The stereochemistry of the hydroboration of ferrocenylalkenes

The hydroboration of π cyclopentadienyl(6,7-dihydro-π-indenyl)iron affords (after oxidation) a considerable amount (34%) of π-cyclopentadienyl(endo-4,5,6,7-tetrahydro-4-hydroxy-π-indenyl)iron. This stereochemical result is a proof of the preliminary complexation of the borane with iron. The regio-selectivity of the hydroboration of cyclic and open ferrocenylalkenes appears to be controlled by both steric and electronic factors.     

Oxiranylcarbinyl rearrangement: the effect of precursor stereochemistry

Rigid, bisected oxiranylcarbinyl mesylates solvolyzed $\text{via}$ an apparent oxiranylcarbinyl cation which rearranged to a 2-oxahomoallyl cation, while the perpendicular isomers gave displacement with inversion.     

The stereochemistry of the methylenecyclopropane rearrangement: evidence against the orthogonal-allylic intermediate

The rearrangements of two optically pure derivatives of Feist's acid were studied: that of dimethyl trans-methylenecyclopropane-2,3-dicarboxylate (TRANS-diester) and trans-2,3-dicyano-methylenecyclopropane (TRANS-dinitrile). The optical purity and configuration of the products, methyl (Z)- and (E)2-carbomethoxyclopropylideneacetate (SYN- and ANTI-diesters) and (Z)- and (E)-2-cyanocyclopropylideneacetonitrile (SYN- and ANTI-dinitriles), establish that the rearrangements occur with predominant, but not exclusive, inversion of configuration at the migrating center. Investigation of the interconversions of (SYN)- and ANTI-diesters and dinitriles reveal that racemic product is not obtained, as would be expected from an orthogonal-allylic diradical intermediate, but that the enantiomer corresponding to antarafacial migration is slightly favored. All of the stereochemical results are explicable by application of the Doering-Sachdev diradical transition state hypothesis     

Oxy-function promoted hydroboration of conjugated dienes with controlled allylic borane rearrangement

Hydroboration of conjugated dienes is promoted by the hydroxy and methoxy groups, which also control the rearrangement of the initially produced allylic boranes.     

On the absolute stereochemistry of the photochemical rearrangement of 2,5-cyclohexadienones to lumiketones

Specification of the absolute configurations of starting material and product in the photochemical rearrangement of a 2,5-cyclohexadienone to a bicyclo[3.1.0]hexen-2-one (lumiketone) permits unambiguous determination of the stereochemistry at two crucial stages in the generally accepted reaction mechanism, namely the electrocyclic ring closure to give a 3,5-bonded intermediate, and the subsequent [1,4]-sigmatropic shift. The stereochemical course of the latter process in turn allows specification of whether the rearrangement is occurring directly from an electronically excited intermediate or a ground state zwitterion. An unconstrained monocyclic chiral cyclohexadienone has been studied in which steric constraints are minimal, and in which the system should have free choice of available pathways. The absolute configuration of the resolved dienone was determined by X-ray crystallographic analysis of a derivative containing a chiral center of known configuration. The absolute configuration of the resultant optically active diastereomeric lumiketones has been assigned from the Cotton effects observed on the corresponding dihydrolumiketones, assuming these chiroptical effects are governed by a normal Octant Rule instead of the Inverse Octant Rule which empirically correlates such data for cyclopropyl ketones lacking polarizable substituents. The limitations on these assignments are discussed, as well as alternative efforts to obtain completely unambiguous stereochemical assignments for the lumiketones. On the basis of these assignments, it is concluded that the sigmatropic shift proceeds with inversion of configuration at the migrating carbon, consistent with reaction via a ground state zwitterion.     

On the stereochemistry of the BF3-catalyzed rearrangement of (+)-isophorone oxide

The title reaction proceeds with partial or extensive racemization depending on the solvent used.     

The di-π-methane rearrangement : The effect of 1,4-diene structure and conformation on the reaction efficiency and stereochemistry

The photochemistry of the stereoisomeric 1,3-dimethyl-3(2-phenylethenyl)cyclohexenes has been explored Direct irradiation of the cis- and trans-β-styrylcyclohexenes leads to di-π-methane rearrangement, producing the endo- and exo-3,7-dimethyl-8-phenybicyclo[5.1.0]oct-2-enes, and cis -trans isomerization, interconverting the 1,4-diene containing substrates. Triplet sensitized photolysis of both substituted cyclohexenes leads exclusively to cis-trans isomerization. Results from low conversion direct irradiations of the cis- and trans-β-styrylcyclohexenes indicate that the singlet rearrangements are stereospecific, and lead to formation of the 8-exo and 8-endo-phenylbicyclic octenes, respectively. The relationship between di-π-methane structure and triplet reaction efficiency, and the effect of conformation on the rearrangement stereochemistry are discussed.     

