Intermediates in Addition Reactions

The intermediate in addition reactions is a vinylic carbocation, which forms less readily than an alkyl carbocation. 

The conversion of 1-propanol over H-ZSM-5 or H-Y was not found to yield any C3 oxygenated products for a range of reaction conditions and the products are mainly propene and butenes. This confirms that the introduction of the carbon-carbon double bond into the reactant molecule significantly affects the reactivity. Conversion of 2-propanol over H-ZSM-5 was found to give significant selectivity to acetone at low flow rates and this indicates that this could be a possible reaction intermediate. In addition, reaction of propene oxide over H-ZSM-5, under comparable conditions to those utilised for allyl alcohol, produced significant selectivities of both acetone and allyl alcohol. 

Cyclic bromonium ion An intermediate in addition reactions, in which bromine bridges both ciirhons of a double bond. 

When we discussed cationic addition reactions in Chapter 9, we included a section on the polymerization reactions of alkenes (p. 384). Dienes form quite stable intermediates in addition reactions and are especially prone to polymerization. 

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

Carbinolamines are formed by nucleophilic addition of an amine to a carbonyl group and are intermediates in the for mation of imines and enamines Carbocation (Section 4 8) Positive ion in which the charge re sides on carbon An example is tert butyl cation (CH3)3C Carbocations are unstable species that though they cannot normally be isolated are believed to be intermediates in certain reactions... 

The intermediate in these reactions in the case of the addition of YX is consistent with the addition of Y to the 1-position to form an aHyUc intermediate to which X adds to produce either the 1,2- or 1,4-product. 

Small and Browning agree with Schbpf and Winterhalder that thebainone methyl enolate (formed by 1 6 addition in which the oxygen bridge is ruptured and ethylenic linkages produced at C —C and C —C ) may be the primary intermediate in tWs reaction since it is hydrogenated... 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

There are very few homolytic reactions on triazolopyridines. A suggestion that the ring opening reactions of compound 1 involved free radical intermediates is not substantiated (98T9785). The involvement of radical intermediates in additions to ylides is discussed in Section IV.I. The reaction of radicals with compound 5 and its 1-substituted derivatives gives 4-substituted compounds such as 234 (96ZOK1085). A more detailed study of the reaction of the 1-methyl and 1-phenyl derivatives with r-butanol and ammonium persulfate produced 4-methyl substitution with a silver nitrate catalyst, and the side chain alcohol 235 without the catalyst (96ZOK1412). 

A mechanism for this reaction has been proposed [75], The first key intermediate in the reaction is the copper(I) acetylide 42. The additional ligand may be solvent or H2O. The acetylene moiety in 42 is activated for a 1,3-dipolar cycloaddition with the nitrone to give intermediate 43, with introduction of chirality in the product. A possible route to ris/traws-41 might be via intermediate 44. Finally, the cis isomer is isomerized into the thermally more stable trans-41. It should be mentioned that the mechanism outlined in Scheme 6.32 was originally proposed for a racemic version of the reaction to which water was added. 

The addition of acetate ion to an arenediazonium ion is also an O-coupling. As the equilibrium of this reaction lies very much to the side of the starting ions (Suschitzky, 1967), and since diazoacetates play an important role as transient intermediates in homolytic reactions of A-nitrosoacetanilides, we will discuss diazoacetates in that context (Sec. 10.10). 

A number of studies have been reported concerning azide-isocyanide condensations to give tetrazoles. Early work by Beck and co-workers 18, 19) describes the addition of various isocyanides to metal azido species [Au(N3)4]", [Au(N3)2]", Au(PPh3)N3, and M(PPh3)2(N3)2, M = Pd, Pt, Hg. The products are carbon-bonded tetrazolato-metal complexes. It is not known whether metal isocyanide complexes are intermediates in these reactions. More recently inverse reactions with azide ion addition to metal isocyanide complexes were carried out, with similar results. From... 

OS 87] [R 35] [P 67] Generally, on-line analysis allows one to determine product concentrations, giving proper conversion [72, 74], A limit is given by intermediates which also absorb in the same spectral range, also shifting the maximum of the absorption curve. It was assumed that the product pinacol does not contribute to the absorption, only the reactant and the intermediates. In addition, a delay arises between the end of irradiation and on-line analysis as the reaction mixture has to pass a conduit between the two locations. Hence the reaction proceeds further and this is dependent on the flow rate and residence time. 

In addition, another computational study in the frame of DFT, using the hybrid functional MPW1K,53 had suggested that o-QM may be an intermediate in the reaction of the peroxy radical (HO2") with the benzyl radical at the ortho-position (Scheme 2.16),54 which should be significant in atmospheric processes and low-temperature combustion systems (T < 1500 K). 

The stability of molecules depends in the first place on limiting conditions. Small, mostly triatomic silylenes and germylenes have been synthesized successfully at high temperatures and low pressures, 718). Their reactions can be studied by warming up the frozen cocondensates with an appropriate reactant, whereas their structures are determined by matrix techniques 17,18). In addition, reactions in the gas phase or electron diffraction are valuable tools for elucidating the structures and properties of these compounds. In synthetic chemistry, adequate precursors are often used to produce intermediates which spontaneously react with trapping reagents 7). The analysis of the products is then utilized to define more accurately the structure of the intermediate. 

Jug and co-workers investigated the mechanism of cycloaddition reactions of indolizines to give substituted cycl[3,2,2]azines <1998JPO201>. Intermediates in this reaction are not isolated, giving evidence for a concerted [8+2] cycloaddition, which was consistent with results of previous theoretical calculations <1984CHEC(4)443>. Calculations were performed for a number of substituted ethenes <1998JPO201>. For methyl acrylate, acrylonitrile, and ethene, the concerted [8+2] mechanism seems favored. However, from both ab initio and semi-empirical calculations of transition states they concluded that reaction with nitroethene proceeded via a two-step intermolecular electrophilic addition/cyclization route, and dimethylaminoethene via an unprecedented two-step nucleophilic addition/cyclization mechanism (Equation 1). 

The simple oxidative addition product Ru(CF3)(CdCF3)(CO)2(PPh3)2 is a likely intermediate in this reaction. Several stable, zero-valent CF2 complexes have been isolated using Cd(CF3)2, and they are included in Table VII (see Section V,C,2,b). 

A 1,2-diazetidine has been proposed as an intermediate in the reaction of pyridazine-3,6-dione (12) with styrene.87 The observed product was thought to arise from addition of water to the 1,2-diazetidine, although the alternative more likely explanation involving a dipolar intermediate (cf. Scheme 5) was apparently not considered. In the photochemical reaction of styrene with DEAZD, a 1,2-diazetidine structure was tentatively assigned to a minor product.88 Attempted photochemical [2 + 2] cycloaddition of DEAZD to other olefins failed to give any 1,2-diazetidines.88... 

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