Indian Journal of Chemistry Vol. 42A, January 2003, pp. 11 - 18 Papers Methyl and vinyl substituted IH-, 2H-, and 3H-phospholes and their Diels-Alder reactions with ethylene: A semiempirical AMI study . K Geetha & G Narahari Sastry* Department of Chemistry, Pondicherry University, Pundicherry 605 014, India Email: gnsastry@ya hoo . com Received 23 July 2002; revised 30 September 2002 The equilibrium geo metries and the relative stabilities of a ll the possible isomers of pho spho Ie , methylphosphole a nd vin ylphosphole have been estimat ed using the se miempiri ca l AM I method. The Di els-Alder trans iti on stat es and products between eac h of th ese iso mers, with ethylene as dien op hile, have been loca ted. AM I results show that 2H-pho sphole is more stable th an 3H-phosphole by 3.6 kca l mol-I which is in turn more stable than I H-pho sphole by abo ut 6.0 kcal mol- I. The Di els-A ld er activation energies of 2H-phospholes are mu ch lower compared to those involving I H- and 3H-p hosphol es . Either methyl or vin yl substitution does not a lt er the broad features in th e activation and reac ti on energ ies compare:: to the parent co mpound. Ever since the synthetic ava ilability of phospholes and their substituted analogs, their stability, aromaticity, a nd reactivity have generated considerable interest ' - 6 . Diels-Alder reactions of phospholes were proved to be an important route to access phosphorous containing polycyclic compounds 5 - t I. In various aromaticity indices, phosphole (1) is the least aromatic among the five membered heterocycles: pyrrole, furan, thiophene and phospholeI 2 -1 4 . The lower aromaticity of phosphole (1) is expected to increase their diene character of phosphole enhancing the reactIv Ity towards cycloaddition reactions compared to the other heterocycles. Functionalization of the Diels-Alder adducts seems to provide access to novel frameworks with potential industrial applications 5 . 6 . The relative stabilities of 1 H- , 2H- and 3H- phospholes and their substituted analogs were important factors especially as the [1 ,5] sigmatropic shift s, which result in the inter-conversions among these isomers, occur in a more facile manner in these molecules ' · 3 . Computational studies have played important roles in understanding the Diels-Alder reactivity of phospholes 8 . lo . 2H-phospho les were shown to have lower activation energies compared to their I H- and 2H-analogs. The relative stabilities of the substituted phospholes were also studied. However, theoretical studies of the Diels-Alder reactions on the substituted phospholes were scarce. Considering the fact that several substituted phospho\es are available and also were taking part in the Diels-Alder reac tions, it should be interesting to see how the substituents affect th e structures, stabilities and reactivities of phospholes. In this study we aim to understand the per turbations caused by an electron donating group, CH 3 , and an electron withdrawing group, -CH=CH 2 , on th e relative stabilities of all the possible isomers of methyl and vinyl phos pho les. Scheme I depicts all th e 0 p /0 0 I H 2 3 R OJ:) 10 p R p p I R 4 6 R R 10 f) , 0--, f,?: R H 7 8 9 10 R o 0-,9 , JO R H 11 12 13 14 Scheme 1- 1H-, 2H- 3nd 3H- phosphol es 3nd th eir me th yl and vin yl substitution at various pos iti ons.
Transcript
Indian Journal o f Chemistry Vol. 42A, January 2003, pp. 11 - 18
Papers
Methyl and vinyl substituted IH-, 2H-, and 3H-phospholes and their Diels-Alder reactions with ethylene: A semiempirical AMI study
. K Geetha & G Narahari Sastry*
Department of Chemistry, Pondicherry University, Pundicherry 605 014, Indi a
The equilibrium geometri es and the re lative stabilities of all the possible isomers of phospho Ie, methylphosphole and vinylphospho le have been estimated using the semiempirical AM I method. The Di els-A lder transiti on states and products between each of these isomers, with ethy lene as dienophile, have been located. AM I results show that 2H-phosphole is more stab le than 3H-phosphole by 3.6 kcal mol- I which is in turn more stable than I H-phosphole by about 6.0 kcal mo l- I. The Diels-A lder activation energies of 2H-phospholes are much lower compared to those involving I H- and 3H-phospholes. Either methyl or vinyl substitution does not alter the broad features in the ac tivatio n and reac tio n energies compare:: to the parent compound.
