It looks like what you have in mind could be implemented like this:
function det = myDet(A)
if isequal(size(A),[2 2])
det = A(1,1)*A(2,2)-A(1,2)*A(2,1);
else
det = 0;
top_row = A(1,:);
A(1,:) = [];
for i = 1:size(A,2)
A_i = A;
A_i(:,i) = [];
det = det+(-1)^(i+1)*top_row(i)*myDet(A_i);
end
end
end
But note that there is nothing special about the 2-by-2 case, so you could let the recursion go all the way down to the scalar case:
function det = myDet(A)
if isscalar(A)
det = A;
return
end
det = 0;
top_row = A(1,:);
A(1,:) = [];
for i = 1:size(A,2)
A_i = A;
A_i(:,i) = [];
det = det+(-1)^(i+1)*top_row(i)*myDet(A_i);
end
end
