J. Org. Chem. 1986,51, 337-342 CO) (Reference 9 reports an 1805-cm-' IR band for CFz=N in CF,=NCHFCF,); 'H NMR 6 3.96 (s, CH30);19FNMR 4 -29.3 (very br A branch, d, J F F = 84 Hz, 1F, N=CF,), -42.8 (very br B branch, d, JFF= 84 Hz, 1F, N=CF2), -82.2 (m, 3 F, CF3),-83.0 (A branch, d, Jm = 143 Hz, 1F, CF,O), -83.3 (m, 3 F, CF3),-84.9 (br, 2 F, CF,O), -91.2 (B branch, d, J F F = 143 Hz, 1 F, CFZO), -100.6 (m, 1F, =NCF), -130.7 (m, 2 F, CF,), -132.9 and -133.8 (m's, 1 F, CF of two racemates). Similar chemical shifts for CFz=NRF have been reported previously (e.g., ref 26). Anal. Calcd for C10H3F16N04:C, 23.78; H, 0.60; N, 2.77. Found: C, 23.69; H, 0.64; N, 2.61. Methyl Perfluoro-5-(methylimino)-2-methyl-3,6-dioxanonanoate (22) and Methyl Perfluoro-5-(methylimino)-2methyl-3-oxapentanoate (23). A mixture of 21.5 g (0.043 mol) of imine 21, 25 mL of hexafluoropropene cyclodimer, and 20 g (0.12 mol) of hexafluoropropene epoxide was heated at 200 "C in a 100-mL metal tube for 8 h. Distillation afforded 3.1 g (21%) of imiie 23, bp 40-41 "C (20 mm): IR (neat) 2970 (saturated CH), 1780 (C=O, CF=N), 1300-1100 cm-' (CF, CO); 'H NMR 6 4.00 (5, OCH,); 19FNMR $I -31.3 (9, J F F = 14 Hz, 1F, N=CF), -57.3 (d, J F F = 14 Hz, 3 F, CF,N), -70.7 (A branch d of d, J F F = 168, 14 Hz, 1 F, OCFF), -80.0 (B branch d of d, J F F = 168,lO Hz, 1 F, OCFF), -82.4 (5, 3 F, CF3), -129.6 (t, Jm = 12 Hz, 1 F, CF). Anal. Calcd for C7H3F10N03:C, 24.79; H, 0.89. Found C, 24.82; H, 0.93. Imine 22,4.2 g (19%), bp 47-58 "C (10 mm), was contaminated with a little isocyanate impurity: IR (neat) 2970 (saturated CH), 1790 (C=O), 1750 (C=N), 1300-1100 cm-l (CF, CO) with a band at 2280 cm-' for impurity (N=C=O); 'H NMR 6 3.96 (s, CH,O); 19FNMR $I -54.8 (m, 3 F, CF,N), -66.2 (A branch d of m, J F F = 168 Hz, 1 F, =CCFFO), -76.9 (B branch d of p, Jm = 168,12 Hz, 1 F, =CCFFO), -81.7 (t, J F F = 8 Hz, 3 F, CF3), -82.5 (5, 3 F, CF,), -88.4 (br, 2 F, CF,O), -129.1 (t,J F F = 12 Hz, 1 F, CF), -129.4 (s, 2 F, CF2). Anal. Calcd for Cl0H3Fl6NO4:C, 23.78; H, (26) Ogden, P. H.; Tiers, G. V. D. Chem. Commun. 1967, 527.
337
0.60. Found: C, 23.88; H, 0.56. N-Methoxycarbonyl (Trifluoromethy1)trifluoroaziridine (24). A solution of 25.0 g (0.25 mol) of methyl azidoformate in 500 mL of hexafluoropropene was irradiated with a Hanovia 679-A-36 lamp in a quartz vessel at -37 "C for 18 h. Fractionation afforded 11.4 g (20%) of 90% pure aziridine 24, bp 88 "C. The other component of the mixture was identified as methyl azidoformate. An analytical sample of 24 was obtained by preparative GC: 'H NMR 6 3.95; '9 NMR 4 -74.8 (m, 3 F, CF,), -121.5 (m, 2 F, CF2),-171.3 (m, 1 F, CF). Anal. Calcd for C5H3F6N02: C, 27.12; H, 1.36; N, 6.78. Found: C, 27.42; H, 2.12; N, 6.52. Methyl Cyanodifluoroacetate (26) and Methyl 3-Isocyanatotetrafluoropropionate(27). A mixture of 18.5 g (0.10 mol) of methyl 3-azidotetrafluoropropionate, 100 mL of acetonitrile, and 8.55 g (6.5 ml, 0.05 mol) of nickel carbonyl under nitrogen reacted vigorously after a short induction period. The temperature was maintained at 25 "C with an icewater bath until gas evolution subsided, and the mixture was then stirred at 25 "C for 3 days. Volatile5 were removed under reduced pressure and fractionated to give 3.7 g of a mixture of the known nitrile 26 (identifed by IR)and the isocyanate 27, bp 50-69 "C (150 mm), followed by 4.1 g (20%) of methyl 3-isocyanatotetrafluoropropionate, bp 69-70 "C (150 mm); IR 3020, 2970, and 2860 (saturated CH), 2280 (NCO), 1785 (CO), 1250-1100 cm-' (CF); 'H NMR 6 3.97 (8, OCHJ; 19FNMR 4 -83.3 (m, 2 F, CF,NCO), -120.3 (t, J F F = 3.8 Hz, 2 F, CF,C=O). Anal. Calcd for C5H3F4N03:C, 29.87, H, 1.50; N, 6.97; F, 37.79. Found C, 30.17, H, 1.73; N, 7.11; F, 37.56. Registry No. 1, 99605-38-6; 2, 86414-01-9;3, 99605-39-7; 4, 99605-40-0; 5, 99605-41-1; 7, 86414-17-7; 8, 99605-42-2; 10, 99605-43-3; 11, 99605-56-8; 12, 99605-44-4; 13, 99617-52-4; 14, 99605-45-5; 16, 99605-46-6; 17, 99605-47-7; 18, 86414-03-1; 19, 99605-48-8; 20, 99605-49-9; 21, 99605-50-2; 22, 99605-51-3; 23, 99605-52-4; 24, 99605-53-5; 26, 99605-54-6; 27, 99605-55-7; perfluoroallyl fluorosulfate,67641-285; 3-azidotetrafluoropropionate, 99617-53-5;norbornene, 498-66-8; methyl azidoformate, 1516-56-9.