Acyclic stereoselection. 21. Synthesis of an ionophore synthon having four asymmetric carbons by sequential aldol addition,claisen rearrangement and hydroboration

Allylic alcohol 7, obtained by a synthesis involving stereoselective aldol addition to an unsaturated aldehyde, is transformed by the Ireland variant of the Claisen rearrangement into unsaturated acid 9. Subsequent elaboration of this material gives homoallylic alcohols 13 and 14, which are hydroborated to obtain primarily 3 and 4. It is shown that the hydroboration is intermolecular, rather than intramolecular.     

The transition metal catalyzed hydroboration of enamines

The addition of catecholborane (HBcat, cat = 1,2-O₂C₆H₄) to 9-vinylcarbazole can give either the branched or linear hydroboration product depending upon the judicious choice of metal catalyst used in these reactions. Analogous reactions with pinacolborane (HBpin, pin = 1,2-O₂C₂Me₄) and HBBzpin (Bzpin = 1,2-O₂C₂Ph₄) using catalytic amounts (5 mol%) of either Rh(acac)(dppb) or [Cp^∗IrCl₂]₂ gave the linear hydroboration product selectively. Hydroborations of 1-pyrrolidino-1-cyclopentene and 1-pyrrolidino-1-cyclohexene were complicated by a competing dehydrogenative borylation pathway. The branched isomer was not observed to any significant extent in these reactions, suggesting that the directing effect of the nitrogen atom is negligible. Although catalyzed additions of HBcat to 1-vinyl-2-pyrrolidinone gave complicated product distributions arising from competing reactions, addition of HBpin effectively generated the corresponding linear hydroboration product in good yields.     

Rearrangement of lithiated S-alkyl O,O-dialkyl thiophosphates: scope and stereochemistry of the thiophosphate-mercaptophosphonate rearrangement

S-Alkyl O,O-dialkyl thiophosphates are prepared by alkylation of the triethylammonium salt of O,O-diisopropyl thiophosphoric acid. S-Benzyl thiophosphate was metallated at temperatures of ≥-45 °C by trityllithium and LiTMP (lithium 2,2,6,6-tetramethylpiperidide) and S-alkyl thiophosphates only by LiTMP to give dipole-stabilised carbanions which rearrange to α-mercaptophosphonates in yields of up to 45%. Metallation occurs with a high primary kinetic isotope effect (k(H)/k(D) up to ≈50). When the lithium (R)-N-isopropyl-1-phenylethylamide was used to induce the isomerisation of S-pentyl thiophosphate an α-mercaptophosphonate with an ee of 22% was isolated. (R)-S-[1-D(1)]hexyl O,O-diisopropyl thiophosphate was rearranged to a dextrorotary α-mercapto-[1-D(1)]hexylphosphonate, whose (R)-configuration was determined by chemical correlation. The thiophosphate-mercaptophosphonate rearrangement proceeds with retention of configuration.     

The markovnikoff hydroboration of a trisubstituted olefin

Hydroboration-oxidation of akuammicine $\text{1}$ mainly gave the tertiary alcohol $\text{2}$ Hydroboration-oxidation of double bonds is a well known anti-Markovnikoff process^1,2. In the course of applications of this method to the indole alkaloid field, the following Markovnikoff hydration of a trisubstituted olefin was surprisingly observed :

     

Vinylmetalloids : II. The high regioselectivity of dialkylboranes in the hydroboration of vinyltrimethylsilane

Dialkylboranes such as disiamylborane, dicyclohexylborane and 9-borabicyclo[3.3.1]nonane add cleanly to vinyltrimethylsilane (I) with high regiospecificity (? 95%) to give the corresponding β-silylethylboranes, valuable intermediates in the synthesis of functionalized organosilanes. Oxidation of these adducts provides a convenient, high-yield route to 2-trimethylsilylethanol (IIβ). Reduction of acetyltrimethylsilane (IX) with borane-tetrahydrofuran provides a simple route to the isomeric 1-trimethylsilylethanol (IIα). These new developments illustrate the utility of borane reagents in the regiospecific syntheses of trimethylsilylethanols (II).     

雪松烯的硼氢化反应及其产物构型的测定

(-)-α-雪松烯是由天然雪松醇制得, 它与硼烷反应得到二异雪松基硼烷. 其红外光谱显示是二聚体. 在用H~2O~2/NaOH氧化之后, 得到异雪松醇, 其绝对构型由X-ray结构分析得到确定. 根据这一结果, 硼氢化的立体化学和二异雪松基硼烷的构型得到确定.     

Asymmetric synthesis, stereochemistry and rearrangement reactions of naturally occurring 7'-hydroxylignano-9,9'-lactones

The asymmetric synthesis of a series of (7'S,8R,8'R)-7'-hydroxylignano-9,9'-lactones is presented, among them the mammalian lignan (7'S)-hydroxyenterolactone and (7'S)-parabenzlactone, allowing the stereochemistry of natural occurring (-)-parabenzlactone to be re-assigned. A hydroxylactone rearrangement and its possible mechanisms are discussed. Finally a brief survey of the current naming and numbering variants of 7'-hydroxylignano-9,9'-lactones is presented, along with a suggestion for harmonization of the nomenclature.