Insertion selectivity

As a tool to improve the regio- and stereoselectivity of C-H insertion, activation of a specific C-H bond of substrates to be inserted seems to be appropriate in conjunction with the manipulation of carbene character. These two tools for the improvement of insertion selectivity will provide us with useful tools of the C-C bond formation by carbenes and carbenoids. 

For application in organic synthesis, the regiochemistry of insertion of carbenoids into un-symmetrical zirconacydes needs to be predictable. In the case of insertion into mono- and bicydic zirconacydopentenes where there is an wide variety of metal carbenoids insert selectively into the zirconium—alkyl bond [48,59,86], For more complex systems, the regiocon-trol has only been studied for the insertion of lithium chloroallylides (as in Section 3.3.2) [60]. Representative examples of regiocontrol relating to the insertion of lithium chloroal-lylide are shown in Fig. 3.2. 

The relatively high tendency of aluminium alkyls to undergo j -hydrogen transfer, coupled with olefin insertion barriers that are much higher than those for transition metals, makes it unlikely that very efficient high-MW olefin polymerization catalysts based on aluminium will be discovered. Even for the reaction of aluminium alkyls with ketones, using ligands to tune towards insertion selectivity will be difficult. 

Dirhodium(II) compounds are reported to be the most suitable catalysts for insertion. Selectivity is higher and yields are greater with dirhodium(II) carboxylates or carboxamidates than with copper catalysts, whereas Ru catalysts are not known to facilitate C-H insertion. As expected by a process that is basically electrophilic, electron-donating substituents that are adjacent to the site of insertion activate that center for C-H insertion ril4]. In addition to electronic influences, however, conformational effects that are basically steric in origin can also control reaction selectivity [115]. 

Cyclohexylisonitrile inserts selectively into the endocyclic vinyltitanium bond of a-methylenetitanacyclobutene complexes 37, although the reaction fails for many other isonitriles (Equation 12) <19960M1176>. No insertion occurs upon treatment with nitriles or ketones, in contrast to the established reactivity of typical titanacyclobutenes <1996CHEC-II(lb)887>. The reaction of fert-butylisonitrile with 1,9-anthracenediyl-zirconocene complex 38 proceeds exclusively by insertion into the terminal ring zirconium-carbon bond, presumably due to steric considerations (Equation 13) <2000JA9880>. 

Components of a photosystem can be inserted selectively into the lipid wall or the inner cavity of the vesicle. For this purpose the lipid and components insoluble in water are dispersed together in aqueous solution by sonification. This leads to an occlusion of water insoluble components within the lipid bilayer. The vesicle membrane is sufficiently stable and impermeable for a number of ions. This allows one to prepare, by gel-filtering, the media of different ionic composition inside and outside the vesicle as shown in Fig. 2b. Such asymmetry of chemical content can be preserved for a rather long time (from several hours to several days). Recently the surfactant molecules with double bonds, which can be polymerized after vesicle preparation, were used for further enhancement of vesicle stability [37-39]. Such polymerized vesicles are stable for several months. 

The synthesis of (+)-estrone methyl ether (36) illustrates the enantioselective construction of a polycyclic target by the use of chiral auxiliary control to establish the first cyclic stereogenic center [14], In this case, the specific design of the naphthyldiazoester 32 directed Rh-mediated intramolecular C-H insertion selectively toward one of the two diastereotopic C-H bonds on the target methylene. The new ternary center so created then biased the formation of the adjacent quaternary center in the course of the alkylation. The chiral skew in the product cyclo-pentanone (35) controlled the relative and absolute course of the intramolecular cycloaddition, to give the steroid (+)-estrone methyl ether (36). 

The stereochemistry of nitrene insertion into unactivated C—bonds has been studied using substituted cyclohexanes as substrates. For arylnitrenes which usually exhibit triplet reactivity, the reaction is nonspecific, but most other nitrenes undergo stereospecific C—insertion. For example, benzqylni-trene inserts selectively into the tertiary C—bond of toth cis and tra/u-1,4-dimethylcyclohexane with retention of configuration. Similarly with cis- and rra/is-l,2-dimethylcyclohexane as substrate, ethoxycarbonyl-, methatiesulfonyl- and cyano-nitrenes all insert with retention of configuration at the tertiary C—bond. 

