Rhodium with catecholborane

Catalytic Asymmetric Hydroboration. The hydroboration of olefins with catecholborane (an achiral hydroborating agent) is cataly2ed by cationic rhodium complexes with enantiomericaHy pure phosphines, eg, [Rh(cod)2]BE4BINAP, where cod is 1,5-cyclooctadiene and BINAP is... 

While Josiphos 41 also possessed an element of atom-centered chirality in the side chain, Reetz reported a new class of ferrocene-derived diphosphines which had planar chirality only ligands 42 and 43, which have C2- and C -symmetry, respectively.87 Rhodium(i)-complexes of ligands (—)-42 and (—)-43 were used in situ as catalysts (0.75 mol%) for the hydroboration of styrene with catecholborane 1 for 12 h in toluene at — 50 °C. The rhodium/ i-symmetric (—)-43 catalyst system was the more enantioselective of the two - ( -l-phenylethanol was afforded with 52% and 77% ee with diphosphines (—)-42 and (—)-43, respectively. In both cases, the regioselectivity was excellent (>99 1). With the same reaction time but using DME as solvent at lower temperature (—60 °C), the rhodium complex of 43 afforded the alcohol product with an optimum 84% ee. 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

Rhodium-catalysed asymmetric cyclization/hydroboration followed either by Pd-catalysed arylation or by oxidation was applied to the synthesis of a number of chiral, non-racemic carbocycles and heterocycles. Thus, reaction of enyne (28) with catecholborane, catalysed by a 1 1 mixture of [Rh(COD)2]+ Sbly,- and (S)-BINAP (5 mol%), followed by Pd-catalysed arylation with /7-IC6H4CF3, afforded benzyli-denecyclopentane (29) in 65% yield with 88% ee.46... 

Catalytic hydroboration of perfluoroalkenes 68 with catecholborane provides either terminal 69 or internal alcohols 70 regioselectively <19990L1399>. The regioselectivity is controlled by a judicious choice of catalyst. The anti-Markovnikov alcohol can be obtained with very high selectivity by using cationic rhodium catalysts such as Rh(COD)(DPPB)+BF4, while neutral Rh catalysts such as Wilkinson s catalyst provide the Markovnikov product (COD = cyclooctadiene Equation 3) <19990L1399>. 

Catalytic hydroboration is a new methodology of great synthetic potential. The reaction is usually carried out with catecholborane in the presence of rhodium, palladium, iridium and ruthenium compounds.2 In contrast to olefins, very little is known on catalytic hydroboration of conjugated dienes and enynes. Our earlier studies on the uncatalyzed monohydroboration of conjugated dienes,6 reports on the hydroboration of 1-decene with catecholborane catalyzed by lanthanide iodides,7 and monohydroboration of 1,3-enynes in the presence of palladium compounds,8 prompted us to search for other transition metal catalysts for monohydroboration of conjugated dienes and enynes 9 10... 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

Complexes of cationic rhodium compounds with asymmetric phosphane ligands catalyze the hydroboration of prostereogenic alkenes with catecholborane (see Section D.2.5.2.1.4.). The product alcohols are of 7-96% cc (Table 3). Enantioselectivity is excellent for the hydroboration/oxidation of styrenes but low for stilbenes. The small number of examples studied to date precludes generalizations, however, compared to the uncatalyzed reaction, opposite regioselec-tivity is observed for the addition to styrenes. 

Asymmetric Hydroboration. Rhodium complexes are known to catalyze hydroboration of alkenes with unreactive borane derivatives, e.g. catecholborane and oxaborolidine. This reaction proceeds enantioselectively by use of BINAP as a ligand for neutral " or cationic rhodium complexes. Reaction of styrene with catecholborane followed by oxidation affords (R)-1-phenylethanol in 96% ee in the presence of (R)-BINAP and [Rh(cod)2]Bp4 (eq 5). ... 

