Butyl isocyanates 2+2 cycloaddition reactions

The reactivity of zirconium imido complexes supported by a dibenzotetraaza[14]annulene (taa) macrocycle ligand toward a range of unsaturated substrates has been examined. Reaction with t-butyl isocyanate cleanly generated the cycloaddition product Zr N(2,6-( -Pr)2C6H3)C(0)N(t-Bu) (Me4taa) (equation 21). ... 

Often the initially formed bonds at low temperature are not the ones that are isolated at room temperature. Also the electronic configurations play a part in product formation. For example, in the [2+2] cycloaddition reaction involving two carbodiimides the more nucleophilic carbodiimide attacks the more electrophilic carbodiimide giving rise to the formation of only one reaction product. The latter reactions proceed stepwise, while sometimes concerted reactions are observed. Sterical hindrance also plays an important role in product formation. We have utilized A-methyl-A -t-butylcarbodiimide as a probe in determining the structure of the derived cycloadducts, because the reaction always proceeds via addition across the C=N bond with the methyl substituent. For example, in the [2+2] cycloaddition reaction with benzoyl isocyanate the reaction proceeds across the C=0 bond of the isocyanate, because t-butyl isocyanate is the only product generated in the retro reaction... 

Niobium ketene hydrides 11 react with t-butyl isocyanide to give t-butyl isocyanate and L2Nb(H)=C=CPh2. An intermediate 12 in this reaction is most likely formed via a [2-1-1] cycloaddition reaction . 

The 1,3-dipolar character of triazaallenium salts, from now on referred to as l,3-diaza-2-azoniaallene salts, is evidenced by the many [3+2] cycloaddition reactions these types of compounds can participate in. The l,3-diaza-2-azoniaallene salts are generated in situ and trapped with suitable dipolarophiles. For example, l,3-diaza-2-azoniaallene salts undergo stereospecific [3+2] cycloaddition reactions with alkynes and olefins. However, they fail to react with isocyanates, isothiocyanates and azo compounds. Also, [3+2] cycloadditions to carbodiimides and cyanamides are observed. In contrast, nitriles fail to react. 1,3-Diaza-2-azoniaallene salts are obtained in the oxidation of 1,3-disubstituted triazenes with t-butyl hypochlorite. The resultant Al-chlorotriazenes react with antimony pentachloride to form the salts as reactive intermediates. Above —25°C, l,3-diaza-2-azoniaaUene salts disproportionate into diazonium salts and azo compounds. 

Two pathways were considered a dipolar cycloaddition across S(l) and N(4) (path (a)) and a dipolar cycloaddition across S(l) and the exocyclic nitrogen N(exo) (path (b)) (Figure 5). The reactions are, however, facilitated by more electrophilic isocyanates (RSO2NCO > RCONCO > ArNCO > Alkyl NCO) disfavoring path (a), and the observed reactivity order of the starting heterocycles ((85a) > (85b) (85c)) is in better agreement with path (b). The expected intermediate (86) was successfully trapped when the bulky t-butyl group was introduced in position... 

The reaction of phenyl isocyanate with t-butyl-A-(2,6-dimethylphenyl)imidoyl isocyanate proceeds similarly. A double [4+2] cycloaddition occurs in the reaction of carbonyl diisocyanate with aliphatic isocyanates to give l,3,5-triazino[2,l-b]-l,3,5-oxadiazine tetrones 471. ... 

Furthermore, exploration into fuUy intermolecular version led to the discovery of a Ni/PEtj catalyst system that mediates the cycloaddition of various alkynes and isocyanates (Scheme 2.26) [22]. The reaction of trimethylsilyl(TMS)-methyl alkyne with ethyl isocyanate affords two regioisomeric products, 88 and 88, in a ratio of 52 40 (isolated yields). Interestingly, the reaction with t-butyl-methyl alkyne led to exclusive formation of one regioisomer, 89. However, 10 mol % catalyst loading and 60 °C were required to achieve high conversions. Similar to the reaction of... 

Density functional theory (DPT) calculations were performed to elucidate the relative energies of the intermediates and products of this reaction. There are two reaction pathways that could generate the bis(imido) product 9 and the oxo-imido species 8 from U(N Bu)2(I)2(OPPh3)2 (Paths 1 and 2, Scheme 14.3). See color plate section. The first pathway involves the [2 - - 2] cycloaddition of the C = N bond of the aryl isocyanate to form an AQV-bound ureato intermediate (13), which can isomerize to form species 14 with the -NPh group trans to the terf-butyl imido moiety. Compound 14 can then eliminate BuNCO to generate the unsymmetrical bis(imido) complex 9 (top line in Scheme 14.2). Alternatively, AO-bound carbamate intermediates 15 and 16 could form that result from the [2 + 2] cycloaddition of the C = O bond of the aryl isocyanate across the U = N imido bond. Elimination of a substituted carbodiimide would generate the oxo-imido complex 8 (bottom line in Scheme 14.2). 

Miscellaneous Reactions. Isobutene, as the carbenium ion source for Ritter reactions, produces Al-r-butyl amides (eq 34). Isobutene undergoes highly regioselective cycloaddition with isocyanate to form azetidinones and with nitrones to yield isox-azolidines (eqs 35 and 36). Allyl thioethers are formed by the electrophilic addition of Benzenesulfenyl Chloride to isobutene (eq 37). Alkenylphosphonous dichlorides and alkenylthionophosphonic dichlorides are also formed by addition of PCI5 and P2S5 (eqs 38 and 39). ... 

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