Dendrimers ) are macromolecules that exhibit a defined structure and not only a high degree of order but also a high degree of complexity. From a topological viewpoint, dendrimers contain three different regions core, branches, and surface (Fig. 1). A most important feature of dendrimer chemistry is the possibility to insert selected chemical units in predetermined sites of their architecture. Further, because of their three-dimensional structure, dendrimers exhibit internal dynamic cavities where ions or molecules can be hosted. 

It has been proposed that poisons, such as CO and COz, insert selectively into the propagative transition metal-carbon bond. The reactions involved are believed to proceed in the following sequence (for CO as the poison) ... 

Exposure of an azirine to carbon monoxide in the presence of a catalytic amount of [Pd(PPh3)4] gives the bicyclic P-lactam (23) in reasonable yield. Attempts to synthesize monocyclic P-lactams from azirines has so far failed, but aziridines can be carbonylated to -lactams. )epending on the organome-tallic reagent used two different products can be obtained (Scheme 26). Using catalytic quantities of [Rh2(CO)4Ch] carbon monoxide inserts selectively into the more-substituted C—N bond, whereas using a stoichiometric amount of [Ni(CO)4] it is the less-substituted C—N bond that is carbonylated. ... 

Diazo compounds insert selectively into the alkenyl C—Al bond , which leads to allylic Al compounds ... 

A single substituent allylic to the monosubstituted olefin is sterically large enough to make the initial insertion selective (Eqs. 79,80) [38]. Either substrate diastereomer can be prepared, leading to different steric interactions in the transition states (Figs. 13,14), and different product stereochemistry patterns. 

The ability to insert selectively metal cations into macromolecule ions in the gas phase is attractive both for the study of the intrinsic interactions of metal ions with macromolecules and for maximizing the structural information available from tandem mass spectrometry. For example, singly-charged, singly-sodiated peptide cations have been shown to fragment next to the C-terminal residue [124], whereas the protonated version fragments at various locations along the peptide backbone. The insertion... 

The reaction of the metallacycle 231 with hetererocumulenes of the type X=G=Y (X, Y = O, S, NR) has been studied in detail. " " These molecules insert selectively in the Ni-G(aryl) bond of 231 (Scheme 70). It has been proposed that the selectivity is controlled by the facile displacement of the PMes ligand located cis to the aryl. 

Yi W, Huang S, Zhang J, Chen Z, Zhou X. Reactivity of scorpionate-anchored yttrium alkyl primary amido complexes toward carbodiimides. Insertion selectivity of Y—NHAr and Y—CH2Ph bonds. Organometallics. 2013 32 5409-5415. 

Spigos et al. [112] adopted a strict protocol that resulted in a remarkably low number of complications. The protocol included broad-spectrum antibiotics started 8-12 hours before the procedure and continued for 1-2 weeks, local antibiotics (such as gentamicin) suspended in the solution used to deliver the particulate embolic agents and administered through the angiographic catheter, strict attention to sterility (whole-body povidone-iodine bath or wide surgical scrub at the site of catheter insertion), selective catheterization with the catheter tip beyond pancreatic branches, effective pain control with narcotics or epidural anesthesia for 48 hours (which prevents the splinting that may... 

Click the Insert Selection button on the Chopper toolbar. An event appears on the timeline corresponding to the selection area in the Chopper. 

By making the arrow longer than the selection area, you can insert events in a periodic fashion with a time interval between them by repeatedly clicking the Insert Selection button. 

Click the Insert Selection button. The example uses six repetitions. 

Thus, the propagation steps are consecutive and rapid insertions of coordinated monomer into the growing polymer chain. Both the position of the coordinated monomer and the location of the regenerated vacant site can significantly impact the catalyst polymerization stereoselectivity, an issue that will be discussed in Section 3.21.5. While there is just one possibility for ethylene coordination and insertion, there are four possible modes for 1-alkene (a-olefin) coordination and insertion. Selection among these modes determines the... 

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