Ever since the synthetic availability of phospholes and their substituted analogs, their stabi lity, aromaticity, and reactivity have generated considerable interest '-
6.
Diels-Alder reactions of phospholes were proved to be an important route to access phosphorous containing polycyclic compounds5
-t I. In various
aromaticity indices, phosphole (1) is the least aromatic among the five membered heterocycles: pyrrole, furan, thiophene and phospholeI 2
- 14
. The lower aromaticity of phosphole (1) is expected to increase their diene character of phosphole enhancing the reactIvIty towards cycloaddition reactions compared to the other heterocycles . Functionalization of the Diel s-Alder adducts seems to provide access to novel frameworks with potential industrial applications5
.6
.
The relative stabilities of 1 H-, 2H- and 3Hphospholes and their substituted analogs were important factors especially as the [1 ,5] sigmatropic shifts, which result in the inter-conversions among these isomers, occur in a more facile manner in these molecules ' ·
3. Computational studies have played
important ro les in understanding the Diels-Alder reactivity of phospholes8
.lo
. 2H-phospholes were shown to have lower activation energies compared to their I H- and 2H-analogs. The relative stabilities of the substituted phospholes were also studied. However, theoretical studies of the Diels-Alder reactions on the substituted phospholes were scarce. Considering the fact that several substituted phospho\es are avai lab le and also were taking part in
the Diels-Alder reactions, it should be interesting to see how the substituents affect the structures, stabilities and reactivities of phospholes.
In this study we aim to understand the perturbations caused by an electron donating group, CH3, and an electron withdrawing group, -CH=CH 2, on the relative stabilities of all the poss ibl e isomers of methyl and vinyl phospholes. Scheme I depicts all the
0 p /0 0
I H
2 3
R
OJ:) 10 p R p p I R 4 6
R R
10 f) ,0--, f,?: R H
7 8 9 10
R o 0-,9 ,JO R H
11 12 13 14
Scheme 1- 1 H-, 2H- 3nd 3H- phospholes 3nd the ir methy l and vinyl substitution at various pos itio ns.
12 INDIAN J CHEM, SEC A, JANUARY 2003
dienes considered ir. the study . These subs tituents, in add itio n to exerting the e lectronic effects, also are ex pected to increase the kinetic stability through steric protection . However, the present study is restricted to treat the phospholes as di enes, with ethy lene as d ienoph ile. Of course, dimeri zat io n is a certai n alternative, which is interesting to explore, and the results of that study wil l be publi shed separately . Methyl and viny l substituents were chosen as idealized ones to mimic the electron donating and wi thdrawing effects respect ively and th is makes the tota l number of dienes cons idered incl uding parent phospholes to 25. Frontier orb ital anaiys is and the type of e lectron demand were analyzed fo r each of the reactan t pairs. T he s tructures and energetics of the transition states and products were discussed.
Computational Methods Diels-Alder reaction has been a challenging
. . l b ' I 15-19 Wh' l reacti on to mooe y computat lona means . 1 e quite a higher level of theory including electron correlation is desirable, such calculations become very expensive as the sys tem size increases . Although semi empiri cal methods are not quite adequate in distinguishing between stepwise and concerted mechanisms, AM 120 calcul at ions were expected to y ie ld quite reasonab le results for this class of reactions . Therefore , we resorted to adopt this co mputationally viable methodology in th is s tudy. Even though the present study may not yield numbers o f high quantitative accuracy, it is expected to provide qualitatively correct results .
