Allylboration

Finally, several examples of intramolecular allylboration have been reported 51,92 -95. These reactions are highly stereoselective and afford only a single diastereomer in each case. 

The data reported in Table 3 for the 2-butenylborations of 2-(dibenzylamino)propanal shed additional light on this transition state model. The ( )-2-butenylboration of 2-(dibenzyl-amino)propanal evidently proceeds preferentially (90%) by way of transition state 9, suggesting that the bulky dibenzylamino substituent destabilizes transition state 8 (X = NBn2 > CH3). On the other hand, the (Z)-2-butenylboration of 2-(dibenzylamino)propanal is relatively non-selective, compared to the excellent selectivity realized in the (Z)-allylborations of a-chloro- or x-alkoxy-substituted chiral aldehydes. This result suggests that an increase in the steric requirement of X destabilizes transition state 11 such that significantly greater amounts of product are obtained from transition state 10. 

This point is also strikingly demonstrated in the enantiotopic group and diastereofacial selective allylboration of the me so complex 5 that provides the (45,65)-diastereomer with 45 1 diastereoselectivity and >98%ee85b. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

In a second set of examples, it was shown that the stereoselectivity of the aldehyde allylborations of 9-[( T)-l-trimethylsilyl- or l-trimethylstannyl-2-butenyl]-9-borabicyclo[3.3.1]nonane is controlled to a significant extent by conversion to an ate complex by treatment with butyllithium, MT-butyllithium or pyridine19. 

Chiral, nonracemic allylboron reagents 1-7 with stereocenters at Cl of the allyl or 2-butenyl unit have been described. Although these optically active a-substituted allylboron reagents are generally less convenient to synthesize than those with conventional auxiliaries (Section 1.3.3.3.3.1.4.), this disadvantage is compensated for by the fact that their reactions with aldehydes often occur with almost 100% asymmetric induction. Thus, the enantiomeric purity as well as the ease of preparation of these chiral a-substituted allylboron reagents are important variables that determine their utility in enantioselective allylboration reactions with achiral aldehydes, and in double asymmetric reactions with chiral aldehydes (Section 1.3.3.3.3.2.4.). 

Z)-l-Methyl-2-butenylboronate 7 undergoes an exceptionally enantioselective reaction with benzaldehyde (99% ee), propanal (79%. 98% ee), 2-methyl-2-propenal (85%, 99% ee), and ( )-2-methyl-2-pentenal (81 %, 99% ee)10 38. Excellent enantioselectivity is also realized in reactions of the analogous chiral a-methyl-) y-disubstituted allylboronate27 40. Whether the l,2-dicyclohexyl-l,2-ethanediol auxiliary plays a beneficial role in this reaction, as suggested above for the asymmetric allylboration reactions of 6, has not yet been determined. 

Fig. 9.1. Most favorable si and re transition structures for allylboration of acetaldehyde. The si TS is favored by 1.75kcal/mol, which is attributed to an electrostatic attraction between a formyl carbonyl oxygen lone pair and the acetaldehyde carbonyl carbon. In the re TS, there is a repulsive interaction between lone pairs on the formyl and acetaldehyde carbonyl oxygens. Reproduced from J. Am. Chem. Soc., 124, 10692 (2002), by permission of the American Chemical Society. 

Visual models, additional information and exercises on Allylboration can be found in the Digital Resource available at Springer.com/carey-sundberg. 

Recently the scope of the allylboration has been expanded by the discovery that it is catalyzed by certain Lewis acids, especially Sc(OTf)3.52 The catalyzed reaction exhibits the same high diastereoselectivity as the uncatalyzed reaction, which indicates that it proceeds through a cyclic TS. 

Allyl tetrafluoroborates are also useful allylboration reagents. They can be made from allylic boronic acids and are stable solids.63 The reaction with aldehydes is mediated by BF3, which is believed to provide the difluoroborane by removing a fluoride. The addition reactions occur with high stereoselectivity, indicating a cyclic TS. 

Allylboration with Lewis acid-catalyzed Lewis acid—catalyzed SnCl2- mediated... 

Hanessian reported the synthesis of enantiomerically pure or highly enriched allylglycine and its chain-substituted analogs from the reaction of the sultam derivatives of O-benzyl glyoxylic acid oxime with ally he bromides in the presence of zinc powder in aqueous ammonium chloride (Eq. 11.41).72 Brown noticed the critical importance of water in the asymmetric allylboration of /V-trimethylsilyIbcnzaldimines with B-allyldiisopinocampheylborane.73 The reaction required one equivalent of water to proceed (Eq. 11.42). 

The Hall group [52] has developed a new three-component domino reaction of 1-aza-4-borono-1,3-butadiene 4-152, a dienophile and an aldehyde to give a-hydroxy-methylpiperidine derivatives. In the first step, a hetero-Diels-Alder reaction takes place, which is followed by allylboration. As an example, reaction of 4-152 with the maleimide 4-153 in the presence of benzaldehyde furnished 4-154 in yields of up to 80% using the three substrates in a 1 2 1 ratio (Scheme 4.32). 

As demonstrated by Hoffmann and coworkers, hydroformylation can also be combined with an allylboration and a second hydroformylation, which allows the formation of carbocycles and also heterocycles [213]. A good regioselectivity in favor of the linear aldehyde was obtained by use of the biphephos ligand [214]. Reaction of the allylboronate 6/2-76 having an B-configuration with CO/H2 in the presence of catalytic amounts of Rh(CO)2(acac) and biphephos led to the lactol 6/2-80 via 6/2-77-79 (Scheme 6/2.17). In a separate operation, 6/2-80 was oxidized to give the lactone 6/2-81 using tetrabutyl ammonium perruthenate/N-methylmorpholine N-oxide. 

Scheme 6/2.17. Domino hydroformylation/allylboration/hydroformylation process.

Compounds of this type, listed in Table 9, are available by interaction of triallylboranes with certain polyyne derivatives of silicon and tin. Their formation involves 1,1- and 1,2-allylboration of triple and double carbon-carbon bonds. 

As was mentioned above (see Section 12.13.4.2), the 1,1-organoboration of alkynylsilanes, -stannanes, and -germans with trialkylboranes and boracyclanes occurs under very mild (Sn), mild (Ge), or harsh (Si, > 100 °C) reaction conditions <1995CCR125>. Similar reactions with triallylboranes proceed in general under milder conditions in comparison with trialkylboranes and furthermore a competition between 1,1- and 1,2-allylboration often takes place... 

The reaction of trimethylsilylacetylene with triallylborane in CDCI3 proceeds stepwise and involves 1,1- and 1,2-allylboration <1968IZV1923, 1999JOM(580)234>. Vinylborane 116 readily cyclizes into compounds 117 and 118 (Scheme 49). 

The thermal reaction of dipropynylsilanes with AII3B also involves 1,1- and 1,2-allylboration of the triple bond (or vice versa) (Scheme 50). l-Sila-3-bora-2-ethylidene-4-cyclopentene derivatives 119 are formed quantitatively at room temperature. They undergo an intramolecular 1,2-allylboration on heating for 3h to give the corresponding bicyclic compounds 120 quantitatively <2002JOM(649)232>. 

The reaction of the tetrayne pentasilane with triallylboranes furnishes a mixture of silole 121 and compound 122 in a 1 5 ratio (Scheme 51). An excess of AII3B did not induce further allylboration with the remaining C C bonds even upon prolonged heating <2002JOM(649)232>. 

Table 9 Bicyclic compounds with bridgehead boron atom via allylboration of 1 -alkynylsilanes and -tins...

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

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