Addition Compounds of Alkali Metal Hydrides. 28. Preparation of Potassium Dialkoxymonoalkylborohydrides from Cyclic Boronic Esters. A New Class of Reducing Agents Herbert C. Brown,* Won Suh Park,l Jin Soon Cha,'* and Byung Tae Cholb Richard B. Wetherill Laboratory, Purdue University, West Lafayette, Indiana 47907
Charles A. Brown Department of Interfacial Science, IBM Research Laboratories, San Jose, California 95193 Received August 13, 1985
The reaction of cyclic boronic esters poaseasing a wide range of steric requirements with excess potassium hydride to form the corresponding potassium dialkoxymonoalkylborohydrides was explored. In cases involving a less hindered diol such as ethylene glycol, 2,3-butanediol, or 1,3-propanediol, the reaction is slightly exothermic and quite facile, being complete in less than 1h at 25 "C. In cases involving a highly hindered diol such as pinacol, the reaction is very sluggish, even at 65 "C. The stability of the potassium dialkoxymonoalkylborohydrides is strongly dependent upon the steric bulkiness of the alkyl groups of the boronic ester. Thus, for R = n-hexyl, 3-hexyl, tert-butyl, or thexyl, the addition product is quite stable to disproportionation. However, for R = methyl, the corresponding borohydride is unstable, undergoing rapid redistribution to form a white precipitate. The stable potassium dialkoxymonoalkylborohydridesthus formed reduce 2-methylcyclohexanone with moderate stereoselectivity, giving the cis isomer preferentially, with selectivities of 73-84%.
Trisubstituted borohydrides such as trialkylborohydrides and trialkoxyborohydrides constitute a highly attractive class of reducing agents in organic synthesis.
Trialkylborohydrides have been synthesized by several different routes, such as the reaction of trialkylboranes with alkali metal hydrides2 (LiH, NaH, KH), or with
(1) (a) Postdoctoral research associate on Grant ARO DAAG-29-82K-0047 supported by the United States Army Research Office. (b) Postdoctoral research associate on Grant ARO DAAG-29-85-K-0062 supported by the United States Army Research Office.
(2) (a) Brown, C. A. J. Am. Chem. SOC.1973, 95, 4100; (b) J. Org. Chem. 1974,39,3913. (c) Brown, C. A.; Kriihnamurthy,S. J. Organomet. Chem. 1978, 156, 111. (d) Brown, H. C.; Khuri, A.; Kim, S. C. Inorg. Chem. 1977, 16, 2229. (e) Brown, H. C.; Krishnamurthy, S.; Hubbard, J. L. J. Am. Chem. SOC.1978,100, 3343.
0022-3263/86/1951-0337$01.50/0
0 1986 American Chemical Society
338 J. Org. Chem., Vol. 51, No. 3, 1986
Brown et al.
Table I. I’B NMR Spectral Data and Physical Properties of Cyclic Boronic Esters
entry 1 IC
Id 2a
2b 2c 3a 3b 4a 4c 4d 5a 5b 5c
5d
cyclic boronic ester 2-methvl-l,3,2-dioxaborolane 2-meth;l-4;4;5,5-tetramethyl-1,3,2-dioxaborolane Z-methyl-1,3,2-dioxaborinane 2-n-hexyl-1,3,2-dioxaborolane 2-n-hexyl-4,5-dimethyl-1,3,2-dioxaborolane 2-n-hexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 2-(3-hexyl)-1,3,2-dioxaborolane 2-(3-hexyl)-4,5-dimethyl-l,3,2-dioxaborolane 2-tert-butyl-1,3,2-dioxaborolane 2-tert- butyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 2-tert-butyl-1,3,2-dioxaborinane 2-thexyl-1,3,2-dioxaborolane 2-thexyl-4,5-dimethyl-l,3,2-dioxaborolane 2-thexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 2-thexyl-1,3,2-dioxaborinane
tert-butyllithium: or with lithium aluminum hydride in the presence of trieth~lenediamine,~ or with lithium trimetho~yaluminohydride.~ They have proven to be powerful selective reducing agents.6 On the other hand, trialkoxyborohydrides were synthesized by treatment of the corresponding trialkoxyboranes with alkali metal hyd r i d e (NaH, ~ ~ ~ KH) ~ ~ and proved to be very mild reducing a g e n t ~ . ~Accordingly, ~f development of a general procedure for the syntheses of mixed alkoxyalkylborohydrides has been of considerable interest. Recently we reported a general synthesis of potassium 9-alkoxy-9-boratabicyclo[3.3.l]nonanes, K-9-OR-9-BBNH,s a class of stable dialkylmonoalkoxyborohydrides,by treating the corresponding borinic esters with excess potassium hydride in THF (eq 1). However, there has not been reported any
B-OR
t (excess)KH 7 25 THF
104
@(
Kt
(1)
R = I - P r , r u - B u , t - B u , t - A m , T h x , Et3C
