L-alkenyl boranes

The stereoselective synthesis of the title compounds has been achieved. Thexylborane on reaction with 2 molar equivalents of 1-iodo-l-alkyne at 0 °C proceeds to near completion (88% for 1-iodo-l-hexyne) to form fully substituted organoborane (33), which upon treatment with 2 molar equivalents of sodium methoxide at 0 °C readily produces frunx-1,2,3-butatrienes (Eq. 102) The same reaction, however, with either 1-chloro or 1-bromo-l-alkynes is sluggish to form the thexyl-l-halo-l-alkenyl-borane . 

J3-[(Z)-l-Bromo-l-alkenyl]borane derivatives offer potential for the conversion of 1-bromo-alkynes into ketones. Treatment of the adducts from the acetylene and dialkylboranes or alkylbromoboranes with sodium methoxide gives alkenyl-borinate or -boronate esters, either of which may be oxidized to the ketone (Scheme 14). When using dialkylboranes, thexylalkylboranes may be used with advantage. ... 

Trimethylamine-tcrt-butylborane has also been used successfully in reactions with terminal alkynes (161) and nitriles (162), affording tert-butyldi(l-alkenyl)boranes and dimeric alkylideneamine-Icrt-butylboranes (XV), respectively. 

Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The... 

The mildness of these reagents tolerates the presence of various functional groups such as ester, ether, halogen, and nitrile. The stereospecific cis nature of hydroboration gives exclusively the tram alkenylboranes, often also in high regioisomeric purity (Eq. 53). On the other hand, highly pure (Z)-l-alkenyl-dialkylboranes are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamyl-borane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 55)106). 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of l-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

ALLYLIC ALCOHOLS B-l-Alkenyl-9-borabicyclo[3.3.1 nonanes. Benzene-selenenic acid. Borane-Dimethyl sulfide. 

A stereospecific synthesis of B-( )-l-alkenyl-9-BBN derivatives that are not available by hydroboration reactions has been developed. The 9-BBN dimer has featured in thermal isomerization and thermolysis studies and similar studies on dimesityl-3-hexyl-borane show that, in comparison to the diphenyl compound, it is quite resistant to thermal isomerization and an explanation is offered. ... 

B-Alkenyl-9-borabicyclo(3,3,l)nonane (B-alkenyl-9-BBN) adds across the carbonyl group of a simple aldehyde to afford in good yields, the corresponding allylic alcohol S2). The reaction proceeds with complete retention of vinylic borane stereochemistry resulting in the /raw-disubstituted olefin linkage in the final product. The reac-... 

The trialkylborane is derived from monoalkyl-(l,l,2-trimethylpropyl)borane by hydroboration. Care must be taken in the oxidation step leading to the a-chiral ketones and buffering is necessary to avoid racemization. However, even with these precautions, 2-6% racemization is observed in some cases9,20. The intermediate alkenyl-(l,l,2-trimethylpropyl)borinates are relatively resistant to oxidation thus prolonged reaction times or higher temperatures are sometimes required to complete the reaction via the haloalkyne route. 

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