Stereocontrol in Catalyzed and Uncatalyzed Hydroborations

11 Stereocontrol in Catalyzed and Uncatalyzed Hydroborations Kevin Burgess and Michael J. Ohlmeyer

Downloaded by YORK UNIV on November 8, 2012 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch011

Chemistry Department, Rice University, Houston, T X 77251

Catalyzed hydroborations are potentially valuable in organic syntheses. This value is illustrated with examples of enantioselective hydroboration of prochiral alkenes with catecholborane in the presence of optically active rhodium complexes, and with stereocomplementarity in catalyzed and uncatalyzed hydroborations of chiral alkenes. Stereocomplementarity is rationalized in terms of secondary orbital effects that perturb incipient Dewar-Chatt bonding. This theory may have repercussions in other areas besides hydroboration chemistry.

IVi

A N Y B O R O N H Y D R I D E C O M P O U N D S A D D T O A L K E N E S under mild con-

ditions without intervention of catalysts; others do not. For instance, hydroboration of alkenes and alkynes by boron hydride cluster molecules (1-7) and by borazine (8) occurs at convenient rates only in the presence of various transition metal complexes. Reactions of boron hydride clusters and borazine are of little consequence to practical organic chemists, but Noth's discovery (9) that hydroborations involving catecholborane could be catalyzed exposed a range of possibilities. Hydroboration of alkenes and alkynes with unreactive boranes via transition-metal-mediated reactions are potentially valuable when one is restricted by the chemo-, regio-, or stereoselectivity of conventional hydroborations. Consequently, we (10-14) and others (15-18) became interested in exploiting catalyzed transition metal reactions in organic synthesis. This chapter describes efforts to control absolute and relative stereochemistry via catalyzed hydroborations. It focuses on features that complement conventional methodology.

0065-2393/92/0230-0163$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

164

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

Enantioselective Hydroborations Previously, all methods for hydroboration of prochiral alkenes with control of absolute stereochemistry relied upon reagent-controlled diastereoselectivity (19-26) (i.e., reactions of optically active boron hydrides) to give diastereomers in unequal amounts (Scheme I).

uncatalyzed reaction


Me Y ^ Me

B

L

#

2

+

R

V^BL Me

#

2

0 x j d a t j 0 n

-

ratio of these diastereomers represents the diastereoselectivity oi the reaction

Me

Me

ratio of these enantiomers gives the enantiomeric excess oi the product Scheme I. Uncatalyzed hydroboration of prochiral alkenes.

Several disadvantages are associated with such processes: • optically active boranes are air- and water-sensitive; they must be prepared and isolated before use; • optically active boranes also tend to be high-molecular-mass compounds, so relatively large quantities must be synthesized to hydroborate alkenes of low molecular mass; • diastereoselectivities are variable; and • separation of products from chiral auxiliary residues is problematic. A conceptually superior approach to this problem is the use of optically active transition metal catalysts to accelerate hydroborations of relatively unreactive boranes (Scheme II). This approach may give one enantiomer preferentially, and hence the processes are enantioselective (i.e., diastereoselective interactions within a catalytic cycle).


11.

R

V

BURGESS & O H L M E Y E R

^ C H

Stereocontrol in Hydroborations

165

catalytic rhodium complex/ homochiral ligand

2

HB(OR)2 Me

B(OR) + 2

Me

R

N^^B(OR)

2

oxidation -

Me

ratio of these

enantiomers represents


the enantioselectivityOf the reaction

γ^ΟΗ Me

+

V

^

O

Me

H

+ HO

enantiomers gives the enantiomeric excess of the product

ratio of these

catechol

HB(OR)2

-

Η - Β

χ

π

catecholborane Scheme 11. Hydroboration of unreactive boranes catalyzed by optically active transition metal catalysts.