synthesis of stable dialkoxymonoalkylborohydrides. Preliminary experiments revealed that the products from simple boronic esters, KRB(OR’)2H,underwent rapid redistribution. However, the use of cyclic esters from diols was more promising. Accordingly, we studied the reaction of cyclic boronic esters with KH for the synthesis of stable potassium 2-alkyl-1,3,2-dioxaborolane hydrides (KDBLH) (3)(a)Corey,E.J.; Becker,K.B.;Varma, R.K.J. Am. Chem. SOC. 1972,94,8616. (b)Corey,E.J.; Albonico,S.M.;Koelliker,U.;Schaaf, T.K.;Varma,R.K.Zbid. 1971,93,1491.(c) Corey,E.J.; Varma,R. K. Zbid. 1971,93,7319.(d)Brown,H.C.;Kramer,G.W.;Hubbard,J. L.; Krishnamurthy,S. J. Organomet. Chem. 1980,188, 1. (4)Brown,H.C.;Hubbard, J. L.;Singaram,B.Tetrahedron 1981,37, 2359. (5)Brown,H.C.;Krishnamurthy,S.;Hubbard,J. L.J . Organomet. Chem. 1979,166,271. (6)(a) Krishnamurthy,S.Aldrichimica Acta 1974,7,55. (b) Brown, H.C.;Dickason,W.C.J. Am. Chem. SOC.1970,92,709. (c) Krishnamurthy,S.; Brown,H.C.Zbid. 1976,98,3383;(d)J. Org. Chem. 1976,41, 3064. (e)Brown,H.C.;Kim,S. C. Synthesis 1977,635. (0Krishnamurthy,S.;Vogel, F.;Brown,H.C.J. Org. Chem. 1977,42,2534. (7)(a)Brown,H.C.;Mead, E.J. J . Am. Chem. SOC.1953,75,6263. (b)Brown,H.C.;Mead,E.J.; Shod,C.J.Zbid. 1966,78,3616. (c)Brown, C. A.;Krishnamurthy,S.;Kim,S. C.J. Chem. SOC.,Chem. Commun. 1973,391. (d)Brown,H.C.;Nazer,B.;Sikorski,J. A. Organometallics 1983,2,634.(e)Brown,H.C.;Cha,J. S.;Nazer,B.Inorg. Chem. 1984, 23, 2929. (0Brown,H.C.;Cha,J. S.;Nazer,B.;Kim,S.C.;Krishnamurthy,S.;Brown,C.A. J . Org. Chem. 1984,49,885. (8)Brown,H.C.;Cha,J. S.;Nazer,B.;Brown,C.A. J. Org. Chem. 1985,50,549.
IIB NMR chem shift, 6 34.2 33.5 30.2 34.7 34.7 34.4 35.3 34.9 35.5 34.8 31.1 35.4 35.1 34.7 31.4
bp, OC/torr 75-771750 120-1221747 44-46/30 84-86/17 90-921 16 98-10018 74-76/16 80-82116 51/17 60-61120 49-501 16.5 70119 70-73/14 87/17 79-80116
n20D
1.3892 1.4003 1.4028 1.4285 1.4173 1.4228 1.4215 1.4126 1.4084 1.4105 1.4211 1.4272 1.4204 1.4274 1.4368
or potassium 2-alkyl-1,3,2-dioxaborinane hydrides (KDBNH) (eq 2) as a class of dialkoxymonoalkylborohydrides.
1-
n . 2 , KDBLA = 3. KDBNH
n
Results and Discussion A series of cyclic boronic esters such as 2-alkyl-1,3,2dioxaborolanes and 2-alkyl-1,3,2-dioxaborinanes were synthesized according to established procedures and their reaction with excess potassium hydride studied. The stability of the resulting potassium dialkoxymonoalkylborohydrides was also examined by llB NMR spectra and measuring the number of moles of H2evolved by hydrolysis of aliquots of the reagent solution at appropriate time intervals. The stable potassium dialkoxymonoalkylborohydrides were characterized by IR and llB NMR spectroscopy and their stereoselectivities as reducing agents examined with 2-methylcyclohexanone. Synthesis of Cyclic Boronic Esters. Representative cyclic methylboronic esters, 2-methyl-1,3,2-dioxaborolane (la),2-methyl-4,5-dimethyl-1,3,2-dioxaborolane (lb), 2methyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IC), and 2-methyl-l,3,2-dioxaborinane (ld),were synthesized by the reaction of methylboronic acid with the corresponding diols in pentane. Methylboronic acid, in turn, was prepared by the carbonylation of borane-dimethyl sulfide complex, followed by hydrolysis of the resulting methylboroxine, according to a procedure developed in this l a b ~ r a t o r y . ~
1 , R-Me
2,R = n-Hex
a
b
C
d
3,R83-Hex 4.R* t - BU 5. R=Thx
n-Hexyl- and 3-hexylboronic esters such as 2-n-hexyl1,3,2-dioxaborolane(2a), 2-n-hexyl-4,5-dimethyl-1,3,2-dioxaborolane (2b), 2-n-hexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2d),2-(3-hexyl)-1,3,2-dioxaborolane (3a), and 2- (3-hexy1)-4,5-dimethyl-1,3,2-dioxaborolane (3b) were (9)Brown,H.C.;Cole,T.E. Organometallics 1985,4,816.
J. Org. Chem., Vol. 51, No. 3, 1986 339
Addition Compounds of Alkali Metal Hydrides
Table 11. Reaction of Potassium Hydride with Cyclic Boronic Esters in THF entry
la IC
Id 2a 2b 2c 3a 3b 4a 4c 4d 5a 5b 5c 5d
cyclic boronic ester
2-methyl-1,3,2-dioxaborolane 2-methyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolane 2-methyl-l,3,2-dioxaborinane 2-n-hexyl-l,3,2-dioxaborolane 2-n-hexyl-4,5-dimethyl-1,3,2-dioxaborolane 2-n-hexyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolane 2-(3-hexyl)-1,3,2-dioxaborolane 2-(3-hexyl)-4,5-dimethyl-1,3,2-dioxaborolane 2-tert-butyl-1,3,2-dioxaborolane 2-tert-butyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 2-tert-butyl-l,3,2-dioxaborinane 2-thexyl-1,3,2-dioxaborolane 2-thexyl-4,5-dimethyl-1,3,2-dioxaborolane 2-thexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane Z-thexy1-1,3,2-dioxaborinane
temp, "C
time, h
25 25 25 25 25 65 25 25 25 65 25 25 25 65 25
0.25 2.0 b 1.0 1.0
stability of addition products unstable" unstable" stable stable
C
1.0 1.0 1.0 d 1.0 1.0 1.0
stable stable stable stable stable stable
C
1.0
stable
White precipitate formed. *Boronic ester disappeared, but none of the desired addition product formed. No reaction. Slow formation of the desired addition product was observed, but the product was unstable at 65 O C .
