Bond formation, isocyanate with

Hydroxy-THISs add regioselectively to the C=N bonds of isocyanates or isothiocyanates. The initially formed cycloadducts eliminate carbonyl sulfide with formation of 4-hydroxy- or 4-mercaptoimidazolium hydroxide inner salts (21) (Scheme 21). 4-Hydroxyimidazolium hydroxide... 

The described fluorous-tag strategy has also been applied to the synthesis of biaryl-substituted hydantoins (Scheme 7.81) [94]. 4-Hydroxybenzaldehyde was converted into the corresponding perfluorinated species, which was then subjected to a reductive amination. The resulting amine was treated with an isocyanate to produce the fluorous-tagged urea, which spontaneously cyclized to form the corresponding hydantoin. Finally, the fluorous tag was detached by a Suzuki-type carbon-carbon bond formation to furnish the desired target structure in good yield. 

For the reaction of phosphane oxide with isocyanate, the rate-determining step is the formation of the oxazaphosphetane 45 via P—O—bond formation of the intermediate betaine (44), since the stable and energetically favorable P=0 double bond is broken here. Subsequent rapid decomposition of the oxazaphosphetane 45 into iminophosphorane and carbon dioxide occurs. Within the actual aza-Wittig step, the intermediate betaine (46) is generated in a rate-determining step by nucleophilic attack of the iminophosphorane nitrogen on the carbonyl C. By P —O-bond formation, betaine (46) is then converted into an oxazaphosphetane (47), which decomposes... 

The heat of formation of [MolCOlg] has been determined as -960 + 12 kJ mol by measuring its heat of decomposition. The Mossbauer parameters for the 100 keV transition of in [W(CO)g] and some tungsten(vi) complexes have been measured and discussed in terms of known bonding and structure. Secondary ions [M (CO) ] (M = Mo, m = I or 2 M = W, m = 1—4 n = 0—14) formed by ion-molecule reactions have been observed in the mass spectra of the hexacarbonyls. A mixt u re of [Cr(CO) ] and [MolCO) ] vapours affords [CrMo(CO) ] ( = 5—7). [MofCOl ] and [WICO) ] catalyse the condensation of isocyanates with aldehydes to give imines in high yields. ... 

One principal access to 1,2,4-oxadiazoles consists of N—O bond formation by electrocyclic ring closure of nitrenoids N—CR —N—CR =0. For instance, heating acyl isocyanates (159) with trimethylsilyl azide alfords high yields of 1,2,4-oxadiazoles (Scheme 68) <79JCS(P1)185,80JOC5130). Similarly, elimination of HCl from A -chloroamidines (160) (Scheme 69) <84BCJ1I6> and ther-... 

Reactions involving the [4 + 1 + 1] principle, an example of which is shown in equation (136), are rather uncommon and of strictly limited utility [3 + 2 + 1] and [2 + 2 + 2] processes, on th,e other hand, are well known. Representative [3 + 2+1] three-bond formation processes are given in equations (137)—(141), from which it can be seen that the common situation is where ammonia, a substituted amine or formamide constitutes the one-atom fragment. Many [2 + 2 + 2] atom fragment syntheses are known and some are familiar reactions. Thus, the cobalt(I)-catalyzed condensation of nitriles and isocyanates with alkynes gives pyridines and 2-pyridones, often in excellent yield (e.g. equation 142), while the cyclotrimerizations of nitriles, imidates, isocyanates, etc., are well established procedures for the synthesis of 1,3,5-triazine derivatives (e.g. equation 143). Further representative examples are given in equations (144)-(147), and the reader is referred to the monograph chapters for full discussion of these and other [2 + 2 + 2] processes. Examination of the... 

Bond formation between oxygen and sulfur occurs when the AM2-hydroxyethyl)thiourea (155) is oxidatively cyclized with bromine to give the 1,2,4-oxathiazine (3) (77TL4245), and when the intermediate (183), from reaction of ketones with fluorosulfonyl isocyanate (FSI), is cyclized with base to give (184) <80AG(E)13l). 

