Boranes borinic acid esters

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Trimethylamine oxide Borinic acid esters from boranes... 

Borines s. Boranes, Phosphite borines. Phosphorous triamide borines Borinic acid esters... 

A mixture of triisobutylborane, o-tolyl borate, and borane in tetrahydrofuran stirred 2 hrs. at 100° under Ng, cooled, pentane added followed by 1-pentene and LiAlH4 in ether at 0°, stirred 0.5 hr. at this temp., then ice-cold NaOH-soln. added diisobutyl-n-pentylborane. Y 85%. Also isolation of the intermediate o-tolyl diisobutylborinate (Y 84%), and f. borinic acid esters as well as borinic acids s. H. C. Brown and S. K. Gupta, Am. Soc. 95, 2802 (1971). 

Isolation of free boronic acids using an aqueous work up may lead to low yields, especially for small or polar ones, which tend to be water-soluble even at a low pH (Section 1.4). In such cases, it is often better to isolate the desired boronic acid as an ester. In an improved procedure that does not involve an aqueous work-up. Brown and Cole reported that the reaction of several types of organolithium intermediates with triisopropylborate was very effective for the synthesis of arylboronic esters [180]. To help minimize the possible formation of borinic acids and boranes by multiple displacements (i.e.. Equation 28 in Figure 1.19), the reaction protocol involves the slow addition of the organolithium to a solution of triisopropylborate in diethyl ether cooled to -78 °C. The use of smaller borate esters such as trimethylborate gave large proportions of multiple addition products (i.e., borinic acid and borane). With triiso-... 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

The reaction undergone by alcohols with trialkyl- and triaryl-boranes in the presence of pivalic acid, to give borinic esters, and the thermal cyclization of bis(dialkylborinates) to boronates, are discussed briefly in Section II. Many borinates have been prepared in quantitative yield from mono-, di-, oligo-, and poly-saccharides,338,108 and mixed... 

Non-fluxional vinylic boranes do not react with carbonyl compounds. Nevertheless, the vinylic borane 29 reacts with acetone (2 h under reflux) yielding the Z-isomer of the homoallylic borinic ester 30b. The cw-configuration of the reaction product 30b corresponds to the following sequence of transformations (Scheme 2.11). The [1,7]-H shift in the vinylic borane 29 gives the allylic Z, Z-isomer 28d which immediately reacts with acetone before the equilibrium among the allylic isomers is established. On the other hand, cyclopentanone reacts directly with 29 under mild conditions yielding Z, Z-1,3,5-heptatriene 31 and borinic ester 32 (Scheme 2.12). Apparently 29 reacts with the enol form of cyclopentanone, and a direct splitting of the B-Q ,2 bond takes place. Similar reaction of 29 with acetic acid was used for the preparative synthesis of previously unknown hydrocarbon 31 (Scheme 2.12) [35]. 

---