Allylboration of aldehydes

A theoretical study of allylboration of aldehydes shows that (i) an initial complex may form, but if so, it is weak, and predicted reactivity trends are unchanged whether it is taken into account or not, and (ii) electron delocalization from the aldehyde oxygen to the boron p atomic orbital governs the reaction. ... 

Alternatively, the allylboration of aldehydes 200 with chiral allyl boronates 199a-d (conveniently prepared from camphorquinone in four steps) also provided the optically active homoallylic alcohols 201a-d with high ee in the presence of achiral Lewis acid catalysts. These boronates are relatively unreactive with aldehydes at low temperatures in the absence of Lewis acid catalyst. However, they furnish low to moderate ee for the allylation at higher temperatures. Hall and co-workers were able to increase the reactivity of the allyl boronates at low temperatures by the addition of strong Lewis acids such as Sc(OTf)3 and obtained the homoallylic alcohols with high ee at low temperatures (Equation 9) <2003JA10160>. 

The double allylboration of aldehydes using l,3-bisboryl-2-methylenepropane provided -symmetric 1,5-diols with excellent ee (Equation (166)).641,642 ( )-l,5-diols (Equation (167))643 or (Z)-l,5-diols (Equation (168))643 were diastereoselectively synthesized via double allylboration of aromatic or aliphatic aldehydes with 7-borylallylboranes. 

Brown, H. C., Bhat, K. S., Jadhav, P. K. Chiral synthesis via organoboranes. Part 32. Synthesis of B-(cycloalk-2-enyl)diisopinocampheylboranes of high enantiomeric purity via the asymmetric hydroboration of cycloalka-1,3-dienes. Successful asymmetric allylborations of aldehydes with B-(cycloalk-2-enyl)diisopinocampheylboranes. J. Chem. Soc., Perkin Trans. 1 1991, 2633-2638. 

Allylboration of aldehydes and ketones with B-crotyl-9-BBN affords only the rearranged product (Eq. 6.8 Table 6.7) [1]. Monomeric formaldehyde gives 2-methyl-3-buten-1 -ol. 

Intermolecular allylboration of aldehydes proceeds with high simple diastereoselectivity to give either anti- or syn-B-methyl homoallylic alcohols 5 and 6. (J)... 

All in all, we found that intramolecular allylboration of aldehydes proceeds with predictable and reliable stereoselectivity when forming 6-membered tetrahydropyrane or piperidine rings which are adorned with adjacent vinyl and hydroxy substituents. 

Asymmetric allylboration of aldehydes containing an adjacent ester group with B-allyldiisopinocampheylborane, followed by hydrolysis and cyclization, provides the corresponding co-allyllactones in high yields and > 92% enantiomeric excess. Hydrogenation of these lactones provides the corresponding w-propyllactones without any loss of optical purity (Scheme 5) (7). 

In conclusion, we have reviewed the utility of the products from allylboration of aldehydes with B-allyldiisopinocampheylborane for the synthesis of lactones of varying ring sizes. This is yet another example of the versatility of organoboranes in asymmetric syntheses. 

Lachance, H. Xu, M. Gravel, M. Hall, D. G. Scandium-Catalyzed Allylboration of Aldehydes as a Practical Method for Highly Diastereo- and Enantioselective Construction of Homoallylic Alcohols. /. Am. Chem. Soc. 2003, 125, 10160-10161. 

Ishiyama, T Ahiko, T Miyaura, N. Acceleration Effect of Lewis Acid in Allylboration of Aldehydes Catalytic, Regiospecific, Diastereospecific, and Enantioselective Synthesis of Homoallyl Alcohols. /. Am. Chem. Soc. 2002, 124,12414-12415. 

Grayson, M. N. Rellegrinet, S. C. Goodman, J. M. Mechanistic Insights into the BINOL-Derived Rhosphoric Acid-Catalyzed Asymmetric Allylboration of Aldehydes. J. Am. Chem. Soc. 2012,134,2716-2722. 

The role of boron trifluoride etherate in the allylboration of aldehydes using (R)-pinanediol has been studied computationally, considering the alternatives of attachment of BF3 to the chiral ligand or to the aldehyde. ... 

A new protocol has been introduced to improve selectivity in allylboration of aldehydes. The readily available a-substituted allyl or crotyl pinacol boronic esters often give low A/Z-selectivity. Addition of -butyllithium followed by trapping of alkoxide with TFAA generates an allyl borinic acid, which gives very high -selectivity, and in some cases, this is the opposite of the standard conditions. The borinic ester intermediate was characterized by B-NMR. 

Boland and Hertweck synthesized the chloroallylborane 17 via the deprotonation of allyl chloride 150 with lithium dicyclohexyl amide 151 and subsequent reaction of the resulting (Z)-chloroallylanion 152 with Ipc2BOMe and BF3 Et20 (Scheme 25.18). Allylboration of aldehydes with 17 provide... 

Barrett and Seefeld prepared the aminoallylborane 19 via the reaction of lithiated aUyl diphenylam-ine 180 with IpC2BOMe 26. Allylboration of aldehydes with 19 and oxidative woikup provided homoallylic alcohols 181 in anti-stereochemistry (Scheme 25.26). 

Other prominent allylboranes that provide significant levels of enantio- and diastereoselectivity toward the allylboration of aldehydes include those by Masamune and Soderquist. Masamune s reagent is synthesized in four steps via the hydroboration of trimethylsilylbutadiene 222 followed by methanolysis to obtain the racemic B-methoxyborolane 224. Kinetic resolution with A-Methylpseudoephedrine 225 yields the remarkably air-stable crystalline complex 226 that can be separated from the solution at -20°C in almost quantitative yield in >95% de. The solution contains the enantiomerically pure borolane 227. Either of these borolanes could be reacted with aUyl magnesium bromide to provide the corresponding allylboranes 228 (Scheme 25.35). 

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