Substituent effects cross-coupling

An effective cross-coupling of alkynes using silicon-tethered Fe(CO)s-promoted cyclocarbonylation was shown to provide a seven-membered ring dialkoxysilane 302, which subsequently upon treatment with Me3NO in acetone at 0°C was converted to cyclopentadienones 303 with variable substituents (Scheme 54) <2002OL2837>. 

Thorough analysis of the coupling constants suggests that vicinal and cross-ring coupling can be valuable when used for stereochemical assignments in thietane oxides and dioxides, provided one takes into consideration the conformational changes and the substituent effects . 

If the stannane bears substituents at the position geminal to tin, the reaction requires activation by Cu1 (Section 9.6.3.2.1). The cross-coupling of aryl (or heteroaryl) stannanes with aryl (or heteroaryl) halides or triflates is a general route to various biaryls and their analogues, particularly useful in those cases when the Suzuki-Miyaura reaction is less effective for... 

Apart from Cu1, Zn11 salts or Zn metal were also shown to be effective, thus bringing forward an alternative efficient protocol (36) which may be useful for the applications in which the contamination of products by Cu cannot be tolerated. Such protocols can also be advantageous in cross-coupling with acetylenes bearing electron-withdrawing substituents, such as propiolate esters,115 which usually give unsatisfactory results under the standard conditions. 

Cross-coupling reactions between organoboranes and heteroaryl halides are effectively catalyzed by Pd(0) in the presence of a base. Couplings in simple pyrimidines are illustrated by the reaction between 2-chloropyrimidine and 2- or 3-thiophene- and selenophene-boronic acids which give the corresponding 2-substituted pyrimidines (921) (Scheme 72). In 2,4-dichloro- or 2,4-dibromo-pyrimidine it is the 4-halo substituent which is the more reactive. 

The stannylboration of alkynes was followed by cross-coupling of the G-Sn bond in the synthesis of stereodefined 1-alkenylboron compounds 143 (Equation (48)).261 There was a large accelerating effect of boryl substituents on thermal ring closing to the methylene cyclobutenes at 140 °G. 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

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