Crotyl-9-BBN

To a solution of 0.18 g (1 mmol) of /V-benzylideneaniline in F.t,0 is added 0.194 g (1.1 mmol) of 9-(2-butenyl)-9-borabicyclo[3.3.1]nonane (crotyl-9-BBN) at — 78 °C. The reaction is quenched at O C with several drops of coned [ICl. The mixture is stirred overnight at r.t.. and a 3 N aq soln of NaOH is added at 0°C to make the solution basic. The mixture is extracted twice with ht20. dried, condensed, and filtered through a short column of silica gel (hexane/Et20 10 1) to remove the 9-BBN residue. 

The addition of crotyl-9-BBN to the chiral imino ester 2 provides a syrc-selective synthesis of optically active amino esters.1... 

The equilibrium position of the allylic rearrangement depends on the steric requirements of the a- and y-carbons. Thus, e.g., the H-NMR spectra of tricrotylborane " (XXIII) and B-isoprenyl-9-BBN (XXIV) do not reveal the presence of their regioisomers XXV and XXVI. These observations, however, do not preclude allylic rearrangement of XXIII and XXIV. Variable-T NMR examination of B-crotyl-9-BBN and XXIV indicates that the cis-trans isomerization arising via allylic rearrangement is rapid on that time scale at 40 and 140°C, respect vely ... 

Allylboron compounds have been the most widely studied of the type 1 allyl organometallics. Of these, the dialkylcrotylboranes isomerize most readily and often require handling at temperatures below -78 °C for isomerization to be suppressed. Thus, it is probably more appropriate to view such compounds ie.g. crotyl-9-BBN) as type III crotyl organometallics. The boratropic isomerization of dialkylcrotylboranes, however, is sensitive to steric factors. For example, the ( )- and... 

The reactions of crotyl-9-BBN (47) and pyruvate esters have been studied. As was observed in reactions with glyoxylates, stereoselectivity depends on the steric bulk of the ester group (Scheme 10). Interestingly, the stereochemistry of the major product (54) suggests that the CO2R substituent adopts an equatorial position in the cyclic transition state like (20). 

Relatively few studies of the reactions of allylboronates and ketones have appeared. The reaction of (85) and ethyl pyruvate, for example, was conducted under 6 kbar pressure at 45 C for 80 h to give a 9 1 mixture of diastereomers (86a) and (86b). The stereochemistry of this reaction parallels that seen with crotyl-9 BBN (Figure 10) in that the structure of the major isomer is consistent with a transition state in which the —C02Et unit adopts an equatorial position. The same result could occur, however, via a boatlike transition state with an axial —C02Et group. 

Yamamoto and coworkers have studied the reactions of crotyl-9-BBN (47) and achiral aldimines (Scheme 15). These reactions occur at much lower temperature than those involving crotylboronates because of the greater reactivity of (47). No clear stereochemical pattern, however, is apparent in the data. Assuming that (47) reacts preferentially as the ( )-crotyl isomer, one would expect anti dia-stereomer (93) to be the major product via transition state (95a Scheme 16) by analogy to Hoffmann s results with oximes and oxime ethers (Schemes 8 and 14). Only in entries 1, 6, 7 and 8 of Scheme 15, however, is this stereochemical result realized. Yamamoto argues that syn diastereomer (92), the major... 

In the final analysis, it appears that the diastereoselectivity of the reactions of imines and crotylboranes (47, 96) depends on the relative rates of crotyl transfer e.g. 95a —> 93) versus imine isomerization that leads to competitive pathways (e.g. 94d - 92). When R is an aryl group, the rate of crotyl transfer is probably faster than competitive imine isomerization. When R is an alkyl group, however, the relative rates are probably inverted. The driving force for imine isomerization is probably that complex (95a) is not very stable owing to the bulky 9-BBN unit positioned syn to R, while the complex of crotyl-9-BBN (47) and a (Z)-aldimine e.g. 94d) is probably much more stable. It is conceivable, therefore, that the overall rate of reaction via (94d) can be much faster than via (95a) even though the (Z)-imine cannot be detected in solution. Additional research is clearly needed to clarify the stereochemical course of these reactions. 

Insight into the structure of several allyl/crotyl organometallic reagents has been obtained spectroscopically. Kramer and Brown have examined the NMR spectra of 3-allyl-9-BBN and 3-crotyl-9-BBN... 

Figure 3 Equilibrium between cis (a) and trans (c) P-crotyl-9-BBN involving the intermediacy of the p-methallyl...

Some examples of combined syn-anti and diastereofacial selectivity employing /V-n-propyl- and N-isopropyl-aldimines, derived from a-phenylpropionaldehyde, and crotyl-9-BBN, -magnesium and -zirconium reagents have been reported by Yamamoto et al. Cram selectivity, which is observed in the analogous reactions of these chiral imines with allyl organometallics (see Section 4.3.2.1.2i), is preserved as ratios of Cram anti-Cram products are consistently about 8 1. Anti selectivity is also observed but the ratios do not exceed 7 3. The weak anti selectivity parallels that observed in reactions of crotyl-9-BBN with branched a-alkylaldimines. Since syn-anti selectivity is influenced more by the a-substituent than by the A-substituent of the aldimine, more synthetically useful levels of combined syn-anti and diastereofacial selectivity might be expected in other series of a-substituted aldimines. 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

Similarly, B-crotyl-9-BBN reacts with 9-BBN in less than 30 min at room temperature and affords after oxidation 1,3-butanediol (95%), a small amount of 1,2-isomer (3%), and traces of 2,3- and 1,4-diols (Eq. 5.37) [7]. 

Allylboration of aldehydes and ketones with B-crotyl-9-BBN affords only the rearranged product (Eq. 6.8 Table 6.7) [1]. Monomeric formaldehyde gives 2-methyl-3-buten-1 -ol. 

Table 6.7 Yields of homoallylic alcohol from B-crotyl-9-BBN [ 1 ] ...

The utility of a-silyl- and a-stannyl-substituted crotyl-9-BBN [4] has been extended to realize the stereoregulated synthesis of four [8a] carbon units (Table 6.26). Consequently, the reaction of a-silyl- or -stannyl-substituted cro-tyl-9-BBN with aldehydes in the presence of certain bases (such as pyridine, n-butyllithium, or sec-butyllithium) provides the high regulation of the stereochemistries over four consecutive acylic carbon atoms the threo relation between C-1 and C-2 and the ds-configuration at C-3 and C-4 (Scheme 16.28) [8aj. 

It is significant to mention that use of base is essential to realize the high stereoregulation. The base probably forms the ate complex and influences the reactivity and selectivity of crotyl-9-BBN derivatives. n-Butyl or sec-butyl bases, sometimes, cause the migration of the butyl groups. 

Table 6.26 Stereoregulated reaction of a-silyl or a-stannyl substituted crotyl-9-BBN with aide-hydes [8a]...

Yamamoto and coworkers [8b] have reported the diastereoselective C-C bond formation for the synthesis of tertiary alcohols. The reaction of B-crotyl-9-BBN with pyruvates produces the threo (anti) isomer as the major product. By increasing the steric bulk of the ester groups of pyruvate, the threo (anti) isomer is obtained predominantly or exclusively (Eq. 6.22 Table 6.27) [8b]. On the other hand, reaction of allenic organometallics gives mainly the erythro (syn) isomer. 

The reaction is extended with crotyl-9-BBN [19], The chiral iminoester reacts with crotyl-9-BBN (Eq. 9.7) in essentially quantitative yield to give Cram-erythro amino acid ester. 

---