synthesized by the hydroboration of the corresponding olefins with dibromoborane-dimethyl sulfide complex, followed by direct treatment of the resulting alkyldibromoborane with the corresponding diols in pentane, according to a published procedure.1° As representative cyclic tert-butylboronic esters, 2-tert-butyl-1,3,2-dioxaborolane (4a), 2-tert-butyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4c), and 2-tert-butyl-1,3,2-dioxaborinane (4d) were prepared either by the reaction of tert-butylboronic acid, t-BuB(OH),, with the corresponding diols in pentane or by trans-esterification of t-BuB(OMe), with the corresponding diols. Those compounds, t-BuB(OH), and tBuB(OMeP), were, in turn, prepared by addition of tBuMgCl to a threefold excess of B(OMe)3to give [t-BuB(OMe)3]-MgC1+,followed by decomposition of the "ate" complex with aqueous HC1 and anhydrous HC1 in EE, respective1y.l' Thexylboronic esters such as 2-thexyl1,3,2-dioxaborolane (5a), 2-thexyl-4,5-dimethyl-l,3,2-dioxaborolane (5b), 2-thexyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolane (5c),and 2-thexyl-l,3,2-dioxaborinane(5d) were synthesized by direct reaction of thexylborane with the corresponding diols.12 The physical properties and 'lB NMR spectral data of these cyclic boronic esters are summarized in Table I. All of the boronic esters exhibited llB chemical shifts in the range of 6 30-36 downfield relative to BF3.0Et2 as a reference. Moreover, alkyldioxaborolanes always showed 3-4-ppm higher chemical shifts than the corresponding alkyldioxaborinanes, consistent with the results observed previously for similar boronic esters.loJ3 Reaction with Potassium Hydride. The cyclic boronic esters, 2-alkyl-1,3,2-dioxaborolanesand 2-alkyl(10) Brown, H. C.; Bhat, N. G.; Somayaji, V. Organometallics 1983, 2, 1311. (11) Since it was possible to prepare [t-BuB(OMe),]-MgCI+,which can be easily transformed to t-BuB(OMe), or t-BuB(OH),, by ut3.equiv of B(OMe), in the reaction with t-BuMgC1 (without complication of ~ Cgenerality ~~), of the further alkylation to give [ ( ~ - B U ) ~ B ( O M ~ ) , ] - Mthe reaction for the preparation of boronic acids possessing highly hindered alkyl groups is being studied (unsuccessfulwhen R = Me): t-BuMgC1 + 3[B(OMe),]
EE
[~-BUB(OM~)~]-M~CI+ + 2[B(OMe)3]
(12) The reaction of monoalkylboranes with diols to produce the corresponding cyclic boronic esters is being studied since it was general for thexylboronic esters: Thx8Hz t O P O H
-
t 2H*t
ThxS')
' 0
(13) Goetz, R.; Nath, H.; Pommerening, H.; Sedlak, D.; Wrackmeyer, B. Chem. Ber. 1981,114, 1884.
1,3,2-dioxaborinanes,were allowed to react with vigorously stirred suspension of a modest excess of potassium hydride (free of oil) in THF at 25 "C or 65 "C. The course of the reaction was monitored by withdrawing aliquots of the mixture at appropriate time intervals and observing their llB NMR spectra. The cyclic boronic esters exhibit signals between 6 3Q-36 whereas, the corresponding borohydrides exhibit signals between 6 0-10. Consequently, the reactions could be easily followed by the disappearance of the cyclic boronic ester signal with the concurrent appearance of the borohydride signal. The results are summarized in Table 11. The reactivity of the cyclic boronic esters with potassium hydride varied markedly with the steric requirements of the boronic esters, especially that of the dialkoxy moiety. Thus, in those cases where the dialkoxy group was a less hindered diol such as ethylene glycol (a), 2,3-butanediol (b), or 1,3-propanediol (a),the reaction of the boronic ester with potassium hydride was slightly exothermic and the reaction was complete within an hour at 25 "C (entry la,c, 2a,b, 3a,b, 4a,d, 5a,b,din Table 11). However, in cases where the dialkoxy group was from a highly hindered pinacol ( c ) ,the reaction of the boronic esters, 2-nhexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and 2thexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, with potassium hydride did not take place even in refluxing THF for several days (entries 2c and 5c in Table 11). The reaction of 2-tert-butyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane took place very sluggishly at 25 "C (ca. 46% after 5 days) and the reaction could not be driven to completion, even by long refluxing in THF, without decomposition of the desired product. Evidently the reaction conditions required for the synthesis are too drastic for the survival of the fragile addition product, potassium 2-tert-butyl4,4,5,5-tetramethyl-1,3,2-dioxaborolane hydride (entry 4c in Table 11). Stability of the Dialkoxymonoalkylborohydrides. The stability of the addition products, potassium 2-alkyl-1,3,2-dioxaborolanehydrides (KDBLH) and potassium 2-alkyl-l,3,2-dioxaborinanehydrides (KDBNH) were studied by taking aliquots of the solution in THF and measuring the number of moles of H2evolved on hydrolysis and by taking the I'B NMR spectra at appropriate time intervals. The stability varied with the steric bulkiness of the alkyl groups. Thus, when the alkyl group possesses low steric requirements, such as methyl, the corresponding potassium dialkoxymonoalkylborohydrides formed were unstable regardless of the steric bulkiness of the dialkoxy groups (entries l a and IC in Table 11) and formed white precipitates rapidly at 25 "C. Even at 0 "C, the boro-
340 J. Org. Chem., Val. 51, No. 3, 1986
entry
la' IC'
2a' 2b'
3a' 3b' 4a' 4d' 5a' 5b' 5d'
Brown et al.
