June 5, 1961
HYDROBORBTION WITH
TRIMETHYLANINE
184'; therefore, it appears likely that the acid of m.p. 182187" consisted of 2,2,4-triphenylbutanoic acid with less than 2 mole yoimpurity. A portion (2.00 9.)of the acid of m.p. 182-187' was recrystallized from benzene and then sublimed in vacuo to give 1.80g. of acid, m.p. 186.1-187.1". Thisacid gave no depression of m.p. when mixed with an authentic sample of 2,2,4-triphenylbutanoic acid. The authentic sample of 2,2,4-triphenylbutanoic acid was prepared from 1,1,3-triphenylpropanol-l which was made from 2-phenylethylmagnesium bromide and benzophenone after the method of Kharasch and This alcohol was converted to its methyl ether and the ether was cleaved by Na-K alloy by the procedure of Ziegler and co-workers6 ex(19) RI. S Kharasch and .T. G. Burt, J . Org. Chem., 16, 150 (1951).
[COSTRIBUTION FROK
THE
2541
&BUTYLBORANE
cept that instead of shaking with. the alloy, stirring for 6 hours on a high-speed stirrer15 was employed. Carbonation of the reaction mixture gave after the usual isolation and purification procedure 2,2,4-triphenylbutanoic acid, m.p. 186.1-186.9' (recorded5m.p. 181").
Acknowledgments.-This work was supported in part from grants from the Engineering Experiment Station, Georgia Institute of Technology, Project E-162. Grateful acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Rayonier Corporation for partial support of this work.
ROHM& HAASCo., REDSTONE ARSENAL RESEARCH DIVISION,HUNTSVILLE, ALA.]
Amine Boranes. VIII. The Hydroboration of Terminal Olefins, Dienes and Terminal Acetylenes with Trimethylamine &Butylborane' BY M. FREDERICK HAWTHORNE~ RECEIVED FEBRUARY 2, 1961 Trimethylamine t-butylborane reacted with terminal olefins to afford a convenient synthesis of t-butyldialkylboranes. Similarly, 1,4-pentadiene, 1,3-butadiene and 2-methyl-l,3-butadiene formed 1-t-butyl-I-boracycloalkaneswhen treated with trimethylamine t-butylborane. Divinyl ether and dimethyldivinylsilane afforded similar cyclic derivatives which contained an additional heteroatom. Terminal acetylenes and trimethylamine t-butylborane provide a route to t-butyl di-( 1alkeny1)-boranes of unknown stereochemistry.
The recent application of pyridine borane3 and trimethylamine borane4s6to the hydroboration6 of olefins suggested the possibility of similar reactions with the recently described trimethylamine alkylborane^.^ A s previously reported,' the latter materials were prepared by the lithium aluminum hydride reduction of alkylboronic anhydrides in the presence of trimethylamine. Hennion, McCusker and Rutowskia have convincingly demonstrated that the mixed trialkylborane, t-butyldiisobutylborane, is relatively stable toward disproportionation. This enhanced stability presumably arises from the large steric requirement of the t-butyl group in the transition state which leads to disproportionation. Brown and Zweifelg have employed a sterically encumbered dialkylborane in their study of terminal acetylene hydroboration. This reagent was di-(3-methyl-2-butyl)-boraneprepared from diborane and 2-methyl-2-butene. This same reagent was also employed to provide steric control of olefin hydroboration. lo Thus the stabilization of mixed trialkylboranes and the moderation of the facile hydroboration reaction have been accomplished by the use of bulky alkyl substituents on boron. In view of these results and our own past experiences
with trialkylamine alkylboranes, i t appeared to be possible to utilize a sterically moderated member of that series of reagents in selective hydroboration reactions. Such reactions could conceivably lead to the preparation of novel boranes. Trimethylamine t-butylborane was chosen as the hydroboration reagent and employed with such model substrates as terminal olefins, dienes and terminal acetylenes. Results and Discussion The reversible thermal dissociation of trimethylamine-t-butylboranel would produce t-butylborane. If this process were carried out in the presence of an unsaturated substrate the free t-butylborane might be easily captured to produce a hydroboration product. Since t-butylborane contains two boron-hydrogen bonds, this process could be repeated once again with the release of trimethylamine.l 1
(1) Previously presented in part as a Communication t o the Editor, J . A m . Chem. SOC.,82, 748 (1960). (2) Visiting Lecturer, Harvard University, Fall, 1960. (3) M. F. Hawthorne, J . 0 7 g . Chem., 23, 1788 (1958). (4) R . Koster, Angew. Chem., 69, 684 (1957). (5) E. C. Ashby, J . A m . Chem. SOL.,81, 4791 (1959). (6) H . C. Brown and B. C. Subba Rao, J . 078. Chem., 22, 1136
t-CdHgBH??( CH3)3
(1957).
