Insertion Mode

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

In cases where a block of sugar irnits is transferred it is usually inserted at the nonreducing end of the polymer, which may be covalently attached to a protein. Notice that the insertion mode of chain growth exists for lipids, polysaccharides, and proteins. 

A mass spectrum of the methyl ester of N-trifluoroacetylmelphalan was reported, without analysis of the fragmentation pattern [41]. The mass spectrum of mel-phalan-d2 (direct insertion mode) showed a molecular ion at ra/e 306 (3% relative intensity) and a base peak at m/e 232 (M-CH(NH2)C00H)+. A linear relationship was demonstrated between the composition of melphalan/melphalan-d2 mixtures and the peak intensities at m/e 230 and m/e 232 l44]. 

Deramond, E., J.-M. Savariault, and J. Galy. 1994. Silver insertion mode in 3-AgxV205 tunnel structure. Acta Cryst. C50 164—166. 

Regiospecificity of Ziegler-Natta and related coordination catalysts for a-olefin polymerisation implies non-equivalence of the two insertion modes, primary and secondary [schemes (39) and (40) respectively], owing to electronic and/or steric factors. 

As there are two reactive sites on Ci and C3 of the allyl group a zr-allyl group present at the end of the polymer chain allows for two insertion modes 1,2- and 1,4-insertion. If BD is exclusively polymerized in either of these modes the two poly(butadiene) isomers l,2-poly(butadiene) and 1,4-poly(butadiene) are obtained (Scheme 29) [297]. 

The action of HFA on a silylphosphane has been reported (5). Both possible insertion modes have been found. Compound 21b undergoes an intramolecular Arbuzov rearrangement, which is evident from the large i9F-3ip and coupling constants (5). 

Figure 14 Alternative propylene insertion modes in diimine-nickel catalysts (RLS = rate-limiting step).

Figure 2 Proposed mechanism of butadiene hydrocyanation with Ni catalysts showing the alternate insertion modes that prepare the way for introduction of the second CN group...

Solid samples can be introduced into electrothermal atomizers either as such or as slurries. Each insertion mode has its own features, advantages and disadvantages. 

The variables that influence the solid sampling process depend on the particular sample insertion mode used. 

It should be noted that Ci-symmetric metallocenes show a stereoselectivity increase with elevated polymerization temperature and lowered monomer concentration, a behavior opposite to that displayed by the C2-symmetric metallocenes. The regioregularity of PP samples prepared with most Ci-symmetric metallocene catalysts is fairly high. The predominant monomer insertion mode is 1,2. Isolated 2,1 and/or 3,1 units can be observed with <0.5 mol%o. 

Ramalingam and coworkers have performed detailed computational studies at various levels of theory with respect to the insertion of HCIC and CbC singlet carbenes into the carbon-hydrogen bond of small alkanes and identified two insertion modes (Fig. 6.8)." Of these, the a approach has been found to be preferred over the n mode at all levels of theory because the k attack leads to the eclipsed conformation (compare transition state 143 to 144, which is a secondary saddle point). In the initial phase of the insertion process there is a net charge flow from the alkane to the carbine. Barriers for the insertion... 

Although Pol V replicates undamaged templates with relatively low fidelity (10 3 to 10-4) [76], one striking quality is Pol V s ability to accurately bypass UV photoproducts (e.g., inserting dATP opposite thymine-thymine (TT) cyclobutane pyrimidine dimers (CPDs) [76]). Analysis of insertion tendencies opposite a variety of adducts/lesions led to the observation that Pol V seems to have two insertion modes (i) correct dNTP insertion and (ii) default dATP insertion [37]. UV light is a frequently encountered form of DNA damage for which a translesion synthesis DNA polymerase might be important and since TT CPDs are the major UV lesion... 

Boyd-Boland and Pawliszn reported the first application of SPME to the analysis of herbicide residues in 1995, for the simultaneous determination of nitrogen-containing herbicides in soU, water, and wine samples.Herbicides have been extracted following the three extraction modes (DI, HS, and in-tube), but direct insertion mode was the most used for these compounds. Krutz et al. have recently published an exhaustive review dealing with SPME for herbicide determination in environmental samples. 

Scheme 5. Four Possible Insertion Modes of a Prochiral Olefin Such as Propene, into the Mt-Growing Chain (here simulated by a methyl group) Bond ...

If the olefin is unsymmetrically substituted, the structure of the product will evidently depend on the direction of insertion thus, for propylene the two insertion modes (2) and (3) may in principle be formulated ... 

Markownikoff s rule is frequently applied to this problem, and one of the insertion modes is designated as Markownikoff mode, and the other as anti-Markowni-koff mode. There is, however some confusion in the literature as to which is which. 

The two insertion modes are designated a and b, to make the scheme applicable to various kinds of substituted ethylenes. For the case of propylene (X = CH3), a is the anti-Markownikoff mode, and we have seen that, for electronic reasons, this mode is generally favored in the case of group VIII catalysts, in particular in the presence of donor ligands (Table 6). Actually, if no particularly bulky ligands are present in catalytic Ni species, the aa route is by far predominant, and 4-methylpentene-l, and positional isomers therefrom, are the main products of dimerization -66)... 

Scheme 1.18. Olefin insertion modes into M-H bond and ft-W eUmination.

The hydroformylation of propylene provides two types of products, n- and isobutyraldehydes depending on the insertion modes of propylene into the M-H bond. As shown in Scheme 1.18a and b, where R = H, the anti-Markovnikov type addition of M-H to the double bond in (a) gives the linear propyl, whereas the Markovnikov type addition gives the isopropyl group bound with the metal. Further insertion of CO yields the linear and branched acyl groups. 

Butadiene may insert into the M-R bond by 1,2-insertion or 1,4-insertion mode. The latter mode giving the poly-cA-1,4-butadiene is particularly important for producing materials for synthetic rubber tires. For isoprene polymerization the situation is further complicated because of the presence of the methyl substituent. Since the physical properties of poly-cw-l,4-isoprene are quite similar with those of natural rubber, knowing the means to control the polymerization behavior is of particular industrial importance. 

A common example of an input interface interpretation error occurs with many word processors where the user may think they are in insert mode but instead they... 

Recently, a few theoretical works also concentrated on ODHP reaction and yielded some interesting but contradicting results. Curtiss et al. [65] have performed both cluster and periodic slab model calculations to investigate the activation of propane over V2O5 (010). However, they only concentrated on the oxygen insertion mode (i.e., TS5 in Fig. 5) and arrived at a calculated barrier for... 

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