B-allenyl-9-BBN

Table 9.16 Synthesis of homopropargylic alcohols from B-allenyl-9-BBN.

B-Allenyl-9-BBN has also been shown to react cleanly and efficiently with other electrophiles [27]. Not surprisingly, aldehydes show the highest reactivity. In a competition experiment between benzaldehyde and acetophenone at -78 °C, the aldehyde adduct predominated by more than 30 l(Eq. 9.22). Competition experiments with other carbonyl compounds showed a similar bias for aldehyde adducts. 

Acid chlorides were also shown to be reactive electrophiles. B-Allenyl-9-BBN affords tertiary bis-homopropargylic alcohols in satisfactory yields upon reaction with acetyl or benzoyl chloride (Eq. 9.23). 

A tertiary homopropargylic alcohol could also be prepared by treatment of ethyl acetate with two equivalents of B-allenyl-9-BBN. However, the reaction proceeded slowly and was not general for other esters, which proved to be unreactive, as were tertiary amides and alkyl halides. However, homopropargylic amines could be prepared in high yield and with minimal allenic byproduct through allenylboration of imines with B-allenyl-9-BBN (Eq. 9.24). 

Brown and coworkers [3,4] discovered that B-allenyl-9-BBN [3] readily prepared from B-chloro-9-BBN [5] and allenylmagnesium bromide [6] (Eq. 6.13) undergoes a facile condensation with aldehydes and ketones to yield exclusively the corresponding homopropargylic alcohols in excellent yields [3, 4]. B-Alle-... 

The reagent B-allenyl-9-BBN is highly stable and can be distilled and stored either in neat condition or as a 1 -M solution in hexane at 0 °C under an atmosphere of nitrogen, and no detectable change is observed even after 1 month [4]. Allenylboration of aldehydes or ketones yields the corresponding borinate ester, which under the usual alkaline hydrogen peroxide oxidation conditions afibrds homopropargylic alcohols in excellent yields (Scheme 6.11. Table 6.14) [3,4]. 

Table 6.14 Allenylboration of representative carbonyl compounds with B-allenyl-9-BBN at 25 °C in EtjO [3]...

Unlike trialkylboranes, B-allenyl-9-BBN reacts with a,(3-unsaturated carbonyl compounds in 1,2-fashion to give, exclusively, the olefinic homopropargylic alcohols (Table 6.14) [3]. 

Both acetyl and benzoyl chlorides react with 2 equiv of B-allenyl-9-BBN (Scheme 6.12) in ether to afford the corresponding tertiary homopropargylic alcohols cleanly and in excellent yields [3] and without the contamination of allenic side products. 

Carbonyl compound Temp ( O (reaction time, h) Product with B-allenyl-9-BBN (%) Product with allenyl magnesium bromide (%)... 

In addition, its high regioselective allenylboration toward diethylketone [A] and 4-f-butylmethylketone [B] as compared with the behavior of allenylmagne-sium bromide and di( -butyl)allenylboronate makes B-allenyl-9-BBN, a valuable reagent [3]. 

Table 6.16 Comparison of the allenylboration of [A] and [B] with allenylmagnesium bromide, di(n-butyl)-allenylboronate and B-allenyl-9-BBN [4]...

Brown and coworkers [11] have demonstrated that B-allenyl-9-BBN reacts vigorously with imines (Eq. 12.5) at room temperature and affords the correspon -ing homopropargylic amines in excellent yields along with a smaU amount o (2-4%) of the allenic amines (Table 12.3) [11]. 

Wang et al [2] have reported that treatment of the readily available 3-(fert-butyl-dimethylsilyl)-l-(trimethylsilyl)-l-propyne [4] with n-butyllithium, followed by B-MeO-9-BBN and 4/3 BFj-OEtj [5] yields B-allenyl-9-BBN. The condensation of B-allenyl-9-BBN with hexanal or benzaldehyde and followed by the elimination step of the Peterson olefination [3] affords the conjugated enynes (Scheme 24.15) [2]. 

Allenyls-9-BBN required for the synthesis of these unsaturated compounds are obtained by the following reaction sequence. Allenylsilanes [6] are lithi-ated with tert-butyllithium [7], followed by treatment with B-OMe-9-BBN and... 

Insertion Reactions. Trimethylsilyldiazomethane undergoes net insertion between the B-C bond of borinate and boronate esters. Thus, olefin hydroboration, followed by treatment with TMSCHN2, oxidation, and desilylation offers a method for hydroxymethylation of alkenes in fair to moderate yield (eq 54). Stable, chiral allenylboranes (IR and IS) are prepared by reaction of TMSCHN2 with 5-MeO-9-BBN followed by resolution with pseudoephedrine and reaction with allenyl magnesium bromide (eq 55). Compounds IR and IS are useful for the asymmetric allenylboration of aldehydes with predictable absolute stereochemistry. The precursors to 1 are then readily recovered during work-up. ... 

The 10-TMS-9-BBD system (1) is readily prepared frtmi many 9-BBN derivatives through the clean insertion of the stable, commercially available reagent, TMSC1B42. Resolution of the chiral bicyclic system is efficiently accomplished (40%) through its pseudoephedrine complex (8) vidiich is readily converted to the B-H (3), B-allyl (9) ot B-allenyl- 10-TMS-9-BBD(13) derivatives. These new reagents have been demonstrated to function as highly useful... 

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