Diastereo- and enantiomerically pure allylboronates: their synthesis and scope

Allylboronates are highly attractive reagents for allyl additions. Enantiomerically pure, stable reagents with a stereogenic centre in alpha-position to boron are especially versatile, albeit often difficult to synthesize. Starting from boron-containing allyl alcohols 6 and 7, which are discussed in detail herein, a set of reagents were obtained via [3,3]-sigmatropic rearrangements and consecutive transformations in the side chain. The configurations could be established first by chemical correlation, but also by X-ray crystallography (16, 18, 34, and 39). Allyl additions were performed resulting in the formation of predominantly (Z)-configured homoallylic alcohols (31, 43-45) with high enantiomeric excess. Detailed investigations on the matched-mismatched interaction between the reagents 15/16 (and ent-15/ent-16, respectively) and isopropylidene glyceraldehyde 42d are presented.     

An NMR study of the allyl rearrangement in boron compounds

The temperature dependence of the PMR spectrum is reported for allyl compounds of boron, nitrogen, and silicon. The allyl rearrangement is observed in triallylboron, allylboracyclopentane, and triallylboron pyridinate. The rate of the rearrangement has been measured as a function of temperature to give the activation energy. A scheme is proposed for the rearrangement in boron compounds.     

Lewis acids catalyze the addition of allylboronates to aldehydes by electrophilic activation of the dioxaborolane in a closed transition structure

This study addressed the mechanism of the recently reported Lewis acid-catalyzed manifold for additions of allylboronates to aldehydes. A series of control experiments using various reagents and conditions, and kinetic studies by low-temperature NMR spectroscopy, were performed to investigate the origin of the activation provided by the best catalyst, Sc(OTf)3. The results obtained are strongly supportive of a mechanism involving electrophilic boron activation by coordination of the metal ion to one of the boronate oxygens in a closed bimolecular transition state.     

Secondary deuterium kinetic isotope effects in irreversible additions of allyl reagents to benzaldehyde

The competitive kinetics of additions of allyl to benzaldehyde-h and -d from allyltributyl tin, from diisopropyltartrylallyl boronate, and from allyl bromide and zinc dust in aqueous tetrahydrofuran have inverse secondary deuterium kinetic isotope effects, SDKIEs. These inverse SDKIEs are in contrast to the normal SDKIEs that were obtained with allyl lithium and allyl Grignard, suggesting rate-determining single-electron transfer in these cases. By various MO calculations the transition state for addition of allyl boronate occurs with substantial B-O bond formation and little C-C bond formation. The magnitudes of the SDKIEs with the other two allylating reagents, when compared with reasonable equilibrium isotope effects for the addition, suggest transition states with substantial C-C bond formation.     

Rate of hydrogenation of vinyl and allyl compounds of carbon,silicon, germanium,and tin on skeletal nickel

Russian Chemical Bulletin - As in radical additions, vinyl compounds of silicon are hydrogenated more rapidly than allyl compounds. Vinyl and allyl double bonds in germanium and tin compounds...     

Spectrophotometric determination of trace and ultratrace levels of boron in silicon and chlorosilane samples

Summary Spectrophotometric methods for the determination of boron in the low g/g and ng/g range in high-purity silicon and dichloro- and trichlorosilanes were investigated in detail. The procedures established involve dissolution of silicon samples and the hydrolyzed products of chlorosilane samples in hydrofluoric acid-containing reagents followed by evaporation of the silicon matrix as H₂SiF₆. The boron retained in the treated sample solution was then determined by a spectrophotometric method using carminic acid as a chromatic reagent. Special effort has been paid to the control of the analytical blank and reproducible determination of boron. The results indicate that addition of mannitol and proper control of the evaporation process are effective in preventing volatilization of boron during the evaporation of silicon matrix and can thus attain high recovery of boron and reproducible analysis. Through meticulous control of the analytical blank and experimental conditions, the limit of detection for boron determination with the established method can be as low as ng/g levels. Application of the methods to the determination of boron in various stages of purification of silicon and trichlorosilane as well as in borophosphosilicate film was conducted.     