Our preliminary work (10) proved that enantioselective hydroborations of substrates 1-5 are possible (Chart I and Table I). The ligand 2,3-0isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane (DIOP) was used simply because it can be prepared cheaply and easily. It is usually not the best ligand for asymmetric induction, but enantiomeric excesses of up to 76% were obtained nevertheless. Hydroborations of substrates 4 and 5 with appreciable enantioselectivity are particularly significant because 1,1disubstituted alkenes are notoriously difficult to hydroborate with any control of absolute stereochemistry (25). Enantioselection must be increased if these processes are to be of prac tical value. Fortunately, it seems likely that better optical yields can be obtained by using other catalyst systems. Table I indicates slight improve ment when the ligand 2,2'-bis(diphenylphosphino)-l,r-binaphthyl (binap) is used (entries 3 and 6), and other ligands remain to be tested. Recently Hayashi et al. (16) found that catalyzed hydroborations of styrenes can occur in a Markovnikoff sense with good control of absolute stereochemistry. A n other group (18) reproduced many of the results presented in Table I.



products:

substrates:

HO.

(6)

(D

HO (7)

(2)

OH

Ph OH ( 8 )

Y^Ph

(3)

Ph

(9)

Ph

^CH (4)

Ph

Me

Me^


2

Me (10)

t-Bu

(5)

Me^ ^CH

t-Bu 2

11.


167


Table I. Enantioselective Hydroborations Absolute Enantiomeric Temperature Catalyst l Entry" Substrate (°C) Solvent Product Excess (%) Configuration* 1 1 lfl, 2R 40 6 23 A/C H 2 1 IR, 2R 6 31 5 A/C H 3 IR, 2R 1 6 43 5 B/C H 4 1 46 IR, 2R -5 6 A/THF 5 57 IR, 2R 1 -25 A/THF 6 6 1 64 IR, 2R -25 B/THF 6 7 1 55 IR, 2R -40 A/THF 6 8 IR, 2R 2 76 7 -25 A/THF 9 S £-3 7 8 20 A/C He h

6

6

e

6

6

6

d

d


e

10 11 12 13 14

£-3 Z-3 Z-3 4

5 5 -25 -5 -5

5

A/C H A/C H A/THF A/THF A/THF 6

6

6

6

8 8 8 9 10

~0 17 19 27 69

e

S

se

R

"Reaction times were 48-72 h. A: in situ [Rh(COD)Cl] · 2(R,R)-DIOP. B: in situ [Rh(COD)Cl] · 2(R)-binap. 'Determined by H N M R analysis of Mosher's ester derivatives and Ή N M R experiments with Eu(heptafluorobutyrylcamphorate)3, to within ± 5 % . ''By inference from previous entries. 'Not determined. b

2

[

Substrate-Controlled

Diastereoselectivity

Hydroboration of chiral allylic alcohol derivatives can give syn or anti dia stereomers, as shown in the scheme in Table II. Still and Barrish (27) proved that conventional hydroborations of these substrates are anft-selective. We (11, 13) and Evans et al. (15) independently decided to explore catalyzed hydroborations of such substrates. Table II shows the data obtained. Variation of the protecting group X effectively "tunes" steric and electronic charac teristics of the O X substituent. For instance, replacing the acetate group (entry 2) with a trifluoroacetate (entry 3) increases the electron-withdrawing capacity of the O X and causes a three-fold increase in syn selectivity. C o n versely, substitution of pivolate (entry 5) or trityl (entry 6) for acetate (entry 2) primarily increases steric demands of the O X substituent; syn selectivity also increases. One might conclude that syn selectivity is largest when the O X substituent is a large σ-acceptor. Evans et al.'s results (15) with silylprotected ethers (entries 7 and 8) support this inference. Table III gives Still and Barrish's results (27), augmented with two of our own. For uncatalyzed hydroborations of the same substrates, they are all anfi-selective. Thus the transition-metal-mediated reactions are stereocomplementary insofar as they give the syn isomer preferentially. Such behavior underlines the potential of catalyzed hydroborations as a synthetic method. It is necessary to speculate about the mechanism of transition-metalmediated hydroborations to explain differences between conventional and


