Boranes allyl

Optically active TV-sulfonylamino alcohols derived from D-camphor or norephedrine were found to be efficient chiral ligands for the enantioselective allylboration of iV-silylimines (Equation (170)) 646-648 B-Allyl(diisopinocampheyl)borane allylated iV-diisobutylaluminum imines with 87% ee (Equation (171)).649,650... 

Hydrogenation of olefins, enols, or enamines with chiral tVilkinson type catalysts, e.g., Noyort hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

L3.3.3.3.1.1.1. From Allylic Organometallic Reagents and Electrophilic Boranes (a) Allylboranes and Allylboronates... 

Allyllithium reagents have also been used in the synthesis of (Z)-y-alkoxyallylboronates 23 2 5. Stereoselectivity is excellent in these reactions since the (Z)-y-alkoxyallyl carbanions prepared by metalation of allyl ethers are stabilized by chelation. The (Z)-y-alkoxyallyl(diisopinocam-pheyl)boranes are prepared at low temperature by an analogous procedure and must be used at — 78 "C otherwise reaction diaslereoselectivity suffers owing to the facile isomerization to the -isomer26. 

The tartrate ester modified allylboronates, the diisopropyl 2-allyl-l,3,2-dioxaborolane-4,5-di-carboxylates, are attractive reagents for organic synthesis owing to their ease of preparation and stability to storage71. In the best cases these reagents are about as enantioselective as the allyl(diisopinocampheyl)boranes (82-88% ee with unhindered aliphatic aldehydes), but with hindered aliphatic, aromatic, a,/l-unsaturated and many a- and /5-alkoxy-substituted aldehydes the enantioselectivity falls to 55-75% ee71a-b... 

Hydrolysis of 1-boronylallylstannanes, available by stannylation of alkenyl and allyl boranes. gives (Z)-allylstannanes stereoselectively34. 

For reviews, see Hoffmann, R.W. Niel, G. Schlapbach, A. Pure Appl. Chem., 1990, 62 1993 Pelter, A. Smith, K. Brown, H.C. Borane Reagents Academic Press NY, 1988, p 310. For a review of allylic boranes, see Bubnov, Yu.N. Pure Appl. Chem., 1987, 21, 89, For an example that proceeds with asymmetric induction, see Buynak, J.D. Geng, B Uang, S. Strickland, J.B. Tetrahedron Lett., 1994, 35, 985. 

Treatment of 83 with an electrophile such as methyl sulfate, allyl bromide, or triethyloxonium borofluoride, followed by oxidation of the resulting vinylic borane... 

The formation of ethers such as 1806 by EtsSiH 84b can also be catalyzed by trityl perchlorate to convert, e.g., benzaldehyde in 84% yield into dibenzyl ether 1817 [48]. The combination of methyl phenethyl ketone 1813 with O-silylated 3-phenyl-n-pro-panol 1818, in the presence of trityl perchlorate, leads to the mixed ether 1819 in 68% yield [48] (Scheme 12.15). Instead of trityl perchlorate, the combination of trityl chloride with MesSiH 84a or EtsSiH 84b and sodium tetrakis[3,5-bis-(trifluoro-methyl)phenyl]borane as catalyst reduces carbonyl groups to ethers or olefins [49]. Employing TMSOTf 20 as catalyst gives very high yields of ethers. Thus benzaldehyde reacts with O-silylated allyl alcohol or O-silylated cyclohexanol to give the... 

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. 

Allylic derivatives are particularly important in the case of boranes, silanes, and stannanes. Allylic boranes effect nucleophilic addition to carbonyl groups via a cyclic TS that involves the Lewis acid character of the borane. 1,3-Allylic transposition occurs through the cyclic TS. 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

The allylation reaction has been extended to enantiomerically pure allylic boranes and borinates. For example, the 3-methyl-2-butenyl derivative of (Ipc)2BH reacts with aldehydes to give carbinols of greater than 90% e.e. in most cases.39... 

P-Allyl-to-(isopinocampheyl)borane exhibits high stereoselectivity in reactions with chiral a-substituted aldehydes.40 The stereoselectivity is reagent controlled, in that there is no change in stereoselectivity between the two enantiomeric boranes in reaction with a chiral aldehyde. Rather, the configuration of the product is determined by the borane. Both enantiomers of (Ipc)2BH are available, so either enantiomer can be prepared from a given aldehyde. 

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