Hydroboration cyclopentenes

Scheme 9 The thermal migration of umymmetrical hydroborated cyclopentenes...

In a 200-ml three-necked flask fitted with a dropping funnel (drying tube) is placed a solution of 13.4 g (0.12 mole) of 1-octene in 35 ml of THF. The flask is flushed with nitrogen and 3.7 ml of a 0.5 M solution of diborane (0.012 mole of hydride) in THF is added to carry out the hydroboration. (See Chapter 4, Section I regarding preparation of diborane in THF.) After 1 hour, 1.8 ml (0.1 mole) of water is added, followed by 4.4 g (0.06 mole) of methyl vinyl ketone, and the mixture is stirred for 1 hour at room temperature. The solvent is removed, and the residue is dissolved in ether, dried, and distilled. 2-Dodecanone has bp 119710 mm, 24571 atm. (The product contains 15 % of 5-methyl-2-undecane.) The reaction sequence can be applied successfully to a variety of olefins including cyclopentene, cyclohexene, and norbornene. 

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

Organoborane intermediate from hydroboration of 1 -methyl cyclopentene... 

Asymmetric hydroboration. Details are available for the preparation of the reagent and for the asymmetric hydroboration of methyl 2,4-cyclopentadiene-l-acctalc (I) (ogive (2R, 5R)-mcthyl 5-hydroxy-2-cyclopentene-1-acetate (2). ... 

Table 1. Thermal rearrangement of organoboranes resulting from the hydroboration of tetrasubstituted cyclopentene derivatives of type 34.

C. AByrmetvic hydroboration tuith (+)-di-3-pinanylboTvme to form methyl (lR,SR)-5-hydroxy-2-cyclopentene-l-aeetate (Note 4). The (+)-d1-3-pinanyl-borane is prepared from (-)-a-plnene by a modification5,6 of the method of Brown (Note 14). 

Asymmetric hydroboration.2 Hydroboration of the chiral cyclopentene 2 with a number of hindered boranes followed by alkaline hydrolysis usually results in three hydroxy... 

Asymmetric hydroboration.1 Hydroboration of 1-phenyl-1-cyclopentene with IpcBH2 (100% ee) results in a dialkylborane (1) containing the traws-2-phenylcyclopentyl group of 100% ee. However, hydroboration of prochiral trisubstituted alkenes usually results in alkylisopinocampheylboranes of 50-85% ee. Most of these products are solids, and selective crystallization (usually from ether) can give the optically pure dialkyboranes. In some cases resolution can be achieved by allowing the impure borane to age for several... 

The hydroboration of dimethyldivinylstannane can lead to stanna-cyclopentanes and -cyclopentenes through a series of intramolecular rearrangements.19 Tetravinylstannane similarly gives the diborylated spirostannane. 

Several new synthetic routes to acorane sesquiterpenoids have been published during the past year cf. Vol. 8, p. 76). In the first of these routes spiroannulation of the pyrrolidine enamine (201) derived from cyclopentene aldehyde (200) provided an enone (202) which was subsequently converted into (-)-acorenone (203) (Scheme 19). In an alternative synthetic route,Robinson annulation of the aldehyde (204) with methyl vinyl ketone provided a spiro-ketone (205) which served as a common intermediate in the synthesis of (-)-acorenone (203) and (-)-acorenone-B (211) (Scheme 20) cf. Vol. 7 pp. 63, 64). To explain the conversion of (206) into (209), it has been suggested that the intermediate 1,2-borane borate (207) undergoes elimination followed by hydroboration. 

Coupling reactions of alkyl boranes, formed by hydroboration of alkenes, with unsaturated halides (or triflates or phosphonates) is possible, and this reaction is finding increasing use in synthesis. For example, coupling of the alkyl borane derived from hydroboration (with 9-borobicyclo[3.3.1]nonane, 9-BBN) of the alkene 200 with the alkenyl iodide 201 gave the substituted cyclopentene 202, used in a synthesis of prostaglandin Ei (1.205). This type of B-alkyl Suzuki coupling reaction is very useful for the synthesis of substituted alkenes. 

The cyclopentene 7 can be prepared from 2,3,5-tri-O-benzyl-D-ara-binofuranose (sugar carbons numbered) in five steps and high overall yield (Vol. 33, p. 241 see also Org. Lett, 1999,1,1463). This intermediate has been used to prepare (—)-pentenomydn (8) in a further five steps, the first of which was a hydroboration-oxidation which occurred with 2 1 regioselectivity in the desired sense. ... 

NEW Regiochemistry of Hydroboration/Oxidation of an Acyclic Alkene, p. 394 Regio- and Stereochemistry of Hydroboration/Oxidation of a Cyclopentene, p. 394... 

The kinetics for hydroboration of aikenes are conducted in various solvents such as carbon tetrachloride, hexane, cyclohexane, benzene, and THE 9-BBN exists predominantly as the dimer (9-BBN)2 [2]. After the addition of olefins, at 25 °C, the aliquots from the reaction mixture are removed after appropriate intervals of time, quenched with an excess methanol, and analyzed by GLC for residual olefin. All operations are performed under nitrogen until identical rates are observed for more reactive olefins such as 1-hexene, 2-methyl-1-pen-tene, 3,3-dimethyl-1-butene, and cyclopentene, and variation of olefin concentration does not alter the rate. These results establish that the reaction is first order (Eq. 4.1). Typical data for cyclopentene and cyclohexene are presented in Table 4.1 [1]. 

Table 4.1 Rate data and rate constants for the hydroboration of cyclopentene (0.400 M) and cyclohexene (0.400 M) with (O-BBNy (0.200 M) in carbon tetrachloride at 25 C [ 1 ]...

The relative rates of hydroboration [7] of cyclic alkenes depend on the presence of strain [11] on the carbon-carbon double bond. The double bond of cyclopen-tene is considerably more strained than the double bond of cyclohexene, and this strain is responsible for higher reactivity of cyclopentene. 

The competition experiments [1] of a mixture of hexanal and 2-heptanone reveal the preferential reduction of aldehyde of over 85% (ktexanai/ iepatanone = 27) Cyclohexanone undergoes reduction at a faster rate than does cyclopentanone by a factor of 3.2. Cyclopentene is hydroborated with total exclusion of an ester, an acid, and an epoxide. Acid chlorides are reduced, quantitatively, without any significant attack on the esters (<2%). 

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

What product will result from hydroboration/oxidation of 1-methyl-cyclopentene with deuterated horane, BD3 Show both the stereochemistry (spatial arrangement) and the regiochemistry (orientation) of the product. 

PG synthesis during the period under review. In one of these approaches, designed to examine the feasibility of joining the two optically active forms of (82) and (83) by a mixed hydroboration-rearrangement sequence, Corey and Ravindranathan have shown in a model system that the cyclopentene (85) couples smoothly with the rearranged borane (84) from cyclohexylthexyl-borane and bromoacetylene (83 R = ThP) to produce the silylated tetra-hydropyranyl derivative (86). Application of this approach to PG synthesis is in hand. 

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