The structures of a ll the phospholes considered were optimized using AM I method and the natures of the stati o nary points were characterized by frequency calculati ons. All the reactions considered in the study are assumed to follow an asynchronous pathway, due to the unsymmetri cal diene-dicnophi Ie combi nations, and concerted mechanism. Of course, an alternative stepwi se mechanism certainly exists for all the reactions considered but that pathway is expected to lie abo ut 4 -6 kcallmol over the concerted one and therefore we have not considered this alternati ve in the present study . T hen, the transition state structures and the products were located for each of the di enes considered, with ethylene as the dienophile. The saddle poi nts located on the concerted pathway were characteri zed as true transi ti on s tates, possessing one imagin ary frequency in all cases, while the reactants and products were characterized as min ima. The normal modes corresponding to the imaginary frequency were verified by MOPLOT program
package21. In a ll the cases the reaction vector corresponds to the concerted transi tio n state. The frontier orbital energies and the quan tum of charge transfer from the diene to dienophile were also computed at AM 1 leve l. All the calculations were done using the Gauss ian 98 suite of programs22 . While the semiempirical methods. AM 1 and PM 3 have been app lied o n phospho rous containing compounds and the qual itative agreement is not very good with higher levels of theory23-25 . However, previous studies conducted in our laboratory indicate that the qual itative trends are expected not to vary drastically fro m what is obtai ned at sem iempi rica l AM I level of theory 1:;.16.26.27.
Results and DisclJssion The equilibrium geomet ri es of the reactants and
their relati ve stabilities followed by a discussion o n the geometries of the trans itio n states and products are given. Frontie r orbital energ ies along with the type o f electron demand and quantum of ciurge transfer are dealt with next. Finally , the acti vation energies and reactions energies are discussed.
Reactant geo fll etries and energies The optimized geometri es of the parent and the
me thy l substituted phospholes are depicted in Fig. I and similarly the viny l substituted ones are g iven in Fig. 2. A close look at Figs 1 and 2 indicates that the skeleta l bond lengths in the parent and both methy l and vinyl subst ituted analogs are very simil ar, indicating no drasti c changes in the geometries of phospholes upon substitutio n by e ither e lectro n donating or wi thdrawing groups. Among the geometric para meters, the most signi ficant changes are seen in the bond lengths with the substituted bonds, and in mos t of the cases the extended conjugation is seen in the geometries . While the conjugation is strai ghtforward in the case of viny l substituent, the methyl substituent experiences si milar
effect through the pseudo n-orbita ls, albe it to a different extent. Ex pectedly , the exten t of conjugati on is higher wi th the vinyl substituen t, and the C(ring)C(methyl) bond lengths are in the range of ! .46 1-1.472A, whi le the C(ring)-C(vinyl) bo nd lengths are in the range of 1.434- 1.447 A .
The relative energies of the parent as well as methyl and viny l substituted phospho lcs are g iven in Table l. These AM I relative e nergy trends for the paren t phospholes (1-3) are in good agreement w ith the MP2 results repo rted earlier; a lso it is interesting to note that HF results are in disag reement w ith the
GEETHA et al. : SEMIEMPIRICAL AM I STUDY OF PHOSPHOLES
1.590// \\1.364 1.592// \\ 1.357 C 1.468 p ' c3 P 3 -"""": '
1 7~ c / 1 .485 1.8~ c; 1.487 • <; 1 1.499
9M 10M
P 1.738c
P 1.733c
1.587// \\ 1.342 1.600// \'~ 1.343
c c2 1.463 c4 " / 2
4~ / ~.492 c3
1.498 1.492 c3
1.507
1.512 \
13M 14M
Fig. I- AM I optimized bond lengths of I H-, 2H- and 3 H-phl~spholes and their methyl substituted ana logs.
13
Table I- Relative energies of parent, methyl and vinyl substituted I H-, 211-, 3H-phospholes obtained at AM I level. All values are given in keal mol- I
MP2 values2. Thi s indicates the importance of
including dynamic electron correlation to obtain the correct relati ve energy orderi ng for the SLl bsti tLl ted pbospholes. Dienes
I
2 3
4M
SM 6M 7M 8M 9M 10M 11M 12M 13M 14M
/'0..£
6.0
0.0
3.6
3.3
6. 1
6.3
0.3
0.5
0.0
4.8
4.0
3.9
8.2
3.7
Dienes /'0..£
4V
SV 6V
7V
8V
9V IOV
]]V
12V
13V
14V
6.1
6.0
7.2
0.3
0.9
0.0
5.6
4.1 4.0
lOA
3.2
Geomet ries of transition structures and products Tables 2 and 3 depict the geometri es of all the
transition states and products for the methyl and vinyl substituted phospho les (and parent) respectively (see scheme 2 for designation and nomenclature). In al L there are 3 parent phospholes and 11 different reactant pairs each for methyl and vinyl substituted isomers . In six reactant pairs, 1, 4, 5 , 6, 10 and 13, there are two possible routes, namely, syn and anti. When the substituent is disposed toward the diene moiety, the corresponding transi tion state/product is designated as syn. The anti products and transitio n slat.es are
14 INDIAN J CHEM, SEC A, JANUARY 2003
1.467 C2-C3
1345// \\1.345
C,~ C
1.460 C2-C3 1357'1 \\ 1.347
C.~1.434C / C
1.759 p /'.7~ 1 1 .325 / 1.733 C::;:;::::-cs
• 4V
c. 11 1.337
Cs 1 .437' 1 .450 c,-c2 1.610// 11 1.357
p /. C3
1 .7~C, 1.479
7V
1.337 c-' , / ' s 1 766' / 1 748 . P .
