Palladium-catalyzed borylation

The first synthesis of arylboronic esters 209-215 via the coupling of bis(pinacolato)diborane(4) with easily prepared aryldiazonium tetra-fluoroborate salts was reported. The palladium-catalyzed borylation reaction proceeds efficiently under mild reaction conditions in the absence of a base to afford various functionalized arylboronic esters in moderate to high yields (Scheme 34).121... 

Ahiko, T.-a., Ishiyama, T., Miyaura, N. Asequence of palladium-catalyzed borylation of allyl acetates with bis(pinacolato)diboron and intramolecular allylboration for the cyclization of oxo-2-alkenyl acetates. Chem. Lett. 1997, 811-812. 

Murata, M., Oyama, T., Watanabe, S., Masuda, Y. Synthesis of alkenylboronates via palladium-catalyzed borylation of alkenyl trifiates (or iodides) with pinacolborane. Synthesis 2000, 778-780. 

Willis, D. M., Strongin, R. M. Palladium-catalyzed borylation of aryidiazonium tetrafluoroborate salts. A new synthesis of aryiboronic esters. Tetrahedron Lett. 2000, 41, 8683-8686. 

Broutin, P.-E., Cerna, I., Campaniello, M., Leroux, F., Colobert, F. Palladium-Catalyzed Borylation of Phenyl Bromides and Application in One-Pot Suzuki-Miyaura Biphenyl Synthesis. Org. Lett. 2004, 6, 4419-4422. 

Sumimoto, M., Iwane, N., Takahama, T., Sakaki, S. Theoretical Study of Trans-metalation Process in Palladium-Catalyzed Borylation of lodobenzene with Diboron. J. Am. Chem. Soc. 2004, 126, 10457-10471. 

Murata, M. Oyama, T. Watanabe, S. Masuda, Y. Palladium-catalyzed borylation of aryl halides or triflates with dialkoxyborane a novel and facile synthetic route to arylboronates. J. Org. Chem. 2000, 65, 164. 

The concept is applied to hydroxy-directed cyclization of 1,6-enynes (Scheme 5.16) [16]. Palladium-catalyzed borylative cyclization of 10 affords homoallylic alcohol 11 with high stereoselectivity under the direction of the hydroxy group. 

The reaction of certain palladium-heteroatom complexes to alkenes and alkynes is a versatile tool for the synthesis of alkanes and alkene having heteroatoms attached. In particular, the various B-B, B-Si, and B-Sn compounds can be used for palladium-catalyzed borylation of alkenes and alkynes (Scheme 5-2). Borostannylation takes place at ambient temperature, whereas silylboration " only proceeds at a temperature above 80 °C due to the slow oxidative addition of a B-Si bond to a palladium(O) catalyst. Both reactions selectively provide cz j-products via addition of silicone or tin to the internal carbon and boron. The reactions are compatible with various functional groups for both terminal and internal alkynes. Cross-coupling reaction of boranes with organic halides selectively occurs at the terminal C-B bonds to provide regiodefined and stereodefined alkenylboron, alkenylsilicon, and alkenyltin compounds. 

R = 4-CI, 4-CN, 4-NM62, 4-NHAc, 4-CH2CN, 4-COMe, 4-C02Me, 3-COMe X = Br, I, OTf Scheme 3.8 Preparation of aryl boronic esters by palladium-catalyzed borylation. 

Baudoin has prepared a range of sterically crowded orfho-substituted functionalized aryl boronates of type 33 as advanced intermediates for the synthesis of rha-zinilam analogs [14]. This was accomplished by a palladium-catalyzed borylation of ortho-substituted phenyl halides with pinacol borane (30) using a sterically crowded phosphine ligand 34 that significantly improved the yields of borylated products in comparison with dppf. With halides such as 2-bromonitrobenzene and 2-bromoacetophenone, however only httle or no pinacol boronates were obtained (Scheme 3.9). 

Scheme 3.10 Synthesis of heteroaryl boronic esters by palladium-catalyzed borylation.

A related theoretical study on palladium-catalyzed borylation of iodobenzene with diboron was reported by Sakaki et al. [73]. Specifically, the mechanism for the transmetalation step between the Pd complexes Pd(X)(Ph)(PH3)2 (X = OH, F) and diboron B2(eg)2 (eg = -OCH2CH2O-) was investigated. Other theoretical studies, but now within the context of organic acids as electrophiles, were reported by Goo/ en et al. [74, 75]. The palladium catalyzed Suzuki reaction of carboxylic anhydrides with arylboronic acids was analyzed, showing that in those cases monophosphine complexes were also involved in the transmetalation process. 

