Boranes. alkyl-aryl, reactions

Group 13/15 compounds have a long-standing history in inorganic chemistry and have been known for almost two centuries. First reports on such compounds go back to 1809, when Gay Lussac synthesized F3B4—NH3,1 the historical prototype of a Lewis acid-base adduct, by reaction of BF3 and NH3. Since this initial study, numerous Lewis acid-base adducts of boranes, alanes, gallanes and indanes MX3, MF13 and MR3 (M = B, Al, Ga, In X = F, Cl, Br, I R = alkyl, aryl) of the type R3M <- ER 3 (E = N, P, As) (Type A) have been synthesized and... 

Early findings by Suzuki and co-workers [109] showed that the palladium-catalyzed iminocarbonylative cross-coupling reaction between 9-alkyl-9-BBN derivatives, t-butylisocyanide, and arylhalides gives access to alkyl aryl ketones 132 after hydrolysis of the corresponding ketimine intermediates 131. Presumably, the concentration of free isocyanide is kept to a minimum by its coordination with the borane. Formation of an iminoacylpalladium(II) halide 130 by insertion of isocyanide to the newly formed arylpalladium complex followed by a transmetallation step afford the ketimine intermediates 131 (Scheme 8.52). 

An alternative indirect but efficient method for the bromination of all three groups of tri(primaiy alkyl)boranes involves initial reaction with mercury(II) acetate followed by in situ bromination. Alkenyldialkylboranes react with bromine to give bromoalkenes via an addition-elimination mechanism. The meftod of elimination controls the stereochemistry of the product bromoalkenes (Scheme 1). For reasons which are not clear, exactly opposite stereochemical results are obtained from (aryl-ethenyl)dialkylboranes as compared with (alkylethenyl)dialkylbotanes (Scheme 1). ... 

Oxazaphospholidine boranes 82 react regio- and stereoselectively with alkyl lithiums or aryl lithiums in THE at —78°C, with formation of acyclic phosphinite boranes 83. Various substituents R =n-alkyl, c-alkyl, aryl, or ferrocenyl were introduced into aminophosphine boranes 82 in high yield (93-97%) and with high diastereoselectivity dr >98 2). The reaction proceeded with retention of configuration at phosphorus. RecrystaUization of aminophosphine boranes 83 in propanol gave the diastereoisomerically pure products [53]. Acid methanolysis of aminophosphine boranes 83 led to the formation of phosphinite boranes 84 with inversion of configuration on the E-center to yield the compotmds 84 in high... 

Examples of electrophilic addition of secondary phosphines to alkenes or alkynes were described. [114, 124, 125, 135]. Glueck [124-126] reported enantioselective tandem reaction of alkylated/arylation of primary phosphines catalyzed by platinum complex, proceeding with formation of chiral phosphaace-naphthenes. Palladium-catalyzed hydrophosphination of alkynes 219 tmder kinetic resolution conditions gave access to 1,1-disubstituted vinylphosphine boranes 220. However, despite screening several chiral ligands, temperatures, and solvents, the... 

As the overall cross-coupling reaction proceeds with inversion of stereochemistry and reductive ehmination is well known to undergo retention of stereochemistry, the result imphes that transmetaUation in this reaction proceeds predominantly with retention of stereochemistry. In addition to this study, in 1998, Woerpel and Soderquist [102] independently studied the stereochemistry of transmetaUation for the Suzuki-Miyaura cross-coupling reactions of alkyl boranes with aryl or alkenyl hahdes. Their deuterium labehng study revealed that the transmetaUation of alkyl boranes 163 or 166 proceeds with retention of stereochemistry to give products 165 or 167. Soderquist proposed a closed four-membered cyclic transition state 168 to account for the retention of stereochemistry observed during the reaction. 

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

Secondary amines are formed by reaction of trisubstituted boranes with alkyl or aryl azides. The most efficient borane intermediates are monoalkyldichloroboranes, which are generated by reaction of an alkene with BHCl2 Et20.190 The entire sequence of steps and the mechanism of the final stages are summarized by the equation below. 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

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. 

Polystyrene-bound trialkylboranes, which can be prepared by hydroboration of support-bound alkenes with 9-BBN, undergo palladium-mediated coupling with alkyl, vinyl, and aryl iodides (Suzuki coupling Entries 1 and 2, Table 5.3 for vinylations, see Section 5.2.4). Because boranes are compatible with many functional groups and do not react with water, these coupling reactions could become a powerful tool for solid-phase synthesis. To date, however, few examples have been reported. 

---