Generation of allyl Grignard reagents via titanocene-catalyzed activation of allyl halides

A protocol for the generation of allyl Grignard reagents via the catalytic activation of allyl halides is described herein. Subsequent nucleophilic addition to carbonyl derivatives provided the desired homo allylic alcohols in excellent yields (84-99%). Evidence suggests that titanocene dichloride catalyzes the formation of an allyl Grignard species which reacts solely with the carbonyl electrophile as evidenced by the complete absence of Würtz coupling. This methodology will have wide-ranging applicability in the generation of highly reactive organometallic reagents.     

Intermolecular oxonium ylide mediated synthesis of medium-sized oxacycles

Detailed in this account are our efforts toward efficient oxacycle syntheses. Two complementary approaches are discussed, with both employing chemoselective allyl ether activation and rearrangement as the key step. Vinyl substituted oxiranes and oxetanes provide a single step access to dihydropyrans and tetrahydrooxepines. Oxiranes proved to be poor substrates, while oxetanes were slightly better. An alternative approach using substituted allyl ethers proved successful and addressed the limitations encountered in the ring expansions.     

Regioselectivity in the palladium-catalyzed addition of carbon nucleophiles to carbocyclic derivatives.

The regioselectivity of Pd-catalyzed malonate additions and arylations to cycloalkenyl esters can be predicted by completing a stereochemical analysis of the Pd-pi-allyl complex. The Pd-catalyzed malonate additions which have the greatest degree of regioselectivity are in which substituents have a steric influence in blocking the incoming nucleophile. Cyclopentenyl substrates displayed lower regioselectivity than the cyclohexyl counterparts presumably due to increased planarity of the system. Arylations using tin and hypervalent silicon reagents were compared.     

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol or aldehyde oxidation level using allyl acetate as an allyl metal surrogate

Protocols for highly enantioselective carbonyl allylation from the alcohol or aldehyde oxidation level are described based upon transfer hydrogenative C-C coupling. Exposure of allyl acetate to benzylic alcohols 1a-i in the presence of an iridium catalyst derived from [IrCl(cod)]2 and (R)-BINAP delivers products of C-allylation 2a-i. Employing isopropanol as terminal reductant, exposure of allyl acetate to aryl aldehydes 3a-i in the presence of an iridium catalyst derived from [IrCl(cod)]2 and (-)-TMBTP delivers identical products of C-allylation 2a-i. In all cases examined, exception levels of enantioselectivity are observed. Thus, enantioselective carbonyl allylation is achieved from the alcohol or aldehyde oxidation level in the absence of any preformed allylmetal reagents. These studies define a departure from preformed organometallic reagents in carbonyl additions that transcend the boundaries of oxidation level.     

Silylstannylation of allenes and silylstannylation-cyclization of allenynes. Synthesis of highly functionalized allylstannanes and carbocyclic and heterocyclic compounds

Catalyzed by Pd(0), trialkylsilyltrialkylstannane (R(3)Si-SnR'(3)) reagents undergo highly selective additions to 1,2-dien-7-ynes and 1,2-dien-8-ynes to give 2-vinylalkylidenecyclopentanes with silicon and tin substituents on the double bonds. Similar additions of distannanes and borostannanes show that the reactions with silylstannanes are superior in terms of ease of handling of the bifunctional reagents and the isolation of the products after the reaction. The chemo- and regioselectivities are controlled by the enhanced reactivity of the allene unit, while the (Z)-geometry of the exocyclic stannylvinylidene is a consequence of the syn-carbometalation and subsequent reductive elimination from Pd with retention of configuration at the vinyl carbon. Synthesis of highly functionalized pyrrolidines and indolizidines and the reluctance of certain kinds of allenynes and silicon-tin reagents to undergo the cyclization illustrate the scope and limitations of the reaction. Based on the isolation of intermediates, a mechanism for the formation of the cyclic compounds is proposed. Model transition states to explain the stereoselectivity in cyclization of substituted allenynes are provided. Further elaboration using the vinyltin and vinylsilane moieties should lead to highly functionalized carbocyclic and heterocyclic compounds. Under similar conditions, addition of silylstannanes to highly functionalized allenes gives E-allylstannanes with high stereoselectivity. Functional groups such as THP- and silyl-ethers, lactones, beta- and gamma-lactams, alpha,beta-unsaturated esters, olefins, and substituted acetylenes are tolerated under the reaction conditions.     

