Palladium-catalyzed cross-coupling stereoselective

Palladium-catalyzed cross-coupling of alkenylalanes with E) or (Z) alkenyl halides leads to efficient, stereoselective syntheses of 1,3-dienes, as exemplified below. 

The palladium-catalyzed cross coupling of allyl chloride 6 with vinyltributyltin occurs with net retention of configuration if the reaction is conducted under conditions that favor oxidative syn addition4, though the degree of stereoselectivity is somewhat lower than for its catalytic phenylation using a tin reagent (see Table 27). 

An indium-mediated radical cyclization sequence has been used to synthesize stereoselectively 3-alkylideneoxindoles [65, 66]. The generation of predominantly the i -isomer, such as seen with 96 below, is attributed to the strong coordination of the indium to the carbonyl of the oxindole intermediate, and the transformation of various iodo-ynamides to the cyclized oxindoles occurred in good yield. Selective approaches to the E- and disubstituted 3-alkylideneoxindoles involving a tandem palladium-catalyzed cross coupling reaction were also highlighted in this report. 

It is also of interest that the palladium-catalyzed cross-coupling between lithium alkynylzincates and chiral enantiopure l,2-(Z)-vinyHc teUurides that are generated in situ by the regioselective hydrotelluration of the corresponding propargyHc tert-butyldimethylsilyl (TBS) ethers, can be achieved [223]. The reaction is carried out in the presence of 10mol% of Pd(PPh3)4 and Cul, as a catalyst, in a mixture of THF and DMF at rt. Under these conditions, Z-enynes such as 308 are obtained stereoselectively (Scheme 4.70). 

Hoshino, Y Ishiyama, T. Miyaura, N. Suzuki, A. 1988. A stereoselective route to alkenyl sulfides through the palladium-catalyzed cross-coupling reaction of 9-Alkyl-9-BBN with 1-Bromo-l-phenylthioethene or (E)- and (Z)-2-Bromo-l-phenylthioalkenes. Tetrahedron Lett. 29 3983-3986. 

Katayama, H., Nagao, M., Ozawa, F. et al. (2006) Stereoselective synthesis of cis- and trani-OUgo(phenylenevinylene)s via palladium-catalyzed cross-coupling reactions The Journal of Organic Chemistry, 71,2699. 

Heinze, P.L., Burton, D.J. (1988) Palladium-catalyzed cross-coupling of perfluoroalke-nylzinc reagents with aryl iodides. A new, simple synthesis of a,, -trifluorostyrenes and the stereoselective preparation of 1-arylperfluoropropenes. The Journal of Organic Chemistry, 53, 2714—2720. 

Palladium-catalyzed cross-coupling reactions have been largely employed in the synthesis of dienic structures. The very general Suzuki-Miyaura methodology has found a successful application in the stereoselective synthesis of ethyl substituted ( , )-dienoic esters and dienones [54]. This coupling involved a series of vinylboronates 58 and vinyltriflates (or nonaflates) 59, and led, in good yield, to the expected esters and ketones 60. However, a partial isomerization of the electrophilic partner 59 occurred (Scheme 34). 

Enynes can also be synthesized in excellent yields with high regio- and stereoselectivity by transition metal catalyzed cross-coupling reaction. Thus, ( >l-alkenyl-disiamylboranes react with 1-halo-1-alkynes in the presence of catalytic amount of tetrakis(tripheny]phosphine)palladium to afford conjugated trans enynes (Eq. 100)145). 

Functionalized alkenyl diamino- and dialkoxyboranes have been produced regio-and stereoselectively through addition of carbon- or heteroatomgenerated from bromotrichloromethane, thiols, phosphines and tributyltin hydride) to ethynylbis(diisopropylamino)boranes. The synthetic utility of these reactions was illustrated by the preparation of stereodefmed (Z)- or ( )-alkenylboronic esters via palladium-catalyzed crossStille reaction and a Suzuki coupling under basic conditions can be further conducted (Scheme 9.14) [33]. 

---