( 7 ) M. F. Hawthorne, J . A m . Chem. S O L .81, , 5836 (1959). (8) G. F. Hennion, P. A. McCusker and A. J. Rutowski, ibid., SO, 617 (1958). (9) H . C. Brown and G.Zweifel, ibid., 81, 1512 (1959). (10) H. C. Brown and G. Zweifel, ibid., 82, 3222 (lQOO), 8B, 3217 (1QCO).
:+
t-C4Hy6H&(CH3)38 t-CaH9BH S(CHJjI t-C,HyBHz RCH=CH2 + t-CIHsBH( CH*CH?R) t-CaHgBH(CHzCH2R) RCH=CHz + t-CdHsB(CH2CHzR)a
+
+
+ 2RCH=CHz
+
~ - C ~ H QCHzCHzR)z B(
+ IY(CHJ
j3
t-Butyldialkylboranes from Terminal Olefins.In contrast to the reactions of pyridine boranes and trimethylamine b ~ r a n e ~with . ~ olefins, trimethylamine t-butylborane reacted readily with such simple olefins as ethylene and propylene a t temperatures as low as fifty degrees. The amine (11) Since the higher molecular weight trialkylboranes have low vapor pressures and form weak complexes with trimethylamine, this result was anticipated3
2532
111. FREDERICK HAWTHORNE
Substrate
Ethyle~ie Propylene I-Butcne Isobutene 1,3-Butadiene ?-Methyl-1,3butadiene 1,4-Pcntadiene D i v i t i y l ether Diinethyldivinylsi1;ine I-Cutyne I-Pentyne I-Hexyne ' M.P. 65-68'.
t-Butyl diethylborane t-Butyldi-f l-propyl)-boraiie t - B u t y l d i - ; 1-butyl )-!mrane i - B u t y l d i - i i-hutyl). borane 1-t-Butyl- 1 -bnrocyclopentane 1-t-but~l-3-1licthy.1-I -borocyclopentane 1-l-Rutyl-1-borocyclohexane l-t-Butyl-l - l o r u - l - o x a c ~ c l t ) ~ i e x a n e 1-t-B11;Yj-I-t,or!r-4;~-di~i:ethyl-4silncyclohex,!ne t - B u t y l d i - ( 1-btitciiyl)-borane t-Butyldi-f l-l)ei:teriyl)-bor3ne t-Bctyldi-; 1-hexeny1)-borane C and H analyses unavailable.
