Ethyl 4-iodobenzoate

Add slowly a solution of ethyl 4-iodobenzoate in THF (2 mL) to the catalyst and then the red coloured solution of the zinc compound. Wash the excess of zinc dust with THF (2 mL) and add the resulting solution to the reaction mixture. 

Hiyama has used the ingenious sttategy of inttamolecular activation to facilitate the cross-coupling of readily accessible and stable silanes with aryl iodides (Equation 109) <2005JA6952>. The thienylsilane 223 could be reacted with ethyl 4-iodobenzoate in the presence of PdCl2 and the iminophosphine ligand 224 to give the product in 93% yield. 

Synthesis of tetramethylchroman analogue (64) was similar to that of thiochroman (63) except that the key ethynyl intermediate (72) was prepared from acetophenone (70). Formation of a phosphoester from the lithium enolate salt of (70) was followed by elimination of the phosphate group to give acetylene (72). Coupling of (72) with ethyl 4-iodobenzoate proceeded as with (71) and gave (64). 

A solution of ethyl 4-iodobenzoate (210, 276 mg, 1 mmol), pinacol borane (351, 190 mg, 1.5 mmol, 1.5 eq.) and triethylamine (0.42 ml, 0.30 g, 3 mmol, 3 eq.) in dioxane (4 ml) was flushed with nitrogen, and Pd(dppf)Cl2 (22 mg, 3 mol%) was added. The reaction mixture was stirred at 80 C under a nitrogen atmosphere for 2 h. After being cooled, the reaction mixture was poored into water (20 ml) and extracted with dichloromethane (3x20 ml). Combined organic layers were washed with water and brine, dried (Na2S04), filtered, and evaporated. From the crude product, 218 mg (79%) of pure 4-ethoxycarbonylphenyl pinacolboronate (367) was obtained. 

Besides these paUadium/phosphine catalysts, and similar to arylzinc nucleophiles, Pd(0) nanoparticles (PdNPs) obtained from Pd(OAc)2 also catalyze the cross-coupling between alkynylzincs and aryl iodides [142]. Optimal results are obtained with a combination of 0.5 mol% of hgand-free Pd(OAc)2 catalyst and 1 equiv. of nBu4NBr in THF. In contrast, however, with arylzincs, the reaction does not take place at rt and heating to 60 °C is required. As an example, ethyl 4-iodobenzoate (300) is readily transformed into the aUcynylated product 301 (Scheme 4.67). 

A similar coupling reaction of alkenyl iodides with alkenyldi(cyclopentadienyl) zirconium or ethyl 4-iodobenzoate with alkenyl 2-pyrindinyldimethylsilanes affording the corresponding coupling products was also reported (eqs 22 and 23).19 20... 

Scheme 2-19. I/Mg exchange reaction performed with ethyl 4-iodobenzoate (29).

Transition metal-free homocoupling of ethyl 4-iodobenzoate... 

A 25-mL flame-dried Schlenk tube flushed with argon is charged with ethyl 4-iodobenzoate (2.9 mmol, 773 mg), 1,2-dimethoxyethane (dme) (5 mL) and cooled to -20 °C. Isopropylmagnesium chloride (2.9 mmol, 1.33 mL of a 2.1-M solution in tetrahydrofuran) is then slowly added and the reaction mixture is stirred at -20 °C until GC analysis of reaction aliquots indicates complete exchange. Subsequently, a solution of copper(I) cyanide di(lithium chloride) (2.8 mmol, 2.8 mL of a 1-M solution in tetrahydrofuran) is added and the reaction mixture is stirred for 20 min. A solution of (2,2-diphenylvinyl)trifluoromethanesulfonate (330 mg, 1 mmol) and tris(acetylacetonato)iron (38 mg, 0.1 mmol) in 1,2-dimethoxyethane (3 mL) is added at once at -20 °C and the reaction mixture is stirred at room temperature for 1 h. The reaction is quenched with sat. aq. ammonium chloride and extracted several times with diethyl ether. The combined organic layers are washed with a 2 1 mixture of aq. ammonia and aq. ammonium chloride, and sat. brine, and are dried over magnesium sulfate and concentrated under reduced pressure. Purification by column chromatography (pentane-diethyl ether, 99 1) affords the product as a yellow oil 254 mg (77%). 

The nitro-substituted arylmagnesium species 328 is best prepared using steri-cally hindered mesitylmagnesium bromide 329 [179]. Thus, the reaction of the zinc derivative of328 with ethyl 4-iodobenzoate (THF, -40 °C to rt, 3 h) in the presence of Pd(dba)2 (5 mol%) and tfp (10 mol%) provides the biaryl 330 in 68% yield (Scheme 4.73) [179]. 

---