Addition of DBU to a solution of 149 in THF induced an elimination reaction accompanied by loss of a molecule of CO2 and provided the unstable amine 150, which was converted in situ into isocyanate 151 by reaction with phosgene and triethylamine. After filtration to remove hydrochloride salts, the solution of 151 was treated with samarium (II) iodide in the presence of lithium chloride. These conditions, which had been previously determined to be optimal for spirooxindole generation on a model system, provided compound 152 as an inseparable 7 1 mixture of diastereoisomers [43]. The major component of this mixture was determined by NOE analysis to have the required configuration, which is consistent with bond formation from the less hindered, convex face of 151 (Scheme 35). 

Heterounsaturated monomers that undergo coordination polymerisation or copolymerisation with other monomers can be divided into two classes monomers with a carbene-like structure such as isocyanides and carbon monoxide which are coordinated by n complex formation with the transition metal atom at the catalyst active site, and monomers such as isocyanates, aldehydes, ketones and ketenes which are coordinated via 5-bond formation with the metal atom at the catalyst active site. 

The reaction between 41 and tri-ferf-butylsilyl isocyanate with its bulky substituent leads to the five-membered heterocyclic system 100109. The formation of this ring requires cleavage of the Si=Si multiple bond, but nothing is as yet known about the reaction mechanism (equation 20). 

As was pointed out earlier, either central bond cleavage or addition to a side bond can account for the formation of the various products obtained in electrophilic addition reactions. It is highly likely that, in reactions where the cyclobutane ring is retained in the product, it is the central bond which is cleaved. However, when cyclopropylcarbinyl derivatives are obtained, both attack on a side bond as well as cleavage of the central bond followed by skeletal rearrangements are conceivable mechanistic pathways. The literature does record one report which purportedly disproves the side bond fission process in the reaction of chlorosulfonyl isocyanate with bicyclobutane The results of this study, however, have been reinterpretedleaving the question moot. 

Although all the reactions discussed in this section can contribute to the cross-linking of the adhesive and the chemical bonding of the wood particles, the initial reaction of isocyanate with adsorbed water molecules on the wood surface results in immediate wetting of wood with the formation of polyurea. Even on impervious surfaces like glass or metal, the reaction of isocyanate with adsorbed water results in intimate contact after chemical reaction, including poly-... 

IR spectroscopic evidence for covalent urethane bond formation in the reaction between isocyanates and wood has been obtained. Isolation of holocellulose by the sodium chlorite method, isolation of lignin by the H2SO4 procedure, and subjecting both to IR spectroscopy indicated that isocyanates reacted with both cellulose and lignin (194),... 

Reactions of phosgene at the Si-N bond frequently result in the formation of isocyanates with concomitant cleavage of the Si-Cl moiety, Equation (10.28) [1408a,1409] ... 

By contrast, phenyl isocyanate and isothiocyanate and carbon disulphide led to addition-elimination reactions of the Wittig type. The isocyanate reaction in particular gave a complex mixture of products, with ring-closure in the probable intermediate (21) occurring by P—N rather than P—O bond formation. 

Most investigators agree that the strength and durability of isocyanate-bound wood panels are due to the chemical reaction of the isocyanate group with wood hydroxyls as illustrated by the above equation. Thus, the multifunctional isocyanate molecule forms a chemically bonded bridge between two or more adjacent wood particles. This reaction is only one of several involving isocyanates that can and probably do occur in a hot press during formation of particleboards when isocyanate binder is used. A very important reaction is that of isocyanate with water to produce a very unstable carbamic acid which immediately decomposes to form a primary amine and COg ... 

Stereochemistry, substituent effects and activation parameters of most ketene reactions are consistent with a one-step cycloaddition polar effects of substituents and solvents, as well as the isotope effect, often require, however, that a fair amount of charge separation (that is, unequal bond formation) characterises the transition state. It has been kinetically proved that cycloadditions of enamines to ketenes can also proceed through a dipolar intermediate this is so for the reaction between dimethylketene and N-isobutenylpyrrolidine . In the latter case, the rate coefficient for the formation of the intermediate strongly depends on solvent polarity itacetonuriie/ cyclohexane = 560. Use of the Same criteria used for ketenes (as far as experimental data allow it) in the case of the 1,2-cycloadditions of fluorinated olefins results, instead, in the conclusion that a two-step biradical mechanism is operating. Results for 1,2-cycloaddition of sulfonyl isocyanates to olefins, cases (g) and (h) in Table 17, give indications of dipolar intermediates during the course of these reactions. 

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