosylaminopyridines 4-144 [49] by a hetero-Diels-Alder reaction of pyrimidines as 4-143 with dimethyl acetylenedicarboxylate followed by extrusion of methyl isocyanate to give the desired compounds (Scheme 4.30). This approach represents a new method for the synthesis of 2-aminopyridine nucleoside analogues. In addition to the pyridines 4-144, small amounts of pyrimidine derivatives are formed by a Michael-type addition. 

As part of a mechanistic and synthetic study of nucleophihc carbenes the spirocyclic 4(5/l)-oxazolone 18 has been obtained from benzoyl isocyanate (Scheme 6.1) Thermal extrusion of nitrogen from the 1,3,4-oxadiazoline 14 produced the carbonyl ylide 15 that fragmented via loss of acetone to the aminooxycarbene 16. Spectroscopic data [gas chromatography-mass spectrometry (GC-MS), infrared (IR), proton and C-13 nuclear magnetic resonance ( H and NMR)] of the crude thermolysate was consistent with 18. The formation of 18 was rationalized to result from nucleophihc addition of 16 to benzoyl isocyanate followed by cyclization of the dipolar intermediate 17. Thermolysis of 19 and 21 under similar reaction conditions afforded 20 and 22 respectively, also identified spectroscopically as the major products in the thermolysate. 

Thermal studies on l,3-oxazetidin-2-ones, as expected, show the major pathway to be extrusion of carbon dioxide to give imines (70BAU1479,80S571). These systems are surprisingly stable for example, for (V-phenyl-l,3-oxazetidin-2-one AH = 128.9 kJ mol-1 and AS = 54.4 J K-1 mol . There is one report of fragmentation to give ketone and isocyanate... 

Dipolar cycloaddition of anhydro pyrido[2,l-b][l,3]thiazinium hydroxides (128) with aryl isocyanates and dimethyl acetylenedicarboxylate gave pyrido[l,2]pyrimidines (129) and quinolizine-l,2-dicarboxylates (130), respectively (76CB3668). 1,4-Dipolar cycloaddition of pyrido[2,l-h][l,3]thi-azinium betaine (131, R = Me) with 1-diethylamino-l-propyne afforded cycloadduct 132, from which quinolizin-4-one 133 formed by a rapid cheletropic extrusion of carbonyl sulfide (93TL5405 95T6651). 1,4-Dipolar cycloaddition of anhydro 4-hydroxyl-2-oxo-6,7,8,9-tetrahydro-2//-pyrido-[2,l-b][l,3]thiazinium hydroxides (131) and 4-phenyl-l,2,4-triazoline-3,5-dione yielded 135 via 134 [94H(39)219 95H(41)1631] and 136 (95T6651). 

Compounds (46a) and (46b) lose formaldehyde and thioformaldehyde, respectively, before nitrogen extrusion from intermediate unsubstituted tetrazole (46c) the tetrazole then extrudes N2 to form cyanamide. For (49a,b) and (50b,c,e-h) the predominant reaction is 3 + 2-cycloreversion to azides and isocyanates or isothiocyanates, respectively. 

The second pathway, followed by 1,4,2-dioxazolones, proceeds by extrusion of C02 and rearrangement to the corresponding isocyanates 54. The question of a nitrene intermediate or a concerted C02 extrusion and relevant mechanistic aspects have been discussed in CHEC-II(1996). Also, various thermolytic reactions of other fully conjugated l,4-(oxa/thia)-2-azole derivatives, in particular azonylenes 11 (X = S Y = 0 Z = CC12), azonylimines 10 (X = 0 Y = S Z = NR), and imino dioxazolidine derivatives 12 (X = Y = 0 W = Z = NR) have been discussed in CHEC-II(1996) <1996CHEC-II(4)506>. 

The preparation of metal nitrides with N3 reagents typically employs d° metal complexes as starting materials. However, the reactions of r-butyl isocyanate with metal-oxo complexes of OsVI and RuVI represent rare examples of the use N3- reagents with d2-metals. It has been postulated that reaction of the isocyanate with metal-oxo 3 affords a four-membered ring intermediate 4, followed by the extrusion of carbon dioxide to yield r-butyl metal imide 5 (Scheme 1). Elimination of isobutylene from this complex then produces the metal nitride and the isobutylene. 

