Boron allylation

To explain the stereochemical outcome of the reaction of allylic boron reagents with carbonyl compounds, Houk and Li carried out calculations on the transition structures of the model reaction of formaldehyde and allylboronic acid6 (Scheme 3.V). The bimolecular complex formed initially between allylboronic acid and formaldehyde would rearrange via a six-membered transition state to form an intermediate. Calculations show that chair transition state A is 8.2kcal/ mol more stable than twist-boat transition structure B, clearly confirming that the six-membered chairlike transition-state model is a legitimate scheme to predict the stereochemical outcome of the boron allylation reaction. 

The absolute configuration observed for the homoallylic alcohol 84 derived from the (R,R)-allylborane reagent 83 can be rationalized on the basis of a chairlike six-membered transition state33 (Scheme 3.1dd). The major enantiomer is formed via transition state A, in which the boron allyl group attacks the re-face... 

For a comprehensive review of synthetic applications of asymmetric boron allylation reactions, see Chemler, S. R. Roush, W. R. in Modem Carbonyl Chemistry Otera, J., Ed. Wiley-VCH Weinheim, Germany, 2000 Chapt. 11. 

Therefore CH2-R groups are not bulky enough to provide selective transaddition of the boron-allylic fragment to heterocycle. 

The first enantioselective allylation of imines [19-21, 39, 44] were reported by Itsuno only in 1995 [164]. Investigations of a range of chiral borane and boronate allylating reagents led to the identification of N-trimethylsilyl... 

Allylation with allyl borates takes place smoothly under neutral conditions. Allylic alcohols are also used for allylation in the presence of boron oxide by in situ formation of allylic borates[125]. Similarly, arsenic oxide is used for allylation with allylic aleohols[126]. In addition, it was claimed that the allyl alkyl ethers 201. which are inert by themselves, can be used for the allylation in the presence of boron oxide[127]. 

Perfluoroallyl fluorosulfate is prepared by the treatment oiperfluoropropene with sulfur tnoxide m the presence of boron catalysts [2, 3, 4, 5, 6, 7] (equation 2) Perfluoroisopropyl allyl ether reacts similarly to give 58% polyfluoroallyl fluorosulfate in a cis/trans ratio of 6 4 [S] Sultones are the exclusive products without catalyst. Polyfluoroolefins such as 2-hydropentafluoropropylene [9], (2,3-dichloropropyl)tri-fluoroethylene [70], perfluoropropene [2, i], perfluoroisopropyl alkenyl ethers [S], and acyclic polyfluoroallyl ethers [77] undergo sulfur trioxidation to regioselectively produce the corresponding P-sultones in high yield... 

Synthesis of some alkaloids using allyl boron derivatives 99KGS1015. 

Wlien tlie diiral molybdenum -K-allyl-substituted enone 147 was treated witli litliium dimetliylciiptate, formation of adduct 148 witli fait selectivity was observed tSdieme 6.29) [69], Interestingly, bigber selectivities were obtained in tlie presetice of boron ttlbuotlde etlierate. It is assumed tliat Lewis acid coordination induces tlie s-trans reactive conformation 149 [64], Consequently, nudeopb de attack anti to tlie molybdetiLim ftagmetit sbould afford tlie major diastereomer 148. 

Allylboronates prepared from simple diols display appreciable reactivity, but eyelie boronate derivatives prepared from 1,2- or 1,3-diols display considerably less. The commonly employed pinacol esters are among the least reactive members of this class. 2-Allyl-3-methyl-l,3,2-oxaza-... 

Schlosser generated dimethyl [( )-l-(tetrahydro-2//-pyran-2-yloxy)-2-butenyl]boronate (1) via metalation of l-(tetrahydro-2//-pyran-2-yloxy)-2-butene at — 75 °C followed by treatment of the allylic anion with fluorodimethoxyborane13. 

The stereochemical features of the reactions of racemic 1-substituted (Z)-2-butenyl-boronates 2 are considerably different from those of the 1-substituted 2-propenyl- and 1-substituted ( )-2-butenylboronates discussed above. Transition state 5 (see p 1470) is destabilized by allylic interactions between X and the (Z)-methyl substituent26, and consequently diastcrcomcr 10 is the major product via transition state 6 (sec the following table)4,15. 

An interesting and stereoselective synthesis of 1,3-diols has been developed which is based on Lewis acid promoted reactions of /f-(2-propenylsilyloxy (aldehydes. Using titanium(IV) chloride intramolecular allyl transfer takes place to give predominantly Ag/r-l,3-diols, whereas anti-1,3-diols, formed via an / / /-molecular process, are obtained using tin(IV) chloride or boron trifluoride diethyl ether complex71. 

Lewis acids, particularly the boron trifluroride diethyl ether complex, are used to promote the reaction between allyl(trialkyl)- and allyl(triaryl)stannanes and aldehydes and ketones52-54. The mechanism of these Lewis acid promoted reactions may involve coordination of the Lewis acid to the carbonyl compound so increasing its reactivity towards nucleophilic attack, or in situ transmetalation of the allyl(trialkyl)stannane by the Lewis acid to generate a more reactive allylmetal reagent. Which pathway operates in any particular case depends on the order of mixing of the reagents, the Lewis acid, temperature, solvent etc.55- 58. 

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