Allyl borates

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]. 

Systematic studies of the reactions of tartrate allyl-boronates with a series of chiral and achiral alkoxy-substituted aldehydes show that conformationally unrestricted a- and /f-alkoxy aldehyde substrates have a significant negative impact on the stereoselectivity of asymmetric allyl-boration. In contrast, con-... 

In almost the same manner frans-disubstituted hydrooxepans can be obtained from a tandem hydroformylation/allyl boration of (-E)-alkoxyallyl-boronates as a mixture of anomers (Scheme 24) [79]. 

Intermolecular Allylboration. A tandem aza[4+2] cycloaddition/allyl-boration three-component reaction has been designed based on the prece-dented carbocyclic [4- -2] cycloaddition/allylboration and a snbsequent one-pot variant. Thns, the thermal reaction between hydrazonobutadienes 138, A-substitnted maleimides, and aldehydes provides polysnbstituted a-hydroxy-alkylpiperidines 141 via the cyclic allylboronate intermediate 139 and the proposed chairlike transition stmctnre 140 (Eq. 103). Monoactivated dienophiles like acrylates fail to react with heterodienes 138 bnt the scope of aldehydes is very broad both ahphatic and aromatic aldehydes are snitable, inclnding electron-rich ones. An inverse electron-demand variant to access the corresponding dihy-dropyran derivatives via the intermediacy of enantiomerically enriched pyranyl allylic boronate 76 has been snbsequently developed (see Eq. 64). ° ... 

Coupling with allyl acetates gives allyl boronates,14 which exhibit a high diastereoselectivity in the intramolecular allyl boration of carbonyl compounds15 (Scheme 2). 

Allyl borates,89 imidates,90-91 alcohols,77-92 arsenites93 and O-allylisourea94-93 have also been found to function as leaving groups that allow neutral functionalization by similar mechanisms, although the chemistry of these compounds has not been widely explored. 

Regio- and stereo-selective allylation of sulfonylimines has been carried out with trifluoro(allyl)borates and allylstannanes, using palladium-pincer complexes as catalysts.47 Syn products predominate, in contrast to the corresponding reaction of aldehyde electrophiles DFT calculations have been employed to probe the mechanistic differences. 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

Suzuki couplings. Pd-catalyzed reactions of B-allyl borate complex derived from 6-methoxy-9-borabicyclo[3.3.1]nonane with aryl triflates give allylarenes. This technique broadens the scope of the Suzuki coupling to allow transfer of Me, TMSCHj, and alkynyl groups which has eluded conventional manipulations. 

Unless there is a chiral ligand on boron, assemblies a and b of Scheme 5.2 are enantiomeric and the product will be racemic. If the ligand is chiral, then the transition structures are diastereomeric and the products will be formed in unequal amounts under conditions of kinetic control (Chapter 1). Figure 5.1 illustrates several chiral boron reagents that have been tested in the allyl boration reaction, with typical enantioselectivities for each. 

Allylation of carbonyl compounds is another very useful carbon-carbon bond forming asymmetric transformation in organic synthesis. This transformation yields the homoallylic alcohols that have proven to be valuable reagents and intermediates that have found numerous applications in natural product total synthesis. In particular, asymmetric allyl-boration of aldehydes employing tartrate- and pinane-derived reagents has been widely exploited. Although the asymmetric allylation reaction is well documented and widely used in solution phase, the asymmetric variant of the allylation of carbonyl compounds on the solid support has remained largely unexplored. ... 

The addition of certain trialkylboranes to but-2-enyl-lithium before reaction with aldehydes has been shown to lead to formation of the threo-zdducts (48) (Scheme 21),presumably via the ( )-allylic borates (49), with threo ierythro... 

To also add an application to aromatic compounds we cite the Heck-hydroformylation sequence [57] (see 121) and to extend these directed additions into the silicon and boron field a regioselective silylformylation-allyl silylation sequence [58] and a hydroformylation-allyl boration hydroformylation cascade are included. 

A double hydroformylation is also employed in the allyl boration process. The aldehyde group of 128 obtained in the first hydroformylation operates in the intramolecular allyl boration to give rise to the vinylpiperidine 130, which on introduction of the next formyl group ring closes to hemiacetal 129 [59a, b]. 

A study of hydroboration of cyclohex-2-enones and cyclohex-2-enols has been reported. An allylic borate is first produced in both instances. The results indicate high regiospecificity and stereospecificity in the trans-1,2-diols formed. For cyclohex-2-enone where only one allylic borate is possible the product ratios derived from attack on the intermediate allylic borate (115)... 

Strong inductive and polar directive effects of the allylic borate group the stereospecificity at C-3 is due to a steric effect of the t-butyl group. The steric effect of the t-butyl group controls the stereochemistry of products in the hydroboration of 4-t-butylcyclohex-2-enone (119) in particular the traits directive effect of the allylic borate is overcome. Hydroboration of cyclohexa-l,3-diones gave predominantly trans-cyclohexane-l,2-diols in yields of ca. 45—50%. This... 

The Schaus group also reported related reactions involving asymmetric allyl-boration of acyl imines [59], asymmetric three component Petasis condensation reaction of secondary amines/glyoxylates/alkenyl boronates [60], as well as addition of aryl, vinyl, and alkynyl boronates to acyl imines (Fig. 15) [61]. In the later reaction, a two point coordination transition state was proposed to account the observed facial selectivity. 

Nuclear Magnetic Resonance Studies.— The relationship between B n.m.r. chemical shifts in compounds of the general type R3B and n.m.r. chemical shifts for the corresponding cationic centre in the ions RaC is not so simple as had been previously reported. Nevertheless, the comparative data are useful. n.m.r. chemical shifts have been quoted as evidence for the hyper-conjugative interaction of allylic borates, which may therefore be represented as shown in structure (40), or more generally as shown in (41). ... 

Another breakthrough came from Antilla et al., who employed a chiral phosphoric acid as an efficient catalyst for the reduction of prochiral ketones (83). As an extension of their success in the enantioselective allylation of aldehydes with allyl borate [84], they hypothesized that a chiral phosphoric could activate and direct the borane reduction of ketones. Initial studies showed that the asymmetric reduction truly works and, after screening various chiral phosphoric acids, found that 6 stands out as the most suitable catalyst. The enantioselectivity is further improved by using 4-(dimethylamino)pyridine (DMAP) as an additive (ee up to 95%) (Scheme 32.30). To gain mechanistic insight into this reduction, the authors performed several parallel B NMR experiments the results suggest that the complex formed between 6, catechol borate, and DMAP might be the real catalyst for the asymmetric reduction. 

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