complexes of unsubstituted borane rerpired much higher temperatures and longer reaction times for complete utilization in the, hydroboration reaction. This greater reactivity of the t-butvlhorane derivative might be attributed to the greater rate of dissociation of its trimethylamine complex.l ? The products of these reactions 1:-ith simple olefins were trimethylamine and the corresponding t-butyldialkylboranes. These reactions were routinely carried out without solvent and the trialkylborane product was easily purified bv fractional distillation at reduced pressures. Table I presents analytical and preparative data for the products obtained from ethylene, propylene, 1-butene arid 2methylpropene. The infrared spectra of the products contained no evidence of a B-H group but only bands coininonly attributed to alkyl group absorptions. The HI nuclear magnetic resonance spectra were not easily interpreted since the alkyl groups gave resonance lines which often closely coincided with the relatively strong t-butyl group resonance. However, the spectrum obtained with t-butyldiisobutylborane was superimposible upon the spectrum obtained by Davies, Hare and m'hite13 except that the small and ambiguous resonance reported by these authors did not appear.'*$ Due to the fact that the isobutyl group in t-butyldiisobutylborane and similar species13 presents a resonance line a t higher field than that which arises from the f-butyl group. the products obtained in this study were seen to be free of isobutyl group contamination. Such contamination could arise from the thcrmal isomerization of the t-butyl group. The hiyhest field resonance line observed with all products, other than tbutJ.ldiisobutylborane, was of very high intensity and centered near 5.87 p.p.m. higher Geld than benzene. This resonance line is assigned to the f butyl group since it is essentially congruent lvith (12) S u c h an acceleration is not unexpected if t h e trimethylamine Ibutylborane niolecule is suhject t o "E-strain" not present in t h e trirnethvlaniine borzne species. This "B-str?in" noiild be lost upon dissociation. See H. c. Frown a n d R. X. Johnnnesen, J. A m . Chfi,z. Soc., 7 6 , I f i (19.5:;), f o r :L .;umm:iry of steric striii,s , ' I , ; ) A G. l).,vies, D. (i,€[are am1 R. F. 1 1 . \%'bite, J . Chem. S o d . ,
ioio ~ i v i o ~ il II 1 '2. :>riv.,tc cornmunicxtion Dr. Day.-ies ( ~ ho 5 examined our sprctriini) suggested t h a t this small !inr rri:*y be due t o an oxidation grorliict. ; s n d not restricted rotation as :ne\-ic>uslyproposed."
TJOI.
-, _, .22
15.19
1
15.04
0
--I ,:I3
83
79.i 2
14.94 79.12 1 4 . 9 4 77,4G 13.82
78.29
13,s:
78.29 1.'3 . 8; b
(55.93 12.73 13.0;' 81.55 13.20 52.04 13.34 Si), 93
that observed by Davies, Hare and IVhite.l3 These data shoiv that simple terminal olefins are hydroborated in the terminal position without r e a r r n n p ment of the attached t-butyl group. 1-f-Butyl-1-boracycloalkanes and 1-t-Eutyl-1boracyclohexanes with an Additional Intracyclic Heteroatom.-The successful hydroboration of terminal olefins described above clear!y illustrated the bifunctional character of t-butylborane. Thus, the reaction of t-butylborane n-ith dienes was a lngical extension of this reaction which was expected to yield cyclic products. Cyclic boranes of this type have been previously described b:Kostcr. l5
/? C'II?
/ t-C41-IaR' '\
CRiCHz I I
1
+ N(CHaI3
C€I2CII?
Butadiene, isoprene and 1,Gpentadiene were found to react rdpidlv with trimethylamine tbutylborane to prodcce easily purified and air sensitive products. These products were assigned l-tthe structures l-i-but~l-l-borac~-clopentane, butyl-l-bora-3-n1eth~-lcyclopenta1ie and 1-t-butyl1-Soracyclohexane, respectively. The preparative reactions were condtzcted a t atmospheric pressure and a t 65. without solvent. Table I presents the pertinent preparative and characterization data. The H' nuclear niegiietic resonance spectra of these substances exhibited the resonance line attributed to the t-butyl group b i d e ultra) plus undefined resonances at slightly lower field. In addition, the spectrum obtained with the 1,4-pentadienc product contained a small shoulder on the high field side of the strong t-butyl group resonance line. This shoulder is attributed to the 4-methylene group of l-t-butyl-l-borcc!rclohexaneproduct rather than (15) R Koster, .4?2gew Chem , 620, 521 (l9>9) briefly regorts t h e preparation of 1-alkyl- 1- boracyclopentanes a n d t h e analogous cyclohexane (lerivatives.
