Hydroboration acyclic alkenes

Hydroboration of alkenes is another example of a stereospecific reaction, in which different stereoisomers of the starting compound react to give different stereoisomers of the product. Problem 8-14 considers the different products formed by the hydroboration-oxidation of two acyclic diastereomers. 

BBN-H tolerates many functional groups, and this, coupled with its high regioselectivity, allows the clean synthesis of a number of functionalized organoboranes e.g. equation 26), including many derived from unsaturated heterocyclic compounds. It also shows impressive stereofacial selectivity in the hydroboration of cyclic alkenes (e.g. equations 27-29), - and sometimes in the cases of acyclic alkenes. ... 

NEW Regiochemistry of Hydroboration/Oxidation of an Acyclic Alkene, p. 394 Regio- and Stereochemistry of Hydroboration/Oxidation of a Cyclopentene, p. 394... 

Hydroboration of acyclic and cyclic aryl alkenes with (Bpin) (1.1 equiv.) in the presence of NaO Bn (1-100 mol%) and MeOH (2 equiv.) is catalysed by [CuCl(NHC)], (0.5-5 mol%) NHC = Dries, SDries and ICy, and proceeds with very good conversions and regioselectivity (Scheme 2.11). 

Asymmetric hydroboration.1 Hydroboration of phenyl-substituted trisubstituted alkenes, cyclic or acyclic, followed by oxidation results in alcohols with an optical purity of 80 100%, with the (S)-configuration at the hydroxylaled carbon predominating with reagent prepared from (+)-oc-pinene. 

Acyclic diastereoselective hydroboration,J Hydroboration-oxidation of terminal alkenes substituted at C4 by a large and a medium alkyl group can proceed asymmetrically with BH3 S(CH,)2 or, even more selectively, with thexylborane. Example ... 

An equivalent reaction has been achieved via the treatment of hydroborated bisalkenes with alkaline silver nitrate solution (Table 1.4).22,23 This method has been used to synthesize a number of small and medium-size carbocyclic rings in moderate to good yield. The selectivity for terminal cyclization observed for 1,6-heptadiene and 1,7-octadiene indicates that, in these cases, hydroboration of each of the alkenes occurs independently to yield acyclic boranes. It has, however, been found that both cyclic and acyclic boranes react under these conditions to yield the ring-closed products (Scheme 1.3). 

Hydroboration of isoprene with catecholborane at 1 1 molar ratio in tetrahydrofliran in the presence of 1 molar % of NiCbCdppe) at room temperature was completed in 6 h. An interesting observation is a lower reactivity of 1-decene compared with isoprene under these conditions. It is the first case of higher reactivity of 1,3-diene than 1-alkene in the hydroboration reaction. It is also an indirect indication that borane is not involved in the reaction, since acyclic conjugated dienes are less reactive than 1-alkenes toward borane. Hydroboration of representative conjugated dienes in the presence of NiCUCdppe) was carried out on a preparative scale and the results are shown in Table 2.9 The mono-hydroboration products were isolated by distillation and oxidized to the corresponding unsaturated alcohols. 

With acyclic asymmetric compounds high levels of diastereoselection are achieved in the hydroboration/oxidation of those alkenes with preexisting allylic stereogenic centers. 

To achieve diastereoselectivity in electrophilic additions to a double bond in acyclic compounds, there must be a facial preference for attack. An A strain provides such an element for conformational control, as exemplified by hydroboration of the alkene shown below.The hydration of a double bond via hydroboration involves (1) anti-Markovnikov addition of the B-H bond, (2) cis addition of the B-H bond, (3) addition of the B-H bond from the less hindered side of the double bond, and (4) oxidation with retention of configuration. 

Two equivalents 1-alkyne with thexylborane produces the thexyldialkenylboranes . Thexylalkylalkenylboranes can be obtained either by sequential hydroboration of an alkene-alkyne pair with thexylborane [Eq. (ct)] or via hydroboration of an alkene with thexylchloroborane, followed by reduction and hydroboration of an alkyne [Eq. (cw)]. Cyclic trialkylboranes are synthesized via hydroboration. Thus, HjB-THF reacts 2 3 with simple acyclic dienes to give predominantly the dumbbell-shaped organoboranes. When heated, they undergo isomerization to the more thermodynamically stable isomers. These empirical rules predict their thermal behavior (a) the thermal... 

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... 

Alkane synthesis via coupling of organoboranes was discovered very early during the development of organoborane chemistry. The procedure is simple and involves the hydroboration of an alkene followed by treatment with basic silver nitrate (equations 108-110). Both acyclic and cyclic alkanes can be prepared through this procedure. 

This general procedure for the synthesis of alkenes from the enamines, when applied to the acyclic enamines derived from the acyclic ketones by modification of hydroboration-elimination procedure, permits a facile, diastereospecific conversion to either (Z)- or (E)-alkene at will (A) The hydroboration of enamine with 9-BBN, followed by treatment with methanol gives (Z)-alkenes of 99% isomeric purity. (B) The hydroboration of the same enamine with borane-methylsulfide, followed by methanolysis and oxidation with neutral hydrogen peroxide gives ( )-alkenes of 99% isomeric purity (Scheme 24.2) [13a]. 

Asymmetric hydroboration of internal olefins was developed by Perez, Fernandez and co-workers,with [B(pin)]2 as boration reagent. Ligands 126 provided the best chiral induction (up to 59% ee) (Scheme 13.6). Improved results were obtained by Hoveyda and co-workers using 127 and 128 with acyclic and cyclic internal alkenes. 

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