Allylic catalyzed hydroboration

Catalyzed hydroboration has proven to be valuable in controlling the stereoselectivity of hydroboration of functionalized alkenes.169 For example, allylic alcohols... 

Hydroboration of alkenes1 (15, 91). The Rh(I)-catalyzed hydroboration provides a highly diastereoselective reaction in a synthesis of a poly ether antibiotic. Thus the derivative (1) of an acyclic allylic alcohol is converted to the primary alcohol 2 by hydroboration with catecholborane (CB) catalyzed by ClRh[P(C6H5)3]3 with 94 6 selectivity. Note that hydroboration of 1 with disiamylborane (12, 484) proceeds with the opposite selectivity at Cio (8 92). 

The palladium-catalyzed hydroboration of allyl phenyl ether 456 is followed by cyclization of the corresponding triflate to afford the chroman core of the tocopheryls 457 (Equation 186) <1998JA9074>. The intramolecular hydroarylation of l-(but-3-enyloxy)-3,5-dimethylbenzene to afford 4,5,7-trimethylchroman can be accomplished using a RuCh/AgOTf catalytic system (Equation 187) <20040L581>. 2,2-Dimethylchromans 458 are formed by a Mo(CO)6 catalyzed intramolecular cyclization of aryl prenyl ethers 459 (Equation 188) <1998S256>. 

The hydroboration of allylic amine or alcohol derivatives can be used for the preparation of alkylzinc reagents with excellent diastereoselectivity (Equation (34)). Rhodium-catalyzed hydroborations are also compatible... 

In metal-catalyzed hydroborations, the addition is controlled by the catalyst and may result in different regio- and stereoselectivity compared to the uncatalyzed process. This has been observed for additions to styrenes30, allylic alcohols, and amines31,32. However, these directive effects are not yet fully understood33. 

Very recently, three alternatives to the /-selective hydrosilylation-oxidation of 6 and its analogs have been reported. Two of them involve catalyzed hydroboration occurring with moderate to good selectivity11,12, and the other involves conversion to a vinyl allyl ether which then undergoes a double (in ter-intramolecular) hydroboration in which the intramolecular step is highly stereoselective13. These methods are discussed in Section D.4.2.2. 

Diastereoselective rhodium-catalyzed hydroborations of allylic alcohol derivatives give results complementary to those observed in the uncatalyzed reaction with 9-Bombicyclo[3.3.1]nonane. The syn selectivity of the catalyzed reaction increases as the bulk of the R group increases (syn anri = 79 21 for R = TBDPS) (eq 9) ... 

Pyridine and related aromatic (quinoline, quinazoline) P,N derivatives (11, 12) have been created for Rh-catalyzed hydroboration-oxidation [44] or -amination [45]. Other pyridine-related auxiliaries have been synthesized for Pd-assisted allylic alkylation [46] in test conditions furnishing the substitution product in up to 93 % ee. The QUIPHOS ligand 13 has been tested in Pd-assisted allylic amination (up to 94 % ee) [47], allylic alkylation of -ketoesters (up to 95 % ee) [48], and Cu-catalyzed Diels-Alder reaction between an acryloyl derivative and cyclopentadiene [49]. 

Stereoselectivity in the hydroboration of allylic cyclohexenyl derivatives is presented in Table 1. Electronegative substituents direct the boron atom to the adjacent tram-2-position. The stereochemical course of rhodium-catalyzed hydroboration of these derivatives with 1,3,2-benzodiox-aborole is also governed by steric factors, however, the regioselectivity is such that the boron atom is placed in the fra .v-3-position. Phosphinites, which complex with the hydroborating agent, direct the boron atom to the cts-2-position23. 

Catalyzed hydroboration. Hydroboration by CB can be catalyzed by a number of Rh(l) and lr(l) catalysts, particularly RhCl[P(C6H5),)2 and lr(cod)(PCy.iXpy]PF<,. Advantages arc that selective hydroboration owing to steric factors can be improved and the regioselectivity can be enhanced. Moreover, the Rh(l)-catalyzcd hydroboration of acyclic and cyclic allylic alcohols proceeds with high diastcrcosclcctivity, opposite to that observed with 9-BBN (equations I and II). 

