A-Insertions

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

CO is a representative species for Q, -insertion its insertion into C—Pd bonds affords acylpalladium complexes such as 15. Mechanistically, the CO insertion is 1.2-alkyl migration to coordinated CO. This is an important step in carbonyiation. SO , isonitriies, and carbenes are other species which undergo a.a-insertion. 

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A. Insertion Reactions into Metal-Carbon Bonds... 

An alternative proposal for the propagation of this reaction is shown in Scheme 7-24, which includes (a) insertion of CO into the Pd-S bond of 118 to provide 119, and (b) reaction of 119 with 115 to give 116 with regeneration of 118 through a o-metathesis-like transition state 120. According to this mechanism, the ArS group in 118 and the RiN group of 115 are incorporated simultaneously. 

Fig. 5. Possible mechanisms for the MMO hydroxylation step. Pathway A insertion of the oxygen atom of Q into the C-H bond B concerted addition of the C-H bond to Q followed by reductive elimination C, D homolytic attack of Q on the C-H bond E reaction of the peroxo species with substrate.

Glebova, Z.I. and Zhdanov, Y.A., Insertion of a methylene group into the C-P bond of carbohydrate-containing (acyloxy)imino phosphonates, Zh. Obshch. Khim., 62, 2390, 1992. 

Figure 2. Proposed reaction mechanism for (a) insertion, (b) chain termination and (c) chain branching in the case of the Brookhart Ni-bis-imine polymerization catalyst. Large bulky substituents have been removed for clarity...

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

The concept behind this reaction can be traced to the work of Werner and coworkers who demonstrated the clean a-insertion of phenyl and other organic groups into rhodium vinylidenes to give vinyl-rhodium complexes (Scheme 9.14) [29]. 

Scheme 9.14 Oxidation state-dependent stereoselectivity in a-insertions into a Rh vinylidene.

Notably, the proposed stereoselectivity of a-insertion seems to run opposite to that observed for some related processes (vide supra). Such systems differ from the present system, though, because they involve an octahedral Rh(III) center, whereas intermediate 88 is a Rh(I)-square-planar complex. In Werner s original stoichiometric studies, the same mechanistic dichotomy was evident (Scheme 9.14) - square planar Rh(I)-complexes underwent cisoid insertion in the absence of other factors and transoid addition in the presence of an oxidant (HX). In intermediate 88, coordination of the alkene moiety to the rhodium center may also play a role in directing insertion. 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

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The following mathematical manipulations have been used several times already The boundary concentration CA/B is replaced by CB/A. Since flux equations, and finally get ... 

FIGURE 2 RNA editing of the transcript of the cytochrome oxidase subunit II gene from Trypanosoma brucei mitochondria, (a) Insertion of four U residues (pink) produces a revised reading frame, (b) A special class of guide RNAs, complementary to the edited product, may act as templates for the editing process. 

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