Organometallic selectivity

3-tri-n-butylstannyl pyridine with 3-diethylborylpyridine ° and the synthesis of a nncleoside boronic acid analogue.  

Negishi couplings on 2-bromo-5- and -6-tri-n-butylstannyl pyridines are possible due to the relatively high reactivity of 2-bromopyridines (the isomeric 5-bromo-2-stannylpyridine gave only low yields under the same conditions) and the low temperature of the reaction.  

For Suzuki reactions using haloboronic acids and another halide, the obviously best course is to have a more reactive halide in the substrate than in the boronate, so there are quite a few examples of chloro-heteroaryl-boronic acids coupling efficiently with aryl bromides. This difference is clearly shown by the coupling reactions of 2-chloro- and 2-bromopyridine-5-boronic acids with bromo-heterocycles.  

High-yielding Suzuki substitutions of halogen in halo-boronates are also possible using Al-methylimino-diacetic acid (MIDA) to form the protected boronate, which resists coupling under anhydrous conditions. This approach has been applied to aryl and heteroaryl (only thiophene and benzofuran, shown below) systems, and also for polyene synthesis.  

In couplings of 5-metallo-2-chlorotliiazoles, stannanes are preferred to boronates due to the relative instability of the latter. The corresponding zinc derivatives are unsatisfactory.  

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Top S, Vessieres A, Cabestaing C, Laios I, Leclercq G, Provot C, Jaouen G (2001) Studies on organometallic selective receptor modulators (SERMs). Dual activity in the hydroxy ferrocifen series. J Organomet Chem 637 500-506... 

Jaouen G, Top S, Vessieres A, Leclercq G, McGlinchey MJ (2004) The first organometallic selective estrogen receptor modulators (SERMs) and their relevance to breast cancer. Curr Med Chem 11 2505-2517... 

Keywords Bond activation Computational chemistry DFT Dispersion correction Mechanism Modelling Organometallics Selectivity... 

A similar pincer catalyst is the key component of the alkane metathesis system of Fig. 12.7. This consists of three steps run in tandem with two catalysts present. The Ir pincer catalyst first dehydrogenates the alkane, with selective formation of the terminal alkene. This selectivity is expected from the usual organometallic selectivity pattern of initial terminal CH oxidative addition followed by (3 elimination. A Schrock... 

Hard carbon nucleophiles of organometallic compounds react with 7r-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal 7T-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

Acyl halides react with organometallic reagents without catalysts, but sometimes the Pd-catalyzed reactions give higher yields and selectivity than the Lincatalyzed reactions. Acyl halides react with Pd(0) to form the acylpalladium complexes 846, which undergo facile transmetallation. 

Reactions via organometallic intermediates achieve position selectivity on the basis of prior substitution, for example through halogen-metal e.xchange... 

The stereoselectivity of organometallic additions with carbonyl compounds fits into the general pattern for nucleophilic attack discussed in Chapter 3. With 4-r-butylcyclohex-anone, there is a preference for equatorial approach but the selectivity is low. Enhanced steric factors promote stereoselective addition. 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

Reactivity and selectivity of organometallic reagents addition to C=N bond with participation and formation of heterocycles 98CRV1407. 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Throughout each chapter, clear structures, schemes, and figures accompany the text. Mechanism, reactivity, selectivity, and stereochemistry are especially addressed. Special emphasis is also placed on introducing both the logic of total synthesis and the rationale for the invention and use of important synthetic methods. In particular, we amplify the most important developments in asymmetric synthesis, catalysis, cyclization reactions, and organometallic chemistry. 

The selective insertion of diphenylacetylene in the cyciopaiiadated sulfide complex 1 leads to the stable organometallic complex 2, which can be depalladated with silver(I) tetrafluoroborate to give a mixture of the dibenzothiepinium salt 3 and the dibenzo[Z>,z ]thiepin 4.91 Demethyla-tion of 3 to yield 4 is complete after refluxing overnight in chlorobenzene. The synthetic scope of this method for thiepin derivatives is limited due to their thermal instability, but the method is very suitable for the synthesis of 1//-2-benzothiopyrans.91... 

Fluorination of organometallics, e.g., Me3Sn-imidazoles, has been used to improve selectivity with the aromatic—metal bond being broken preferentially (86BSF930). 

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