June 5, 1961
HYDROBORATION WITH TRIMETHYLAMINE &BUTYLBORANE
to contamination of the analytical sample by the B -isobutylisomer. Infrared spectra of each of these products was devoid of absorptions characteristic of boronhydrogen bonds or carbon-carbon double bonds. Furthermore, the boiling points of purified products were sufficiently low to rule out the unlikely possibility of dimer formation. Divinyl ether and trimethylamine t-butylborane were allowed to react without solvent a t atmospheric pressure and a t 70". As the reaction proceeded a volatile crystalline solid was seen to form. This material proved to be the principal reaction product. Purification was carried out by distillation or by recrystallization from pentane a t low temperatures. The purified product (m.p. 6568') was exceedingly reactive toward traces of oxygen but glove-box techniques allowed a determination of its molecular weight and H1 nuclear magnetic resonance spectrum. Attempts to obtain its C and H analyses were fruitless with the techniques available. A boron analysis was obtained by the analysis of aliquots of a solution of known concentration prepared in acetonitrile in the glove-box. These analytical results are shown in Table I. The molecular weight was obtained in benzene solution by the cryoscopic method (found 132 and 144, theory 140). The H1 nuclear mngnetic resonance spectrum obtained was in excellent agreement with the assigned structure.
-
(CH,=CH)
2 0
I-CjH,
Two triplets of equal area were observed a t f2.6S and +4.96 p.p.m. relative to a benzene internal standard. These triplets may be assigned to the two methylene groups bonded to oxygen (low field triplet) and the two methylene groups bonded to boron (high field triplet). The t-butyl group was observed as a single sharp peak a t +5.56 p.p.m. An estimate of resonance peak areas (4 : 4 : 9) agreed with this assignment. Dimethyldivinylsilane and trimethylamine tbutylborane reacted in a similar fashion to afford an easily purified liquid product. -Analyses proved this product to be 1: 1 adduct.
- +
CH3\
I - CIH,BHzN(CH?)3
+
-
I
/H3
C t i 'CH,
I
+ N(C113)3
The H' nuclear magnetic resonance spectrum was characterized by the t-butyl group resonance a t +5.75 p.p.m., a poorly defined triplet centered a t +5.38 p.p.m. and an additional pair of sharp peaks with shoulders centered near +6.70 p.p.m. relative to benzene. The low field triplet may be assigned to the two methylene groups which are bonded to boron. The remaining absorptions
2543
which appear a t high feld may be assigned to the two methyl groups and the two methylene groups which are bonded to silicon and give nearly identical resonances. The infrared spectrum of this compound was compatible with the assigned structure in that no B--E1 or carbon-carbon double bond absorptions were seen. t-Butyldi-(1-alkeny1)-boranes from Terminal Acetylenes.-The reactions of trimethylamine tbutylborane with the terminal acetylenes 1-butyne, 1-pentvne and 1-hexyne were examined. In each case the reaction was strongly exothermic and requircd the use of tetrahydrofuran as a temperature moderating solvent. The selected reaction temperature was GO" and the reaction products were separated and purified by fractional distillation a t reduced pressures. Table I records the preparative and characterization data obtained in these reactions.
+
t-CaH9kH2k(CH3)3 BRCHECH --+ t-CdHgB( CH=CHR)? 4- K( CHI)*
Aside from the analytical data presented in Table I the infrared absorption spectnm and the H1 nuclear magnetic resonance spectrum of each of the products supported their assigned structures. In the infrared, each product displayed strong carboncarbon double bond absorption a t 6.20 p and strong absorption a t 10.0 p. The H1 nuclear magnetic resonance spectra contained the strong resonance line of the t-butyl group a t 5.92 p.p.m. higher field than ben7ene and an unresolved peak a t 0.50 p.p.m. higher field than benzene. The latter peak is attributed to the two hydrogen atoms attached to unsaturated carbon atoms, Other resonance lines were observed Tvhich were not clearly resolved but were nonetheless compatible with al!
