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of alkyl-l,3,2-dioxaborinanes (I-V) in order to study their stereochemical peculiarities. We expected that because of the planar configuration of the boron atom, the ...

STEREOCHEMISTRY

OF

HETEROCYCLES

XLIX.* INVESTIGATION OF T H E C O N F O R M A T I O N O F ALKYL-I,3,2DIOXABORINANES B Y P M R S P E C T R O S C O P Y UDC 547.87'244 : 541.63 : 543.422.25

V. V. K u z n e t s o v , A. I. Gren', A. V. Bogat-skii, S. P. E g o r o v a , and V. I. S i d o r o v

A n u m b e r of previously undescribed a l k y l - l , 3 , 2 - d i o x a b o r i n a n e s w e r e synthesized by condensation of substituted 1,3-diols with alkylboron dichlorides or dibutyl isopropylborate. It was shown by PMR s p e c t r o s c o p y that the 2,5-dialkyl-l,3,2-dioxaborinane molecules a r e conformationally homogeneous a n d do not contain an axial substituent in the 5 position, w h e r e a s the 2 - i s o p r o p y l 5 , 5 - d i m e t h y l - l , 3 , 2 - d i o x a b o r i n a n e molecules exist in a state of r a p i d r i n g inversion, and intr• duction of methyl substituents in the 4, 4, and 6 positions of the 1,3,2-dioxaborinane ring leads to distortion of the ring conformation and conformational heterogeneity of the investigated s a m ple. The observed r e g u l a r i t i e s a r e explained f r o m the position of intensive " o x y g e n - b o r o n " e l e c t r o n exchange in the h e t e r o r i n g . It is concluded that the 2,5-dialkyl-l,3,2-dioxaborinane molecules have p r i m a r i l y a conformation with a semiplanar f o r m .

S y s t e m a t i c studies with r e s p e c t to the conformational analysis of 1,3-dioxanes and 1,3-d~hianes [2, 3] have made it possible to establish a number of principles resulting f r o m the p r e s e n c e of 1,3-nonbonding i n t e r actions in the ring. Taking the r e s u l t s of t h e s e studies into account, we undertook the synthesis of a number of a l k y l - l , 3 , 2 - d i o x a b o r i n a n e s (I-V) in o r d e r to study their s t e r e o c h e m i c a l p e c u l i a r i t i e s . We expected that because of the planar configuration of the boron atom, the number of nonbonding 1,3 interactions would be diff e r e n t than in 1,3-dioxanes. The synthesis of compounds of this type is also of i n t e r e s t f r o m a p r a c t i c a l point of view, since 1,3,2-dioxaborinanes a r e e_xtremely p r o m i s i n g as additives for gasolines and hydrocarbon l u b r i cants [4]. F r o m the few available studies devoted to the s t e r e o c h e m i s t r y of individual r e p r e s e n t a t i v e s of this s e r i e s [5-8] it is known that the methyl group in 2 - p h e n y l - 5 - m e t h y l - l , 3 , 2 - d i o x a b o r i n a n e is equatorially oriented [5]. The introduction of a second methyl group in the 5 position of the dioxaborinane r i n g leads to rapid ring inv e r s i o n with a r a t h e r low b a r r i e r of 7-8 k c a l / m o l e [5, 6]. The molecules of 2-substituted 4 , 4 , 6 - t r i m e t h y l TABLE 1 R4 H~.Z._O

~'2",f 'B-..