1.335 Cs c6
1.451 / 1.446 c,-c2
1.592// \\ 1.367 p c3
1 .7~ C;1.477
BV
5V 6V
1.444 C 1.437C 11 c, - C2
1596//' ~\1369 C 1.59}! \\ 1.357 C 1 440~ • P C3
p~ /. 3-C 1.337 ~ ~ 489 1 785 c, 1 487 s 1.814 f' .
Scheme 2--The designati on o f bond leng ths and the na mes for transition states and products of methyl and viny l substituted I H-. 2/-1 - and 3/-1 -phospho les.
obtained when the substituents are disposed away from the diene. Therefore, a total of 36 reactions, 16 each for methyl and vi nyl, in addition to 4 for the parent, were studied and all the transi tion states and products were obtained. The 6n-electron delocali zation at the transition state structure was an important feature for the Diels-Alde r reactions. The bond lengths r I, r2, r3 and r7 are the ones to be checked to ascertain the extent of de localization at the transition state structures. The geometric parameters depicted in Tables 2 and 3 indicate that the transition states benefit from the 6n-electron delocalization typical for the Diels-Alder reactions.
FMO al/.alysis The energies of frontier molecular orbitals i.e.,
HOMO and LUMO of all the dienes considered in this study, the HOMO-LUMO and LUMO-HOMO energy gaps between the diene and ethylene along with the quantum of charge transfer values from diene to dienophile at the transiti on state (qc r) arc given in Tables 4 and 5. The normal electro;, demand is the
GEETHA et al.: SEM IEMPIRI CAL AM I STUDY OF PHOSPHOLES
Table 2-The important optimi zed bond lengths (i n A) of the transitio n states (TS ) and products (PI') in the Die ls-A lder reac tio ns of die nes 1, 2, 3, 4M- 14M with e thylene obtained at AM I level. The values given in parentheses are for the aI/I i fo rm
Structure
loTS
I-Pr
2-TS
2-Pr
3-TS
3-Pr
4M-TS
4M-Pr
SM-TS
SM-Pr
6M-TS
6M-Pr
7M-TS
7M-Pr
8M-TS
8M-Pr
9M-TS
9M-Pr
lOM-TS
lOM-Pr
llM-TS
llM-Pr
12M-TS
12M-Pr
13M-TS
13M-Pr
14M-TS
14M-Pr
rl
1.397
( 10402)
1.500
( 1.500)
1.388
1.5 13
1.388
10498
1.396
( 1.404)
10498
( 1.500)
1.400
( l AOS)
1.499
( 1.500)
1.395
( 10400)
10498
( 1.499)
1.386
1.510
1.392
1.51 8
1.394
1.517
1.385
( 1.387)
1.512
(1.513)
1.394
1.504
1.389
1.502
1.386
( 1.389)
10497
( 10499)
1.394
10497
r2
10406
( 1.406)
1.352
( 1.353)
1.406
1.340
1.660
1.586
10408
( 10406)
1.352
( 1.354)
10404
( 10402)
1.351
