Olefin capture

From alkyl complex p0, the olefin can be captured to form the Jt-complex TCo, and inserted via 1,2- or 2,1-insertion route. In the model applied here we consider the olefin capture and its insertion as one reactive event i.e. we assume a pre-equilibrium between the alkyl and olefin complexes, described by an equilibrium constant Kcompi = [icq] / [Pol = exp (AGcomp . / RT). This... 

From alkyl complex Po, the olefin can be captured to form the rt-complex tto, and inserted via 1,2- or 2,1-insertion route. In the model apphed here we consider the olefin capture and its insertion as one reactive event, i.e., we assume a pre-equihbrium between the alkyl and olefin complexes, described by an equilibrium constant =[Tto]/[Po]=exp (AG ,p ./RT). This corresponds to neglecting the barrier for the monomer capture. Such an approach is valid for the late-transition metal complexes, e.g., the diimine catalysts studied in the present work, where the resting state of the catalyst is a very stable olefin Ti-complex [16] and the olefin capture barrier and the related n-complex dissociation barrier is much lower than the insertion barriers. This assumption allows one to speed up the simulation otherwise many olefin capture/dissocia-tion steps, not important for the final result of the simulation, would be happening before insertion takes place. It follows from the above considerations that the insertion rate is given by Eq. (8), and the equation for the isomerisation vs insertion relative probability (Eq. 4) includes the isomerisation and insertion rate constants, the equilibrium constant, K omph <>nd the olefin pressure, polefin,- Finally, the relative probability for the two alternative insertions is given by Eq. (4) it depends on the two rate constants ratio only. 

More general procedures for additions of halogen fluorides to highly fluori-nated olefins involve reactions with a source of nucleophilic fluoride ion, such as an alkali metal fluoride, in the presence of aposttive halogen donor [62 107, lOff, 109, 110, 111] (equations 11 and 12) These processes are likely to occur by the generation and capture of perfluorocarbamonic intermediates Tertiary fluormated carbanions can be isolated as cesium [112], silver [113], or tns(dimethylamino)sul-... 

Tnmethyl(trifluororaethyl)tin can also be prepared via in situ formation and capture of tnfluorometbide by trimethyltin chlonde [13, 14] (equation 9) This tin analogue has been used as a precursor for difluorocarbene either by thermal decomposition or by reaction with sodium iodide m 1,2-dimethoxyethane This carbene generation procedure has been used to study difluorocarbene selectivity with steroidal olefins [75] (equation 10). 

The nucleophilic attack of nitrogen bases leads to a variety of products as the result of addition or addition-elimination reactions The regioselectivity resembles that of attack by alcohols and alkoxides an intermediate carbanion is believed to be involved In the absence of protic reagents, the fluorocarbanion generated by the addition of sodium azide to polyfluonnated olefins can be captured by carbon dioxide or esters of fluonnated acids [J 2, 3] (equation I)... 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

The measurement of these angles for a series of [PdClj(NHC) P(OR)3 ] complexes permitted to evidence the remarkable flexibility of NHCs due to rotations aronnd the iV-substituent bonds [82]. This flexibility, captured by the allows NHCs to respond actively to the steric requirements of co-ligands. This is further confirmed by ab initio molecular dynamics simulations aimed at understanding the variability of ( )j and in a series of NHCs containing Ru-complexes relevant to olefin metathesis [83]. 

The one-electron oxidation of enol silyl ether donor (as described above) generates a paramagnetic cation radical of greatly enhanced homolytic and electrophilic reactivity. It is the unique dual reactivity of enol silyl ether cation radicals that provides the rich chemistry exploitable for organic synthesis. For example, Snider and coworkers42 showed the facile homolytic capture of the cation radical moiety by a tethered olefinic group in a citronellal derivative to a novel multicyclic derivative from an acyclic precursor (Scheme 8). 

RIES from diazoalkanes is also sensitive to the dihedral angle between the migrating a-H and the C-N bond of the diazo moiety.57 For example, the A values for the pyridine capture of the photolytically generated carbenes from 46 and 47 are in the ratio of 1.7 1. Similarly, the carbene from 46 is more efficiently generated and trapped in methanol, whereas the photolysis of 47 in methanol affords twice as much olefin (by 1,2-H RIES) compared to the photolysis of 46. These phenomena are attributed to conformational factors that favor RIES during the photolysis of 47, with the proximal excited state represented as a pyramidalized 1,3-C-N=N diradical.57... 

Whereas Hegedus [335] and Danishefsky [336] were the first to discover a tandem Heck reaction from o-allyl-A -acryloylanilines leading to tricyclic pyrrolo[l,2-a]indoles or pyridino[l,2-a]indoles [336], it has been the fantastic work of Grigg to unleash the enormous potential of this chemistry. Grigg and his co-workers parlayed their Pd-catalyzed tandem polycyclization-anion capture sequence into a treasure trove of syntheses starting with IV-allyl-o-haloanilines [337-345], Diels-Alder and olefin metathesis reactions can be interwoven into the sequence or can serve as the culmination step, as can a wide variety of nucleophiles. An example of the transformation of 289 to 290 is shown below in which indole is the terminating nucleophile [340],... 