%Mo" R4

bp, "C R~ R' (ram)

R'

R~

i-C3Hr i-C3H7 i-CaHr i-C~Hr t-C4H9

i-C3H; H

d/-O

Empirical riD:~ formula

o

U~ t

II II[ IV v

H

72 (5)

0,8937 1,4317 CgH,~B02 C~H~.~B02 CsFI,~BO2 08815 1,4265! CgHt~BO2 10:842~ 1,4112 i CIoH2~BO.~

C6Hx3IH I[-I I 1o2 (3) 1o,8951 1,4360 CH3 [CH3IH 1,58--59(13) I0,898c 1,4218

H

H

H

H

CH~[

CH~

38 (8)

48(I0)

I

B, %

626130

[found caIc. 4,8 7,1 6,1 5,6

5,2 7,1 6,5 6,0

42 33 40 62

*See [1] for communication XLVIH. I. I. Mechnikov Odessa State University, Odessa 270000. P h y s i c o c h e m i c a l Institute, Academy of Sciences of the Ukrainian SSR, Odessa 270080. T r a n s l a t e d f r o m Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 2630, January, 1978. Original a r t i c l e submitted August 23, 1976; r e v i s i o n submitted April 18, 1977.

0009-3122/78/1401-0019

$07.50 9 1978 Plenum Publishing Corporation

19

TABLE 2. Chemical Shifts and S p i n - S p i n Coupling Constants of A lkyl- 1, 3, 2-dioxaborinanes (all3) HA I-IA B--R

tt a

5, ppm

J, Hz

5~I1a 5-(CII~):

III IV

1,0013,5013,80 1,2013,77]4,271 1,011 3,46 I

--

6-CHz

-- 3,5013,8( -- 3,77[4,27 [ 3,46 1,84 3,75 ----

o~ - -

4,35

"191 -1 - I

I~-JAB[3/.~aI31Ba ~L~bI3.'~

3,75]

1,55

1,851

1,85 4,38 --

4,8,o 0t,l~176 o8,,o

1,50

1,3,2-dioxaborinanes a r e conformationally l e s s labile than 1,3-dioxane molecules [7, 8], although r i n g inversion evidently also o c c u r s for t h e m [6]. However, the lack of s y s t e m a t i c studies of the s t e r e o c h e m i s t r y of compounds of this type does not make it possible to e s t i m a t e the e f f e c t of substituents on the p r i m a r y conformation of the 1,3,2-dic~aborinane ring. The r e a s o n s r e s p o n s i b l e for the r e a l i z a t i o n of only the 5e c o n f o r m e r of 2-substituted 5 - m e t h y l - l , 3 , 2 - d i o x a borinane or inversion of the r i n g of 2-substituted 595-dimethyl- and 4 , 4 , 6 - t r i m e t h y l - 1 , 3 , 2 - d i o x a b o r i n a n e s a r e not examined in the p a p e r s cited above. A l k y l - l , 3 , 2 - d i a x a b o r i n a n e s I-V (Table 1) w e r e synthesized by the methods in [9, 10]. The IR s p e c t r a of all of the synthesized compounds contain an intense band at 1335-1345 cm -1, which c o r r e s p o n d s to the s t r e t c h i n g vibrations of the B - O bond, an intense band at 1300 cm -1, which is c h a r a c t e r i s t i c for the s t r e t c h i n g vibrations of the B - C bond [11], and a n u m b e r of bands of variable intensity at 980-1180 cm -1, which c o r r e s p o n d to the vibrations of the C - O bond. T h e 3J4, 5 v a l u e s and the magnetic nonequivalence of the r i n g methylene protons (Table 2) in the PMR spect r a of I and II indicate the absence of r i n g inversion and the r e a l i z a t i o n of only one p r i m a r y conformation with an e q u a t