Hydroboration cycloalkenes

The following cycloalkene gives a mixture of two alcohols on hydroboration loi-lowed by oxidation. Draw the structures of both, and explain the result. 

This route can be used to prepare enantiomerically enriched organozinc reagents by asymmetric hydroboration (see Section 4.5.3), followed by exchange with diisopropyl-zinc. Trisubstituted cycloalkenes such as 2-methyl or 2-phenylcyclohexene give an enantiomeric purity greater than 95%. The exchange reaction takes place with retention of configuration.137... 

Z-151, and hydroboration/protonation affords -cycloalkene. In 2002, Trost reported an alternative procedure for the synthesis of the -olefin from an alkyne by hydrosihlation... 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

Methylenecycloalkanes.1 A cycloalkene can be converted into a methylene-cycloalkane by the following steps hydroboration with 9-BBN and carbonylation-reduction2 to give a /i-(cycloalkylmethyl)-9-BBN derivative, which is then allowed to react with an aldehyde1 to form the final product. 

A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

Since the configuration at the migrating carbon is retained, the simple and substrate-induced diastereoselectivity of the overall amination reaction of substituted cycloalkenes is that observed in the hydroboration step (Table 1). 

For tetrasubstituted alkenes and cycloalkenes, hydroboration to the trialkylborane stage is difficult. In these cases, it is possible to produce cleanly the monoalkylboranes RBHo. 

Z-2-Butene undergoes hydroboration with 98% enantioselectivity with (Ipc)2BH. Other Z-disubstituted alkenes give good enantioselectivity (75-90%) but f-alkenes and simple cycloalkenes give low enantioselectivity (5-30%). ... 

Because only syn addition occurs, hydroboration-oxidation is stereoselective—only two of the four possible stereoisomers are formed. As we saw when we looked at the addition of H2 to a cycloalkene, syn addition results in the formation of only the pair of enantiomers that has the added groups on the same side of the ring. 

Hydroboration of cyclic trimethylsilyl eno ethers synthesis of cycloalkenes. Cycloalkanones can be converted into the corresponding cycloalkenes by conversion to the trimethylsilyl enol ether, hydroboration, and treatment with acid, usually aqueous HCl. The sequence is illustrated for cyclohexanone (equation I). 

Hydroboration. Hydroborations with this reagent have been reviewed Brown concludes that the regio- and stereoselectivity of 9-BBN surpass those of other hydroboration reagents. An example of the marked stereospecificity of hydroboration with 9-BBN is the reduction of 1-substituted cycloalkenes to trc/i5-2-methylcycloalkanols (equation I). The same selectivity is shown in the hydroboration of 1,3-dimethylcycloalkenes (equation II). In fact this method is... 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

Boron.—Preparation of Organoboranes. The year 1977 was one of consolidation in hydroboration. Detailed investigations of the reactions of 9-borabicy-clo[3,3,l]nonane (9-BBN) with cycloalkenes and non-conjugated dienes were reported in full. The reagent shows a remarkable degree of positional and stereochemical selectivity during hydroboration. Details of the applications of borane-dimethyl sulphide have appeared, and hydroborations using dimethyl... 

The relative reactivities of different alkenes towards 9-borabicyclo[3,3,l]nonane (9-BBN) and di-isoamylborane have been studied. Di-isoamylborane is the more selective for cycloalkenes, the relative rates of hydroboration of cyclop>entene, cyclo-heptene, and cis-cyclo-octene being 1.0 18.6 68.6, respectively. Hydroboration of substituted cyclo-octenes using diborane is complicated by rapid isomerization of the initially formed alkyl-borane, leading to mixtures of alcohols on oxidation. However, 9-alkyl-9-borabicyclo[3,3,l]nonanes are thermally stable, and hydroboronation of 1-alkylcyclo-octenes using 9-BBN leads sp>ecifically to trans-2-alkylcyclo-octanols after oxidation. ... 

Hydroboration-oxidation of a cycloalkene is both regioselective and stereoselective, but it is not enantioselective (both enantiomers are produced in equal amounts). 

The hydroboration of 3-alkyl cycloalkenes with diborane or disiamylborane form significant amounts of the ds-1,2-isomer. However, it is important to note that none of this isomer is produced when 3-alkyl cycloalkenes are hydroborated [17] with 9-BBN. This amazing selectivity shown by 9-BBN with 1-methyl and 3-methylcycloalkenes gives, exclusively, the fra s-dimethylcydoalkyl-9-BBN derivative [18]. It is evident from 1,3-dimethylcyclopentene, which is exclusively converted into f-2,f-5-dimethylcyclopentanol (Eq. 5.6). 

The reaction is also general procedure for the conversion of cyclic ketones through the hydroboration-elimination sequence (Eq. 24.6) to corresponding cycloalkenes [13]. 

A combination of hydroboration and intramolecular cross-coupling provides a convenient method for the synthesis of five- and six-membered cycloalkenes and benzo-fused cycloalkanes and pyran derivatives (Scheme 31.1) [2]. 

The sulphur and nitrogen heterocycles (80) and (81) have been prepared by hydroboration-carbonylation of bis-olefinic precursors, and the latter case is reported to be the first example of such a synthesis in the presence of a reducible functional group. Another hydroboration-carbonylation sequence has been used in the preparation of methylene cycloalkanes from cycloalkenes, and is shown in Scheme 11. ... 

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