Stereoselective allylations of erythrulose derivatives under anhydrous conditions

We have investigated a number of nucleophilic additions of allylating reagents to several α,α′,β-trioxygenated ketones (O-protected erythrulose derivatives). Reagents based on lithium, magnesium, copper and titanium gave low to medium stereoselectivities and did not display any recognizable trend in the sense of stereoselection. In contrast, reactions involving silicon and tin derivatives were highly stereoselective and gave rise to essentially a single diastereoisomer, the structure of which depended on the type of protecting group. Thus, α,β-di-O-benzylated derivatives experienced almost exclusive addition to the carbonyl Si side, whereas α,β-O,O-alkylidene derivatives (dioxolane acetals) yielded the opposite diastereoisomers as a result of addition to the Re side. These results suggest the intermediacy of α-chelates in the additions of silicon and tin reagents to the di-O-benzylated derivatives. In contrast, the opposite stereoisomers, formed in the reactions of dioxolanes, are believed to be formed through Felkin–Anh transition states, pointing again to the reluctance of acetal oxygens to participate in chelated intermediates.     

Microwave assisted synthesis of enantiomerically pure allylboronates

Stable allylboronates with a stereogenic centre α to the boronic ester moiety represent versatile reagents for stereoselective synthesis of homoallylic alcohols. Use of microwave irradiation in desilylation and sigmatropic rearrangement reactions allows rapid synthesis of α-chiral allylboronates utilized in the highly diastereo- and enantioselective synthesis of (Z)-configured homoallylic α-hydroxy esters by allyl additions to ethyl glyoxylate.     

Activation catalytique des réactifs de grignard par des complexes du nickel en série organosiliciée : I. Réactions de substitution avec les réactifs de grignard non réducteurs

A new method of Si-C bond formation is described which involves coupling reactions between non-reducting Grignard reagents and Si-H bonds catalyzed by Ni^II complexes. The steric environment of the silicon atom has little effect on the reaction.Cobalt complexes are moderate catalysts, and those of iron, copper, zirconium and titanium are inactive.The reactions proceed with retention of configuration at the silicon atom. This use of Grignard reagents avoids the awkward preparations of unsaturated lithium or sodium analogues (allyl, crotyl, benzyl, vinyl).     

Indole-olefin-oxazoline (IndOlefOx)-ligands: synthesis and utilization in asymmetric Rh-catalyzed conjugate addition

The synthesis and utility of novel indole-olefin-oxazoline (IndOlefOx)-ligands are described. The use of these ligands was demonstrated in rhodium catalyzed asymmetric conjugate additions between 2-cyclopentenone, 2-cyclohexenone, and 2-cycloheptenone with different boron reagents with good yields and enantioselectivities of up to 94%.     

2-Halo-diketopiperazines as chiral glycine cation equivalents

A range of (2S,5S)-5-isopropyl-2-halo-N,N′-bis-(p-methoxybenzyl)-piperazine-3,6-diones 8 (Cl), 11, 12 (F) and 13 (Br) have been prepared, either via electrophilic halogenation of the corresponding lithiated diketopiperazine, or via transhalogenation from fluoro-11 and 12. The product distribution and stereoselectivity of additions of allyltrimethylsilane, sodium thiophenolate and a range of organomagnesium reagents to chloro 8 are reported. In the reactions with Grignard reagents the observed stereo- and regioselectivities are dependent on the reagent employed, with C-3 carbonyl addition products predominating upon addition of allyl or methylmagnesium chloride and stereodivergent formal C-2 addition predominating with benzyl or isopropylmagnesium chloride. A model to account for the different reactivity and stereoselectivity in these reactions is proposed.     

