Boranes with aryl halides

Imamoto reported that Pd-catalyzed coupling of phosphine-borane with aryl halides is useful for preparation of asymmetric phosphines. Phosphines can be easily isolated from phosphine-boranes by exchange reaction with amines such as pyrrolidine and DABCO [10]. Lipshutz found that aryl nonaflates ( Nf = nonafluorobutanesulfonate) and triflates are good substrates for coupling with BHs-stabilized diaryIphosphines. Selective coupling with nonaflate without... 

Phosphorylation of At2PH-BH3 with aryl halides proceeds under mild conditions using Pd-Cu catalyst and P-chiral phosphine-boranes of high enantiopurity are prepared by this method. As an effective ligand, MePPh2 is used with Pd(OAc)2. The phosphine-borane 50 was prepared in 68 % yield by phosphorylation of optically active (Sp)-methylphenylphosphine-borane (47) via 48 with the iodide 49 in the presence of the Pd(OAc)2-MePPh2 catalyst and Cul as a cocatalyst at 0°C for 3 days. The reaction proceeded with retention of stereochemistry and the asymmetrically substituted phosphine 51 with 99% ee was obtained [15]. 

Two kinds of nucleophiles undergo catalytic cross-couphng with aryl halides hard nucleophiles, which are associated to a metalhc countercation (organometallic derivatives, Eq. 1 of Scheme 12), and soft nncleophiles, which are associated to a metalloid center (organostannane and borane derivatives, Eq. 2 of Scheme 12). 

Several of the new reactions of unsaturated boranes are notable. Vinylboranes can be induced by a Pd catalyst to couple with aryl halides, giving substituted styrenes in high yield,and with vinyl bromides to give E,E-dienes stereo-specifically. " Allylboranes (60) give eryt/iro-alcohols (61) in high stereoselectivity on reaction with RCHO, and S-alkynyl-9-BBN react with RN=C=0 to give (62). ... 

A direct synthesis of substituted aryl boronic esters has been recently reported by Masuda [12,13]. Thus, coupling of pinacol borane 30 with aryl halides or triflates in the presence of a catalytic amount of PdCl2(dppf) and a base like triethylamine allows the preparation of aryl boronates having a variety of functional groups such as carbonyl, cyano, nitro, and acylammo of type 31 in high yields. The product distribution 31 versus 32 (reduced product) is strongly dependent on the choice of the base employed. In the presence of triethylamine as a base, selective formation of boronates 31 was observed with negligible amount of the reduced aromatic compound 32 (Scheme 3.8). 

COUPLING OF ARYL HALIDES WITH SILANES, STANNANES, GERMANES, AND BORANES 388... 

Although more hydrolytically sensitive than the phosphine boranes, diorganochlorophosphines can be more accessible than diorganophosphines and are not pyrophoric. Thus, the reaction of a chlorophosphine with an aryl halide or aryl triflate in the presence of zinc as a reducing agent and (DPPE)NiCl2 as catalyst provides a convenient procedure for P—C coupling (Equation (49)).150 A related nickel-catalyzed process driven by electrochemical reduction has also been reported 151... 

Like simple aryl halides, furyl halides take part in Suzuki couplings as electrophiles [41, 42]. Young and Martin coupled 2-bromofuran with 5-indolylboronic acid to prepare 5-substituted indole 37 [43]. Terashima s group cross-coupled 3-bromofuran with diethyl-(4-isoquinolyl)borane 38 to make 4-substituted isoquinoline 39 [44]. Similarly, 2- and 3-substituted isoquinolines were also synthesized in the same fashion [45]. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

In a modified version of the Suzuki reaction arylboronates or boranes are utilized instead of arylboronic acid. Under the action of phosphine-free palladium catalysts NaBPh4 and tra(l-naphtyl)borane were found suitable phenyl-sources for arylation of haloaromatics in fully or partially aqueous solutions at 20-80 °C with good to excellent yields (Scheme 6.12) [32-34]. Aryl halides can be replaced by water-soluble diaryliodonium salts, At2IX (X = HSO4, BF4, CF3COO) in the presence of a base both Ar groups take part in the coupling [35]. 

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

Palladium-catalyzed cross-coupling of (2-ethoxyvinyl)borane readily prepared by the hydroboration of ethoxyethyne, with aryl and benzylic halides provides a convenient method for conversion of such halides into aldehydes with two more carbon atoms (Eq. 18)57). 

Trialkylboranes including 9-alkyl-9-BBN and alkyl(disiamyl)borane readily undergo the cross-coupling with 1-alkenyl or aryl halides or triflates [8] (Scheme 20). The reaction is limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN or HB(Sia)2 is the most convenient to furnish the desired boron reagents. Trialkylboranes thus obtained are... 

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