Oxazole N-oxides having a 4-methyl substituent are attacked by acetic anhydride to yield 4-acetoxyoxazoles (equation 21). The combined action of benzoyl chloride and potassium cyanide leads to compounds of the Reissert type, e.g. (177). The reaction of 4-methyloxazole Yoxides with phenyl isocyanate gives 5-hydroxy-4-methylene-l-phenyl-4,5-dihy-droimidazoles by cycloaddition, extrusion of carbon dioxide and recyclization (Scheme 12) with 4-phenyloxazole JV-oxides the reaction takes a different course, yielding imi-dazooxazolidinones (Scheme 13). 

Kaugars and Rizzo have found that 5-alkylamino- or 5-arylamino-l,2,3,4-thiatriazoles react with isocyanates to give 3-oxo-A4-l,2,4-thiadiazolin-5-ylureas (65) (79JOC3840). The structures were verified by independent synthesis and by H and I3C NMR spectroscopy. In the presence of triethylamine the exothermic reaction went to completion in a few hours. The reaction may either take place by attack at the amino group to give (63), followed by N2 extrusion to form the reactive dipole (64 equation 31), or the reaction may be initiated by attack at the ring in the 4-position to give an intermediate thiatriazoline, which, as discussed above, reacts with heterocumulenes (see Section 4.28.2.3.l(iii)). As expected, 5-(dialkylamino)thiatriazoles were not found to react with isocyanates. 

The cycloaddition of isomiinchnones with acetylenic dipolarophiles followed by the extrusion of an alkyl or aryl isocyanate (RNCO) has proven to be an effective method for the synthesis of substituted furans. The Ibata group investigated the bimolecular 1,3-dipolar-cycloaddition of aryl-substituted isomiinchnones with a number of acetylenic dipolarophiles [50]. Aryl diazoimides of type 1 were heated in the presence of a catalytic amount of Cu(acac)2 and the appropriate acetylenic dipolarophile. Formation of the substituted furan was found to be temperature-dependent higher temperatures (ca. 120°C) were needed for complete conversion to the furan. It was reasoned that the extrusion of methyl isocyanate was not as facile as the loss of carbon dioxide from sydnones and miinchnones [50]. 

Non-aryl substituted isomiinchnones also undergo the same transformation but under less rigorous conditions. Thus, when acyclic diazoimides 19 and 20 were subjected to Rh2(OAc)4-catalyzed decomposition in the presence of DMAD, cycloaddition followed by extrusion of methyl isocyanate occurred to give the substituted furans 94 and 95 [35]. 

Instead of losing methyl isocyanate, the extrusion of a tethered alkyl isocyanate occurred when the bicyclic diazoimide 96 was used. The rhodium(II) acetate-catalyzed reaction of 96 in the presence of DMAD produced furano-isocya-nate 98 in 85% yield. The anticipated cycloadduct 97 was not isolated, but instead underwent a subsequent [4+2] cycloreversion under the reaction conditions to give the observed product. The initially formed furanoisocyanate 98 was... 

Several additional examples of the intramolecular cycloaddition of unactivated acetylenes with isomiinchnones were reported by Maier [30]. This cycloaddition approach represents an efficient method for providing rapid access to annulated furans present in several sesqui- and diterpenes, such as the panicu-lides [51],furanonaphthoquinones [52],furodysin,andfurodysinin [53,54].The decomposition of acyclic acetylenic diazoimides 102 and 103 with Rh2(OAc)4 resulted in cycloaddition and retro-Diels-Alder extrusion of methyl isocyanate to give annulated furans 104 and 105 in good yield. The overall transformation is closely related to the intramolecular Diels-Alder reactions of acetylenic oxa-zoles extensively studied by Jacobi and coworkers [55]. 

The reaction can be run in reverse at high [THF], with bis-(trimethylsilyl) carbodiimide replaced by THF. The rate is independent of [THF], consistent with rate-limiting extrusion of carbodiimide, i.e., with the operation of a similar dissociative mechanism in reverse. Similarly, addition of trimethylphos-phine to 35 results in extrusion of the carbodiimide to form the phosphine-stabilized zirconaaziridine [49]. In contrast, the isocyanate insertions are effectively irreversible. 

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