Unstable adducts

The dipolar cycloaddition of 2-diazopropane to l-methyl-3-phenylpyridazin-6(l//)-one takes place through an unstable adduct which thermally decomposes to a 1,2-diazepinone, a pyridazinone and diazanorcaradiene derivative (Scheme 46). 

Reaction of the morpholine or piperidine enamine of cyclopentanone, however, gives an unstable adduct which rearranges under the reaction conditions and an aqueous work-up to give the ring expanded ketone 2-chloro-2-cyclohexen-l-one (203) (138,139). 

The electrochemical investigation on an Hg electrode of the interaction between [Ir(Me2-phen)(cod)] and 02 gives evidence of formation of an unstable adduct.619... 

Similarly, treatment of a hexane solution of (77-C5Me5)2TiCl with CO at room temperature gave the thermally unstable adduct (17-C5Me5)2Ti(CO)(Cl) (52) as evidenced by again a high energy metal carbonyl band at 2000 cm-1 (98). Complex 52 slowly disproportionated at... 

A complementary method was reported 3 years later by Hino, in which a readily enolizable diketopiperazine 48 was directly converted to the epidisulfide by deprotonation with sodium hydride and exposure to sulfur monochloride [38]. As with the Trown method, this method was limited to a specific class of substrates, namely, ones possessing a 1,3-dicarbonyl motif at each of the reactive centers, yet it has also seen subsequent applications in total synthesis [39, 40]. In 1972, Schmidt was able to significantly broaden the scope of the enolate thiolation method by introducing elemental sulfur as the electrophilic agent [41]. In contrast to Hino s method in which formation of a highly reactive, unstable adduct requires readily... 

A more complex structure is that of leinamycin 45 (Scheme 15), a material with potent cytotoxic and antitumor properties, isolated from a Streptomyces sp. A 1,2 dithiolane-3-one ring is spiro fused to a complex macrolactam96 (and references therein). Leinamycin has the remarkable ability to cleave DNA. In brief, leinamycin reacts with a thiol and, after a profound rearrangement, forms an episulfonium ion. This ion alkylates the N7 position of guanosine residues in double stranded DNA an unstable adduct is depurinated by hydrolysis of the glycosidic bond between the alkylated base and a deoxyribose residue. Some structurally less complex l,2-dithiolane-3-one 1-oxides have a similar DNA cleaving ability.97... 

Tropone (255) cycloadds to allene 71c to construct a 5,7-fused ring system, probably via a concerted mechanism [109]. The reaction of 71c with azaheptafulvenes 256 took place much more readily at room temperature to produce the relatively unstable adducts 257, which isomerized gradually to the stable pyrroles. 

The fra/i.s-but-2-cn-1,4-dionc (8.4 mmol) and the methylene compound (8.4 mmol) are stirred with Na2CO, (98 mg) and TEBA-CI (20 mg) in PhH (10 ml) at 75-80°C until the reaction is complete, as shown by TLC analysis (ca. 24 h with P-keto esters and ca. 48 h with cyclohexa-l,3-dione). Et20 (50 ml) is added and the organic phase is washed well with brine. Evaporation of the ethereal solution gives the unstable adduct but, if the etheral solution is saturated with HC1 gas, the adduct is converted into the furan (overall yields >45%). 

Carbenes insert into the N=N bond of azo compounds and the unstable adducts rearrange spontaneously to produce benzopyrazoles (e.g. Scheme 7.31) [37,38], The reactions of azo compounds with dichlorocarbene is also catalysed by tertiary amines [36] and, under such conditions, azoxyarenes produce the benzimidazoles, presum-... 

Phenyliodonium sulfate 289, which is obtained from the reaction of iodo-sobenzene and sulfur trioxide at -50°C, behaves as a 1,4-dipole in the reactions with several alkenes (86ZOR450) to give intermediate [4 + 2] cycloadduct 290. Decomposition of this unstable adduct affords cyclic sulfates 291 (Scheme 73). Likewise, another 1,4-dipole 292 undergoes cycloaddition reaction to give cyclic sulfone 293 (88ZOR888). The 1,4-dipole 292 thermally decomposes to give 294 (86DOK1374) (Scheme 74). 

