Organotin methodology

The organotin group is usually attached to the polymer (usually polystyrene) by using conventional organotin methodology, for example 82... 

A combination of isoxazole and organotin methodologies is utilised in syntheses of flavones and pyrano[3,2- ]chromene-4,6-diones. Cyclisation of the... 

Stannyl derivatives of 2ff-pyran-2-one, accessible from bromopyranones by cross coupling with organotin reagents, themselves take part in Pd(0)-catalyscd cross coupling with enol triflates. This methodology offers a new approach to steroidal pyran-2-ones <96JOC6693>. 

The versatility, predictability and functional-group tolerance of free radical methodology has led to the gradual emergence of homolytic reactions in the armory of synthetic chemistry. Tin hydrides have been successfully employed in radical chemistry for the last 40 years however, there are drawbacks associated with tin-based chemistry. Organotin residues are notoriously difficult to remove from desired end products, and this, coupled with the fact that many organotin compounds are neurotoxins, makes techniques using tin inappro-... 

Schreiber and co-workers (436) prepared a library calculated to contain 2.18 million polycyclic compounds through the 1,3-dipolar cycloaddition of a number of nitrones with alkenes supported on TentaGel S NH2 resin (Scheme 1.83). (—)-Shikimic acid was converted into the polymer bound epoxycyclohexenol carboxylic acid 376 (or its enantiomer), coupled to the resin via a photolabile linker developed by Geysen and co-workers (437) to allow release of the products from the resin in the presence of live cells by ultraviolet (UV)-irradiation. A range of iodoaromatic nitrones (377) was then reacted with the ot,p-unsaturation of the polymer-bound amide in the presence of an organotin catalyst, using the tandem esterification/ dipolar cycloaddition methodology developed by Tamura et al. (84,85) Simultaneous cyclization by PyBrop-mediated condensation of the acid with the alcohol... 

Several important improvements of this methodology have been reported since 1994. A broad range of organotins were used in the reactions with aryl-, heteroaryl-, or alkenyliodonium salts in aqueous organic solvents. This reaction (Equation (2)) takes place at room temperature in the presence of phosphine-free palladium... 

While die above reactions represent only a small fraction of die reactions known for palladium, they form the basis of a powerful methodology for building carbon structures. Several variations have been developed which utilize certain types of reactants and give particular types of products. All diese variations, however, contain a common theme. In each case an electron-deficient reagent (e.g., a vinyl halide or aromatic triflate) reacts with an election-rich reagent (e.g., an alkene, an organoborane, or an organotin) witii the formation of a new carbon-carbon bond. In that sense diese reactions are related to die reactions between carbon nucleophiles and carbon electrophiles discussed previously in diis chapter. They are quite different, however, because diey proceed only in the presence of Pd(0). In fact they proceed only in die coordination sphere of Pd(0). The ability of Pd(0) to catalyze diese reactions is nearly unique We will now examine some of die more common processes. 

Radical chain reactions can take place via organotin reagent without hydrogen donors such as allyltin [38, 39]. This methodology has been applied to synthesize some important chiral precursors of natural products (for example pseudomonic acid). Thus, the pentose derivative 35 affords the compound 36 with retention of configuration via the carbohydrate radical R- at C-4 position [Scheme 18]. 

For some time, intermolecular radical additions have been an integral part of the methodological arsenal of preparative organic chemistry. However, from an ecological point of view the methods that have been used to date suffer from a number of drawbacks that present obstacles to their broad-based industrial application. This applies particularly to the commonly used organotin compounds. 

The allylation of carbonyl compounds with allyltin reagents is still an active area of organotin chemistry from the methodological point of view, and also for synthetic applications. For completeness we should add several alternative techniques, such as the development of trifluoromethanesulphonic acid as a Bronsted acid catalyst for the allylation of aldehydes in water123, or the design of fluorous allyltin reagents for the platinum-catalysed allylation of aldehydes124. 

A second set of conhibutions presents specific methodological and technical features of organometallic NMR applications to solids and to NMR under exceptional diemical and experimental conditions. The two chapters on solid state NMR deal with prospects of organometallic compounds focusing on stiuctural and dynamic properties of carbonyl metal complexes and organotin and -lead compounds. 

The radical produced after oxidative decarboxylation may be trapped intramolecularly by an olefin to form five- or six-membered rings. This methodology avoids the use of the toxic organotin reagents that are commonly used in the generation of radicals. Moreover, when RsSnH/AIBN is used, a C-H bond is formed in the radical termination step. Kolbe reaction protocol, on the other hand, allows the radical formed after cyclization to be captured by a different radical, rather than being reduced (cf. 45 and 47). 

The first examples of the coupling of organotin compounds were reported during the period 1976-1977, by the research groups of Eaborni and Kosugi. " The extensive synthetic and mechanistic work carried out by Stille and coworkers since 1978 soon turned this reaction into a standard methodology... 

In contrast to the organotin hydride reactions, the electrochemical process is thought to proceed via the bicyclo[3.1.0]hexanolate. The method constitutes an alternative methodology which avoids toxic organotin compounds. 

It has been reported that subjection of an 11-membered macrocycle involving an alkyne and alkenes to a radical cyclization procedure leads to a tandem cyclization giving rise to the formation of a fused 6,7-bicychc framework [249]. It should be mentioned that the in situ methodology for generation of organotin hydride works... 

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