Table 111. IR and IrB NMR Spectral Data of the Stable Potassium Dialkoxvmonoalkvlborohvdrides potassium dialkoxymonoalkylborohydride IR Y ~ - cm-I ~ , IIB NMR 6, ppm (mult) 9.3 (br, s) potassium 2-methyl-1,3,2-dioxaborolane hydride 2090 potassium 2-methyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane hydride 2020 6.2 (br, s) potassium potassium potassium potassium potassium potassium potassium potassium potassium
2-n-hexyl-1,3,2-dioxaborolane hydride 2-n-hexyl-4,5-dimethyl-1,3,2-dioxaborolane hydride
2020 2020 2020 2020 2010 2040 2010 2010 2040
2-(3-hexyl)-1,3,2-dioxaborolane hydride
2-(3-hexyl)-4,5-dimethyl-1,3,2-dioxaborolane hydride 2-tert-1,3,2-dioxaborolane hydride 2-tert-butyl-1,3,2-dioxaborinane hydride 2-thexyl-1,3,2-dioxaborolane hydride 2-thexyl-4,5-dimethyl-1,3,2-dioxaborolane hydride 2-thexyl-1,3,2-dioxaborinane hydride
hydrides formed, but the white precipitate also formed, albeit more slowly. In the reaction of 2-methyl-1,3,2-dioxaborinane with potassium hydride, only decrease of the boronic ester was observed by 'lB NMR with concurrent formation of a white precipitate, probably due to the extreme instability of the desired addition product (entry Id in Table 11). However, in cases where the alkyl group of the cyclic boronic esters possessed moderate steric bulkiness such as R = n-hexyl, 3-hexyl, tert-butyl, or thexyl, the corresponding potassium dialkoxymonoalkylborohydrides were stable for an extended period of time provided the THF solution of the borohydride reagents were maintained over excess KH under a positive pressure of nitrogen at 25 " C (entries 2a, b, 3a,b, 4a,d, Sa,b,d in Table 11). Apparently in these systems incorporation of the cyclic dioxyboryl group into cyclic boronic esters induced a stabilization of the dialkoxymonoalkylborohydride reagents not achievable in the noncyclic dialkoxy derivatives. The solutions of the stable borohydrides thus formed were analyzed for the ratio of KB:H. They unambiguously revealed the desired 1:l:l stoichiometry. IR and I'B NMR Spectra of the Stable Dialkoxymonoalkylborohydrides. The stable dialkoxymonoalkylborohydrides prepared in THF were characterized by IR and 'lB NMR spectroscopy and the results are summarized in Table 111. These borohydride reagents, KDBLH and KDBNH, in THF display characteristic absorptions in the IR, such as a strong and broad absorption around 2000 cm-l, attributed to the B-H stretching vibration and strong and sharp absorptions around 1300 cm-' and 1100 cm-' attributed to an unsymmetrical B-0 stretching and a symmetrical B-0 stretching absorption, respectively. These absorptions are similar to the B-H stretching absorptions in LiR3BH2eor K-9-OR-9-BBNH8 and the B-0 stretching absorptions in the corresponding cyclic boronic esters. The l'B NMR spectra of these stable borohydride reagents in THF exhibit broad singlet peaks around the 6 0-10 downfield region relative to BF3.0Et2 as a reference. The appearance of broad singlet peaks for these stable potassium dialkoxymonoalkylborohydrides instead of the anticipated doublet peaks is probably due to the relative high viscosity of the solutions. The singlets for these stable dialkoxymonoalkylborohydrides do not resolve into doublets, even after completion of the reaction.14 However, in the case of the borohydrides of some six-membered cyclic boronic esters such as potassium 2tert-butyl-1,3,2-dioxaborinane hydride and potassium 2thexyl-1,3,2-dioxaborinane hydride (4d' and 5d' in Table (14)This phenomenon is different from that of borohydrides of dialkylboranes reported earlier such as dicyclohexylborane (Chx,BH), disiamylborane (Sia2BH),and diisopinocampheylborane(Ipc2BH),where the broad singlets in the presence of u n r e a d dialkylboranes resolve into sharp triplets when all of the dialkylboranes have been utilized: Brown, H. C.; Singaram, B.; Mathew, C. P. J. Org. Chem. 1981,46, 2712.