Substrate-controlled diastereoselective hydroboration of protected chiral allylic alcohols [25-27] or amines [28, 29] with 9-BBN gives almost always anti selective products. On the other hand, catalyzed hydroboration in most of the cases using catecholborane as hydroborating agent tends to be syn selective [28-30] (Eq. 5.9). 

Fig. 5.2 Reactive conformations in catalyzed and 9-BBN by hydroboration of N-tosyl-protected allylic amine derivatives, a Preferential orientation in catalyzed hydroboration of n-tosyl-protected allylic amine derivatives, b Preferential orientation in 9-BBN hydroboration of N-tosyl-protected allylic amine substrate (R = i-Pr)...

Burgess and Ohlmeyer [30] have reported that electronic effects are important in catalyzed hydroboration, e.g., allylic acetates are hydroborated with less syn selectivity than allylic trifluoroacetate is [25], and proposed the general model [A] (Fig 5.4) for catalyzed hydroboration of chiral allylic alcohols. The model predicts that the OCOCF, substituent (good a acceptors) will preferentially orientate anti to the approaching rhodium complex. The largest of the other two substituents on the chiral center will occupy the outside position, and the smallest will reside in the inside (crowded) site and thus, syn selectivity will result... 

Similarly, Evans et al [36] have also found that several classes of allylic alcohols on rhodium-catalyzed hydroboration afford allylic alcohols with high dia-stereoselectivity-sy product and isomer complementing to that furnished by uncatalyzed variant of the reaction (9-BBN)—the anti product. 

The proposed model [B] implies that syn selection in catalyzed hydroboration should decrease as the o-accepting character of the anti substituent decreases. This explains the higher syn selectivity in catalyzed hydroboration of allylic trifluoroacetates as compared with reactions of allylic acetates and carbamates [25]. The less diastereoselectivity of cationic complexes in hydroboration of chiral allylic systems than in the neutral catalyst systems [29, 30] is rationalized, as the latter have more electron density to shed via back-bonding. The anti selectivity in catalyzed hydroboration of the allylic acetate and trifluoroacetate is rationalized in terms of competition between the phenyl and acetate groups for the role of a acceptor. The further evidence is obtained in the case of pentafluo-... 

The enantioselective synthesis of axially chiral P—N ligands was also accomplished by rhodium-catalyzed [2 + 2+-2] cycloaddition. The reactions of 1,6-diynes 75 with diphenylphosphinoyl-substituted isoquinolinyl acetylenes 76 furnished diphenylphosphinoyl-substituted axially chiral 1-arylisoquinolines 77 with high yields and ee values (Scheme 9.28) [23], The new diphenylphosphinoyl-substituted axially chiral 1-arylisoquinoline 77 (Z = NTs, R = Me) was derivatized to the corresponding axially chiral P—N ligand 78 and isoquinoline A-oxide 79 without racemization, which could be used in the rhodium-catalyzed hydroboration and Lewis base-catalyzed allylation, respectively [23],... 

Ring expansion with Tamura et al. s Beckmann reagent, followed by dechlorination with Zn-Cu couple in methanol saturated with ammonium chloride, provided key pyrrolidinone intermediate 197 in 72% overall yield from 193. The intermediate 197 was then converted by using selenium dioxide and tcrt-butyl hydroperoxide into allylic alcohol (62%), which yielded the desired 1,3-diol 198 through rhodium-catalyzed hydroboration and oxidation as a 1 1 mixture of diastereomers in 72% yield. A seven-step reaction sequence then converted the diol 198 into (-l-)-retronecine 199, which was indistinguishable from an authentic sample of the natural product obtained by hydrolysis of natural monocrotaline (Scheme 16.29). ... 

Kloetzing RJ, Lotz M, Knochel P (2003) New P,N-ferrocenyl ligands for rhodium-catalyzed hydroboration and palladium-catalyzed allylic alkylation. Tetrahedron Asym 14 255-264... 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

A review8 with more than 186 references discusses the synthesis of Rh and Pd complexes with optically active P,N-bidentate ligands and their applications in homogeneous asymmetric catalysis. The effect of the nature of the P,N-bidentate compounds on the structure of the metal complexes and on enantioselectivity in catalysis was examined. Allylic substitution, cross-coup-ling, hydroboration and hydrosilylation catalyzed by Rh or Pd complexes with optically active P,N-bidentate ligands are considered. 

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