o r i a l alkyl group attached to 5-C. t t is c h a r a c t e r i s t i c that this s o r t of conformational homogeneity is not o b s e r v e d in the c a s e of 5 - i s o p r o p y l - l , 3 - d i o x a n e s and 1,3-dithianes [2, 3]. In conformity with the data in [5, 6], the PMR s p e c t r u m of HI contains singlet r e s o n a n c e signals of r i n g methylene p r o t o n s and g e m - d i m e t h y l groups attached to the 5-C atom. T h i s c h a r a c t e r of the signals constitutes evidence in favor of r i n g inversion. To explain the o b s e r v e d r e g u l a r i t i e s we a s s u m e p r a c t i c a l l y complete sp 2 hybridization of the r i n g c~ygen a t o m s and, as a consequence of this, the p r e s e n c e of s t r o n g e r (than in 1,3-dioxanes) 1,3-nonbonding interactions of the " a l k y l - u n s h a r e d e l e c t r o n p a i r s of the r i n g h e t e r o a t o m s " type, which hinder r e a l i z a t i o n of a c o n f o r m a tion with an axial orientation of the alkyl group attached to 5-C. In fact, the considerable p - e l e c t r o n exchange [4, 11] between the oxygen and b o r o n atoms should lead to a change in the spatial orientation of the orbitals of the u n s h a r e d e l e c t r o n p a i r s of the r i n g oxygen atoms and to r e i n f o r c e m e n t of the p c h a r a c t e r of the latter. The COB valence angle in 1,3,2-dioxaborinane, which, according to the r e s u l t s of x - r a y diffraction m e a s u r e m e n t s [12], is 120 ~ a value that speaks in favor of an sp2-hybridized oxygen atom, may s e r v e as a confirmation of this conclusion; the p o r b i t a l of the f r e e e l e c t r o n pair is p e r p e n d i c u l a r to the plane passing through the nucleus of the oxygen atom and two of its bonds. On the other hand, the p r e s e n c e of a considerable b a r r i e r to r o t a t i o n about the B - O bond, which, a c c o r d i n g to the data in [13], is 9-12 k c a l / m o l e , should somewhat stabilize a conf o r m a t i o n whose g e o m e t r y allows p a r a l l e l c h a r a c t e r of the orbitals of the u n s h a r e d e l e c t r o n p a i r s of the oxygen a t o m s and a vacant 2pz orbital of the boron atom; i.e., in other words, it should c r e a t e conditions for m a x i m u m n o x y g e n - b o r o n " e l e c t r o n exchange. Analysis of the Dreiding models of the possible conformations of 1,3,2diaxaborinanes p r o v i d e s evidence that the conformation o f the h a l f - c h a i r f o r m (E) is the most favorable in this sense. Data f r o m x - r a y diffraction analysis of 2 - h y d r o x y - 4 , 6 - d i m e t h y l - l , 3 , 2 - d i o x a b o r i n a n e [12] also provide evidence in favor of the E conformation; the value of the dihedral angle f o r m e d by the Ol, B, O s and C4, C 5, and C 6 planes calculated f r o m these data is 132 ~