0.6 (br, s) 9.0 (br, s) 3.4 (br, s) 8.5 (br, s) 8.8 (br, s) 7.0 (br, d)l 9.0 (br, s) 8.7 (br, s) 7 . 2 (br, d)'
Scheme I
U 4a' 4d'
5a' Sd'
5b'
73%
80% 82% 82% 84%
J
U
J
27 % 20% 18% 18% 16%
111), the broad singlets resolved into broad humplike doublets by dilution to ca. 0.1 M. Reduction of 2-Methylcyclohexanone. Using stable dialkoxymonoalkylborohydrides such as potassium 2tert-butyl-1,3,2-dioxaborolanehydride (4a'), potassium 2-tert-butyl-1,3,2-dioxaborinane hydride (4d'), potassium 2-thexyl-1,3,2-dioxaborolane hydride @a'),potassium 2thexyl-4,5-dimethyl-1,3,2-dioxaborolane hydride (5b'), and potassium 2-thexyl-1,3,2-dioxaborinane hydride (sa'),the reduction of 2-methylcyclohexanone was carried out and the stereochemical outcome was studied (Scheme I). Reduction took place cleanly with the borohydrides at 0 "C within 3 h, producing a quantitative yield of cis/ trans-2-methylcyclohexanolafter hydrolysis of the initial reduction product. These borohydrides exhibited moderate stereoselectivities (84-73%) toward less stable cis2-methylcyclohexanolwith a decreasing selectivity as the steric requirement on boron decreases: 5b', 84% > 5a' N 5d', 82% > 4d', 80% > 4a', 73%. Thus, it is clearly demonstrated that these dialkoxymonoalkylborohydrides can be utilized as a new class of reducing agents for carbonyl functionality in organic compounds. Conclusion The reactivity of cyclic boronic esters with potassium hydride to generate the corresponding potassium dialkoxymonoalkylborohydrides is sensitive to the steric requirement of the boronic ester, especially to the dialkoxy group attached to boron. In general, if the dialkoxy group is from diols of moderate steric requirement such as ethyleneglycol, 2,3-butanediol, or 1,3-propanediol, the cyclic boronic ester reacts readily with potassium hydride within an hour a t 25 "C. The resulting potassium dialkoxymonoalkylborohydrides, possessing an alkyl group of
J. Org. Chem., Vol. 51, No. 3, 1986
Addition Compounds of Alkali Metal Hydrides
moderate steric bulkiness such as n-hexyl, 3-hexyl, tertbutyl, and thexyl are stable toward disproportionation for an extended period of time if the solutions are stored over excess potassium hydride under a positive pressure of nitrogen. These stable potassium dialkoxymonoalkylborohydrides reduce 2-methylcyclohexanone readily and quantitatively to the corresponding 2-methylcyclohexanol with moderate stereoselectivities ( 7 3 4 4 % ) toward less stable cis isomers. Thus, the present study provides a convenient syntheses of the stable cyclic potassium dialkoxymonoalkylborohydrides such as KDBLH a n d KDBNH, a new class of reducing agents. With the present study, we have successfully synthesized all four classes of trisubstituted borohydrides. R
I
MR-B-H
T t
MR-B-H
I
R
I MRO-B-H
RO
I
MRO-B-H
I
RO
RO
C
T h e synthesis of the derivatives containing a single substituent, A and D, offered no significant problem. The synthesis of the "mixed" derivatives required use of cyclic substituents, 9-BBN in B and glycols in C. The most favorable stereoselective results have been achieved with the compounds A a n d B.
Experimental Section All glassware used was dried in an oven, assembled hot, and cooled with a stream of nitrogen. All reactions were carried out under nitrogen atmosphere. Experimental techniques used in handling air-sensitive materials are described e1~ewhere.l~ Tetrahydrofuran was dried over a 4-A molecular sieve and distilled from sodium benzophenone ketyl just prior to use. Potassium hydride was used as received from Alfa and was freed from mineral oil according to the published procedure?b Boranemethyl sulfide complex (BMS) and tert-butylmagnesium chloride in diethyl ether (EE) were from the Aldrich Company and were standardized by measurement of the H2 produced by hydrolysis and by the Watson-Eastham titration,16 respectively, prior to use. Methylborate was from a commerical source and was distilled over sodium metal to remove residual methanol. All of the diols used were high grade commercial reagents (Aldrich)and used directly after drying over molecular sieves, type 4A. Phillips high purity n-pentane was also dried over type 4A molecular sieves. The anhydrous HC1 solution in diethyl ether (ca.2.5 M) was prepared from concentrated HC1 and concentrated HZSO4 using a Brown" apparatus." The HC1-ether solution was standardized by titration with standard base. "B NMR spectra were recorded on a Varian FT-80 spectrometer and all 'lB NMR chemical shifts were reported in 6 (ppm) relative to BF,.OEh. 'H NMR spectra were recorded on a Varian T-60A spectrometer with Me4%as an internal standard and all of the chemical shifts were reported in 6 (ppm) relative to Me4Si. IR spectra were recorded on a Perkin-Elmer 700 spectrophotometer. GC analyses were performed on a Varian 1400 FID chromatograph equipped with a Hewlett-Parkard 3390A integrator/plotter. The alcohol products were analyzed by using a 12 f t X 0.125 in. column packed with 15% THEED on 100/120-meshSupelcoport with the use of a suitable internal standard and authentic mixture. Preparation of Methylboronic Esters la, IC, and Id. Methylboronic acid was synthesized via carbonylation of BMS followed by hydrolysis of the resulting methylboroxinewith H 2 0 according to the procedure developed in this laboratory? The methylboronic esters were wepared by the reaction of methylboronic acid with the corresponding diols in pentane according to the published procedure.l0 For 2-methyl-4,4,5,5-tetra(15) Brown, H. C.; Kramer, G. W.; Levy, A. B.; Midland, M. M. "Organic Syntheses via Boranes"; Wiley-Interscience: New York, 1975. (16)Watson, S.C.;Eastham, J. F. J. Organomet. Chem. 1967,9,165. (17) Brown, H. C.; Rei, M. H. J. Org. Chem. 1966,31, 1090.
