Allyl borane, reaction with aldehydes

Crotyl silanes offer the possibility of diastereoselectivity in reactions with aldehydes in the same way as the corresponding boranes. The mechanism is completely different because crotyl trialkylsilanes react via an open transition state as the silicon is not Lewis acidic enough to bind the carbonyl oxygen of the electrophile. Instead, the aldehyde has to be activated by an additional Lewis acid or by conversion into a reactive oxonium ion by one of the methods described above. The stereoelectronic demands of the allylic silane system contribute to the success of this transformation. Addition takes place in an Se2 sense so that the electrophile is attached to the remote carbon on the opposite side of the n system to that originally occupied by silicon and the newly formed double bond is trans to minimize allylic strain. 

Achiral allylic boranes and chiral aldehydes. The reaction of pinacol-derived al-lylboronate 175 with a number of chiral aldehydes proceeds in only modest selectivity (Scheme 10-73) [120], Almost all of the aldehydes examined demonstrate a weak preference for the syn or Cram product. Almost no change in selectivity was observed when the protecting group on the alcohol is changed from a /-butyl-siloxy to a methoxymethyl group. 

Chiral allylic boranes and chiral aldehydes. The possibilities for double diastereo-selection with allylic borane reagents was first demonstrated by Hoffmann in the combination of chiral 2-butenylboronates with chiral aldehydes [123]. Reaction of... 

The allylation reaction has also been extended to enantiomerically pure allylic boranes. For example, the 3-methyl-2-butenyl derivative of bis-(isopinocam-pheyl)borane reacts with aldehydes to give carbinols of >90% enantiomeric excess... 

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 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. 

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. 

Scheme 3-47. Reaction of aldehydes with chiral allyl boranes 135. Reprinted with permission by Am. Chem. Soc., Ref. 41.

To improve the levels of selectivity in additions to chiral aldehydes, it is possible to resort to the tactic of double diastereoselection with the use of chiral allylic boranes and boronates (see section Double Diastereoselection ). Bis(isopinocampheyl) allylic boranes and the tartrate allylic boronates (see following section), in particular, are very useful in the synthesis of polypropionate natmal products by reaction with a-methyl and a-alkoxy functionalized aldehydes. 

Allyl-bis(isopinocampheyl)borane exhibits high stereoselectivity in reactions with chiral a-substituted aldehydes.39... 

Although the allylation reaction is formally analogous to the addition of allylboranes to carbonyl derivatives, it does not appear to occur through a cyclic transition state. This is because, in contrast to the boron in allyl boranes, the silicon in allylic silanes has no Lewis acid character and would not be expected to coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic transition state. Both the E- and Z-stereoisomers of 2-butenyl(trimethyl)silane give the product in which the newly formed hydroxyl group is syn to the methyl substituent.64 The preferred orientation of approach by the silane minimizes interaction between the aldehyde substituent R and the methyl group. 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

There is a vast array of carbon-carbon-bond-forming reactions involving organoboron compounds. This section covers transfer reactions of carbon moieties from boron to an adjacent carbon. Examples include the transfer of CO, CN, carbanions derived from dichloromethyl methyl ether and dichloromethane, and allyl groups in reactions of allylic boranes with aldehydes. 

The use of enantiopure allylic boranes in reactions with achiral aldehydes results not only in high diastereoselection, but also in high enantioselectionP Pure (Z)- and )-crotyldiisopinocampheylboranes can be prepared at low temperature from (Z)- or E) crotylpotassium and S-methoxydiisopinocampheylborane, respectively, after treatment of the resultant ate-complexes with BF3 OEt2. The R-methoxydiisopinocampheylboranes are prepared by reacting (-)-diisopinocampheylborane, derived from (+)-a-pinene, or (+)-diisopinocampheylborane, derived from (-)-a-pinene, with methanol. 

---