1.3.2- Dioxaborolane, 2-

This chiral modifier provides one of the only methods for selective cyclopropa-nation of substrates which are not simple, allylic alcohols. In contrast to the catalytic methods which will be discussed in the following section, the dioxaborolane has been shown to be effective in the cyclopropanation of a number of allylic ethers [67]. This method has also been extended to systems where the double... 

Recent optimization studies reveal that the yield of 2-(2-propenyl)-1,3,2-dioxaborolane-4,5-di-carboxylate esters (i.e., the tartrate ester modified allylboronates) is improved by using triiso-propyl borate as the borylating agent1. The improved yields are directly related to the increased efficiency of the preparation of the intermediate allylboronic acid. 

Z)-2-(2-hulenyl)-4,4,5,5-tetramethyl-i,3,2-dioxaborolane-, yield 78% isomeric purity via fluorodimethoxyborane route4 97% isomeric purity via the triisopropyl borate route16 >99%. 

Tetramethyl-2-(3-trimethylsilyl-2-propenyl)-l,3,2-dioxaborolane Typical Procedure2 11 ... 

Z)-2-(3-Methoxy-2-propenyl)-4,4,5,5-tctramcthyl-1,3,2-dioxaborolane Typical Procedure232 ... 

Z)-4,4,5,5-tetramethyl-2-[3- 2-trimethylsilylethoxy)-2-propenyl -1,3,2-dioxaboroUme yield 45% (Z)-2-[3-(tetralnclr -2H-pyran-2-y/oxy)-2-propenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane yield 52%. 

A second powerful route to functionalized allylboron compounds involves the reaction of an a-haloalkylboronatc and a vinyl organometallic reagent3 4-28-29, 50c-92 04. This method is especially useful for the preparation of allylboron compounds not accessible via the allylorganometal-lic route. Notable examples that fall into this category are ( )-4,4,5,5-tctramethyl-2-[4-(tetrahy-dro-2//-pyran-2-yloxy)-2-butenyl]-l,3,2-dioxaborolane (yield 41 %, 93% E) and (E)- or (Z)-2-(l,l-dimethyl-2-butenyl)-4,4,5,5-tetramethyl-1.3,2-dioxaborolane (yield 77-84%. 98% E or 93% Z). 

A)-2- 5-(/erf-Butyldimethylsilvloxy)-2-pentenvl -4,4,5,5-tetramethyl-l,3,2-dioxaborolane Typical Procedure30 ... 

Tctramethyl-2- (Z)-3-phcnyl-2-propenyl dioxaborolane (3) Typical Procedure96 ... 

A mixture of 1.97 g (10 mmol) of (Z)-2-(3-methoxy-2-propenyl)-4,4,5,5-tetramethyl-l, 3,2-dioxaborolane and 755 mg (13 mntol) of propanal is stirred without solvent for 2d at r.t., and then for 3 h at 50LC. Residual aldehyde is removed in vacuo then the residue is dissolved in 3 inL of CH,C1, and treated with 1.5 g (10 mmol) of triethanolamine for 12 h. The mixture is filtered, concentrated and bulb-to-bulb distilled from a bath at 50 CC/0.01 Torr, and the distillate is chromatographed over silica gel eluting with CI1,C12 yield 1.22 g (94%) d.r. synjanti) 92 8. 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

Several detailed studies of reactions of achiral aiiylboronates and chiral aldehydes have been reported4,52 - 57. Diastereofacial selectivity in the reactions of 2-(2-propenyl)- or 2-(2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes with x-methyl branched chiral aldehydes are summarized in Table 252, 53, while results of reactions with a-heteroatom-substituted aldehydes are summarized in Table 34,52d 54- 57. 

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... 

Excellent double diastereoselection has also been realized in the reactions of (7 )-2,3-[isopro-pylidenebis(oxy)]propanal and chiral 2-butenylboron reagents (Table 8). The best selectivity for the (3R,4R)- and (SS /Q-diastereomers was obtained by using the tartrate ( )- and (Z)-2-butenylboronates. (S.S -D and (R,R)-D, respectively69,81, while (E)- and (Z)-2-butenyl-2,5-dimethylborolane reagents (R,R)-C and (S,S) C provided the greatest selectivity for the (3S, 45)- and (3y ,4S )-diastereomers< 9. Comparative diastereoselectivity data for reactions with the achiral (E)- and (Z)-2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes have also been provided in the table. 

The diisopropyl 2-dimethylphenylsilyl-2-propenyl- and 2-[cyclohexyloxy(dimethyl)silyl]-l,3,2-dioxaborolane-4,5-dicarboxylate reagents 7 and 8 were recently introduced for the enantiose-lective a- and y-hydroxyallylation of aldehydes86. 

A — 55 C solution of [(trimethylsilyl)chloromethyl]lithium (theoretically 51 mmol prepared from 51 mmol of (chloromethyl)trimethylsilanc, 56 mmol of. vet-butyllithium and 56 mmol of TMF.DA in 68 mL of TIIF) is treated with 7.47 g (49 mmol) of 4,4.5,5-tetramethyl-2-(2-propeny))-l,3,2-dioxaborolane. The mixture is cooled to — 78 C and then allowed to warm to r.t. overnight. 50 mL of ice-cold 2 M hydrochloric acid are added and the mixture is extracted with three 60-mL portions of diethyl ether/CFI2CI2 (5 1). The extracts are concentrated and the residue distilled yield 6.36 g (54%) bp 52-55 rC/0.7 Torr. 

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