168


Table II. Catalyzed Hydroborations of Allylic Alcohol Derivatives

CH

(1) catecholborane/catalyst (ii) oxidation and hydrolysis ^

2

Me OH

OH

OH


Me

OH

Me

anti

syn

Entry 1 2 3 4 5 6 7* 8 9

X

Syn:Anti°

H Ac COCF CO-f-Bu THP CPh £-BuMe Si *-BuPh Si CONMe

2.2:1 2.7:1 7.5:1 6.5:1 8.4:1 18:1 24:1 24:1 2.4:1

3

3

2

fa

2

2

NOTE: Reaction took place in tetrahydrofuran (THF), 48 h, with 1 mol % of [Rh(COD)Cl]*-PPh in a 1:4 ratio; workup with hydrogen peroxide-aqueous base gave near-quantitative yields of the diols, contaminated only with trace amounts of triphenylphosphine oxide derived from the catalyst (*H NMR spectrum). The samples were derivatized without further purification. "Stereochemistries were assigned by comparison with authentic samples or by ' H NMR analysis of acetonide derivatives; diastereomeric ratios were determined by capillary gas chromatographic analysis of acetonide derivatives. ''Evans and co-workers (15); reaction took place in catecholborane (3 equiv), 3 mol % RhClffPhJa, 25 °C. 3

catalyzed reactions of chiral allylic alcohol derivatives. Several oxidative additions of B - H compounds to low-valent transition metal complexes are known (28-33). Significantly, Wilkinson's catalyst and catecholborane combine to give [ R h C l H ( B 0 C H ) ( P P h ) ] (34). This product reacts with alkenes to give hydroboration products (9). Consequently, the generalized mechanism shown in Scheme III seems reasonable; pathways that deviate from this are less plausible. Recently we proved that the catalyzed hydroboration of 1-methylcyclohexene is at least highly cis-selective and perhaps stereospecifically cis (35). This fact provides further evidence in support of the mechanism shown in Scheme III. 2

6

4

3

2


11.


169


Table III. Uncatalyzed Hydroborations of Allylic Alcohol Derivatives

CH

(1) 9-BBN hydroboration (ii) oxidation and hydrolysis ^

2

Me


OH

OH

OH

Me anti Entry l 2 3 4 5 6 7 S

b b b

Me syn X

Syn:Anti

H Ac COCF CO-f-Bu THP CPh £-BuMe Si f-BuPh Si

1:11 1:7.5 1:14 1:15.4 1:3.7 1:5.5 1:9 1:6

3

b

3

b b

OH

2

2

fl

N O T E : Reaction took place in 9-BBN, T H F , -78 to 25 °C; workup with hydrogen peroxide-aqueous base and purification via flash chromatography. "Stereochemistries were assigned by comparison with authentic samples or by Ή N M R analysis of acetonide derivatives; diastereomeric ratios were determined by capillary gas chromatographic analysis of acetonide derivatives. ''Still and Barrish (27).

Formation of complex A could be the stereodeterminant step in these reactions. If this is true, the diastereoselectivities in the overall process can be rationalized on the basis of orbital arguments, by considering electronic and steric effects separately. Electronic factors are governed by frontier orbitals involved in the bonding process. The weakest bond in complexation is back donation of d-electron density from the metal to the π * orbital of the alkene, as represented in Chart II (13). Orientations of the adjacent chiral center, which preferentially cause the orbital mixing that stabilizes this interaction, lead to stereoselec tivity. Such stabilization will arise by mixing a σ * orbital with the alkene ΤΓ*, thus producing a lower energy lowest unoccupied molecular orbital ( L U M O ) for interaction with the highest occupied molecular orbital ( H O M O ) (Chart III).


170


oxidative addition


Coordinatively unsaturated rhodium complex; L * ligand.

alkene complex A H

migratory insertion

Scheme III. Generalized mechanism for catalyzed hydroborations.

-H π'

d

LUMO

HOMO

Chart II. Back-bonding in coordination of an alkene to a transition metal.


11.



171


LUMO

d HOMO Chart HI. "Secondary interactions" involving α σ* orbital of the adjacent chiral center enhance the primary interaction.

