Alkynylzinc derivatives

The 7r-type endo-cyclization of metal acetylides (425) to form strained cyclohexynes (426) has been observed556 for the first time in the reaction of alkynylzincates derived from 5-hexynyl tosylates. 

Diynes. A general method for preparation of 1,3-diynes, particularly terminal ones, involves palladium-catalyzed coupling of alkynylzinc derivatives with (E)-l-iodo-2-chloroethylene. This alkene is obtained in 83% yield by reaction of acetylene with iodine monochloride in 6 A HCI. Coupling results in a l -chloro-1,3-cnync (1), which is converted into a l-sodio-l,3-diyne (2), which in turn can be reduced or alkylated to give a 1,3-diyne. 

An alternative method involves coupling the alkynylzinc derivative with vinyl bromide to give a l-en-3-yne, which is converted into a terminal 1,3-diyne selectively by brom-ination of the double bond followed by debromination (equation I). 

A general and convenient route to conjugated diynes, especially terminal diynes from the corresponding chloroenynes, has been developed [21]. Thus, readily available ( )-iodochloroethylene was first reacted with alkynylzinc derivatives in the presence of a Pd-phosphine complex catalyst to produce ( )-chloroenynes 30 in high yield., The chloroenynes 30 could then be readily converted into the corresponding terminal diynes 31 [Eq.(12)]. 

Additions of alkynylzinc compounds to nitrones possessing an easily recoverable mannose-derived chiral auxiliary of type 101 (equation 72) proceed with good steroeselectivity (93-98% dr). ... 

Similar 1,2-migration/cyclization is observed in the reaction of 5-hexynyl tosylate 92a with zincates (equation 44)61. In addition to the exo-cyclization leading to (alkyli-dene)cyclopentane 93a, endo-cyclization occurs competitively to give cyclohexene 94a, which is a major product in the reaction with PthZnl.i. In the reaction of the phenyl-substituted derivative 92b, two endo-cyclization products 94b and 94 (1 1) were obtained as well as exo-cyclization product 93b. A unified mechanism in which the -electrons of the alkynylzincate 95 participate in exo- and endo-cyclization (Scheme 8) has been proposed to rationalize these observations. The intermediacy of cyclohexyne 96 was verified... 

The zincation of the acetylenic sulfonate 35 furnishes an intermediate lithium alkynylzincate which undergoes a 1,2-migration of a methyl group, followed by a cyclization, leading to a cyclopropylidenylzinc derivative. This intermediate can then be quenched with an aldehyde, resulting in the allylic... 

Haloboration of terminal alkynes yielded stereodefined (Z)-2-halo-l-alkenylboron compounds e.g., 114, 115 that allowed further functionalization of the resulting C-Br or G-B bond. 2,2-Disubstituted-l-alkenylboronic esters were stereoselectively obtained when the haloboration was followed by cross-coupling with 1-alkynylzinc chlorides (Equation (18)).192 The iodoboration-conjugate addition sequence gave alkenyl iodides that were used as intermediate for the total synthesis of deoxyepothilone derivatives (Equation (19)).193-195... 

The use of organozinc substrates in coupling reactions is often supported by PdCl2(dppf). On the other hand, the use of the PPhj derivative does not always give the desired products [115]. This is best illustrated in the coupling between 1,2-dihaloalkenes and 1-alkynylzinc reagents (Scheme 1-7), in which Pd(PPh3)4... 

Alkynylzinc addition to ArCHO can also be carried out in the presence of 3, whereas the disilyl derivative 4 catalyzes enantioselective Reformatsky reaction on ketones (with ee up to 90%)" which operates by a ftee radical mechanism (requiring air to initiate the reaction). 

Methylmagnesium bromide modified by ZnCl2 reacts with aldehydes in the presence of the Ti alkoxide derived from 51. It leads to chiral 2-alkanols. A Ti(IV) complex of the C2-symmetric isophthalamide 52 and a diastereomer of 42 catalyze the addition of alkynylzinc reagents to aldehydes, whereas the polymer-linked hydroxy-imine 53 alone is used for the same purpose. ... 

---