( 1.352)
10411
( 104 10)
1.356
( 1.357)
10411
1.344
10410
1.344
1.401
1.339
10408
( 10407)
1.339
( 1.340)
1.671
1.597
1.659
1.585
1.662
( 1.662)
1.586
( 1.587)
1.651
1.584
r3
1.640
1.769
1.674
1.823
1.396
( 10403)
I Al)X
( 1.500)
10400
( 1.406)
1.504
( 1.504)
1.402
( 1.407)
1.505
( 1.505)
1.652
1.783
1.638
1.765
1.645
1.768
1.636
( 1.639)
1.766
( 1.768)
1.671
1.820
1.678
1.822
1.672
( 1.675)
1.822
( 1.824)
1.686
1.838
r4
1.772
( 1.767)
1.836
( 1.836)
1.797
1.8 16
10498
1.534
1.778
( 1.768)
1.843
( 1. 845)
1.78 1
( 1.788)
1.85 1
(1.85 1)
1.770
( 1.765)
1.834
( 1.835)
1.794
1.8 14
1.796
1.8 16
1.795
1.8 14
1.8 17
( 1.8 15)
1.834
( 1.835)
10497
1.534
10499
1.533
1.506
( 1.502)
1.541
( 1.54 1 )
1.501
1.539
r5
1.48 1
1.53 1
1.506
1.557
1.778
( 1.771)
1.843
( 1.845 )
1.772
( 1.767)
1.832
( 1.833)
1.771
( 1.766)
1.835
( 1.836)
1048 1
1.530
10480
1.530
10485
1.536
10491
( 1.485)
1.538
( 1.538)
1.504
1.555
1.509
1.562
1.51 4
( 1.5 11 )
1.564
( 1.565)
1.506
1.555
r6
2.1 04
(2 .110)
1.530
( 1.528)
2.365
1.543
2. 188
1.550
2.104
(2. 11 4)
1.53 1
( 1.526)
2.055
(2.064)
1.530
( 1.527)
2.117
(2. 122)
1.530
( 1.528)
2.366
1.543
2.360
1.543
2.384
1.549
2.367
(2.393)
1.543
( 1.542)
2.1 77
1.549
2.246
1.555
2.1 9 1
(2. 195)
1.551
( 1.549)
2. 126
1.549
1'7
1.390
( 1.388)
1.539
( 1.538)
1.367
1.5 17
1.389
1.540
1.39 1
( 1.387)
1.539
( 1.536)
1.391
( 1.389)
1.537
( 1.536)
1.390
( 1.388)
1.538
( 1.537)
1.367
1.517
1.367
1.5 17
1.369
1.516
1.366
( 1.364)
1.518
( 1.517)
1.388
1.540
1.388
1.539
1.388
( 1.386)
1.54 1
( 1.539)
1.390
1.539
r8
2.206
1.8 15
2.040
1.532
2.1 04
(2. 11 8)
1.531
( 1.526)
2.1 55
(2. 156)
1.536
( 1.534)
2.093
(2. 100)
1.530
(1.528)
2. 199
1.8 13
2.207
1.814
2. 175
1.813
2.209
(2.222)
1. 8 14
( 1.8 12)
2.051
1.532
1.999
1.532
2.042
(2.055)
1.534
( 1.53 1)
2.089
1.538
15
flow of electron from diene to dienophile and inverse electron demand is the flow of electron fro m dienophile to diene. Except for 1, 4M, SM and 6M, where the quantum of charge transfer at the antitransition state is predicted to be a positive value, a ll
other IH-, 2H- and 3H-phospoles and their methyl and vinyl substituted analogs follow in verse electron demand i.e., the quantum of charge transfer from diene to dienophile at the transition state are In agreement with the electron demand .