To establish a strong secondary interaction, one group on the asymmetric center must be oriented anti to the approaching metal so that the corre sponding σ * orbital can mix with the IT system of the alkene (36). If a σacceptor group and a σ-donating group compete for this position then, in terms of electronic factors, the σ-acceptor group will adopt the anti position because it has a low-energy σ * orbital available to interact with the I T * of the alkene. The σ * orbital associated with the electron-donating group is of relatively high energy and would overlap less. Consequently, if one of the substituents is hydrogen and an "inside crowded model" (37) applies (as would be expected for formation of a IT complex), then the reactive conformer shown in structure 1 (Chart IV) would be used to account for the stereo selectivity (based on electronic factors). For the specific case of allylic alcohols this translates to the projection shown in structure 2 (Chart IV), which indicates that catalyzed hydroborations should be st/n-selective. Steric effects are operative in addition to the electronic effects already discussed. Consider a situation in which the chiral center bears large (L), medium (M), and small (S) substituents that are electronically equivalent. The L group will orient anti to the approaching metal complex; the M substituent will preferentially adopt the outside position (37), which is less encumbered than the inside site, a space best occupied by the smallest group, S (structure 3). Structures 2 and 3 imply that syn diastereoselectivity in catalyzed hydroborations will be highest when the O X substituent is a large σ acceptor group; this is so (see Table I). Houk et al. (38) analyzed stereoselectivity in conventional hydrobora tions. We conclude that steric considerations for conventional hydroborations



172


Structure 1. Electronic bias for orientation of an electron withdrawing group (EWG) and an electron donating group (EDG) in complcxation of a chiral alkene.

Structurel. Interpretation for a chiral allylic alcohol (R = alkyl, X = a protecting group)..

ML, -BR

H - ^ ^ £ = C H

2

Structure 4. Preferential orientation in conventional hydroboration based upon steric demands of the substituents.

Structure 3. Preferential orientation in π-complexation based upon steric demands of the substituents.

HBL

9

HBL

2

Structure5. Preferential orientation in conventional hydroborations basal on electronic demands of the substituents.

2

Structure 6. Preferred reactive conformation in uncatalyzed hydroborations of allylic alcohol derivatives Chart IV.


11.



173


ρ LUMO

π HOMO Chart V. Primary interaction in uncatalyzed hydroborations of chiral allylic alkenes. of chiral allylic alcohols (structure 4) are similar to those for the corresponding catalyzed processes but that opposite diastereofacial selectivities in catalyzed and uncatalyzed hydroborations originate from frontier orbital differences for the transformations. Chart V shows the primary interaction involved in a conventional hydroboration. Chart VI illustrates how mixing of a σ orbital of the α chiral center can enhance this interaction by destabilizing the π system of the alkene. Electron-donating substituents orient anti to the approaching borane in uncatalyzed hydroborations (structure 5) because this places a high-energy σ orbital in the correct position to mix with the alkene ττ system. (Relatively

8ρ LUMO

σ Chart VI. Secondary interactions in uncatalyzed hydroborations of chiral allylic alkenes enhance incipient bonding by destabilizing the HOMO.



174


low-energy σ orbitals are associated with electron-withdrawing groups.) Structure 6 applies this model to the specific case of chiral allylic alcohols, a model that rationalizes anti selectivity. The foregoing reasoning accounts for stereocomplementary behavior in catalyzed and uncatalyzed hydroborations of the allylic alcohol substrates shown in Table I. However, Table IV shows data for hydroborations of phenyl-substituted allylic alcohols. At first glance, these data appear to con tradict the theory given here because the catalyzed processes are anti-se lective (11). These are special cases because aryl groups are good σ-acceptors and are quite large. They tend to orient anti to the approaching rhodium complex in reactive conformations, apparently in preference to O A c groups (entry 1). Phenyl and O C O C F functionalities seem to be comparable in terms of combined electronic and steric features. Consequently, anti selec tivity in entry 2 is less than in entry 1. 3

To confirm these ideas we prepared analogous fluorinated substrates and tested them. Replacement of phenyl with pentafluorophenyl greatly in creases the aromatic group's σ-acceptor abilities and makes it slightly bigger. These factors enhance anti selectivity (entry 4). Conversely, anti selection in uncatalyzed hydroborations of these substrates decreases, as expected (entries 3 and 5).

Table IV. Hydroborations of Phenyl-Substituted Allylic Alcohols

(i) hydroboratbn (ii) H2CVNaOHj

aq)

OH

OH

OH

Me

OH

Me

syn

anti

Entry

Ar

X

Method"

Syn: Anti

1 2 3 4 5

Ph Ph Ph

Ac

catalyzed catalyzed uncatalyzed catalyzed uncatalyzed

1.0:3.5 1.0:1.5 1.0:4.5 1.0:6.9 1.0:3.0

COCF

C F 6

5

C F

5

6

Ac Ac Ac

3

"Catalyzed hydroborations were performed in T H F solutions of 2 mol % of [Rh(COD)Cl) · 4PPh and 3 equiv of catecholborane at 25 °C. Uncatalyzed hydroborations were performed in T H F solutions of 4 equiv of 9-BBN at -78 to 25 °C. Oxidation was with N a O H - H 0 in both cases. Stereochemistries were assigned by formation of acetonides, and analysis was done by gas chromatography. 2

3

2


2

11.


