SOME MECHANISTIC CONCEPTS IN ELECTROPHILIC ADDITION REACTIONS TO C=C BONDS

Transcription

1 SM MANISTI NPTS IN LTPILI AITIN ATINS T = BNS The = ond is considered to e wek se/nucleophile. The high concentrtion of electron density mkes the pi ond Lewis se, ut in order to donte electrons the pi ond must e roken first. In rection which is siclly the reverse of elimintion, n electrophile of generl formul - cn dd to the = doule ond. represents the electrophilic center, nd is the rest of the molecule, or nucleophilic prt. Proton cids such s l fit this description, with = nd = l. Wter cn lso e viewed s n electrophile with = nd =. In generlized representtion, the electrophile - dds to the pi ond, converting the sp 2 crons into sp 3 crons. = lectrophilic prt = Nucleophilic prt xmples: Addition of Br nd wter to the π-ond. Br Br 2 MAKVNIKV INTATIN vs. SN or ANTI AITIN The term Mrkovnikov orienttion refers to the onding preference of nd for cron toms or. The following exmple shows how proton cid cn dd to the π-ond of n unsymmetricl lkene with either Mrkovnikov or nti-mrkovnikov orienttion, depending on the rection conditions used. Mrkovnikov orienttion nti-mrkovnikov orienttion

2 The terms syn ddition nd nti ddition refer to the preference of nd to dd to the π-ond from the sme side of the = plne (syn ddition), or from opposite sides (nti ddition): syn ddition nti ddition The following exmple showing the ddition of wter to 1-methylcyclohexene (deuterium-leled) illustrtes the four possile comintions. 3 syn ddition with Mrkovnikov orienttion 3 nti ddition with Mrkovnikov orienttion 3 2 (4) (3) 3 syn ddition with nti-mrkovnikov orienttion 3 nti ddition with nti-mrkovnikov orienttion Two rections of interest to us, oxymercurtion-demercurtion nd hydroortion-oxidtion, follow pths nd (3) respectively.

3 STPWIS AITIN F LTPILS T π BNS WIT FMATIN F 3-MMB ING INTMIATS Sometimes the electrophilic prt of n electrophile - pproches the = π-ond to simultneously ond to oth crons nd form three memered ring intermedite with deprture of s leving group. In second step, cn ct s nucleophile to perform ckside ttck on one of the crons, prompting ring opening tht results in net nti ddition of - to the = ond. - nti ddition The oxymercurtion-demercurtion rection (ddition of wter in the presence of mercury slts), hlogen ddition, nd the nti-hydroxyltion (vi epoxides) rection re exmples of this outcome. NT AITIN F LTPILS T π BNS WIT FMATIN F 4-MMB ING TANSITIN STATS The electrophile - cn lso pproch the = π-ond in such wy tht oth nd ond to ech of the two crons simultnesouly. This requires the formtion of four-memered ring trnsition stte nd necessrily results in net syn ddition of - to the π-ond. - - trnsition stte syn ddition product The hydroortion-oxidtion (ddition of wter using vi orne) is n exmple of this outcome.

4 M N T MIST F 3-MMB INGS Three-memered rings such s epoxides (cyclic ethers) re strined nd susceptile to ring-opening. This cn hppen for exmple when nucleophile ttcks one of the ring crons, reking the - ond. The lkoxide group (oxygen with negtive chrge) is normlly not good leving group, ut the driving force for the rection is relese of ring strin. epoxide strong nucleophile Wek nucleophiles, which my not e strong enough to open the ring, cn e ssisted y cid ctlysis. Protontion of the oxygen with strong cid enhnces its leving group ility, nd it lso mkes the ring crons more electrophilic (see elow). 3 protonted epoxide 2 wek nucleophile 2 The positive chrge in the protonted epoxide is ctully shred y the oxygen nd the two crons. The resonnce structures show prtil ring opening with positive chrges on ech of the two crons. In n unsymmetricl epoxide, the gretest resonnce contriutor would e the one forming the most stle ction. Tht is, the one where the positive chrge is on the cron ering the most lkyl groups o ction, minor o ction, greter contriutor contriutor

5 The ttck of the nucleophile on the protonted epoxide will therefore tke plce on the cron tht crries the greter mount of positive chrge, even though it is seemingly more stericlly hindered. To visulize this, rememer tht this is not norml sp 3 cron with n idel ond ngle is degrees. Forcing the internl ring ngle to theoreticl vlue of 60 degrees in the epoxide forces the externl ngle to open up. Also, the resonnce structures of the protonted epoxide show tht such crons hve sustntil sp 2 chrcter nd therefore re more plnr thn regulr sp 3 cron preferred ttck preferred product The product then loses proton (in n cid-se rection with wter, for exmple) nd forms vicinl dilcohol, or vic diol. The following exmple shows formtion of the trns diol tht results from ckside ttck of wter on the epoxide. 3 trns diol Severl of the rections from chpter 8 will further illustrte different spects of the chemistry of three memered rings to explin the oserved products.