Propargylic boranes

Table 9.30 Synthesis of allenylsilanes from propargylic boranes.

The reaction of propargylic borane (R = H), however, is dramatically affected by the reaction temperature, and its reaction is carried out at -78 °C and slowly warmed to room temperature. a-Allenic alcohols are obtained as the predominant products from all the aldehydes and certain ketones (Table 6.20) [1]. 

Table 6.20 Reactions of representative aldehydes and ketones with propargylic borane [1]...

Propargylic anions have proved to be a useful source of allenes, and two further variations on the theme have been published during 1982. Thus, Zweifel et al have prepared the functionalized allenes (258) by the reaction of the propargyl-boranes or -allanes (257) with electrophiles, and Yamamoto and... 

Chiral Bromoborane Reagents. Complexes made from chiral l-alkyl-2-(diphenylhydroxymethyl)pyrrolidines and BBra are effective catalysts for asymmetric Diels-Alder reactions. Bromoboranes prepared from chiral 1,2-diphenyl-l,2-bis(arenesulfonamido)ethanes are used to prepare chiral allylic boranes, allenylic borane, propargylic boranes, and enolates. The R-bromodiazaborolidinene (1), prepared... 

However, the classical version of the ABAC, when triallyl-, trimethallyl-, or tricrotylborane are involved in the reaction with RR1CHC=CH (e.g., propargylic ethers), can be applied only for the synthesis of 1-boraadamantane derivatives 35, 3,5-dimethyl- 36 and 4,6-dimethyl-l-boraadamantanes 37 (Scheme 7). Hence, the stmctures of the final cage compounds are rigidly restricted by the stmctures of the starting allylic boranes. 

Asymmetric reduction of a,f -acetylenic ketones. This borane can be used to reduce 1-deulerio aldehydes to chiral (S)-l-deulerio primary alcohols in 90% optical yields. It also reduces a,/ -acctylcnic ketones to (R)-propargylic alcohols with enantiomeric purity of 73-100%. The ee value is increased by an increase in the size of the group attached to the carbonyl group. The value is also higher in reductions of terminal ynones. Alcohols of the opposite configuration can be obtained with the reagent prepared from (— )-a-pinene. 

Fig. 10.24. Asymmetric carbonyl group reduction with Alpine-Borane (preparation Figure 3.27 for the "parachute-like" notation of the 9-BBN part of this reagent see Figure 3.21). The hydrogen atom that is in the cis-position to the boron atom (which applies to both ft- and /T-H) and that after removal of the reducing agent leaves behind a tri- instead of a disubstituted C=C double bond (which applies to ft-, but not / -H) is transferred as a hydride equivalent. In regard to the reduction product depicted in the top row, the designation S of the configuration relates to the aryl-substituted and R to the Rtert-substituted propargylic alcohol.

The CBS reduction has also proven to be an efficient method for asymmetric reduction of a,ft-unsaturated enones14 and ynones15 (Scheme 4.31). The asymmetric reduction of alkynyl ketones affords propargylic alcohols 30 with high levels of enantioselectivity and in moderate to good yields. Optimized reaction conditions for the reduction are the use of THF at — 30° C, 2 equivalents of chiral oxazaborolidine 28b, and 5 equivalents of borane methyl sulfide complex. 

Midland and others reported that B-isopinocampheyl-9-borabicyclo[3.3.l]no-nane [Alpine-Borane (7 )-79] is an effective reagent for the highly asymmetric reduction of alkynyl ketones to afford the propargylic alcohol 8030 (Scheme 4.3z). The reagent (R)-19 is prepared from (+)-a-pinene and 9-borabicyclo[3.3.1]no-nane (9-BBN) and often represented as 19banana. The levels of asymmetric... 

In the same way, reaction of (8) with allenyl- or propargyl-stannanes affords intermediate borane derivatives which, upon reaction with aldehydes, produce the expected adducts with high selectivities (eqs 14 and 15). ... 

The first total synthesis of the neuritogenic spongean polyacetylene lembehyne A was accomplished by M. Kobayashi and co-workers. The single stereocenter of the molecule was introduced via the Midland reduction of a propargylic ketone using an Alpine-Borane , which was prepared from (+)-a-pinene and 9-BBN. 

Here is a simple example in the field of prostaglandin synthesis where 9-BBN was used on a protected optically active propargyl alcohol.12 The starting material is identical to the alkyne 78 that we reacted with Bu3SnH above and the result is the same - cis hydrometallation with the metal atom at the terminus. However that was a thermodynamically controlled stereoselective radical chain reaction while this is a kinetically controlled stereospecific electrophilic addition to give the vinyl borane -87. 

Allylic rearrangement also provides a means of interconverting propargylic and allenic boranes. ... 

Asymmetric reduction of propargyl ketones with (R) or (S) Alpine borane (B-isopinocamphenyl-9-borabicyclo, [3,3,1] nonane(A))... 

Scheme 7.5. Alpine-borane method of asymmetric reduction, (a) Preparation of Alpine-Borane . (b) Reduction of deuterio benzaldehyde [52], (c) Reduction of propargyl ketones...

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