175


Experiments with allyl amine-derived substrates (Table V) prove that the hypotheses presented here have predictive value. N-Tosyl allylic amines were prepared (in optically pure form from amino acids) and hydroborated under catalyzed and uncatalyzed conditions (12). Entries 1, 3, 5, and 7 show that diastereoselectivities in the catalyzed reactions are syn, consistent with


a model similar to that given in structure 2 but with tosylamide functionality replacing the O X groups. The uncatalyzed processes, however, are nonselective (entries 2 and 8) or anfi-selective (entry 6). Catalyzed and uncatalyzed hydroborations of these substrates are, therefore, stereoeomplementary, but syn selectivities in the rhodium-mediated processes are moderate. We predicted on the basis of the theories presented here that increasing the electron-withdrawing ability or the size of the N H T s group should lead to enhanced syn selection. Entries 9 and 10 show that catalyzed hydroborations of the corresponding N-benzylated substrates are indeed appreciably st/n-selective. Hence, we were able to use steric effects to rationally bias product distribution. Table V. Hydroborations of Allylic Amine Derivatives

TsNR

2

CH

(0 hydroboration 2

(ii) HaO^aOHjaq)

Me TsNR

2

OH

TsNR OH 2

Me

Me

syn Entry 1 2 3 4 5 6 7 8 9 10

R

Method"

Syn:Anti

PhCH PhCH t-PrCH t-PrCH i-Pr i-Pr BnOCH BnOCH PhCH t-PrCH

H H H H H H H H Bn Bn

catalyzed uncatalyzed catalyzed uncatalyzed catalyzed uncatalyzed catalyzed uncatalyzed catalyzed catalyzed

7.0:1.0 1.0:1.0 4.0:1.0 2.5:1.0 6.7:1.0 1.0:7.4 4.0:1.0 1.0:1.0 16.0:1.0 10.0:1.0

2

2 2

2

h

anti

R

1

2

2

2

2 2

"Catalyzed hydroborations were performed in T H F solutions of 2 mol % of [Rh(COD)Cl] · 4PPh and 3 equiv of catecholborane at 25 °C. Uncatalyzed hydroborations were performed in T H F solutions of 4 equiv of 9-BBN at -78 to 25 °C. Oxidation was with N a O H - H 0 in both cases. Entry 4 shows an anomalous syn selectivity for an uncatalyzed process. This experiment was repeated several times to check the result. We cannot account for this small syn selectivity at present. 2

3

2

6


2

176



Conclusions Catalyzed hydroborations may be of considerable value in organic synthesis. Refinements of methodologies presented here may supersede reagent-con trolled diastereoselectivity for dictating absolute stereochemistry in hydro borations of some prochiral alkenes. Catalyzed hydroborations of chiral 1,1disubstituted alkenes complement conventional methods for adding B - H to alkenes insofar as the former are st/n-selective. The theory proposed to account for stereocomplementarity in catalyzed and uncatalyzed hydroborations may be applicable to other processes in volving metal ττ complexation. Many reactions of cuprates, for instance, involve transient formation of IT complexes that collapse to give products (39, 40). Others (41) have mentioned possible involvement of d orbitals in S 2 ' reactions of cuprates, but this is a question of orienting a complexing metal relative to a leaving group. Stereoselectivity of cuprate additions to 7-chiral-a^-unsaturated carbonyl compounds is more relevant (42, 43). We suspect that several reaction types can be explained in terms of secondary orbital interactions that perturb incipient Dewar-Chatt bonding. We are actively investigating this area. N

Acknowledgments Financial support for this research was obtained through the Robert Welch Foundation and the National Science Foundation ( C H E 8906969).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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Stereocontrol in Catalyzed and Uncatalyzed Hydroborations

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