To date, however, only few reactions between phosphine and a non-metal halide, in which a chemical bond is formed between phosphorus and a non-metal by HCl condensation, are known. To these, apart from the above-mentioned reactions, belongs also the reaction with CF3SCI which, depending upon the chosen proportions of the reactants, in a sealed tube at -95 °C leads to the formation of (CF3S)2PH or (CF3S)3P Both compounds are not very stable thermally and decompose at 40-50 °C. Tris(trifluoromethylthio)-phosphine forms an unstable adduct with chlorine, which decomposes at 0 °C to give a mixture of PCI3, bis(trifluoromethyl)-disulphide and trifluoromethyl-sulphenyl chloride. 

The addition to alkenes normally leads to unstable adducts that lose carbon dioxide under the reaction conditions. The intramolecular cycloaddition of the sydnone (30) takes place at room temperature, however (Equation (5)) and the cycloadduct (31) has been characterized <86HCA927>. The unstable species formed by the loss of carbon dioxide are also azomethine ylides. It is therefore possible for a second 1,3-dipolar addition to take place, as illustrated in Scheme 6 for the reaction of 3-phenylsydnone with Al-phenylmaleimide <86TL317,92JA8414>. This 2 1 addition has been used as the basis of a synthesis of polyimides. Imides of the type (32) were used as the dipolarophiles and their reaction with 3-phenylsydnone gave linear polymers <87MM726>. 

Such chloramines may occur in trace quantities in many chlorine-treated wastewaters that also contain trace ammonia. NCI3 combines with ammonia to form an unstable adduct, NCls NHs which reacts with excess NH3 producing NH4CI and liberating N2. 

Thermolysis of aryl chloro diazirine (18) in the presence of acetone and a trapping agent such as A -phenylmaleimide gave rise to cycloadducts such as 41. The unstable adduct hydrolyzed during purification resulting in synthesis of bicyclic hemiacetals 42 and 43 as a mixture of endo and exo adducts in 37 and 8% yield, respectively. The exclusive generation of the singlet carbene was confirmed by low-temperature electron spin resonance (ESR) study of the irradiated diazirine. 

In this case, the reduced form of the catalyst builds up with the substrate a relatively unstable adduct AQ, which then decomposes, eventually after further reduction at the electrode surface or in solution. Finally, either the oxidized form P of the catalyst or its reduced form Q is regenerated. As regards the substrate, its reduction follows the aforementioned E.C. (Electrochemical Chemical) mechanism. This type of catalysis is termed chemical catalysis. Finally, it must be pointed out that the clear-cut distinction between redox catalysis and chemical catalysis might be difficult, but amounts ultimately to a detailed study of the nature of the redox reaction. 

Reaction of the isoxazole 168 with the azadiene 169 afforded the isoxazolo[4,5- ]pyridine 170 through the spontaneous loss of nitrous acid and dimethylamine from the unstable adduct 171 (Scheme 12) <1995T7085>. 

At room temperature pyridines react reversibly with halogens and interhalogens, e.g. IC1, to give unstable adducts, which behave as mild halogenating agents. X-Ray diffraction studies of the pyridine-iodine complex have given its structure (73). 

Flavylium salts are oxidized to flavones on treatment with thallium(III) nitrate in methanol. The reaction is considered to involve initial attack at C-2 by methanol. The resulting chromene (484) undergoes reaction with the thallium salt at C-3 and nucleophilic attack at C-4 by methanol to give an unstable adduct (485), which decomposes to the flavone... 

Sullivan and Williams1 have proposed that the novel course oi events involved here may bo rationalized by assuming HS- km- attack at the carbonyl rather than the epoxide function. The unstable adduct generated in this manner is then postulated to undergo the transformations depicted in JSq. (653), giving the observed n-iueroapto-carbonyl products. 

The thermodynamics of complex formation between the chelates ML2 (M = Zn or Hg HL = RC(SH)=CHCOCF3, R = Me or Ph) and py and bipy have been investigated.759 Zinc forms 1 1 complexes with both ligands, the bipy being bidentate. Mercury gives a 1 1 complex with py, but only an extremely unstable adduct is formed with bipy. 

The aminopyrimidine derivatives, cytosine (226), cytidine, and cytidylic acid, form unstable adducts (227), with water these are... 

With HS03, the thiazine (170) gives an unstable adduct, which is hydrolyzed to 171 (Scheme 67) (87MI1). 

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