341
methyl-l,3,2-dioxaborolane(IC), CaH2 had to be used for the removal of water generated in the reaction of methylboronic acid (la): "B NMR 6 with pinacol. 2-Methyl-1,3,2-dio~aborolane~~ 34.2; bp 75-77 "C/750 mm; 'H NMR 6 0.30 (s, 3 H), 4.08 (s, 4 H); n20D 1.3892; IR VB-0 1350, 1020 cm-'; yield, 62%. 2Methyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IC): "B NMR 6 33.5; 'H NMR 6 0.13 (s, 3 H), 1.20 (s, 12 H); bp 120-122 OC/747 mm; n20D 1.4003; IR ~ g - 01350, 1140 cm-'; yield, 51%. 2Methyl-l,3,2-dio~aborinane'~ (Id): 'lB NMR 6 30.2; 'H NMR 6 0.10 (s, 3 H), 1.87 (4, 2 H), 3.90 (t, 4 H); bp 44-46 "C/30 mm; nzoD1.4028; IR vg0 1340, 1100 cm-'; yield, 60%. Preparation of n -Hexylboronic Esters 2a, 2b, and 2c and 3-Hexylboronic Esters 3a and 3b. These n-hexyl- and 3hexylboronic esters were prepared by hydroboration of the corresponding olefins with HBBrz followed by treatment with the correspondingdiols in pentane.1° 2-n-Hexyl-1,3,2-dioxaborolane (2a): "B NMR 6 34.7; 'H NMR 6 0.7-1.7 (m, 13 H), 4.2 (s, 4 H); bp 84-86 OC/17 mm; yield, 75%. 2-n-Hexyl-4,5-dimethyl-1,3,2dioxaborolane (2b): I'B NMR 6 34.7; 'H NMR 6 3.6-4.2 (m, 2 H), 0.8-1.5 (s and m, 19 H); bp 90-92 "C/l6 mm; yield, 80%. 2-n-Hexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane ( 2 4 : "B NMR 6 34.4; 'H NMR 6 0.7-1.7 (s and m, 25 H); bp 98-100 "C/8 mm; (3a): "B NMR 6 35.3; yield, 82%. 2-(3-hexyl)-l,3,2-dioxaborolane 'H NMR 6 0.7-1.7 (m, 13 H), 4.2 (s,4 H); bp 74-76 "C/16 mm; (3b): yield, 71%. 2-(3-Hexyl)-4,5-dimethyl-l,3,2-dioxaborolane "B NMR 6 34.9; 'H NMR 6 0.8-1.5 (s and m, 19 H), 3.6-4.2 (m, 2 H); bp 80-82 "C/l6 mm; yield, 80%. Preparation of tert-Butylboronic Esters (4a, 4c, and 4d). The preparation of 2-tert-butyl-1,3,2-dioxaborolane(4a) is representative. An oven-dried, 250-mL, round-bottomed flask with a side arm, a condenser, and an adpator was attached to a mercury bubbler. The flask was flushed with N2 and maintained under a static pressure of N2 In the flask were placed 17.0 mL (MeO)3B (150 mmol) in EE (57.0 mL) and the flask was kept at 25 "C by using a water bath. A total of 26.3 mL of 1.90 M t-BuMgC1 in was added dropwise via a double-ended needle with EE (50 "01) vigorous stirring. Immediately a white precipitate formed. After completion of addition, the mixture was stirred for 3 h at 25 "C. The reaction flask was brought to 0 "C by using an ice bath and a total of 30 mL of 2 N HC1 was added slowly and stirred for 0.5 h at 25 "C. A clear separation of the water layer and the organic layer was apparent. The "B NMR spectrum of the organic layer showed only the presence of B(OH)3,6 18.1,and t-BuB(OH),, 6 31.4. The organic layer, separated and washed with 2 X 50 mL of degassed water (under N2),showed the presence of ~-BUB(OH)~ only in "B NMR. After evaporation of EE by using a water aspirator, there was added 50 mL of dry pentane and a total of 3.1 g of ethylene glycol (50 mmol) to the resulting white solid, t-Bu(OH),, and stirred for 0.5 h. The solid t-BuB(OH), disappeared and the water separated out. The pentane layer, separated and dried with anhydrous MgS04, yielded 4.48 g of 2-tert-butyl-1,3,2-dioxaborolane (70%, 4a) after evaporation of the solvent: 'lB NMR 6 35.5; 'H NMR 6 1.00 (s,9 H), 4.17 (s,4 H); bp 51 "C/17 mm; nzoD 1.4084; IR vg-0 1340, 1110 cm-'; yield, 82%. 2-tertButyl-1,3,2-dioxaborinane(4d): "B NMR 6 31.1; 'H NMR 6 0.85 (s, 9 H), 1.83 (q, 2 H), 3.92 (t, 4 H); bp 49-50 "C/16.5 mm [lit.18 bp 78-80 "C/72 mm]; n " ~ 1.4211; IR vB-0 1320,1190cm-'; yield, (4c) 83%. 2-tert-Butyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolane was prepared by a Cightly modified procedure. The reaction of t-BuMgC1with 3 equiv of B(OMe)3was quenched with anhydrous HCl in EE to convert the product [t-BuB(OMe)JMgCl+ into t-BuB(OMe)* The EE solution was filtered from MgC12 and t-BuB(OMe)2 was isolated by a fractional distillation from B(OMe)3using a Todd Precise Fractionation Assembly. t-BuB(OMe)2was utilized for trans-esterification with pinacol. MeOH was distilled out f i t , followed by distillation of the desired boronic ester (412) at reduced pressure: I'B NMR 6 34.8; 'H NMR S 0.92 ( ~ , H), 9 1.18 (9, 12 H); bp 60-61 "C/20 mm; nZoD1.4105; IR vB-0 1300,1140cm-'; yield, 86%; MS, m/e M+ 183/184. Anal. Calcd for CloH21B02:C, 65.23; H, 11.52; B, 5.87. Found: C, 64.88; H, 11.90; B, 5.52. Preparation of Thexylboronic Esters 5a-d. The preparation (5a) is representative. A 100-mL, of 2-thexyl-l,3,2-dioxaborolane (18)Brown, H.C.;De Lue, N. R.; Yamamoto, Y.; Marayama, K.; Kasahara, T.; Murahashi, S.; Sonoda, A. J . Org. Chem. 1977,42, 4088.