Hydroboration is especially valuable for the synthesis of stereodefined 1- alkenyl-boronic acids. A general method is the hydroboration of terminal alkynes with cate-cholborane 7 [16, 17] (eq (14)). The reaction is generally carried out at 70 without solvent, but it is very slow in THF solvent. More recent results demonstrate that the hydroboration of alkenes or alkynes with catecholborane is strongly accelerated in the presence of palladium [18]. rhodium [19]. or nickel catalysts [20], thus allowing the reaction to proceed below room temperature. 

Another application of the Wilkinson-type catalyst 18 is the rhodium catalyzed hydroboration of olefins [19]. Various alkenes 20 (internal, terminal, styrenes, etc.) have been successfully hydroborated with catecholborane (21) providing the corresponding boronic esters 22 in nearly quantitative yield. Oxidative work-up (Na0H/H202) led to the corresponding alcohols 23 in 76-90% yield Eq. (11). 

Asymmetric hydroboration of norbornene (27) is a synthetically useful transformation forming optically active norbornanol (28) which is an important chiral synthon. The catalytic enantioselective hydroboration with catecholborane was examined using rhodium complexes coordinated with several chiral phosphine ligands (Scheme 7 and Table 4) [14,15,17,23,24,27]. For this reaction, DIOP (10) and its derivatives 21 and 22, which are modified on the diphenyl-phosphino group, are more enantioselective Hgands than BINAP (7) or chira-phos (9). The highest enantioselectivity was observed in the reaction at -25 °C... 

Asymmetric hydroboration of prochiral olefins can be performed either with a chiral borane in the absence of a catalyst or with catecholborane and a chiral rhodium catalyst [549]. The rich chemistry of the boranes generated by these methods opens the way to a variety of enantioenriched molecules bearing different functionalities [583],... 

Hydroboration with catecholborane catalyzed by a rhodium complex of l-(2-diphenylphosphino-l-naphthyl)isoquinoline (5) is accompanied by high enantio-selectivity. Bis(l-neomenthylindenyl)zirconium dichloride (6) promotes alkylalumination of 1-alkenes through a noncyclic mechanism. 

Hydroborations. Addition of Catecholborane to alkenes is accelerated by Wilkinson s catalyst, and other sources of rhodium-(I) complexes. Unfortunately, the reaction of Wilkinson s catalyst with catecholborane is complex hence if the conditions for these reactions are not carefully controlled, competing processes result. In the hydroboration of styrene, for instance, the secondary alcohol is formed almost exclusively (after oxidation of the intermediate boronate ester, eq 37) however, the primary alcohol also is formed if the catalyst is partially oxidized and this can be the major product in extreme cases. Conversely, hydroboration of the allylic ether (12) catalyzed by pure Wilkinson s catalyst gives the expected alcohol (13), hydrogenation product (14), and aldehyde (15), but alcohol (13) is the exclusive (>95%) product if the RhCl(PPh3)3 is briefly exposed to air before use. The 5yn-alcohol is generally the favored diastereomer in these and related reactions (eq 38), and the catalyzed reaction is therefore stereocomplementary to uncatalyzed hydroborations of allylic ether derivatives. ... 

Other sources of rhodium(I) are equally viable catalysts for hydroborations, notably Rh( ) -CH2CMeCH2)(i-Pr2PCH2CH2P-i-Pr2) which gives a much cleaner reaction with catecholborane than Wilkinson s catalyst. Other catalysts for hydroborations are also emerging. " ... 

The examples of hydrometalations are hydroboration [64,72-77], hydrosilylation [78], etc. [78,79] of unsaturated compounds such as olefins, acetylenes, carbonyls and imines. For example, styrene reacts with catecholboran in the presence of a rhodium catalyst to afford stereoselectively the R isomer of 2-phenylethanol as shown in eq. (18.34) [72]. 

Catalytic hydroboration of alkenylboronic esters with catecholborane in the presence of rhodium complexes and various diphosphine ligands has afforded 1,2-di-boronyl intermediates. Their subsequent oxidation gave the corresponding 1,2-diol with enantioselectivities of up to 79% ee (Scheme 9.55) [115]. 

In this chapter, origin of enantioselectivity in the hydroboration/ oxidation of vinylarenes with catecholborane by cationic rhodium complexes is discussed based on computational work of Fernandez and Bo as... 

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