COMMUNICATIONS TO THE EDITOR
4,504
The Signs of Sitg-H1and Si2Y-F1g Coupling Constants1
I I
0.5
Vol S(i
Sir:
5
4
7
6
8
10
9
11
Number of Corben Atoms in u-Chlaro.l-olkene
Fig. 1.-Plot of t h e r a t e d a t a based on t h e assumption t h a t t h e substituent effect changes by a cotistarit factor per methylene group.
Based on the marked deviation of the point for 5 chloro- 1-pentene from the line defined by the other rate constants, the rate of reaction of 5-chloro-l -pentene may be calculated to be seven to ten times faster than expected in the absence of chlorine participation. Since terminal and nonterminal substituents show similar rate effects,2a the per cent of 1,4-chlorine shift expected for the reaction shown in eq. 1 is 43-15.4 The agreement with the value found, 40yc, is fair. TABLE I RATESOF XDDITIOYOF TRIFLUOROACETIC .\CID 3 0 U-CIILORO1ALKEYES AT 60 (lo" Alkene
k
x
.ec
-1
10-
60"
4-Chloro- 1-butene 5-Chloro-1-pentene 6-Chloro- 1-hexene 7-Chloro-1-heptene 8-Chloro- 1-octene (1-Chloro-1-nonene 11-Chloro-1-undecene
0.125 -11 sodium trifluoroacetate was present t o neutralize ad\-entitious traces of strong acid Alkene concentrations were 0 . 1 or 0.0;3 A ~ I 6 Determined bl- M r . G Thompson. From ref. 3 1'
Previously a 1,l-chlorine shift was found in addition of trifluoroacetic acid to 5-chloro-1-pentyne5 but not in addition of trifluoroacetic acid to 5-chloro-1-pentene. The present demonstration of a 1,l-chlorine shift in an alkene addition supports previous postulate^^,^ colicerning the importance of cyclic halonium ion intermediates in alkene additions and demonstrates t h a t halogen shifts are not a special feature associated with the possible intermediacy of vinyl cations in the reaction of alkynes. ,:1i 1' k< P e t e r a o n a n d C . Allen, J . A m ( ' h e m Soc , 66, 3608 (196:3). 4 ) F o l l , , w i n g t h e usual t r e a t m e n t f o r e s t i m a t i n g a n c h i m e r i c assistance. ant1 u i i n g t h e a v e r a g e d v a l u e . (7 C 10, 2 = 8 .5, fi ion o f assisted r e a c t i o n I k t C 3 , . ,= ( 7 5 8 3 = 0.88. T h e per c e n t ofchlorine ,hiit is ,50'r X 0.88 = 44'.< 'i: I' li P e t c r c o n a n d J E r l u d d e y , .I A t r t . C ' h r m .Tor.. 8 6 , 2865 (1H6:V i
PAULE PETERSOX EDDIE1. P TAO RECEIVED ACGVST A. 1961
Although it is well established theoretically? and experimentally3 that the signs of directly bonded X-H coupling constants are positive (with the exception of H-F),i t is less certain t h a t directly bonded X-Y coupling constants follow the same pattern. Iieceiit 110 calculations by Pople and Santry?" indicate that the X-Y coupling constants for first-row elements are likely to be negative if one of the nucleii is F L Y anti positive in all other cases in agreement with the limited experimental data available a t present4 In this communication we wish to report the signs for directly bonded SiZ9-H1and Si2g-F*9 coupling constants. These have been determined by carrying out { S i 2 Y } H 1 field-sweep double resonance experiments 011 (CH:J?SiHCl and (CH3)&iF molecules, respectively. The procedure followed can be illustrated by considering the case of (CH;I)aSiFin some detail. The proton spectrum for this compound, measured with a Varian DP-MI spectrometer, consists of a doublet centered a t r !).SO f O . ( l l , with J~IL-C--Sj-F1g = i . 5 1 f 0.05 c.p.s. Superimposed about the midpoints of the main proton signals are Si2y satellite doublets5 with J H , - c.si>,7.09 0.05 c.p.s. Allthoughthe Si29spectrum was not recorded. the first-order spectrum would consist of two sets of decets ( J H i - C - S i ? q = 7.09 c.p.s.) separated by J s ~ ? # -between FL~ their midpoints The SizYradiofrequency field was applied a t 11.%I 5 Xlc. sec. with a Kohde and Schwarz Type X V A frequency synthesizer, and frequencies were monitored with a Kohde and Schwarz Type FKhl counter. The Si29 frequency was varied in small increments (5-10 c.P.s.), while repeatedly scanning the proton spectrum until the correct decoupling frequency was obtained. Since J S i Z ~>> -~1 JS 8i29-C i t -is ~ Ipossi, ble to irradiate near the frequency of one set of Siz9multiplets without perturbing transitions of the other set. The relative signs of and J H - C - S ~ - I ~ N can therefore be determined by irradiating one of the SizYmultiplets and observing which pair of satellite lines collapses. For (CH3)&F, irradiation a t the lowest SizYfrequency collapsed the l o w f i e l d satellite, increasing the Si2gfrequency by 280 c . P . s . ; i.e., Js~?Q.-FI~ led to a collapse of the high-field satellite. Thus JSi2q-FlY and J H ~ - ~ have - ~ ~the . . same F ~ ~sign (note, y for Siz9is negative). Since it is likely that the vicinal HI-FI9 coupling constant is positive,6 it can be concluded that Jsi23-~19 is also positive. However, because of the negative magnetogyric ratio of Si?H, the reduced coupling constant?a given by K = ( 2 ~
*
(11 T h i s a i i r k was carried o u t while t h e a u t h o r was a g u e s t worker a t t h e N a t i o n a l Physical I , a h o r a t o r y , Basic Physics Ijivision, T e d d i n g t < ) n ,ISnglancl ( 1 ) ( a ) J . A Pople a n d I ) . P.S a n t r y , JJo:. P h y s , 8 , 1 !1961), (b) h i K a r l ) Ius a n d 1) ?. GI r a.n t . P r o c . Sa!! A c a d . Sci. C , S., 45, 1260 (lSiiY), ( c ) hl K a r p l u s , J . A n i Chrm Soc.. 84, 24.58 ( 1 9 6 2 ) , ( d ) S > f u l l e r , J C h f m P h y c , 36, 3.59 (1962). (31 ($1) P C 1,auterhur a n d I