20

O~A~

0~ B B

\

C

8-

E

D

It follows f r o m the above m a t e r i a l that the ~ p r e f e r r e d n c o n f o r m a t i o n for 2 , 5 - d i a l k y l - l , 3 , 2 - d i o x a b e r i n a n e m o l e c u l e s is the s e m i p l a n a r f o r m or a v e r y strongly c o m p r e s s e d (in the h e t e r o a t o m i c portion of the ring) chair and that the n a t u r e of the 1,3-nonbonding i n t e r a c t i o n s evidently c o n s i s t s in mutual r e p u l s i o n of the axial substituent attached to 5-C and the e l e c t r o n p a i r s of the oxygen a t o m s p e r p e n d i c u l a r to the plane of the h e t e r o a t o m i c portion o f t h e m o l e c u l e . The s a m e r e a s o n is one of the m a j o r r e a s o n s f o r r i n g i n v e r s i o n of r g . The " o x y g e n - b o r o n " e l e c t r o n exchange p r o b a b l y a l s o changes the r e s o n a n c e conditions of the protons of the i s o p r o p y l substituent attached to the boron a t o m in I - I V (Table 2), which shows up in the s p e c t r u m in the f o r m of an a n o m a l o u s singlet instead of a doublet. D a t a on an a n o m a l y of this type a r e not available in the l i t e r a t u r e . This anomaly m a y be due to a change in the shielding constant of the methylidyne proton of the i~opropyl group, evidently u n d e r the influence of the magnetic a n i s o t r o p y of the B - O bond, as a r e s u l t of which it b e c o m e s p r a c t i c a l l y m a g n e t i c a l l y equivalent to the protons of the m e t h y l groups of the i s o p r o p y l group. O n e ' s attention is d i r e c t e d to the c h a r a c t e r of the r e s o n a n c e band of the methylene protons attached to the 5-C a t o m and the proton attached to 6-C in the PMR s p e c t r a of IV and V. The f o r m e r give a multiplet c e n t e r e d at 6 1.85 ppm, while the latter give a low-intensity t r i p l e t at 3.75 p p m (3j= 8.0 Hz) and a complex m u l t i p l e t c e n t e r e d at 4.38 ppm. All of this p r o v i d e s a b a s i s for the a s s u m p t i o n that IV and V a r e e o n f o r m a tionaUy h e t e r o g e n e o u s m L x ~ r e s containing flexible f o r m D. T h i s conclusion is in good a g r e e m e n t with the a b o v e - n o t e d change in the spatial orientation of the u n s h a r e d e l e c t r o n p a i r s of the oxygen a t o m s , which c a u s e s an i n c r e a s e in the c o n f o r m a t i o u a l r i n g s t r a i n b e c a u s e of a " 4 - C H 3 - e l e c t r o n p a i r " shielded interaction in the s e m i p l a n a r c h a i r c o n f o r m a t i o n . T r a n s i t i o n to the D c o n f o r m a t i o n weakens this interaction, as well as the 1,3 i n t e r a c t i o n of the axial CH 3 group with the 6-H proton, although it also leads to a c e r t a i n amount of disruption of the p - e l e c t r o n exchange between oxygen and boron. T h u s the r e s u l t s obtained in this r e s e a r c h make it p o s s i b l e to a s s u m e that the r e a l i z a t i o n of one " p r e f e r r e d ~ c o n f o r m a t i o n with a 5e orientation of the alkyl group of inversion and c o n f o r m a t i o n a l heterogeneity of the 1,3,2-dioxaborinane r i n g a r e d e t e r m i n e d mainly by the p e c u l i a r i t i e s of the " a l k y l - u n s h a r e d e l e c t r o n p a i r s of the r i n g oxygen a t o m s " nonbonding interaction. To obtain additional data on the " p r e f e r r e d " c o n f o r m a t i o n we calculated the dipole m o m e n t s f o r the A-E c o n f o r m a t i o n s of 2 , 5 - d i a l k y l - l , 3 , 2 - d i o x a b o r i n a n e (Table 3). The " p r e f e r a b l e n e s s " of the conformation of the s e m i p l a n a r f o r m for a n u m b e r of 5 - n i t r o - 5 - a l k y l - 2 - p h e n y l - l , 3 , 2 - d i o x a b o r i n a n e s was p r e v i o u s l y proved [14] by this method. However, our data constitute evidence for the low d e g r e e of suitability of the d i p o l e - m o m e n t method for the c o n f o r m a t i o n a l a n a l y s i s of 2 , 5 - d i a l k y l - l , 3 , 2 - d i o x a b o r i n a n e s in view of the s m a l l d i f f e r e n c e s in the dipole m o m e n t s calculated for the v a r i o u s c o n f o r m a t i o n s . EXPERIMENTAL The PMR spectra of solutions of the compounds in chloroform and carbon tetrachloride were measured with an RS-60 spectrometer with hexamethyldisiloxane as the internal standard. The IR spectra of solutions in carbon tetrachloride were recorded with a UR-20 spectrometer. The starting data for the calculation of the dipole moments were: angles OBOo123~ ', COB I19~ ', CCO 109o54 ', and CCC 111~ bond lengths B-O 1.362, C-O 1.459, and C-C 1.510 A [12]; dipole moments B~--O ~ 0.40, B~C ~ 0.25, C-*O N 0.86, and C~-H 0.30 D [14]. TABLE 3. Calculated and Experimental Dipole Moments of the Conformers of 2,5-Diisopropyl1,3,2-dioxaborinane Conformer