The carbonyl oxygen of an ester group, (e.g., in acrylates or vinyl esters), is more basic than a vinyl group and it captures protons (or other cations) from the catalytic system to give a resonance-stabilised cation which does not involve the reaction site, namely the olefinic double bond. Hence, acrylates and vinyl esters do not polymerise cationically. 

The benzannulated analogs were also found to behave in a similar fashion. Attachment of a pendent olefin to the benzannulated enyne-allene system as depicted in 89 allowed the aryl radical in 90 to be captured in a 5-exo radical cyclization reaction leading to 91 and then the dihydrobenz[e]indene 92 (Scheme 20.20) [55, 56]. 

Thermolysis of 164 bearing a pendent olefin at 150 °C also allows the aryl radical in 167 to be captured in a 5-exo fashion, leading to 165 as a 1 1 mixture of diastereo-mers (Scheme 20.33). The corresponding Lewis acid-catalyzed [3,3]-sigmatropic rearrangement promoted by AgBF4 was facile at room temperature, making it possible for 166 to be isolated. Thermolysis of 166 at 75 °C afforded 165 in 80% yield. 

Initially, one carbon atom (a methyl group) is attached to the metal of the catalyst (A in Figure 1). In the first step, it will capture and insert a propylene molecule via either 1,2- or 2,1-insertion route. Thus, one of these insertion events is stochastically chosen this choice, however, is not totally random but weighted by the probabilities of the two reactions. Here the relative probabilities are proportional to the relative rates. Now, if one assumes that the 1,2-insertion has happened in the first step, i.e. the iso-butyl group is attached to the catalyst (B) after insertion. At this stage four different elementary events are possible two alternative insertion routes (1,2- and 2,1-) proceeded by the capture of olefin, the termination reaction,... 

In the sensitized generation of ethoxy-carbonyl-carbene the fight capture by the diazocompound is only 25% A number of photosensitized reactions of triplet carbenes with cis- and trans-olefins are fisted in Table 10. The sensitizers used, as well as their various triplet energies Et, are also given in the table. 

Most of the toluene and xylenes have their origin in catalytic reforming or olefins plants. From there, the processing schemes vary widely from site to site. The schematic in Figure 3-6 captures most of the variations, although its hard to portray that some plants separate the BTXs from each other early in the scheme while others do it at varying places downstream of an aromatics recovery unit. 

Clark TD, Ghadiri MR. Supramolecular design by covalent capture. Design of a peptide cylinder via hydrogen-bond-promoted intermolecular olefin metathesis. J Am Chem Soc 1995 117 12364-12365. 

Mathias and Moore (30-33) described a new synthesis of isomiinchnones 55 via the thermal cyclization of A-(chloroacetyl)lactams (54) (Scheme 10.7). These isomiinchnones can be captured by NPM to give fused 2-pyridones in moderate yields. Cycloadducts from the reaction with DMAD are produced in much lower yields (<17%), and other olefinic dipolarophiles (fumarate, maleate, acrylate, and dicyanocyclobutene) are unreactive. Reaction of 7/-(chloroacetyl)benzamide (57) in the presence of NPM gave 58 in low yield. 

The first report of resin capture in solution-phase chemical library synthesis involved the covalent capture of solution-phase Ugi reaction products onto a functionalized polystyrene resin.73 Excess reactants, reagents, and reagent byproducts were washed away from the resin-captured intermediates. Further manipulation and release afforded purified solution-phase products for screening. More recently the same group reported on resin capture as a technique for the preparation of tetrasubstituted olefin libraries.74 75 As illustrated in Scheme 5, m-vinyl di-boryl esters were reacted with aryl halides (R3ArX) in parallel Suzuki reactions, leading to solution-phase intermediates. Another Suzuki reaction, this time with the... 

In order to produce a high initial rate we suggest that the iron complex must produce radicals by attacking a reactant, and the thiol is the most likely one. This proposal is supported by the recent demonstration by Wallace (25) that ferric octanoate readily reacts with thiols at ambient temperature to give RS radicals which are effectively captured by an olefin, provided the ratio of thiol to iron concentrations is not greater than 10. 

In acetonitrile, the valence band of Ti02 lies at about +2.4 V versus the standard calomel electrode and the conduction band lies at -0.8 V. The hole migrates to the surface where it oxidizes adsorbed olefin (oxidation potential = +1.4 V), producing a surface-bound cation radical. The electron also migrates to the surface where it is captured by adsorbed oxygen (reduction potential = -0.75 V), forming adsorbed superoxide. 

LiBr and in the presence of cyclopentene as a scavenger olefin. The kinetics, determined by monitoring the formation of strong acids (TfOH or HBr), show that the rate of solvolysis of 65 is dependent on [Br-] (at a constant ionic strength). In the presence of Br-, the products are trans- 1,2-dibromides and bromo-solvates of both cyclohexene and cyclopentene. The cyclopentenyl products have been shown to arise from the electrophilic addition of Br2/Br3 to cyclopentene, while trans-l, 2-dibromocyciohexane 67 is formed by Br- capture of the bromonium ion 66 on carbon. The Br2 required for bromination of cyclopentene results from attack by Br- on the bromonium ion 66 on Br+. On the basis of the ratio of the cyclopentyl products to 67, Br- capture of the solvolytically produced bromonium ion 66 (by attack on Br+) is 4-5 times more prevalent than attack on carbon in AcOH, and ca 25 times more preferred in MeOH123. 

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