16 INDIAN J CHEM, SEC A, JANUARY 2003
Table 3-The optimized bond lengths (in A) of the transiti on states (TS) and product~. (Pr) of the Diels-Alder reacti ons of the dienes 4V-14V, with ethylene, obtained at AM I level. Values given in parentheses correspond to the allti form
Structure r I r2 r3 r4 r5 r6 r7 r8
4V-TS
4V-Pr
SV-TS
SV-PI"
6V-TS
6V-Pr
7V-TS
7V-Pr
8V-TS
8V-PI"
9V-TS
9V-Pr
lOV-TS
IOV-Pr
llV-TS
llV-Pr
12V-TS
12V-Pr
l3V-TS
l3V-Pr
14V-TS
14V-Pr
1.395
( 1.402)
1.498
( 1.50 I)
1.402
( 1.408)
1.498
( 1.499)
1.392
( 1.398)
1.497
( 1.497)
1.383
1.509
1.396
1.520
1.398
1.5 19
1.385
( 1.386)
1.51 2
( 1.513)
1.398
1.504
1.391
1.50'2
1.386
( 1.387)
1.497
( 1.498)
1.399
1.497
1.408
( 1.407)
1.352
( 1.354)
1.402
(1.40 I )
1.351
( 1.352)
1.415
(1.414)
1.361
(1.361 )
1.415
1.348
1.41 2
1.348
1.399
1.339
1.407
( 1.407)
1.339
(1.340)
1.674
1.604
1.658
1.586
1.66 1
( 1.661)
1.586
(1.587)
1.647
1.584
1.395
( 1.402)
1.498
(1.501)
1.402
( 1.408)
1.504
( 1.505)
1.407
( 1.41 2)
1.507
( 1.507)
1.656
1.783
1.637
i.764
1.646
1.766
1.636
( 1.637)
1.764
( 1.768)
1.668
1.81 6
1.680
1.820
1.670
( 1.674)
1.822
( 1.824 )
1.689
1.846
1.779
( 1.773)
1.845
( 1.847)
1.780
( 1.776)
1.858
( 1.859)
1.771
( 1.766)
1.833
( 1.835)
1.793
1.8 14
1.795
1.8 16
1.793
1.813
1.823
( 1.817)
1.840
( 1.838)
1.496
1.534
1.498
1.532
1.510
( 1.505)
1.542
( 1.544)
1.502
1.541
1.78 1
( 1.772)
1.845
( 1.847)
1.774
( 1.769)
1.83 1
(1.831)
1.769
( 1.764)
1.836
( 1.836)
1.480
1.530
1.479
1.530
1.488
1.539
1.491
( 1.486)
1.537
( 1.538)
1.505
1.556
1.5 11
1.566
1.515
(1.514)
1.566
( 1.568)
1.507
1.554
2.101
(2. 115)
1.531
(1.525)
2.028
(2.036)
1.530
(1 .527)
2. 139
(2. 137)
1.531
(1.528)
2.386
1.543
2.360
1.543
2.404
1.554
2.365
(2.389)
1.544
( 1.542)
2.162
1.549
2.278
1.560
2. 193
(2.194)
1.550
( 1.548)
2.080
1.549
1.391
( 1.387)
1.539
( 1.536)
1.391
( 1.389)
1.537
( 1.535)
1.390
( 1.388)
1.538
( 1.537)
1.367
1.517
1.366
1.517
1.369
1.516
1.367
( 1.364)
1.517
(1.516)
1.389
1.540
1.388
1.538
1.388
( 1.387)
1.541
( 1.539)
1.392
1.539
2.106
(2. 115)
1.531
(1 .525)
2. 191
(2. 188)
1.539
( 1.536)
2.077
(2.089)
1.530
( 1.528)
2.192
1.8 13
2.2 12
1.8 14
2. 166
1.8 13
2.207
(2.2 11 )
1.8 14
( 1.811)
2.063
1.532
1.984
1.53 1
2.043
(2.047)
1.533
( 1.530)
2.126
1.538
Energetics Lower the activation energies in the reaction of
dienes, more will be the product stability, where an aromatic ring is generated in the product formed. The activation and reaction energies for all the reactions considered in this study are given in Tables 4 and 5. The energetics corresponding to the parent and methyl substituted phospoholes are given in Table 4. Clearly, the 2H-phospholes are the most reactive towards Diels-Alder reactions and also are the most exothermic. This broad feature is unchanged as a function of methyl substitution and the quantitative di fferences are also not very significant. For example,
the maximum deviation due to methyl substitution is 2.4 kcallmol in activation energies and 4.2 kcallmol in reaction energIes. Similar IS the case for vinyl substitution. The reactivity ordering is 2H-phospholes > 3H-phospholes > 1 H-phospholes. The dienes that are aromatic experiences high acti vation energies compared to the less aromatic and non aromatic dienes.