342
J. Org. Chem. 1986,51, 342-346
round-bottomed flask, dried and cooled under N2,was maintained at 0 OC using an ice bath and was charged with 10.0 M boranedimethyl sulfide complex (50 mmol). A total of 4.25 g of 2,3dimethyl-2-butene (50 mmol) was added slowly and the mixture was sitrred for 1 h. Ethylene glycol (3.1 g, 50 mmol) was added dropwise with vigorous stirring with control of the H2 evolution at 25 "C and the mixture was stirred for 1 h. Removal of the volatiles by water aspirator followed by distillation yielded 7.4 g of 2-thexyl-1,3,2-dioxaborolane (95%, 5a): "B NMR 6 35.4; 'H NMR 6 0.77-0.90 (s and d, 12 H), 1.35-1.65 (m, 1 H), 4.08 (s, 4 1.4272; IR YW 1310, 1160 cm-'; yield, H); bp 70 OC/19 mm; 95%. 2-Thexyl-4,5-dimethy1-1,3,2-dioxaborolane (5b): llB NMR 6 35.1; 'H NMR 6 0.80-0.90 (s and d, 12 H), 1.10-1.30 (d and d, 6 H), 1.60-1.90 (m, 1H), 4.00-4.50 (m, 2 H); bp 70-73 "C/14 mm; ~ 1160 cm-'; yield, 92%; MS, m/e M+ nZoD1.4204; IR U B 1310, 184/185. Anal. Calcd for C,oH21B02:C, 65.23; H, 11.52; B, 5.87. Found: C, 65.57; H, 11.36; B, 5.60. 2-Thexyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane(5c): "B NMR 6 34.7; 'H NMR 6 0.77-0.90 (m, 12 H), 1.18 (s,12 H), 1.35-1.65 (m, 1H); bp 87 "C/17 mm; nZoD1.4274; IR ~ g - 01300, 1140 cm-'; yield, 94%; MS, m/e M+ 211/212. Anal. Calcd for C12H%BO2:C, 67.92; H, 11.90; B, 5.09. Found: C, 68.27; H, 12.25; B, 4.79. 2-Thexyl-1,3,2-dioxaborinane (5d): "B NMR 6 31.4; 'H NMR 6 0.77-0.85 (s and d, 12 H), 1.23-2.03 (m, 3 H), 3.90 (t,4 H); bp 79-80 "C/16 mm [lit.ls ~ 1180 cm-'; yield, bp 74-75 OC/13 mm]; nZoD1.4368;IR v B 1300, 92%. General Procedure for the Reaction of Potassium Hydride with Cyclic Boronic Esters. A preparation of potassium 2thexyl-4,5-dimethyl-1,3,2-dioxaborolane hydride is representative. To an oven-dried, lOO-mL, round-bottomed flask, cooled and maintained under N2 as usual, was added 3.5 g of KH (30 mmol) as an oil dispersion with the aid of a double-ended needle. The mineral oil was removed by washing with pentane (3 X 30 mL). To the suspension of KH in 40 mL THF kept at 25 "C by a water bath was added a total of 3.68 g of 2-thexyl-4,5-dimethyl-l,3,2dioxaborolane (20 mmol) with vigorous stirring. The reaction was slightly exothermic and complete within 1 h. The "B NMR spectrum of the clear supernatant after the settling of excess KH
showed a broad singlet at 6 9.8 and the solution IR of the product exhibited a strong B-H stretching absorption at 2010 cm-', indicating the formation of potassium 2-thexyl-4,5-dimethyl1,3,2-dioxaborolane hydride. The hydride concentration was measured, 0.42 M (93% yield), by the number of moles of H2 evolved when the reagent was hydrolyzed with THF-glycerine2 N HCl (1:l:l).The reagent was analyzed for its potassium and boron contents, which were measured as potassium hydroxide (by a standard acid titration) after hydrolysis and as 2,3-dimethyl2-butanol after oxidation with NaOH-H202 by GC analysis, respectively. Concnetrations of K:B:H 0.42 M0.41 M:0.42 M were clearly establishing K:B:H 1:1:1stoichiometry. The solution was stored over excess KH under a positive pressure of N2for a month and showed no disproportionation in "B NMR and no decrease in its hydride concentration. Reduction of 2-Methylcyclohexanone. The reaction of 2-methylcyclohexanone with potassium 2-thexyl-4,5-dimethyl1,3,2-dioxaborolane hydride (5b)is representative. To a 50-mL, round-bottomed flask fitted with a side arm and capped by a rubber septum were placed 0.8 mL of THF and 5.2 mL of solution hydride of potassium 2-thexyl-4,5-dimethyl-1,3,2-dioxaborolane (0.42 M, 2.2 mmol) in THF and the flask was maintained at 0 "C by an ice bath. To this was added 2.0 mL of THF solution of 2-methylcyclohexanone (1.0 M, 2.0 mmol) cooled to 0 "C, and the mixture was stirred for 3 h. The reaction was quenched with H 2 0 and organoborane was oxidized with NaOH-H202. The aqueous layer was saturated with anhydrous K2C03and the GC analysis of the organic layer after addition of cyclopentanol as an internal standard revealed 84% cis- and 16% trans-2methylcyclohexanol.
Acknowledgment. The support of this work by the United States Army Research Office (Grants ARO DAAG-29-79-027, ARO DUG-29-82-K-0047, and ARO DAAG-29-85-K-0062)is gratefully acknowledged. We also acknowledge several earlier exploratory experiments by Drs. Ramachandra G . Naik and V. Somayaji.
Phosphinylhydrazyls R2NNP(0)L2,an ESR Study. Influence of the Captodative Effect on the Three-Electron NN .rr Bond M. Negareche,' Y. Badrudin,2 Y. Berchadsky, A. Friedmann, and P. Tordo* SREP, Universitd de Provence, CNRS I A 126, 13397 Marseille Ceder 13, France
Received April 22, 1985
l,l-Dimethyl-2-phosphinylhydrazyls and 1-tert-butyl-2-phosphinylhydrazyls have been generated from the corresponding hydrazines. Their radical structurea have been assigned on the basis of their ESR spectral parameters. In contrast with the behavior of trialkylhydrazyls or 1,2-dialkylhydrazyls it has been shown that in the phosphinylhydrazyls the larger nitrogen splitting is due to the tervalent nitrogen. Phosphinylhydrazyls have been shown to exist in equilibrium with their dimer form. The radical Me,NNP(O)(OEt), is particularly persistent, and the free activation energy for the rotation about the NN bond has been measured (AG' = 9.8 i 0.5 kcal/mol).
The solution ESR spectra of a large number of triarylhydrazyls have been studied for many years.3 More recently, Ingold e t aL4reported the first solution spectra for some mono- and dialkyl-substituted hydrazyls as well as t h e spectrum of hydrazyl, HNNH2, itself. T h e electronic distribution in the three-electron T bond