Sumimoto M, Iwane N, Takahama T, SakaM S (2004) TheOTetical study of tians-metalation process in palladium-catalyzed borylation of iodobenzene with diboron. J Am Chem Soc 126 10457-10471... 

The palladium-catalyzed stannylboration (90) [124] or silylboration (87) [109, 114] succeeds in the intramolecular carbocyclization of diynes and enynes (Scheme 1-27). It is interesting that a very strained four-membered cycUzation of hexa-l,5-diyne proceeds without any difficulties, similarly to five- or six-membered cycUzation. The boryl group is selectively introduced into the more reactive C=CH rather than C=C for enynes and into the terminal C=CH rather than the internal C=CR for diynes, again suggesting a mechanism proceeding through the first insertion into the Pd-B bond in preference to the Pd-Sn or Pd-Si bond. 

Stannaboration of diynes and enyne provide cycloalkanes incorporating both stannyl and boryl groups, which are otherwise difficult to synthesize. Palladium-catalyzed reactions of 1,4-, 1,5-, and 1,6-diynes with 9 give the corresponding cyclization products in good yields (Equation (99)).247 The reaction is so efficient that even a strained four-membered ring is readily formed. 

In the study of substituent effect on the thermal electrocyclic ring-closing reactions of vinylallenes, two stereo-isomeric boryl-substituted vinylallenes 13 and 14 are synthesized by means of the palladium-catalyzed stannaboration of alkynes (Schemes 59 and 60).248... 

A convenient direct route has recently been described for obtaining regioregular polyalkylthiophenes using a tandem iridium-catalyzed borylation to produce the monomer, and a palladium-mediated coupling to produce the polymer [68]. The treatment of substituted thiophenes with B2pin2 in the presence of [lrCl2(COD)]2/ 4,4 -di-tert-butyl-2,2 -bipyridine (DTBPY) provided the expected monomer in 97% yield (Scheme 7.35). 

Palladium-catalyzed reaction of aryl halides with diboron reagent to produce aryl-boronates. Also known as Hosomi-Miyaura borylation. 

The palladium(0)-catalyzed borylation/Suzuki coupling protocol was developed by Baudoin and co-workers in order to synthesize sterically hindered 2,2 -disubstituted biaryl compounds such as 170 in high yield and in a convenient manner [162,163], Indeed, the catalytic borylation of 2-bromoaniline 167 (R=H) gave the corresponding pinacolboronic ester 168 which was reacted in a one-pot fashion with phenyl iodide 169 in the presence of barium hydroxide to give the functionalized biphenyl 170 in 78% yield. Cleavage of the MOM group and cyclization in the presence... 

The scope and value of the benzannulation reaction is further increased by the substitution pattern of the arene ring, which can be modified by the incorporation of allcynes bearing additional functional groups such as silyl, stannyl, or boryl substituents. These functional groups have been used in various palladium-catalyzed (cross)-coupling reactions [63, 64]. Further structural elaboration may be based on benzannulation followed by nucleophilic aromatic addition [63b]. 

The palladium-catalyzed reaction allowed intramolecular five- or six-membered carbocyclization of diynes and enynes (Equation (40)).245,249 The boryl group was selectively introduced into the more reactive C=CH bond rather than C=C bond of enynes, and into the terminal C=CH bond rather than the internal C=CR bond of diynes. 

Diborons and HBpin have been found to act as the boron nucleophiles in the presence of a base for the palladium-catalyzed cross-coupling reaction of organic electrophiles. Both reactions provide a simple and direct method for the borylation of organic halides and triflates. 

The palladium-catalyzed reaction of diboron with allyl acetates or chlorides is a convenient alternative to transmetallation method for the synthesis of functionalized boronates. The reaction of diboron 119 with allyl acetates309-312 smoothly occurred without the assistance of a base, whereas the presence of AcOK was critical for the coupling with allyl chlorides313 (Equation (57)). The boron atom coupled with the less-hindered terminal carbon giving the thermally stable (if)-allyl boronates. Thus, the (E)- and (Z)-cinnamyl acetate, chloride (entries 1-4), and their secondary derivatives (entries 7 and 8), all afforded an (if)-cinnamyl boronate. The borylation of prenyl acetate was slow (entry 6), but the corresponding chloride (entry 5) and tertiary derivative (entry 9) worked well for the same purpose (Equation (58)).309,310... 

Ishiyama, T., Takagi, J., Kamon, A., Miyaura, N. Palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron with vinyl trifiates -substituted by a carbonyl group efficient synthesis of -boryl- -unsaturated carbonyl compounds and their synthetic utility. J. Organomet. Chem. 2003, 687, 284-290. 

---