Conclusions The present study presents a detailed analysis of the
cycloaddition reactions of the methyl and vinyl substituted five membered heterocyclic 1 H-, 2H- ,
GEETHA el al. : SEMIEMPIRICAL AM I STUDY OF PHOSPHOLES 17
Table 4-The fronti er molecular orbital energies (EHOMO and ELUMO in eV) of dienes 1-3 and 4M- 14M and the FMO energy diffe rence between the dienes and dienophile (EN and EI in eV) along with the quantum of charge transfer from diene to di enophile at the TS (qCT). All values are obtained at AM I level
Diene EIIOMO
- 9.70
2 - 9.24 3 - 9.54
4M - 9.59
SM - 9.36
6M - 9.52
7M - 9.05 8M - 9. 13 9M -8.99
10M - 9.2 1
11M - 9.43 12M - 9.3 1
13M - 9.52
14M - 9. 18
- 0. 11
- 0.66 - 0.40
- 0.02
- 0.09
-0.07
- 0.62 - 0.61 - 0.65
-0.63
-0.35 - 0.38
- 0.38
- 0.42
11.14
10.68 10.98
11.03
10.80
10.96
10.49 10.57 10.43
10.65
10.87 10.75
10.96
10.62
(a) EN = EHOMO (diene) - ELUMO (ethylene). (b) EI = ELUMO (diene) - EIIOMO (ethylene).
(c) The quantum of charge transfer values from diene to di enophile at the wlli-TS.
Table 5-The fronti er orbital energies (EIIOMO and ELUMO in eV) of di enes 4V - 14V and the FMO energy gap I 1\\ ,·l·n the dienes and ethylene (EN and EI in eV) along with th t: qU;lIl1um of charge transfer from diene to dienophile at the TS (qed. All the values are obtained at AM I level
Diene EIIOMO ELUMO
4V
SV
6V
7V 8V 9V
lOY
lIV 12V
13V
14V
- 9.62
- 9.02
- 9.23
- 8.84 - 9.00 - 8.76
- 9.21
- 9.18 - 9.05
- 9.55
- 8.79
-0.05
- 0.46
- 0.36
- 0.92 - 0.72 - 0.91
- 0.67
- 0.45 - 0.58
- 0.42
- 0.79
11.06
10.46
10.67
10.28 10.44 10.20
10.65
10.62 10.49
10.99
10.23
10.50
10.09
10.19
9.63 9.83 9.64
9.88
10. 10 9.97
10.13
9.76
(a) E = EIIOMO (diene) - ELUMO (ethylene). (b) EI = ELUMO (diene) - EIIOMO (ethy lene). (c) The quantum of charge transfer values dienophile at the al/li-TS .
Table 6-Acti vation energy (!'lEt ) and reaction energy (!'lE,) obtained for the Diels-A lder reactions of I H-. 2H- and 3Hphospholes and various methyl substituted phospholes with ethylene at AM I level. The values given 111 parenthesis correspond to the aI/Ii form. All values are given in keal mol- I
Table 7-Aetivati on energy (!'lEt ) and reaction energy (!'lE,) obtained for the Diels- Alder reactions of vinyl substituted I H-. 2H- and 3H-phospohles with e thylene at AM I level. The va lues given in parenthesis correspond to the (II/Ii form. All values are in keal mol- I.
3H-phospholes with ethylene at AM I level. An examination of the geometries indicates that all the methyl and vinyl substituted dienes confirm to [4+2] cycloaddition reaction with ethylene. FMO analysis predicts that methyl and vinyl substituted analogs follow inverse electron demand. Exception IS
observed for dienes 1, 4M, SM and 6M, where the quantum of charge transfer values (qc-r) at the anti transition states are found to be positive. The dienes that are aromatic experience less delocalized transition state and possess high activation energy and those that correspond to non-aromatic diene di splay relatively higher 6rt-electron delocalization at the transition state, possessing low activation energy and more exothermicity. In parent, methyl and vinyl substituted I H-, 2H- and 3H-phospholes, 2Hphospholes are found to be highly reactive which
18 INDIAN J CHEM, SEC A, JANUARY 2003
have lower activation energy compared to I H- and 3 H -phospholes.
Acknowledgement UGC is thanked for support in the form of SAP
DRS program to the department. SUB-D1C and its staff are thanked for extending computational facilities . T.c. Dinadayalane is thanked for useful discussions.
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