Nitrile oxides mechanism

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

The reaction of alkyl nitro compounds with acetyl chloride in the presence of an alkenic compound produced a 2-isoxazoline. The mechanism is believed to proceed via a nitrile oxide and is illustrated in Scheme 112 (B-79MI41613). 

Rahman and Clapp decomposed dinitromethane derivatives in DMF in the presence of alkenes to obtain 2-isoxazolines. Without any alkene present, an acid and KNO2 were obtained. They proposed a mechanism which proceeded via a three-membered ring or a nitrocarbene which rearranged to a nitrile oxide (76JOC122, 75MI41612). 

The mechanism is thought to be analogous to that suggested for the formation of 1,3,5-benzoxadiazepines from nitrile oxides and 2-phenylbenzazete (see Section 4.2.1.4.2.1.). 

Table 10. Molecular mechanics calculations on the INOC reactions of nitrile oxides 98 (energies in kcal/mol)...

Dipolar cycloaddition of C60 with nitrile oxides was modeled at the B3LYP/6-31G(d,p)//AMl level, and its mechanism and regiochemistry were investigated. Theoretically, the reaction can proceed by four types of additions, viz., closed [6,6], open [5,6], closed [5,6], and open [6,6] additions. Analysis of... 

I.3.4.2.2. Nonaromatic Unsaturated Heterocycles Reactions of aromatic nitrile oxides with 1-azirines are followed by the ring opening of the latter to give 4-benzamidoisoxazoles 145 (314). The structure of 145 (R = 4-C1C6H4, Ar = Ar7 = Ph) was established by single-crystal X-ray analysis. A mechanism for the formation of 145 has been proposed, (see Scheme 1.29). 

It is of interest to mention that DFT study performed, prior to experimental observations, revealed for Cu(I)-catalyzed cycloaddition of nitrile oxides to 1-alkynes, a stepwise mechanism involving unprecedented metalacycle intermediates, which appear to be common for a variety of dipoles (382). 

Certain specific steric effects are operative on intramolecular nitrile oxide— olefin cycloadditions. These effects are governed by both ring size and character of substituents. Thus, cycloadditions to the exomethylene group are successful with substituted methylenecyclohexanones 334 (m = 1, 2 n = 2) and gave tricyclic 335 (m = 1, 2), but do not occur with methylenecyclopentanones 334 (m = 1, 2, 3 n = 1). Activation energies calculated by molecular mechanics are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives cA-decalone 336 (403). 

Treatment of y-nitrothioamides 368 with phenyl isocyanate and triethylamine (nitrile oxide generation conditions) leads to a.j3-unsaturated nitriles 369. The mechanism proposed for this reaction is shown in Scheme 1.42, which includes the dehydration stage of the nitrile oxide formed (418). 

A study of the regioselectivity of the 1,3-dipolar cycloaddition of aliphatic nitrile oxides with cinnamic acid esters has been published. AMI MO studies on the gas-phase 1,3-dipolar cycloaddition of 1,2,4-triazepine and formonitrile oxide show that the mechanism leading to the most stable adduct is concerted. An ab initio study of the regiochemistry of 1,3-dipolar cycloadditions of diazomethane and formonitrile oxide with ethene, propene, and methyl vinyl ether has been presented. The 1,3-dipolar cycloaddition of mesitonitrile oxide with 4,7-phenanthroline yields both mono-and bis-adducts. Alkynyl(phenyl)iodonium triflates undergo 2 - - 3-cycloaddition with ethyl diazoacetate, Ai-f-butyl-a-phenyl nitrone and f-butyl nitrile oxide to produce substituted pyrroles, dihydroisoxazoles, and isoxazoles respectively." 2/3-Vinyl-franwoctahydro-l,3-benzoxazine (43) undergoes 1,3-dipolar cycloaddition with nitrile oxides with high diastereoselectivity (90% de) (Scheme IS)." " ... 

The most widely used method for the dehydration of primary nitroalkanes involves their treatment with phenyl isocyanate and triethylamine, introduced in 1960 by Hoshino and Mukaiyama (7). A probable mechanism for the formation of the nitrile oxide is shown in Scheme 6.4. This method is known to be very effective for the preparation of aliphatic or aromatic nitrile oxides. In some cases, the separation of the byproduct A,A -diphenylurea from the reaction mixture may be troublesome. In order to circumvent this problem, 1,4-phenylene diisocyanate was introduced (82,83). The polymeric urea that is formed as a byproduct is largely insoluble in the reaction mixture and can easily be removed. 

The generation of a nitrile oxide bearing a carbamoyl group (i.e., 16) was effected by treating 4-nitro-3-isoxazoline-5-one (15) with a mixture of acetonitrile and water (Scheme 6.5). Although the mechanism of this reaction is not clear, the method allows for the formation of a functionalized nitrile oxide (16) and subsequent cycloaddition under mild conditions (91). 

The above dramatic dependence of regio- and stereoselectivity on the nature of the metal can be explained by the reaction mechanism shown in Scheme 11.49 (167). The nitrone cycloadditions of allylic alcohols are again magnesium-specific just like the nitrile oxide reactions described in Section 11.2.2. Magnesium ions accelerate the reaction through a metal ion-bound intramolecular cycloaddition path. On the other hand, zinc ions afford no such rate acceleration, but these ions catalyze the acetalization at the benzoyl carbonyl moiety of the nitrone to provide a hemiacetal intermediate. The subsequent intramolecular regio- and stereoselective cycloaddition reaction gives the observed products. 

Nitrooxazoles 271a-C also react with electron-rich ynamines to yield isoxazo-lines. °° The proposed reaction mechanism involves the Michael addition of the ynamine to give 275, followed by rearrangement to a nitrile oxide 277. Intramolecular 1,3-dipolar cycloaddition of 277 accounts for the exclusive cis stereochemistry observed in the products 278a-c (Scheme 8.78). 

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