Enantioselective diboration

In comparison with the platinum catalysts, rhodium catalysts are much more reactive to effect addition of bis(catecholato)diboron even to non-strained internal alkenes under mild reaction conditions (Equation (5)).53-55 This higher reactivity prompted trials on the asymmetric diboration of alkenes. Diastereoselective addition of optically active diboron derived from (li ,2i )-diphenylethanediol for />-methoxystyrene gives 60% de (Equation (6)).50 Furthermore, enantioselective diboration of alkenes with bis(catecolato)diboron has been achieved by using Rh(nbd)(acac)/(A)-QUINAP catalyst (Equation (7)).55,56 The reaction of internal (A)-alkenes with / //-butylethylene derivatives gives high enantioselectivities (up to 98% ee), whereas lower ee s are obtained in the reaction of internal (Z)-alkenes, styrene, and a-methylstyrene. 

Use of bis(catecholato)diboron (2.178) in the rhodium-catalysed hydroboration results in enantioselective diboration and a number of simple trans-alkenes such as trans- 3-methylstyrene (2.179) are converted into the 1,2-diol in high ee by diboration/oxidation using (nbd)Rh(acac) in combination with (S)-QUINAP (2.169) as catalyst. 

Silver carbene complexes act as very efficient NHC transfer reagents for the synthesis of different metal-NHC complexes. On the other hand, as far as we know, there is only one example described to date where a silver-NHC complex was used for asymmetric catalysis. In 2006, Ferndndez and coworkers reported the first and only asymmetric catalysis using a chiral NHC-silver catalyst. Enantioselective diboration of styrenes was realized by using silver complex 111 as the catalyst (Scheme 3.70) [99], However, the diol was obtained in low yield and low enantioselectivity (less than 10% ee). 

SCHEME 430 Palladium-catalyzed enantioselective diboration/allylation. 

Synthetic strategy Catalytic enantioselective diboration of pro-chiral allenes Catalyst Tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] / (R,R)-phosphoramidite (ligand I)... 

Keywords Pro-chiral allenes, h/s(pmacolato)diboron, Pd2(dba)3, (/ ,/ )-phosphoratnidite (bgand I), toluene, room temperature, enantioselective diboration... 

The last cascade reaction we report here for the synthesis of a quaternary spirocenter employs a platinum catalyst [21 ]. A regio and enantioselective diboration of 1,3-diene 44 could lead to a chiral bis(boryl) intermediates 45 (Scheme 9.12), which could then react in two subsequent asymmetric carbonyl allylations of succinic dialdehyde, leading to the creation of three stereogenic centers in good diastereomeric (9 1) and promising enantiomeric ratios (88 12) 46. 

Coombs JR, Haeffiier F, Kliman LT, Morken JP. Scope and mechanism of the Pt-catalyzed enantioselective diboration of monosubstituted alkenes. J z4m Chem Soc. 2013 135 11222-11231. 

Morgan JB, Miller SP, MorkenJP. Rhodium-catalyzed enantioselective diboration of simple alkenes. l Am Chem Soc. 2003 125 8702-8703. 

Trudeau S, Morgan JB, Shrestha M, Morken JP. Rh-catalyzed enantioselective diboration of simple alkenes reaction development and substrate scope. J Org Chem. 2005 70 9538-9544. 

Scheme 14.69 Formal dihydro lation via enantioselective diboration.

Diboration of terminal alkenes has also been studied with other d " metals (Fig. 2.12) including the Ag and Au complexes 75-77 and the Pt" complexes 78-79. Styrene is diborylated with 100% selectivity and good conversions in THF (46% for 75 and 94% for 77 at 5 mol%, 60 h) using equimolecular amounts of (Bcat)j. The difference in activity between the Ag and Au complexes has been ascribed to the increased lability of the Ag-NHC bond, which may lead to catalyst decomposition under the reaction conditions, hi both catalytic systems it is believed that the active species involves only one coordinated NHC ligand. Complex 77 is less active than 74 and 75, possibly due to steric reasons. The enantioselectivity of 77 in the diboration of prochiral alkenes is very low [63]. 

The rhodium-catalysed reaction between simple alkenes (50) and bis(catecholato) diboron (51) has been reported to result in the syn addition of the diboron across the alkene. The l,2-bis(boronate) thus obtained was subsequently oxidized to provide the corresponding 1,2-diol (52). In the presence of QUINAP ligand, high enantioselection in the diboration was attained. The reaction was found to be highly selective for trans- and trisubstituted alkenes and can also be selective for some monosubstituted alkenes.59... 

A combination of organocatalysts, the prolinol silylether 17b and an A-heterocyclic carbene (NHC), was used by Cordova and co-workers [64] for the related asymmetric P-boration of cinnamaldehyde. While the secondary amine served to activate the enal through the formation of a chiral iminium ion intermediate, the NHC functioned as a Lewis base forming the nucleophilic adduct with diborate. The product was treated in situ with a stabilized phosphorus ylide affording a synthetically useful chiral allylboronate (Scheme 14.22). A 50 mol% loading of amine was needed to reach reasonable enantioselectivity (60% ee). Lowering of the amine loading to 10 mol% caused a drop in the enantioselectivity (34% ee). 

This interest in catalytic hydroboration led to the development of the transition-metal-catalyzed diboration of alkenes and alk5mes. The diboration of alkenes and alkynes generates bifunctional products, and additions to alkenes have now been conducted with high enantioselectivity. The following sections describe the t es of catalysts used for catalytic hydroboration and diboration of alkenes, alkynes, and dienes, as well as catalytic cycles that accoimt for selectivities and side products formed during these processes. 

Toribatake K, Nishiyama H. Asymmetric diboration of terminal alkenes with a rhodium catalyst and subsequent oxidation enantioselective synthesis of opticaUy active 1,2-diols. Angetv Chem Int Ed Engl. 2013 52 11011-11015. 

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