4118
J . A m . Chem. SOC.1989, 1 1 1 , 4118-4120 Scheme I
Table I oroduct (R =)
RX
entry a b c d e
M e 2a PhCH2 2b Me(CH2)2CH22c M e 2 C H 2d CH,=CHCH, 2e
Me1 PhCH2Br Me(CH2),I Me2CHBr CH2=CHCH2Br
-
1
vield (%) ~
I
~~
74," 796 87," 936 78" 73" 6lC
+
1) O°C/THF/20min
RX 2)
(COhFe
2-6hr
R=(a) Me-
1
"Yields are based on the starting C3H5Fe(C0)31, 1, generated in situ as sodium salt. bYields based on reaction with pure isolated C,H,Fe(CO)CPPN*. (r-C,H5),Fe(C0), is also obtained as reported previ~usly.~
acterized1° a t low temperature (-40 "C) by reaction of 1 with CH31 and PhCH2Br in the absence of PPh,. Treatment of 3a,b with PPh, a t -78 " C results in immediate formation of acyl complexes 4a,b ( I ' , ~< 10 min, -78 "C) which were characterized by ' H N M R spectroscopy." In a first-order process, complexes 4a,b convert to 2a,b via an acyl migration reaction. Rates meas-l, 21 "C, AG* = sured by ' H N M R were k4s-2a = 5.1 X 18.9 kcal/mol for C H 3 C 0 migration and keZb = 6.1 X s-l, 21 OC, AC* = 18.8 kcal/mol for P h C H 2 C 0 migration. No intermediates (i.e., neither 5 nor r-allylhydride species) were detected by lH N M R in the conversion of 4a,b to 2a,b. Free a,@-unsaturated ketones may be readily obtained from 2a-e. Refluxing enone complexes 2b,c in C H 3 C N (2-6 h) gives metal-free enone compounds 6b,c in good yields (70-7776). Alternatively irradiation (GE sunlamp) of enone complexes in CH,CN under mild conditions (0 OC, 2-6 h) leads to 6b,c in somewhat better yields (80-85%).
I
*
PPh3(2eq)/25"C
(b) PhCHz( c ) Me(CH2),(d) Me2CH(e) CHz-CHCHz-
a) Me1 b) PhCH2Br
migration
Y3
0
M
L
m
h!l
regioselectively to the more hindered methyl-substituted carbon of the *-allyl moiety.
hMe ' * ' I
PPh3 (CO)2 Fe
-1) RWO°CfTHFi20min
(C0)3Fe
+
2) PPh&?eq)/25%
26hr
"'$(
PPhj(C0)zFe
Me ME
CH,CN (excess)
2b.S
A:
c
SiLb 2Lb Acknowledgment is made to the National Institutes of Health (GM 28938) for support of this research.
M e / \
reflux or hv
On the basis of the mechanism in Scheme I, if the 16-electron olefin complexes, 5a-e, could be intercepted and displaced prior to its isomerization to 2a-e, then a reaction sequence would be available to prepare not only a,P-enones but also the less readily available unconjugated &y-isomers. This proved possible using the following procedure: After alkylation of anion 1 (R = CH2Ph, -CHMe2, 0 "C, 30 min), the reaction mixture was quenched with excess CH3CN. Irradiation of this mixture at 0 OC for 2-6 h gave the P,y-unsaturated ketones in good yields (75-80%). The products were contaminated by small amounts of a,&unsaturated ketone (