g calc, D ~lexp, D

A

B

C

D

E

2,49

2,49

2,36

2,36

2,49

2,46

21

The # exp value was determined from the experimental dielectric permeabilities measured with a Tangens apparatus. The indivtdualRy of the investigated substances was monitored by gas-liquid chromatography with LKhM7M and LKhM-8M chromatographs; the detector was a catharometer, the phases were E-301, SKTFT-50, and Carbowax-20M, the column temperature was 125-135~ and the c a r r i e r - g a s (helium) flow rate was 3.5 m l / m i n . 1,3,2-Dioxaborinanes I and HI [9]. A ~ e of equimolar amounts of 2,2-dimethyl- [15] or 2-isopropyl1,3-propanediol [16] and dibutyl isopropylborate [17] was refluxed in a nitrogen atmosphere for 5 h, after which it was subjec$~l to vacuum distillation. 1,3,2-Dioxaberinanes II, IV, and V [10]. A solution of 0.05 mole of 2-haxyl-l,3-propanediol (obtained by reduction of the corresponding malonic e s t e r by the method in [16]) or 2-methyl-2,4-pentanediol (synthesized by reduction of diaceton~ alcohol with sodium borohydride by the general method in [19]) in 50 ml of methylene chloride was added dropwise with stirring to a cooled (to 0~ solution of 0.05 mole of alkylboron dichloride [18] in 70 ml of methylene chloride in a nitrogen atmosphere, after which the mixture was heated at 40~ until hydrogen chloride evolution ceased completely (~ 3 h). The methylene chloride was then removed by distillation, and the residue was fractionated in vacuo. LITERATURE

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

22

CITED

A.I. Gren', A. M. Turyanskaya, and A. V. Vaigt, i~ Problems in Stereochemistry [in Russian], No. 6, Kiev (1977), p. 87. A . I . Gren', Doctoral Dissertation, Odessa State University, Odessa (1974). A . V . Bogat-skii, in: Application of Conformational Analysis in the Synthesis of New Organic Substances [in Russian], Odessa (1975), p. 3. P . M . Maitlis, Usp. Khim., 33, 748 (1964). F. Davis, I. Turchi, B. Maryanoff, and R. Hutching, J. Org. Chem., 37, 1583 (1972). D. Carton, A. Pontier, M. Ponet, J. Soulie, and P. Cadiot, Tetrahedron Lett., No. 28, 2333 (1975). W. Woods, I. Bengelsdorf, and D. Hunter, J. Org. Chem., ~ 2766 (1966). W. Woods and P. Strong, J. Am. Chem. Soc., ~ 4667 (1966). R. Mehrotra and G. Srivastava, J. Indian Chem. Soc., 39, 203 (1962). R. Cragg and M. Nazery, J. Chem. Soc., Dalton Trans., No. 13, 1438 (1974). W. Gerrard, Organic Chemistry of Boron, Academic P r e s s (1961). S. Kuribayashi, Bull. Chem. Soc. Jpn., 46, 1045 (1973). P. Finocchiaro, D. Gust, and K. Mislow, J. Am. Chem. Soc., 95, 7029 (1973). T. Urbanski, D. G'tirne, R. Kolinski, H. Piotrowska, A. Janczyk, B. Serafin, M. Szretter-Szmid, and M. Witanowski, Nitro Compounds. Proceedings of the International Symposium, Warsaw (1963), p. 195. W. Whitmore, J. Am. Chem. Soc., 63, 124 (1941). R. NystromandW. Brown, J. Am. Chem. Soc., 69, 1197 (1947). P. Brindly, W. Gerrard, and M. Lappert, J. Chem. Soc., 2956 (1955). B . M . Mikhailov and T. A. Shchegoleva, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, No. 9, 1080 (1957)o J. Prichard and R. Vollmer, J. Org. Chem., 28, 1545 (1963).

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