Allenyl boronates

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Use of tartrate esters as chiral auxiliaries in the asymmetric reactions of allenyl boronic acid also have been reported Ikeda, N. Aral, I. Yamamoto, H. J. Am. Chem. Soc. 1986,108, 483 Haruta, R. Ishiguro, M. Ikeda, N. Yamamoto, H. Ibid. 1982,104, 7667. 

To elucidate the reaction pathway, deuterium-labeled allenyl pinacol boronate 10 was prepared, and the addition reaction with hydrazonoester 6 was conducted in the presence of Bi(OH)3 and Cu(OH)2 (Scheme 4). In both Bi- and Cu-catalyzed cases, the reactions proceeded smoothly (in quantitative yields in both cases). In the Bi(OH)3-catalyzed reaction, a major product was allenyl compound 11, in which the internal position was deuterized. It was assumed that a propargyl bismuth was formed via transmetalation from boron to bismuth, followed by addition to hydrazonoester via y-addition to afford allenyl compound 11. Thus, two y-additions could selectively provide a-addition products [75, 76, 105, 106]. It was confirmed that isomerization of 10 did not occur. Recently, we reported Ag20-catalyzed anti-selective a-addition of a-substituted allyltributyltin with aldehydes in aqueous media [107], On the other hand, in the Cu(OH)2-catalyzed reaction, a major product was propargyl compound 12, in which the terminal position was deuterized. A possible mechanism is that Cu(OH)2 worked as a Lewis acid catalyst to activate hydrazonoester 6 and that allenyl boronate 10 [83-85] reacted with activated 6 via y-addition to afford 12. 

Brown and co-workers developed a novel homoallenyl boronate reagent 169 based on diisopropyl tartrate for the stereoselective homoallenylation of aldehydes 170. The reagent 169 was prepared via homologation of the corresponding allenyl boronate or the alkylation of halomethyl boronate with allenyl Grignard similar to those reported in Scheme 26. The allyl boronate 169 upon reaction with aldehydes furnished the dienyl alcohols 172 with high ee (Scheme 28) <1996JOC100>. 

Chiral allenyl boronic esters (159) add efficiently to both aldehydes and 8-hydroxyketones with high levels of stereoselectivity/ stereospecificity (Scheme 44). Both (+)- and (-)-tartrates may be used to provide the enantiocomplementary products. 

Diethylzinc catalyses allenylation of aldehydes and ketones by an allenyl boronate, via a site-selective B/Zn exchange a balance of allenyl and propargyl alcohols are produced. A mild enantioselective addition of vinylzinc reagents (derived from vinyl iodides and diethylzinc) has been reported, with wide tolerance of functional groups. 

Enantioselective addition of allenyl boron reagents to A/-phosphinoyl i... 

Allenyl boronic ester. In principle, using a chiral Lewis acid catalyst, it should be possible to activate carbonyl compounds asymmetrically. In our early work, we focused... 

In 1988, an ASI workshop on Selectivities in Lewis acid promoted reactions was held in Greece, during which I proposed the mechanism of our asymmetric propargylation reaction using chiral allenyl boronic ester [13]. In an enantioface differentiation process, the chiral nucleophile was added to the carbonyl group of aldehydes, thus allowing the preparation of chiral propargylic alcohols [14]. Based... 

Palladium-catalyzed annulation reactions of conjugate acceptors and allenyl boronic ester provide substituted cyclopentenes in high yields and diastereose-lectivities (Scheme 6.24). These reactions are hypothesized to commence by the conjugate addition of a nucleophilic propargyl-palladium complex. Transmetalation of allenylboronic acid pinacol ester with a Pd(II) catalyst proceeds via an SE2 mechanism to provide the propargyl-palladium complex, which on conjugate attack on the electrophile furnishes an allene intermediate. Finally, endo carbopalladation of the pendant allene and protodepalladation generates the cyclopentene [28]. 

Scheme 6.24 Reaction with allenyl boronic ester.

Enantioselective synthesis of homopropargyl amines can be effected through copper-catalysed reaction of an allenyl boron reagent with aldimines. The first nucleophilic allylation of r-electrophiles by allylboron reagents has been achieved enantioselectively using a chiral rhodium catalyst (Scheme an allylrhodium intermediate has been implicated. Similar additions of R CH=CR BF3K have also been reported. ... 

The use of tartrates as chiral auxiliaries in asymmetric reactions of allenyl bor-onic acid was first reported by Haruta et al.69 in 1982. However, it was not for several years that Roush et al.,70 after extensive study, achieved excellent results in the asymmetric aldol reactions induced by a new class of tartrate ester based allyl boronates. 

The reaction of hydrazonoester 6 with allenyl pinacol boronate 7 was set as a model, and several metal hydroxides (20 mol%) were screened as catalysts in H20-DMF (1 3) at room temperature [104]. It was found that allenyl adduct 8 was produced with high selectivity in the presence of bismuth(III) hydroxide, Bi(OH)3 (Scheme 3). Interestingly, copper(II) hydroxide, Cu(OH)2, preferentially gave propargyl adduct 9. On the basis of these promising results, we further optimized the reaction conditions using Bi(OH)3 and Cu(OH)2 (Table 5). 

Thus, it was found that selective propargylation or allenylation reactions of hydrazonoester with allenyl pinacol boronate proceeded smoothly in the presence of a catalytic amount of bismuth(III) or copper(II) hydroxide in aqueous media. The use of metal hydroxide as a catalyst in organic synthesis is rare, and it is noteworthy that efficient catalysis occurred in aqueous media. In addition, the allenyl adduct was produced with high selectivity in the presence of Bi(OH)3, whereas the propargyl adduct was obtained with high selectivity in the presence of Cu(OH)2 as a catalyst. 

Chiral allenylmetal compounds provide convenient access to enantioenriched homopropargylic alcohols through Se2 additions to aldehydes. The syn adducts can be obtained through addition of allenyl tributylstannanes in the presence of stoichiometric boron trifluoride etherate (BF3-OEt2). The use of allenylmetal halides derivatives of Sn, Zn, and In lead to the anti diastereomers. The former additions proceed through an acyclic transition slate whereas the latter are thought to involve a cyclic transition state, thus accounting for the difference in diastereoselectivity. 

A species such as (8), formed via transmetallation, often undergoes reductive elimination to complete a very useful coupling reaction. Thus, magnesium,boron, zinc and tin are effective. The major asset of this method is the complete retention of the configuration in the alkene, resulting in products of exceptional isomeric purity. The halides involved include aromatic, alkenyl and allenyl. Several reviews of this method have been published. " ... 

Under basic conditions, Pd(II) catalyses the bisfunctionalisation of allenyl esters by boronic acids and aldehydes in a one-pot reaction which yields 4,6-disubstituted 5,6-dihydropyran-2-ones 20. The tandem process involves attack by nucleophiles at the 3-position and by electrophiles at C-4 some uncyclised 5-hydroxypent-2-enoic acid derivatives can also be recovered <05JOC6848>. The aerobic cyclisation of O-alkenyl P-ketoamides affords 2,6-dihydropyran-3-ones (Scheme 7) <05OL5717>. 

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