Asymmetric Catalysis with Silicon‐Based Cuprates: Enantio‐ and Regioselective Allylic Substitution of Linear Precursors

An enantio‐ and regioselective allylic silylation of linear allylic phosphates that makes use of catalytically generated cuprate‐type silicon nucleophiles is reported. The method relies on soft bis(triorganosilyl) zincs as silicon pronucleophiles that are prepared in situ from the corresponding hard lithium reagents by transmetalation with ZnCl₂. With a preformed chiral N‐heterocyclic carbene–copper(I) complex as catalyst, exceedingly high enantiomeric excesses are achieved. The new method is superior to existing ones using a silicon–boron reagent as the source of the silicon nucleophile.     

Organoboron compounds,synthesis of allyl(dialkyl)boranes and diallyl(alkyl)boranes

Highly reactive allyl(dialkyl)-, crotyl(dialkyl)-, 3,3-dimethylallyl(dialkyl)-(= prenyl(dialkyl), and diallyl(alkyl)-boranes were prepared by allylation of esters R₂BOR′, RB(OR′)₂ or thioesters R₂BSR′ (R = alkyl) using allylic derivatives of aluminium, magnesium or boron in exchange reactions.The titled compounds are stable up to 100°C and do not symmetrize even on heating at 100°C for a long time. PMR spectroscopy data show that the characteristic feature of these compounds is a permanent allyl rearrangement, the rate of which increases with an increase in temperature. For allyl(diethyl)-borane at 100°C and 125°C the rates are equal to 2500 and 5000 sec^?1 respectively; activation energy of the rearrangement amounts to 11.8±0.2 kcal mol^?1.The boronallyl bonds in unsymmetrical allyl(alkyl)boranes readily split under the action of water and alcohols, protonolysis being accompanied by allyl rearrangement, crotyl and prenyl compounds are converted into 1-butene or 3-methyl-1-butene, respectively.     

Direct synthesis of B-allyl and B-allenyldiisopinocampheylborane reagents using allyl or propargyl halides and indium metal under Barbier-type conditions

We report the first one-pot process for the asymmetric addition of allyl, methallyl, and propargyl groups to aldehydes and ketones using B-chlorodiisopinocampheylborane ((d)DIP-Cl) and indium metal. Under Barbier-type conditions, indium metal was used to generate allyl- and allenylindium intermediates, and subsequent reaction with (d)DIP-Cl successfully promoted the transfer of these groups to boron forming the corresponding chiral borane reagents. The newly formed borane reagents were reacted with aldehydes and ketones to produce the corresponding alcohol products in high yields and up to excellent enantioselectivity (98% ee). This method produced excellent enantioenriched secondary homoallylic alcohols from the allylation and methallylation of benzaldehyde. Using this method, the methallylation and cinnamylation of ketones afforded the highest enantioselectivities, while the propargylation of both aldehydes and ketones provided low enantiomeric excesses. In addition, this procedure provided the first synthesis of B-allenyldiisopinocampheylborane, which was characterized by (1)H and (11)B NMR spectroscopy. This is the first example of the direct synthesis of allylboranes that contained substitutions from the corresponding allyl bromide and indium, thereby expanding the utility of the DIP-Cl reagent. Hence, a general and straightforward route to these chiral organoborane reagents in one-pot has been developed along with the asymmetric Barbier-type allylation and propargylation of aldehyde and ketone substrates using these chiral organoborane reagents in subsequent coupling reactions.     

Enantioselective formal hydration of α,β-unsaturated acceptors: asymmetric conjugate addition of silicon and boron nucleophiles

The direct enantioselective 1,4-addition of water to α,β-unsaturated acceptors is an open challenge in asymmetric catalysis. Enantioselective conjugate addition of either silicon or boron nucleophiles and subsequent enantiospecific oxidative degradation of the carbon-element bond represents, however, an attractive detour. A single extra step thereby enables an indirect enantiocontrolled construction of (in a broader sense) aldols from α,β-unsaturated carbonyl and carboxyl compounds. While that strategy had been obvious for a long time, it was recent stunning progress in transition metal-catalysed activation of interelement linkages that brought about the solution to that long-standing problem. A concise introduction of existing protocols for stereoselective 1,4-addition of oxygen nucleophiles is followed by a comprehensive summary of the recent rapid advances in transition metal-catalysed (and metal-free) asymmetric conjugate transfer of nucleophilic silicon and boron onto α,β-unsaturated acceptors.